Patent Publication Number: US-10314106-B2

Title: Method and apparatus for channel access in wireless communication system using unlicensed spectrum

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation application of prior application Ser. No. 15/233,215, filed on Aug. 10, 2016, which claimed priority under 35 U.S.C. § 119(e) of a U.S. Provisional application No. 62/204,694, filed on Aug. 13, 2015, in the U.S. Patent and Trademark Office and U.S. Provisional application No. 62/240,270, filed on Oct. 12, 2015, in the U.S. Patent and Trademark Office, the entire disclosure of each of which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a channel access method and apparatus for sensing and occupying an unlicensed band channel in a mobile communication system operating in an unlicensed band. 
     BACKGROUND 
     In order to meet the increasing wireless data traffic demand since the commercialization of 4th generation (4G) communication systems, the development focus is on the 5 th  generation (5G) or pre-5G communication system. For this reason, the 5G or pre-5G communication system is called beyond 4G network communication system or post long term evolution (LTE) system. 
     In order to accomplish high data rates, consideration is being given to implementing the 5G communication system on the millimeter Wave (mm Wave) band (e.g., 60 GHz band). In order to mitigate propagation loss and increase propagation distance, the 5G communication system is likely to accommodate various techniques such as beamforming, massive multiple input multiple output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna. 
     Also, for throughput enhancement of the 5G communication system, research is being conducted on various techniques such as small cell, advanced small cell, cloud radio access network (cloud RAN), ultra-dense network, device to device (D2D) Communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), and interference cancellation. 
     Furthermore, the ongoing research includes the use of hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation and sliding window superposition coding (SWSC) as advanced coding modulation (ACM), filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA). Meanwhile, there is a need of a method for configuring a contention window (CW) for channel sensing to use an unlicensed band. 
     The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
     SUMMARY 
     Aspects of the present disclosure are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present disclosure is to provide a method and apparatus for occupying a channel in the unlicensed band. 
     In accordance with an aspect of the present disclosure, a method of a base station is provided for use in a wireless communication system. The method includes receiving a feedback corresponding to data transmitted in a first subframe arriving after a first sensing period in an unlicensed band from at least one terminal, checking data reception state of the at least one terminal as one of acknowledgement (ACK), negative acknowledgement (NACK), and discontinuous transmission (DTX) based on the feedback, and determining a length of a second sensing period based on the data reception state of the at least one terminal. 
     In accordance with another aspect of the present disclosure, a base station of a wireless communication system is provided. The base station includes a transceiver for transmitting/receiving signals and at least one processor configured to control for receiving, by the transceiver, feedback corresponding to data transmitted in a first subframe arriving after a first sensing period in an unlicensed band from at least one terminal, checking data reception state of the at least one terminal as one of ACK, NACK, and DTX based on the feedback, and determining a length of a second sensing period based on the data reception state of the at least one terminal. 
     Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses various embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  are diagram illustrating a communication system to which the present disclosure is applied according to various embodiments of the present disclosure; 
         FIG. 2  is a diagram illustrating a channel occupancy operation according to a channel operation according to various embodiments of the present disclosure; 
         FIG. 3  is a diagram illustrating an unlicensed band channel access scheme of a wireless fidelity (Wi-Fi) system according to various embodiments of the present disclosure; 
         FIG. 4  is a flowchart illustrating an unlicensed channel access scheme of a licensed assisted access (LAA) system according to various embodiments of the present disclosure; 
         FIG. 5  is a diagram illustrating a contention period configuration method according to various embodiments of the present disclosure; 
         FIG. 6  is a diagram illustrating a method for applying a changed contention period according to various embodiments of the present disclosure; 
         FIG. 7  is a diagram illustrating a contention period configuration method according to various embodiments of the present disclosure; 
         FIG. 8  is a flowchart illustrating a contention period configuration method of an (evolved node B) eNB for channel sensing operation according to various embodiments of the present disclosure; 
         FIG. 9  is a flowchart illustrating a contention period configuration method of a user equipment (UE) for channel sensing operation according to various embodiments of the present disclosure; 
         FIG. 10  is a diagram illustrating an eNB apparatus according to various embodiments of the present disclosure; 
         FIG. 11  is a diagram illustrating a UE apparatus according to various embodiments of the present disclosure; and 
         FIG. 12  is a flowchart illustrating a method of an eNB according to various embodiments of the present disclosure. 
     
    
    
     Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures. 
     DETAILED DESCRIPTION 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     Although the description is directed to the long term evolution (LTE) and LTE-advanced (LTE-A) systems, the present disclosure can be applied to other communication systems operating on both the licensed and unlicensed bands with or without modification. 
     The mobile communication system has evolved to a high-speed, high-quality wireless packet data communication system capable of providing data and multimedia services beyond the early voice-oriented services. in order to support the high speed, high quality wireless packet data transmission service, various mobile communication standards (such as high speed downlink packet access (HSDPA), high speed uplink packet access (HSUPA), long term evolution (LTE), and LTE-A of 3rd generation partnership project (3GPP), high rate packet data (HRPD) of 3rd generation partnership project-2 (3GPP2), and 802.16 of Institute of Electrical and Electronics Engineers (IEEE)) have been developed. Particularly, the development of LTE/LTE-A (hereinafter, referred to as LTE) continues at a considerable pace to improve system throughput and frequency efficiency. The LTE system is capable of increasing data rate and system throughput with carrier aggregation (CA) which makes it possible to use multiple frequency bands. Typically, a current LTE system operates in a licensed band (licensed spectrum or licensed carrier) of which usage right belongs to the network operator. In the case of the frequency bands (e.g. frequency bands below 5 GHz), it has already been occupied by other operators or other communication systems, it is difficult for an operator to secure multiple licensed frequency bands and this makes it difficult to extend the system throughput with the CA technology. In this respect, recent researches are conducted on the technologies of using unlicensed band (unlicensed spectrum or unlicensed carrier) to process the significantly increasing mobile data in the licensed frequency spectrum shortage environment (e.g. LTE in unlicensed (LTE-U) and licensed assisted access (LAA)). Particularly, since the 5 GHz band is an unlicensed band in which relatively small number of communication devices operate in comparison to another unlicensed band of 2.4 GHz and has very broad bandwidth, it is relatively easy to secure additional frequency band. That is, the LTE technology for aggregating multiple frequency bands, i.e. the CA technology, can be used to aggregate licensed and unlicensed band frequencies. This makes it possible to configure a licensed band LTE cell as the primary cell (PCell) and an unlicensed band LTE cell (LAA cell or LTE-U cell) as a secondary cell (SCell) such that the LTE system operates in both the licensed and unlicensed band using the legacy CA technology. Although such a system can be applied to a dual connectivity environment in which the licensed and unlicensed band cells are connected to each other through a non-ideal backhaul as well as the CA environment in which the licensed and unlicensed band cells are connected to each other through an ideal backhaul, the present disclosure is directed to the CA environment in which the licensed and unlicensed band cells are connected to each other through the ideal backhaul. 
     Typically, the LTE/LTE-A system uses orthogonal frequency division multiple access (OFDM) for data transmission. In the OFDM scheme, the modulation signal is positioned in a 2-dimensional space represented by time-frequency grid. The time domain resource is divided into OFDM symbols that are orthogonal. The frequency domain resource is divided into subcarriers that are orthogonal. Accordingly, in the OFDM scheme, it is possible to indicate a smallest unit resource by indicating an OFDM symbol on the time axis and a subcarrier on the frequency axis, and the smallest resource unit is referred to as resource element (RE). Since the REs are orthogonal even after the signal has passed a frequency selective channel, the signals transmitted through different REs can be received by a receiver without causing interference with each other. In the OFDM communication system, a downlink (DL) bandwidth is comprised of a plurality of resource blocks (RBs), and each physical resource block (PRB) consists of 12 consecutive subcarriers on the frequency axis and 14 or 12 consecutive OFDM symbols on the time axis. Here, the PRB is a basic unit of resource allocation 
     The reference signal (RS) is the signal transmitted by the eNB in order for the user equipment (UE) to estimate channels, and the LTE system has common reference signal (CRS) and demodulation reference signal (DMRS) as a dedicated reference signal. The CRS is the reference signal transmitted across the whole DL band such that all UEs can receive for use in configuring feedback information or demodulating the control and data channels. The DMRS is also the reference signal transmitted across the whole DL band such that a specific UE can receive for use in data channel demodulation and channel estimation but not for use in configuring feedback information. Accordingly, the DMRS is transmitted on the PRB in which the UE is scheduled. 
     In the time domain, a subframe consists of two slots, i.e. first and second slots, of which each spans 0.5 ms. The physical downlink control channel (PDCCH) is transmitted in a control channel region, and the enhanced PDCCH (ePDCCH) is transmitted in a data channel region, the two regions being separated on the time axis. This aims to receive and modulate the control channel signal quickly. The PDCCH regions are spread across the whole bandwidth of the downlink (DL) band in such a way that a control channel is divided into small controls channels which are distributed across the whole bandwidth of the DL band. The uplink (UL) is divided into a control channel (PUCCH) and a data channel (PUSCH), and the acknowledgement (ACK) channel corresponding to the DL link data channel and other feedback information are transmitted through the control channel if there is no data channel and, otherwise if there is a data channel, through the data channel. 
       FIGS. 1A and 1B  are diagrams illustrating communication systems to which the present disclosure is applied according to various embodiments of the present disclosure. 
     Referring to  FIGS. 1A and 1B ,  FIG. 1A  is directed to a case where an LTE cell  102  and a license assisted access (LAA) cell  103  coexist in the coverage of a small eNB  101  in a network and the UE  104  (or UE  114  in  FIG. 1B ) communicates data with the eNB through the LTE cell  102  and the LAA cell  103 . In this case, the LTE cell  102  and LAA cell  103  are not limited to any duplex mode, and it may be assumed that the LTE cell  102  as the PCell performs data communication through the licensed band and the LAA cell  130  as the SCell performs data communication through the unlicensed band. However, if the LTE cell is the PCell, uplink (UL) data transmission is permitted only through the LTE cell  102 . 
       FIG. 1B  is directed to a case where an LTE macro eNB  111  for securing a large coverage and a small eNB  112  for increasing data transmission amount are installed and, in this case, the LTE cell  116  and LAA cell  115  are not limited to any duplex mode. At this time, the LTE macro eNB or LTE eNB  111  may be replaced by an LTE small eNB. If the LTE eNB has the PCell, it may be configured that the uplink (UL) transmission is performed only through the LTE eNB  111 . At this time, it is assumed that the LTE eNB  111  and LAA eNB  112  are connected through an ideal backhaul network. Accordingly, the X2 interface  113  allows for fast communication between the eNBs and, although UL transmission is available to only the LTE eNB  111 , it is possible for the LAA eNB  112  to receive the relevant control information from the LTE eNB  111  through the X2 interface  113  in real time. The methods proposed in the present disclosure can be applied to both the systems configured as shown in  FIGS. 1A and 1B . 
     Typically, an unlicensed frequency band or channel is shared by a plurality of devices. At this time, the devices using the unlicensed band may be different system devices. In order for the various devices to operate in the unlicensed band, they follow normal operations as follows. 
     A transmitting device which wants to transmit a signal including data or control information checks whether the unlicensed band or channel is occupied by other devices before attempting to transmit the signal and may occupy the channel or not depending on the channel occupancy state. Such an operation is referred to as listen-before-talk (LBT). That is, the transmitting device has to check whether the corresponding channel is preoccupied according to a predetermined or preconfigured method. The channel sensing method may be predefined or preconfigured. The channel sensing time period may be predefined, preconfigured, or set to a value selected in a predetermined range. The channel sensing time period may be configured in proportion to a predetermined maximum channel occupancy time. At this time, the channel sensing operation for checking whether the channel can be occupied may be configured differently depending on the unlicensed band frequency, area, and country regulation. For example, it is allowed to use the 5 GHz frequency band without any channel sensing operation with the exception of the radar detection operation in the US. 
     A transmitting device which wants to use the unlicensed band senses the corresponding channel through a channel sensing operation (or LBT) to determine whether the channel is occupied by other devices and, if the channel is not preoccupied by any device, it occupies the channel to use. At this time, the device using unlicensed band predefines or configures the maximum channel occupancy time after the channel sensing operation. At this time, the maximum occupancy time may be predefined according to the regulation stipulated per frequency band and area or configured by another device, e.g. the eNB. At this time, the channel occupancy time may be configured differently depending on the unlicensed band, area, or country regulation. For example, the maximum occupancy time for the 5 GHz band unlicensed spectrum is 4 ms in Japan. In Europe, however, the channel can be occupied continuously up to 10 ms or 13 ms. At this time, the devices occupied the maximum occupancy time may perform the channel sensing operation again to re-occupy the channel according to the channel sensing result. 
     A description is made of the unlicensed band channel sensing and occupancy operation hereinafter with reference to  FIG. 2 . 
       FIG. 2  is diagram illustrating DL data and control signal transmission from an eNB to an UE, and this may be applicable to UL signal transmission from the UE to the eNB according to various embodiments of the present disclosure. 
     Referring to  FIG. 2 , an LTE subframe  200  has a length of 1 ms and consists of a plurality of OFDM symbols. The eNB and UE that are capable of communicating on an unlicensed band may occupy the corresponding channel during a predetermined channel occupancy time (or transmission opportunity (TXOP))  250  and  260  for communication. If the eNB which occupied the channel during the channel occupancy time  250  needs further channel occupancy, it performs channel occupancy operation, e g channel sensing period  220 , and occupies the channel or not depending on the channel sensing operation result. At this time, the channel sensing period (or length) may be predefined or configured to the UE by the eNB through higher layer signaling, or may be configured depending on the result of data transmission/reception on the unlicensed band. 
     Also, at least one of the variables for performing the channel sensing operation again may be set to a value different from that of the previous channel sensing operation. 
     The channel sensing and occupancy operations may be configured differently depending on the frequency band, area, or country regulation. A description is made of the channel sensing and occupancy hereinafter in detail operation with an example of Load-based equipment as one of the channel access schemes in the 5 GHz band regulation EN301 893 of Europe. 
     If the eNB need to use the channel additionally after the maximum channel occupancy time  250 , it has to determine whether the channel is occupied by other devices during the minimum channel sensing period  220 . At this time, the minimum channel sensing period  220  may be determined depending on the maximum channel occupancy period as follows. 
     maximum channel occupancy period: 13/32×q, (q=4, . . . , 32) 
     minimum channel sensing period: ECCA slot length×rand (1, q) 
     Here, the ECCA slot length is a smallest channel sensing period unit (or length) predefined or configured. That is, if q=32, the transmitting device may occupy the unlicensed band during up to 13 ms. At this time, the minimum required channel sensing period is selected randomly in the range of 1˜q (i.e., 1˜32), and the total channel sensing period becomes ECCA slot length x selected random value. Accordingly, if the maximum channel occupancy period increases, the minimum channel sensing period increases too. The maximum channel occupancy period and minimum channel sensing period are just examples and may be configured differently depending on the frequency band, area, and country regulation, and changed according to the revision of the frequency regulation in the future. Also, additional operations (e.g. introduction of additional channel sensing period) may be included in addition to the channel sensing operation according to the frequency regulation. 
     If it is determined that the unlicensed band is not occupied by other devices during the channel sensing period  220 , i.e. if the channel is in the idle state, the eNB may occupy the channel immediately. At this time, whether the channel is occupied by other devices during the channel sensing period  220  may be determined based on a predefined or preconfigured reference value. For example, if the signal strength received from other devices during the channel sensing period is greater than a predetermined threshold value (e.g. −62 dBm), it may be determined that the channel is occupied by other devices. If the received signal strength is equal to or less than the threshold value, it may be determined that the channel is in the idle state. At this time, the channel occupancy determination may be performed in various ways such as detecting a predefined signal with the received signal strength as described above. 
     The normal LTE operation is performed in unit of subframe (e.g. signal transmission/reception starts from the first OFDM symbol of a subframe), it may fail to transmit or receive a signal transmitted at an arbitrary OFDM symbol. Accordingly, the eNB which has detected the idle channel during the channel sensing period  220  in the subframe may transmit a specific signal for channel occupancy from time when the channel sensing period  220  ends till right before the first OFDM symbol of the next subframe, i.e. the duration  230 . That is, the eNB may transmit the second signal (e.g. primary synchronization signal(s) (PSS)/secondary synchronization signal(s) (SSS)/cell-specific reference signal(s) CRS and newly defined signal) for the unlicensed band channel occupancy and UE synchronization before transmitting the first signal (e.g. legacy enhanced physical downlink control channel (EPDCCH) and physical downlink shared channel (PDSCH)) in the subframe  210  or  240 . At this time, at least one of the second signals may not be transmitted depending on the channel sensing period end time. If the corresponding channel occupancy start time is set before a specific OFDM symbol, it is possible to transmit the second signal or the first signal after transmitting the third signal (newly defined signal), before the start of the next OFDM symbol. Although the channel sensing operation period is described using the unit of OFDM symbol for explanation convenience in the present disclosure, the channel sensing operation period may be configured regardless of the OFDM symbol of the LTE system. 
     Here, the second signal may be generated by reusing the PSS/SSS used in the legacy LTE system or at least one of the PSS and SSS generated with a sequence different from the root sequence used in the licensed band. The second signal may also be generated with a sequence with the exception of the PSS/SSS sequence necessary for generating unlicensed band eNB identifier (physical cell identifier (PCID)) so as not to be confused with the unique value of the eNB. The second signal may also include at least one of the CRS and channel state information reference signal (CSI-RS) used in the legacy LTE system or a (E)PDCCH or PDCCH or signal modified therefrom. 
     At this time, since the second signal transmission duration  230  is included in the channel occupancy time, the frequency utilization efficiency can be maximized by delivering the least information in the second signal transmitted in the duration  230 . 
     The LTE system operating in an unlicensed band (LAA or LAA cell) has to use a new type of channel access scheme LBT different from the legacy licensed band channel access scheme to share the unlicensed band with other systems (e.g. Wi-Fi) as well as to meet the regulation requirement for use of the unlicensed band. A brief description is made of the channel access scheme of a Wi-Fi system for use of the unlicensed band hereinafter using  FIG. 3 . 
       FIG. 3  is a diagram illustrating an unlicensed band channel access scheme of a wireless fidelity (Wi-Fi) system according to various embodiments of the present disclosure. 
     Referring to  FIG. 3 , if a Wi-Fi application processor (AP)  310  has data to transmit to Station  1  (STA 1 ) or UE  1   315 , it performs a channel sensing operation to occupy the corresponding channel. At this time, the channel sensing operation is performed during a distributed coordination function (DCF) interframe space (DIFS)  330 . Whether the channel is occupied by other devices can be determined in various ways, e.g. measurement of received signal strength and detection of a predetermined signal in the corresponding duration. If it is determined that the channel is occupied by other device  320 , the AP 1   310  selects a random variable  335 , e.g. N, within a predetermined contention window (CW) (e.g. 1˜16). Typically, this operation is referred to as backoff operation. Afterward, the AP 1   310  senses the channel during a predetermined time period (e.g. 9 us) and, if it is determined that the channel is idle, decrements the random variable N  355  by 1. That is, N is updated (N=N−1). If it is determined that the channel is occupied by another device in the time period, the AP 1   310  freezes the variable N  355 . If the data  340  transmitted by the AP 2   320  is received, the STA 2   325  sends the AP 2   320  an acknowledgment (ACK)/negative acknowledgement (NACK)  347  corresponding to the data  340  after short interframe space (SIFS)  345 . At this time, the STA 2   325  may transmit the ACK/NACK  347  without channel sensing operation. After the STA 2   325  transmits ACK  347 , the AP 1   310  is aware that the channel is idle. If it is determined that the channel is idle during the DIFS  350 , the AP 1   310  performs channel sensing during the period (e.g. 9 us) predefined or preconfigured for backoff operation and, if it is determined that the channel is idle, decrements the variable N  355  again. That is, it updates N (N=N−1). At this time, if N=0, the AP 1   310  occupies the channel to transmit data  360  to the STA 1   315 . If the data  360  is received, the STA 1  transmits ACK/NACK acknowledging the receipt of the data to the AP 1   310  after SIFS. If NACK is received from the STA 1   315 , the AP 1   310  may select the variable N for use in the next backoff operation during the extended contention period. That is, assuming the previous contention period [1,16], if the NACK is received, the STA 1   315  may increase the contention period to [1,32]. If ACK is received, the AP 1   310  may initialize the contention period to the initial value (e.g. [1,16]) or decrease or maintain a preconfigured contention period. 
     In the case of a Wi-Fi system, however, one AP (or eNB) and one STA (or UE) communicate in the same period. As denoted by reference number  347  and  370 , the STA (or UE) transmits the data reception result (e.g. ACK or NACK) to the AP (or eNB) immediately upon receipt of the data. At this time, the AP  310  or  320  performs the channel sensing operation for next data transmission after receiving the ACK or NACK from the terminal  315  or  325 . In the case of the LAA system, however, one eNB may transmit data to multiple UEs simultaneously. If the data are received by the UEs at the same time point (e.g. time n), the UEs may transmit ACK or NACK to the eNB at the same time point (e.g. n+4 in frame structure type 1 (FDD)). Accordingly, the LAA eNB may receive the ACK/NACK transmitted by one or more UEs simultaneously unlike the Wi-Fi system. There may be at least 4 ms time difference between the ACK/NACK transmission time point of the UE and the data transmission time point of the eNB. Thus, if the LAA eNB configures (or reconfigures) the contention period based on the ACK/NACK received from the UE as in Wi-Fi, it may receive the ACK/NACK from multiple UEs at a certain time point, resulting in contention period configuration ambiguity. Also, the ACK/NACK transmission delay of the UE makes it ambiguous to determine the (re)configured contention period. The present disclosure proposes a method for an LAA eNB to configure a contention period clearly based on the ACK/NACK information transmitted by the UEs. 
     Although the description is directed to the CA environment for explanation convenience, the present disclosure is not limited thereto but can be applied to the dual-connectivity or stand-alone environment of the unlicensed band operation. 
       FIG. 4  is a flowchart illustrating an unlicensed channel access scheme of a LAA system according to various embodiments of the present disclosure. 
     Referring to  FIG. 4 , a description is made of the unlicensed band channel occupancy method in an LAA system hereinafter with reference to  FIG. 4 . The LAA cell (LAA SCell or LAA eNB) having no data to transmit stays in the idle state at operation  401 . At this time, the idle state is the state where the LAA cell transmits no data in the unlicensed band. For example, after the start state  400 , the idle state  401  is the state where the LAA cell has no data to transmit to the UE or, if it has data to transmit to the UE, suspends transmitting the data to the UE. If it is required for the LAA cell in the idle state to occupy a channel to transmit data or control signal to the UE at operation  402 , the LAA cell performs the first channel sensing operation at operation  403 . At this time, the first channel sensing operation may be configured differently depending on at least one of a time period (e.g. 34 us) preconfigured or configured by another device and type of data or control signal to be transmitted by the LAA cell. For example, in the case of transmitting only the control signal without data, the first channel sensing operation period may be set to a value different from that for the first channel sensing operation execution for transmitting data from the LAA cell to the UE (e.g. shorter than the first channel sensing operation period for the case of transmitting data). At this time, the first channel sensing operation configuration values may be predefined. At least one of the other parameters (e.g. received signal strength threshold for determining whether to sense the channel), as well as the first channel sensing operation period, may be configured differently for the case of transmitting only the control signal and for the case of transmitting data from the LAA cell to a specific UE. At this time, the LAA cell may set the contention period for the second channel sensing operation to an initial value. The first channel sensing operation is the operation of determining whether the corresponding channel is occupied by other devices using various methods including at least one of measuring received signal strength or detecting a predefined signal during the first channel sensing operation period. At this time, the first channel sensing operation-related parameters including the first channel sensing period may be preconfigured or configured by other devices. If it is determined that the channel is idle at operation  404 , the LAA cell occupies the channel to transmit signals at operation  405 . If it is determined that the channel is occupied by other devices at operation  404 , the LAA cell selects a random variable N in the configured contention period [x, y]. At this time, the initial contention period may be preconfigured or (re)configured by the eNB. The contention period may be configured based on various parameters including the number of channel occupancy times, channel occupancy rate (e.g. traffic load), and ACK about data transmitted during the channel occupancy (e.g. ACK/NACK). For example, if it is determined at operation  406  that further channel occupancy is necessary, the LAA may reset or configure the contention period based on the ACK about the data transmitted at operation  414  or at least one of the aforementioned methods. The contention period determination method based on the ACK about the data transmission result at operation  414  is just an example, and it is also possible to configure the contention period based on the value set in the previous channel occupancy and data transmission operation or a preconfigured value. For example, if a NACK is received from the UE after the LAA cell has transmitted data to the UE in the channel occupancy period, the LAA cell may increase or maintain the contention period. If an ACK is received from the UE after the LAA which has occupied the channel in the increased or maintained contention period transmits data, it may be possible to decrease or maintain the contention period or initialize the contention period. Here, the ACK/NACK-based contention period configuration method is just an example, and it is possible to configure the contention period using the aforementioned other criteria. 
     If the random variable N is configured in the preconfigured contention period at operation  407 , the LAA cell may perform the second channel sensing operation using the configured N at operation  408 . At this time, the second channel sensing operation is performed through at least one of received signal strength measurement and predetermined channel detection during the configured period based on the determination criteria different from those for the first channel sensing operation. That is, the second channel sensing operation reference time may be set to a value equal to or less than the first channel sensing time. For example, the first channel sensing time may be set to 34 us while the second channel sensing time is set to 9 us. The second channel sensing operation threshold may be set to a value different from the first channel sensing operation threshold value. 
     If it is determined that the channel sensed at operation  408  is idle at operation  409 , the LAA decreases N by 1 at operation  410 . At time, decrementing N by 1 is just an example, and it is possible to decrement N differently depending on the configuration value or to configure N differently depending on the type or characteristic of the signal. If the decremented N is 0 at operation  411 , the LAA cell occupies the channel to transmit data at operation  405 . Otherwise if N is not 0, the LAA cell performs the second channel sensing operation again at operation  408 . If it is determined that the channel sensed at operation  408  is not idle at operation  409 , the LAA cell performs the third channel sensing operation at operation  412 . At this time, the third channel sensing operation may be configured in the same manner as the first or second channel sensing operation. For example, the first and third channel sensing reference times may be set to 34 us identically. At this time, the first and second channel sensing thresholds may be set to different values. The channel sensing operation reference time and threshold value are just an example, and it is possible to set the third channel sensing operation-related parameters and criteria to values identical to or different from those for the first channel sensing operation. 
     The third channel sensing operation may be configured to incur time delay without channel sensing or channel occupancy operation. Also, the third channel sensing time may be configured to be identical with or different from at least one of the first and second channel sensing times. The LAA cell determines whether the channel is occupied by other devices based on the threshold configured for the third channel sensing operation. If it is determined that the channel is idle, the LAA cell may perform the second channel sensing operation again at operation  408 . If it is determined that the channel is not idle at operation  413 , the LAA cell performs the third channel sensing operation at operation  412 . At this time, at least one of the first to third channel sensing operations may be omitted depending on the type and characteristic of the data or control signal to be transmitted by the LAA cell. For example, in the case of transmitting a control signal (e.g. discovery reference signal (DRS), the LAA cell may perform the first channel sensing operation and occupy the channel immediately according the channel sensing operation result. At this time, omitting at least one of the first to third channel sensing operations in the case of transmitting the DRS is just an example, and it is possible to omit at least one of the first to third channel sensing operations for transmitting in the case of transmitting other control signals. 
     As described above, a contention period may be configured. The contention period may be configured based on at least one of the number of channel occupancy attempts, channel occupancy rate (e.g. traffic load), and ACK about data transmitted during the channel occupancy (e.g. ACK/NACK). In the case of configuring the contention period based on the signal reception ACK of the eNB or the UE, however, if one or more UEs transmit ACK/NACK simultaneously or if the LAA cell transmits ACK/NACK to one or more UEs simultaneously, it is ambiguous to configure the contention period. Thus the present disclosure proposes a method for configuring the contention period based on the ACK transmitted by the eNB/UE in correspondence to the data transmitted by the UE/eNB in the unlicensed band. 
     Embodiment 1 
       FIG. 5  is a diagram illustrating a contention period configuration method according to various embodiments of the present disclosure. 
     Referring to  FIG. 5 , descriptions are made of the DL channel sensing operation and the channel access method from the viewpoints of the LAA cell  505  and the UE 1   510  which is receiving data from the LAA cell  505  hereinafter with reference to  FIG. 5 . The LAA cell  505  performs channel sensing operation  520  to occupy the channel in the idle state. At this time, the channel sensing operation  520  may be performed using at least one of the operations at operations  403 ,  408 , and  413  of  FIG. 4 . At this time, the maximum occupancy time allowed for the channel is denoted by reference number  530 . The signal  525  may be transmitted until the start point of the subframe  515  according to the channel occupancy start time. Afterward, the LAA cell  505  may transmit data scheduled to the UEs in the channel occupancy duration  530 . From the viewpoint of UE 1   510 , it receives data in subframes n, n+1, n+3, and n+4 in the channel occupancy time  530 . At this time, the LAA cell  505  may transmit data to UE 1   510  and UE 2   511  simultaneously using different frequency resources (e.g. different resource blocks). In the FDD system, the UE 1   510  and UE 2   511  receive data in subframe n and transmit data reception results  550  and  555  respectively to the eNB at subframe n+4. The eNB may retransmit the data according to the data reception results. That is, after transmitting the data to the UE 1   510  and UE 2   511  in the channel occupancy time  530 , the eNB may receive the data reception results from the UEs during the period from subframe n+4 to subframe n+8. At this time, the eNB may configure a contention period for channel sensing operation depending on the data reception results from the UEs. 
     Although the description is made based on the transmission relationship between data transmission time of the LAA cell and the data reception result transmission time of the UEs assuming the LAA cell and UE operating in the FDD mode for explanation convenience, the methods according to the above embodiment can be applied to the LAA cell and UE operating in the TDD mode identically according to the predefined time (e.g. Hybrid Automatic-Repeat-Request (HARQ) feedback timing specified in TS36.213). 
     In the case of the LAA cell operating in the FDD mode, the UE 1   510  and UE 2   511  which receive data at subframe n transmit corresponding data reception results  550  and  555  to the eNB at subframe n+4 respectively. The eNB may receive the data reception results from the UEs in the duration [n+4˜n+8] (e.g., for results  560 ,  570 , and  580  with regard to UE 1   510 , and results  565 ,  575 , and  585  with regard to UE 2   511  shown in  FIG. 5 ). At this time, the period for receiving A/N from the UEs for contention period configuration (hereinafter, referred to as contention period configuration reference time) is configured as follows. 
     Method A-1: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to all data transmitted within the channel occupancy time of the LAA cell. 
     Method A-2: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to the data transmitted at the last data transmission timing within the channel occupancy time of the LAA cell. 
     Method A-3: Time at which the LAA cell receives data perception ACKs from part or all of the UEs in correspondence to the data transmitted at the first data transmission timing within the channel occupancy time of the LAA cell. 
     Method A-4: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to the data transmitted at a specific timing within the channel occupancy time of the LAA cell. 
     Method A-5: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to the data transmitted before or after a specific time within the channel occupancy time of the LAA cell. 
     Method A-6: Predefined or configured time 
     Method A-7: Time at which the LAA cell receives data reception results or the latest data reception result from part or all of the UEs before a new channel sensing operation start time 
     Although the channel sensing operation start time has been described under the assumption that a random variable is selected, right before the channel sensing operation start time, to configure the channel sensing period necessary for perform new channel sensing operation, if the real channel sensing operation start time and the time at which the random variable is selected in the contention period for the channel sensing period configuration mismatch, the above described method may be applied based on the time of selecting the random variable within the contention period for configuring the channel sensing period configuration. 
     For the case where the LAA cell does not receive all data reception results from the UEs in correspondence to the downlink (DL) data transmitted in the channel occupancy duration before the new channel sensing operation start time and the case where the LAA cell receives all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may be possible to apply different contention period configuration methods. A description thereof is made hereinafter. If the LAA cell receives all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may configure the contention period for a new channel occupancy period based on the all or part of the data reception results in correspondence to the DL data transmitted in the channel occupancy period (e.g. Method A-1). If the LAA cell does not receive all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may configure the contention period for a new channel occupancy period based on part of the data reception results from the UEs in correspondence to the DL data transmitted in the previous channel occupancy period like Method A-7. 
     It is also possible to configure such that the new channel sensing operation start time of the LAA cell arrives after receiving all of the acknowledgements (ACKs) corresponding to the DL data transmitted in the previous channel occupancy period. 
     Referring to  FIG. 5 , a description is made of Method A-1. The LAA cell  505  receives the data reception result (ACK/NACK) corresponding to the data transmitted by the LAA cell in the channel occupancy duration  530  cell from the UEs in the duration [n+4˜n+8]. Accordingly, the period in which the LAA cell receives the data reception result corresponding to the data transmitted in a specific channel occupancy period can be configured as the contention period configuration time. At this time, the DL data reception result corresponding to the DL data transmitted during a period shorter than a normal LTE subframe (1 ms) may not be included in the contention period configuration reference time for configuring the contention period. If there is at least one NACK among the DL data reception results corresponding to the data transmitted in the channel occupancy period of the LAA cell, the LAA cell may increase or maintain the contention period. If there is no NACK among the DL data reception results corresponding to the data transmitted in the channel occupancy period of the LAA cell, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     A description is made of Method A-2 hereinafter with reference to  FIG. 5 . The LAA cell  505  receives data reception results corresponding to the data transmitted by the LAA in the channel occupancy duration  530  from the UEs in the duration [n+4˜n+8]. At this time, the LAA cell may configure the last data transmission time requiring A/N transmission in the channel occupancy period  530  or the subframe n+8 at which the data reception result corresponding to the data transmitted at last subframe n+4 having the length of 1 ms as the contention period configuration reference time for contention period configuration. At this time, the reception result corresponding to the data transmitted in a time period shorter than a normal LTE subframe (1 ms) is not included in the contention period configuration reference time for configuring the contention period. If at least one NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may increase or maintain the contention period. If no NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     Referring to  FIG. 5 , a description is made of Method A-3. The LAA  505  receives data reception results corresponding to the data transmitted by the LAA in the channel occupancy period  530  from the UEs in the duration [n+4˜n+8]. At this time, the LAA cell configures the first data transmission time required for transmitting A/N in the channel occupancy period  530  or a time point when the data reception result corresponding to the data transmitted at the first subframe n is received as the contention period configuration reference time for configuring the contention period. At this time, the reception result corresponding to the data transmitted in a time period shorter than a normal LTE subframe (1 ms) is not included in the contention period configuration reference time for configuring the contention period. If at least one NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may increase or maintain the contention period. If no NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     Referring to  FIG. 5 , a description is made of Method A-4 and method A-5. The LAA cell  505  receives the data reception results corresponding to the data transmitted in the channel occupancy period  530  from the UEs in the duration [n+4 n+8]. At this time, the LAA cell may configure a duration for receiving the data reception results corresponding to the data transmitted in a specific subframe at the data transmission time requiring A/N transmission in the channel occupancy duration  530  or transmitted before or after the specific subframe as the contention period configuration reference time. For example, the contention period configuration reference time may be configured according to the A/N transmission time relationship of the UE which is predefined between the LAA cell and the UE. That is, in the case of FDD, the A/N transmission time of the UE which is predefined between the LAA cell and UE is 4 ms after data transmission time of the LAA cell. Accordingly, the LAA cell may configure the contention period configuration reference time using the A/N transmission time relationship of the UE based on the channel occupancy period  530 . For example, a subframe before the A/N transmission time relationship of the UE which is defined based on the last data transmission time (or the last subframe having the length of 1 ms) of the channel occupancy period  530  may be configured as the contention period configuration reference time. That is, the subframe before the A/N transmission relationship (4 m) corresponding to the last channel occupancy time n+4 as denoted by reference number  530  of  FIG. 5 , i.e. the A/N transmission time n+4 corresponding to subframe n, may be configured as the contention period configure reference time. That is, if the LAA cell occupies the channel for 10 ms (n˜n+10), it is possible to configure the contention period using the reception result corresponding to the data transmitted to the UE at n+6. If at least one NACK is received in the contention period configuration reference time for configuring the contention period, the LAA cell may increase or maintain the contention period. If no NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     A description is made of Method A-6 hereinafter with reference to  FIG. 5 . The LAA cell  505  may configure the A preconfigured based on the time n when data transmission starts in the channel occupancy period  530  and the time n+4 when the data reception results are transmitted by the UEs or configured by the LAA as the contention period configuration reference time. For example, if A=100 ms, the LAA cell may configure the duration (n+100 or n+104) in the range of 100 ms based on the time n when the LAA transmits data of the time n+4 when the data reception result corresponding to the transmitted data is received as the contention period configuration reference time. 
     A description is made of the Method of A-7 with reference to  FIG. 5 . The LAA  505  operating in the FDD mode receives the channel reception results in the duration [n+4˜n+8] in correspondence to the data transmitted by the LAA  505  to the UEs  510  and  511  in the channel occupancy time  530 . 
     At this time, the LAA cell  505  may configure the contention period for a new channel sensing operation  540  based on the reception results received from the UEs in the duration starting from the previous channel sensing operation start time till right before the new channel sensing operation  540  in correspondence to the DL data transmitted by the LAA in the unlicensed band or the most recently received data reception results  550  and  555 . That is, the LAA cell may configure time n+4 when the reception results corresponding to the DL data transmitted by the LAA cell in the unlicensed band as the contention period configuration reference time for configuring the contention period. In other words, the LAA cell  505  may configure the contention period for the new channel sensing operation  540  based on the data reception results received from the UEs before the new channel sensing operation  540  is started for reoccupying the unlicensed band (e.g. before the random variable N necessary for performing the new channel sensing operation  540  at operation  407  of  FIG. 4  is selected) or the latest one of the data reception results. That is, if at least one NACK  555  is included in the data reception results  550  and  555  or in the most recent data reception results  550  and  555  received between the previous channel sensing period  520  and the new channel sensing period  540 , the LAA cell may configure the contention period to be longer than or equal to the previous contention period. If no NACK is included in the data reception results  550  and  555  or in the most recent data reception results  550  and  555  received between the previous channel sensing period  520  and the new channel sensing period  540 , the LAA cell may configure the contention period to be equal to or less than the previous contention period. At this time, the LAA cell may initialize the contention period for the new channel sensing period  540  to the initial contention period. 
     At this time, the LAA cell may use the A/N information received from part or all of the UEs at the configured contention period configuration reference time for configuration of the contention period. For example, it is possible to configure all UEs transmitting the data reception results in the contention configuration time as the contention period change reference UEs. In another example, it is possible to configure UEs selected, based on the channel quality information (or allocated MCS value), among all UEs transmitting the data reception results in the contention period configuration reference time as the contention period change reference UEs. For example, it may be possible to configure the UEs configured with the lowest MCS or preconfigured or allocated an MCS in the range selected by the LAA as the contention period change reference UE. That is, the UE having a low MCS may be regarded as a victim of significant interferences from the neighboring devices so as to be used as the contention period change reference. It may be also possible to configure the UE which transmits measured channel information to the LAA most recently or which transmits a signal predefined for reporting the UE channel condition (e.g. RSSI) among the UEs transmitting data reception results in the contention period configuration reference time as the contention period change reference UE. The contention period configuration reference UE may be configured based on one or any combination of the above methods. 
     Also, the contention period configuration reference time may be configured in such a way of combining or extending the aforementioned method as well as applying each method without modification. For example, the contention period configuration reference time may be configured based on the channel occupancy time of at least one LAA cells in Method A-1. For example, a period corresponding to two channel occupancy periods  530  and  535  may be configured as the contention period configuration reference time. The aforementioned methods and cases are just examples but the present disclosure is not limited thereto. That is, although Methods A-1 to A-6 are directed to the cases of configuring the contention period configuration reference time for contention period configuration based on one channel occupancy time, it may be possible to configure the contention period configuration reference time corresponding to at least one channel occupancy period as the contention period configuration reference time for the contention period configuration. 
     The LAA cell may configure the contention period for the second channel sensing operation based on part or all of the data reception results transmitted by the UEs using at least one or any combination of the above method. At this time, the contention period change method is described in association with the application to the channel sensing operation based on the ACK/NACK received from the UEs in association with the second channel sensing operation, but it may also be used to change at least one of the first and third channel sensing operation criteria using the contention period change method or at least one of the criteria. For example, it is determined that it is necessary to change the contention period using the methods to be described hereinafter, it may be possible to change at least one of the first or third channel sensing operation criteria (e g channel sensing reference time) according to a predetermined method or set to other values. The LAA cell applies a contention period change method to the second channel sensing operation using the contention period configuration reference time and ACK/NACK received from the contention period configuration reference UE as follows. 
     Method B-1: If at least one NACK is received from the contention period configuration reference UE during the contention period configuration reference time, then maintain or change the contention period applied to the next second channel sensing operation. 
     Method B-2: Maintain or change the contention period applied to the next second channel sensing operation based on the number or ratio of NACKs (or ACKs) received from the contention period configuration reference UE during the contention period configuration reference time. 
     Referring to  FIG. 5 , a description is made of Method B-1 in detail. In the case that the contention period configuration reference time is configured to [n+4˜n+8] corresponding to the channel occupancy time  530  as in Method A-1 and all UEs which transmit data reception results in the contention period configuration reference time as the contention period configuration reference UEs, the LAA cell receives the NACK  555  from the UE 2   511  during the configured contention period configuration reference time and thus may change (e.g. increase) the contention period. At this time, the contention period may be changed exponentially (i.e., 16→32→64→128, . . . , →1024). That is, the contention period may increase exponentially from the minimum contention period value (or initial value) of 16 to the maximum contention period value of 1024. The exponential increment method is just an example, and it is possible to use a linear increment method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. If neither ACK nor NACK is received from the contention period configuration reference UEs in the configured contention period configuration reference time, the LAA cell may assume receipt of NACK and increase the contention period. If neither ACK nor NACK is received from the contention period configuration reference UEs in the contention period configuration reference time configured as above, the LAA cell may reuse the previously configured contention period without change or initialize the content period to the initial length. If no ACK is received from the contention period configuration reference UEs in the configured contention period configuration reference time or if all of the received are ACK, the LAA cell may change (e.g. decrease) the contention period. At this time, the contention period may be maintained as a predetermined length or decreased exponentially (e.g., 1024→512→ . . . →32→16). At this time, the exponential decrement method is just an example, and it may be possible to use a linear decrement method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. The aforementioned contention period value denotes the maximum value of the contention period. However, the minimum contention period value may also be configured in the aforementioned method. For example, a description thereof is made under the assumption of the initial contention period configuration period of [1, 16]. If at least one NACK is received from the contention period configuration reference UE during the contention period configuration reference time, the LAA may change the contention period to be applied to the subsequent channel sensing operation to [1, 32]. Alternatively, it may be possible to change the maximum value of the previous contention period with the maximum value of the contention period. In the above example, the contention period may be set to [16, 32]. Changing from the maximum value of the contention period to the minimum value of the contention period is just an example, and it may also be possible to configure the contention period to the minimum value in various ways. For example, it is possible to set the contention period to a fixed minimum value of 0 or 1 which is predefined. 
     Referring to  FIG. 5 , a description is made of Method B-2 in detail. If the contention period configuration reference time of the LAA cell is set to (n+4˜n+8) corresponding to the channel occupancy time  530  of the LAA cell and if all UEs which transmit data reception results in the contention period configuration reference time as the contention period configuration reference UEs, the eNB receives 1 NACK  555  from the UE 2   511  during the contention period configuration reference time configured as shown in  FIG. 5 . If it is determined that when the number of NACKs is equal to or greater than K (e.g. K=2) or when the percentage of NACKs is equal to or greater than P % (e.g. 10%) the contention period change reference is configured in Method B-2, the contention period may not be changed but be maintained or may be decreased or initialized to the initial length in the example of  FIG. 5 . If the number or percentage of the NACKs received from the contention period configuration reference UEs during the contention period configuration reference time configured as above is equal to or greater than K or P %, the LAA cell may maintain or increase the contention period. The contention period may be increased exponentially (e.g., 16→32→64→128→ . . . 4 1024) or decreased exponentially (e.g., 1024→512→ . . . →32→16). The exponential increment and decrement methods are just examples, and it may be possible to use a linear increment or decrement method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. If neither ACK nor NACK is received from the contention period configuration UEs during the contention period configuration reference time as configured above, the LAA cell may assume the receipt of NACK to increase the contention period or reuse the preconfigured contention period. At this time, the LAA cell may change the contention period even when the condition of Method B-2 is not fulfilled. That is, although the contention period change condition is configured to be fulfilled when the number of NACKs received from the contention period configuration reference UE during the contention period configuration reference time is equal to or greater than 2 in Method B-2, if neither ACK nor NACK is received from the contention period configuration reference UEs during the contention period configuration reference time, the LAA may change the contention period according to the configured threshold or regardless of the configured threshold. Also, if it is configured that when the number or percentage of the received NACKs is equal to or greater than K or P % the contention period is changed in Method B-2, the contention period may be changed according to the number or percentage of the received NACKs. For example, if the percentage of NACKs among the data reception results received from the UEs during the previous data transmission period is P % or if the percentage of the NACKs among the data reception results received during the period starting from the previous channel sensing period start time and ending right before the new channel sensing period start time or starting from the time when generating a random variable (e.g. random backoff number) for the previous channel sensing period and ending right before the time when generating a random variable (e.g., random backoff number) of the new channel sensing period operation is P %, the contention period may increase as much as P % from the previous contention period. In more detail, if an exponential increment method (e.g., 16→32→64→128→ . . . →1024) or an exponential decrement method (e.g., 1024→512→ . . . →32→16) is used for the contention period and if the percentage of NACKs among the data reception results received from part or all of the UEs during the period from the previous channel sensing operation start time to the new channel sensing operation start time is 40% (e.g., 4 of 10 A/N values are NACKs) as in Method A-7, it may be possible to increase the contention period for the new channel sensing operation as much as 40% (22.4, 22, or 23) in comparison to the previous channel contention period (e.g., 16). At this time, the increased contention period may be rounded off to an integer. If the percentage of NACKs of the data reception results received from part or all of the UEs during a period from the previous channel sensing operation start time to the new channel sensing operation start time is higher than a predetermined threshold value (e.g. 50%), it may be possible to increase the contention period (16→32) regardless of the percentage of NACKs. At this time, if the number of NACKs is equal to or less than (or less than) K or the percentage of the NACKs is equal to or less than (or less than) P % (K and P may be 0), the contention period for the new channel sensing operation is set to the same value as the previous contention period or initialized to the initial length. 
     As described above, if neither ACK nor NACK is received from the contention period configuration reference UEs during the contention period configuration reference time (discontinuous transmission (DTX)) the LAA cell assumes receipt of NACK to increase the contention period according to the contention period determination criteria or reuse the previously configured contention period. A description is made of the case where the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the configured contention period configuration reference time in more detail hereinafter. The LAA cell performs DL data transmission to the UE in the channel occupancy period, and the UE which receives the data transmits to the LAA cell the data reception result through one of the licensed band or the unlicensed band through which the data has been received or another unlicensed band. Typically, the LAA cell sends the UE a DL control channel (PDCCH) carrying DL data scheduling information for transmitting DL data (PDSCH). The UE checks the scheduling information included in the UL control channel and performs DL data reception operation as scheduled. However, if the UE does not receive the control channel carrying the scheduling information through the DL control channel, it determines that it has not been scheduled by the LAA cell and thus the terminal neither performs the downlink (DL) data reception operation nor transmits any data reception result. Also, it may be possible that the LAA cell has transmitted the scheduling information to the UE through the DL control channel and the UE checks the scheduling information received through the DL control channel and transmits the data reception result to the LAA cell through a licensed band or an unlicensed band but the LAA cell does not receive the data reception result. At this time, if the UE is configured to transmit the data reception result through the licensed band and if the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX), it may be possible not to increase the contention period for unlicensed band channel sensing but to maintain the previous contention period or use the initial value. Also, if the LAA cell receives the DTX or NACK/DTX from the contention period configuration reference UEs during the contention period configuration reference time, it regards the receipt of the DTX or NACK/DTX as the receipt of NACK or distinguishes between the NACK/DTX and DTX. For example, since LAA cell cannot identify whether the NACK/DTX information is NACK or DTX transmitted by the LAA UE, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as not to be included in determining the contention period. That is, the DTX may be excluded in determining the contention period. For more details on ACK, NACK, NACK/DTX, and DTX configuration and transmission method, see TS36.211, TS36.212, and TS36.213. 
     For example, a UE configured to transmit ACK/NACK information to the LAA with 2 antenna ports and PUCCH format 1b with channel selection can transmit to the LAA cell four types of reception status information of ACK, NACK, DTX, and NACK/DTX. 
     In the UE configuration method based on the PUCCH format 1b with channel selection, the eNB may configure the UE which is capable of supporting carrier aggregation (CA) of up to two carriers to use the PUCCH format 1b with channel selection always when configuring two carriers to the UE. 
     In the case that more than two carriers are configured to the UE capable of supporting CA of more than two carriers, it may be possible to configure to use the UE to use one of the PUCCH format 1b with channel selection and the PUCCH format 3. 
     Meanwhile, the LAA cell may transmit to the UE a control message for configuring the PUCCH format 1b with channel selection. The UE configured with the PUCCH format 1b with channel selection may transmit the reception status information to the LAA cell according to the configuration included in the control message. 
     At this time, the LAA cell may interpret the DTX and NACK/DTX information as NACK or distinguish NACK/DTX and DTX among the ACK/NACK status information types. For example, single the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. At this time, it may be possible to interpret both the NACK/DTX and DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. At this time, it may be possible to interpret both the NACK/DTX and DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. 
     If the LAA UE which is configured with the ACK/NACK information transmission scheme for cell 1 and cell 2 with the PUCCH format 1b with channel selection does not receive PDCCH scheduling PDSCH to the cell 1 and cell 2 (DTX) or if the combination of ACK/NACK information for cell 1 and cell 2 include only DTX or NACK/DTX without ACK and NACK, the LAA cell does not transmit any ACK/NACK information to the LAA cell (not transmission). The LAA UE configured with the PUCCH format 1b with channel selection does not transmit ACK/NACK information, and the LAA cell which has not received the ACK/NACK information from the LAA UE interprets the DTX or NACK/DTX as not receiving ACK/NACK so as not to be included in determining the contention period. At this time, it may be possible to interpret both the DTX and NACK/DTX for the LAA cell as NACK or distinguish between NACK/DTX and DTX. For example, since the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX transmitted by the LAA UE, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as not to be included in determining the contention period. At this time, assuming that cell 1 is a licensed band cell (licensed carrier) and cell 2 as an LAA cell, if the LAA UE configured with the PUCCH format 1b with channel selection determines cell 1 as DTX and cell 2 as NACK/DTX, the LAA UE may transmit no ACK/NACK information to the LAA cell. In this case, the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX for cell 2 of the LAA UE, it interprets the NACK/DTX as NACK so as to be included in determining the contention period. 
     In detail, the LAA UE may be configured with the ACK/NACK information transmission scheme for cell 1 and cell 2 using the PUCCH format 1b with channel selection in the LAA cell as shown below in table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 HARQ-ACK(0) 
                 HARQ-ACK(1) 
                 n PUCCH   (1)   
                 b(0)b(1) 
               
               
                   
               
             
            
               
                 ACK 
                 ACK 
                 n PUCCH, 1   (1)   
                 1, 1 
               
               
                 ACK 
                 NACK/DTX 
                 n PUCCH, 0   (1)   
                 1, 1 
               
               
                 NACK/DTX 
                 ACK 
                 n PUCCH, 1   (1)   
                 0, 0 
               
               
                 NACK 
                 NACK/DTX 
                 n PUCCH, 0   (1)   
                 0, 0 
               
            
           
           
               
               
               
               
            
               
                 DTX 
                 NACK/DTX 
                 No Transmission 
               
               
                   
               
            
           
         
       
     
     As shown in table 1, if no ACK/NACK is received, the LAA UE may assume that the feedback to cell 1 is DTX and the feedback to cell 2 is NACK/DTX. 
     The LAA cell may ignore the DTX to do not include it in the contention period determination criteria and assume the receipt of NACK/DTX as the receipt of NACK. In detail, the DTX may not be used in determining the length of the next LBT because it is not included in the contention period determination criteria. 
     In the case that the UE is configured to transmit the data reception result in the unlicensed band, if the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX), it may be possible to increase the contention period for unlicensed band channel sensing. That is, if it is determined that the reason why the LAA cell receives ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX) is the collision of the devices on the unlicensed band or hidden node which is not detected by the LAA cell or UE, it may be possible to increase the contention period for unlicensed band channel sensing. If the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX) without consideration of the influence of the hidden node, it may be possible to maintain the contention period for the unlicensed band channel sensing. At this time, if the UEs transmit the data reception results in the licensed band, it may be possible to maintain the contention period for unlicensed band channel sensing regardless whether the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX). If the UEs transmit the data reception results in the unlicensed band and if the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX), the UE may assume that no downlink (DL) scheduling information is received from the LAA cell through the DL control channel and increase the contention period for new unlicensed band channel sensing. 
     The contention period may be changed according to the above described criteria and schemes in such a way of comparing the data reception results received from the contention period configuration reference UEs during the configuration period configuration reference time with the contention period channel threshold set by the LAA cell to determine whether to change the contention period and changing the contention period based on the determination result and contention period change scheme. However, since a new channel sensing operation  630  may be executed before the change of the contention period, there is a need of a contention period configuration method for the channel sensing operation  630  executed before the contention period change time point. 
     The LAA cell  600  performs the channel sensing operation  610  during the preconfigured contention period to determine whether the channel is occupied, for data transmission to the UE  605 . If it is determined that the channel is idle, the LAA cell  600  occupies the channel during the configured channel occupancy period  620  or predetermined or preconfigured maximum channel occupancy period to transmit data to the UE  605 . At this time, it is assumed that the contention period configuration reference time of the LAA cell is configured with Method A-2, the contention period configuration reference UEs are all UEs which transmit data reception results during the contention period configuration reference time, and the contention period change method is Method B-1. That is, the LAA cell  600  changes the contention period based on the A/N  674  corresponding to the last subframe among the A/N&#39;s corresponding to data transmitted during the channel occupancy period  620  (and similarly for A/N  675 ,  676  and  677  shown in  FIG. 6 ). The LAA cell  600  may perform the channel sensing operation  630  to secure further channel occupancy period  640  before receipt of the A/N  674  for the channel sensing period  620 , i.e. before the changed contention period application timing. In the typical Wi-Fi operation, the contention period is changed for reoccupying the channel. That is, the contention period may increase or decrease according to the data reception results of the UEs during the channel occupancy period. However, since the LAA cell  600  may perform the channel sensing operation  630  before the contention period is changed, it is necessary to configure the contention period for the channel sensing operation  630  which is performed before the changed contention period application timing. 
     The contention period may be configured for the channel sensing operation  630 , which is performed before the contention period change timing, as follows. 
     Method C-1: Reuse contention period configured for previous channel occupancy 
     Method C-2: Use initial value of contention period 
     Method C-3: Change according to UE reception performance received before channel sensing operation 
     Method C-4: Use predefined contention period 
       FIG. 6  is a diagram illustrating a method for applying a changed contention period according to various embodiments of the present disclosure. 
     Referring to  FIG. 6 , method C-1 is described hereinafter in more detail. As shown in  FIG. 6 , the contention period for the channel sensing operation  630  executed before the changed contention period application timing may be configured to be identical with the contention period used for the latest channel sensing operation  610 . Alternatively, the contention period for the channel sensing operation  630  which is performed before the changed contention period application timing is configured to use the initial value of the contention period as in Method C-2 or to use a predetermined period as in Method C-4. Alternatively, the contention period may be changed based on the A/N information received from the UEs before the channel sensing operation  630  which is performed before the changed contention period application timing. At this time, it may be possible to change the contention period based on the A/N information received from the UEs by excluding at least one of the configured contention period configuration reference time and the contention period configuration reference UE or changing the method. 
     The LAA cell may perform the channel sensing operation  650  to further occupy the channel after the channel occupancy period  640 . At this time, the channel sensing operation  650  may be performed based on the previously changed contention period. 
     In addition, with reference to  FIG. 6 , the LAA cell  600  may use another contention period, other than the previously changed contention period, for the channel sensing operation after the contention period change time point. That is, it may be possible to perform the channel sensing operation by not using the previously changed contention period or using a separately defined contention period according to the type of the signal which the LAA  600  wants to transmit on the occupied channel. 
     For example, it may be possible to perform the channel sensing operation with another contention period configured for the channel occupancy to transmit a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without common data transmission (PDSCH/PUSCH) as denoted by reference numbers  620 ,  640 , and  660  and UE-specific data transmission. For example, in the case of attempting channel occupancy for transmitting a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission, it may be possible to use another contention period  680  different from the previously changed contention period. For example, it may be possible to perform the channel sensing operation using the initially configured contention period or the contention period configured separately for transmitting control information (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission. At this time, it may be possible to occupy the channel to transmit the control channel without extra channel sensing operation. It may also be possible to occupy the channel without at least one of the first to third channel sensing operations. It may also be possible to perform the channel sensing operation for transmitting control information (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission by setting at least one of the first to third channel sensing operation thresholds to the minimum value. If it is necessary to occupy the channel for normal data transmission  695  after the channel occupancy for control channel transmission (e.g., reference element  690 ), the LAA cell  600  may perform the channel sensing operation with the previously configured contention period (e.g., contention period changed through the channel occupancy period  660 ), the contention period  650  used for channel occupancy for previous data transmission, or the initial contention period. 
     Method C-3 is described hereinafter in more detail with reference to  FIG. 6 . The LAA cell  600  operating in the FDD mode may receive the data reception results  670 , 671 , 672 , and  673  corresponding to the data, which have transmitted to the UE  605  during the channel occupancy period  620 , within the period [n+4˜n+8]. Although the description is made with a relationship between the data transmission time of the LAA cell and the data reception result transmission time of the UE under the assumption of the LAA cell and LAA UE, it may be possible to apply all of the methods proposed in the above embodiments to the communication between the LAA cell and UEs operating in the TDD mode according to a predefined timing (e.g., HARQ feedback timing defined in TS36.213). 
     At this time, it may be possible to configure the contention period for the channel sensing operation  630  based on the data reception results received from the UEs till before performing the new channel sensing operation  630  including the previous channel sensing operation  610 . At this time, since the LAA cell  600  does not receive the data reception result from the UEs in the duration from the start time of the previous channel sensing operation  610  till just before the start time of the new channel sensing operation  630 , it may be possible to reuse the contention period configured for the previous channel sensing operation  610 , the initial contention period, or a predetermined contention period as the contention period for the new channel sensing operation  630 . A description is made of a case where the LAA cell  600  performs the channel sensing operation  630  to occupy the unlicensed band as denoted by reference number  640 . The LAA cell  600  receives the data reception results  670 ,  671 ,  672 , and  673  from the UE  605  in the channel sensing and occupancy periods  630  and  640 . If it is necessary to reoccupy the unlicensed band after the channel occupancy period, the LAA cell may perform the new channel sensing operation  650  to reoccupy the unlicensed and. At this time, according to Method C-3, the contention period for the channel sensing operation  650  may be configured based on the data reception results  670 ,  671 ,  672 , and  673  received in the duration from the start time of the previous channel sensing operation  630  to the start time of the new channel sensing operation  650  or the most recently received data reception result  673 . For example, if the data reception result  673  received most recently from the UE in the duration from the start time of the previous channel sensing operation  630  to the start time of the new channel sensing operation  650  is NACK, it may be possible to configure the contention period for the new channel sensing operation  650  by maintaining or increasing the contention period of the previous channel sensing operation exponentially or linearly or selecting one of the preconfigured contention period candidate values. If the data reception result  673  transmitted by the UE is ACK, it may be possible to configure the contention period by maintaining or decreasing the contention period of the previous channel sensing operation  630  exponentially or linearly or selecting one of the preconfigured contention period candidate values. 
     In addition, the LAA cell may configure at least one of the minimum and maximum values of the contention period differently depending on the service quality requirements (quality of service (QoS)) of the data to be transmitted in the channel occupancy period. In more detail, in the case of providing a real time service such as voice of IP (VoIP) and streaming service of which service quality drops due to the data transmission delay, it may be possible to configure the minimum and maximum values of the contention period so as to minimize the contention period for channel sensing required for data transmission. For example, it may be possible to set the minimum and maximum contention period values for VoIP service to 7 and 15 respectively. In the case of providing a file transfer protocol (FTP) service or hypertext transfer protocol secure (HTTP) service of which service quality is less influenced by data transmission delay, it may be possible to set the minimum and maximum contention periods to values greater than those for the VoIP service. For example, it may be possible to set the minimum and maximum values of the contention period for the FTP service to 12 and 1023 respectively. At this time, if the data transmitted from the LAA cell to the UE in the channel occupancy period include two or more different QoSs, the contention period for the channel sensing operation of the LAA cell may be configured differently depending on the QoS. That is, if the LAA cell transmits multiple data having different QoSs to multiple UEs in the channel occupancy period, the contention period for the channel occupancy operation of the LAA cell may be configured differently as follows. 
     Method G-1: Configure contention period according to QoS having the least contention period configuration value among multiple QoSs 
     Method G-2: Configure contention period according to QoS having maximum contention period configuration value among multiple QoSs 
     Method G-3: Configure contention period using contention period configuration values of part or all of multiple QoSs 
     For example, it is assumed that the LAA cell transmit data to two UEs. At this time, UE 1  is receiving data for a VoIP service, and UE 2  is receiving data for a FTP service. The minimum and maximum contention periods for VoIP service are 7 and 15 respectively, and the minimum and maximum contention periods for the FTP service are 15 and 1023 respectively according to QoS. That is, the maximum and minimum contention periods are determined based on the QoS of the service. In the case of transmitting the data for VoIP and FTP services to UE 1  and UE 2  during the channel occupancy period of the LAA cell, Method G-1 determines the contention period for the channel sensing operation necessary for the channel occupancy period based on the QoS having the least contention period configuration values among the contention period configuration values for the VoIP and FTP services (e g minimum and maximum contention period values of 7 and 15). That is, the contention period may be configured with the minimum contention period value of 7 and maximum contention period value of 15. Method G-2 determines the contention period for the channel sensing operation necessary for the channel sensing period based on the QoS having the greatest contention period configuration value among the contention period configuration values for the VoIP and FTP services (e g minimum and maxim contention periods of 15 and 1023). That is, the contention period may be configured with the minimum contention period value of 15 and maximum contention period value of 1023. Method G-3 determines the contention period by averaging the contention period configuration values for the VoIP and FTP services. That is, it may be possible to determine the contention period for the channel sensing operation necessary for the channel occupancy period based on the contention period configuration values for the VoIP and FTP services, e.g. values of (11, 519) obtained by averaging the minimum contention period value (7, 15) and the maximum contention period value (15, 1023). 
     At this time, the QoS of the data transmitted in the previous channel occupancy period and the QoS of the data to be transmitted in the new channel occupancy period may differ from each other. At this time, it may be possible to configure the contention period for the channel sensing operation performed to occupy the channel newly based on the QoS of the data to be transmitted in the new channel occupancy period. At this time, the contention period for the new channel sensing operation may be configured differently depending on the data reception results transmitted in the previous channel occupancy period. A description is made thereof with reference to  FIG. 5 . It is assumed that the FTP service data are transmitted to UE 1  and UE 2  in the previous channel occupancy period  530 , as shown in  FIG. 5 . At this time, if the contention period for the case of performing the channel sensing operation  520  for the channel occupancy period  530  is configured according to Method G-1 among Methods G-1, G-2, and G-3, the minimum and maximum values of the contention period may be set to 15 and 1023 respectively. If the contention period for the channel sensing operation  520  is set to the minimum value of the contention period, the LAA cell  505  selects a random value in the contention period [0, 15] or [1, 15] and senses the channel during the channel sensing operation period corresponding to the selected value. In the case that the LAA cell transmits data to UE 1  and UE 2  in the channel occupancy period  530  and then wants to occupy the channel again during the channel occupancy period  535 , the LAA cell has to perform the channel sensing operation  540 . However, if the VoIP service data are transmitted to the UE 3  and UE 4  in the channel occupancy period  535 , the contention period for the channel sensing operation  540  may be configured with the minimum value of 7 and the maximum value of 15 according to Method G-1. That is, the LAA cell  505  selects a random value in the contention period [0, 7] or [1, 7] and senses the channel during the channel sensing operation period. In the case of using Method A-7, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  to the new channel sensing period  540  or the most recently received data reception result is NACK, the new channel contention period  540  may increase according to Method B-1 or B-2. That is, if the new channel contention period is configured with the exponential increment scheme, it has to increase [0, 15] or [1, 15] to [1, 15] or [0, 31]. In the case of transmitting data having different QoSs, however, the contention period configuration value may vary according to QoS of the data to be transmitted in the new channel occupancy period and the contention period configuration method (e.g. contention period is configured as [0, 7] or [1, 7] with Method G-1) but the number of contention period increments may be maintained. That is, if the VoIP service data are transmitted in the new channel occupancy period, the contention period for the new channel sensing operation may be configured according to the channel sensing period configuration for the VoIP service. In the case of using Method A-7, however, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, the new channel contention period  540  should be increased and thus the contention period for the new channel sensing operation increases from [0, 7] or [1, 7] to [0, 15] or [1, 15]. That is, the contention period for the channel sensing operation may be configured differently depending on QoS, but the number of contention period increments may be configured regardless of QoS. For example, if the contention period is increased from [0, 15] to [0, 31] and then to [0, 63], the number of contention period increments is 2. If the contention period is initialized from [0, 63] to the initial value, the number of contention period increments is 0. That is, in the case that there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, if it is necessary to increase the contention period according to Method B-1 or B-2, it may be possible to configure the new channel contention period  540  by applying the number of contention period increments which is increase by 1 in comparison to the number of previous contention period increments. That is, if the number of contention period increments in the previous channel sensing period  520  is 2, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, and if it is necessary to increase the contention period according to Method B-1 or B-2, it may be possible to configure the new channel contention period  540  by applying the number of contention period increments of 3. That is, the contention period becomes [0, 127]. 
     Embodiment 2 
     Referring to  FIGS. 4 and 5 , descriptions are made of the DL channel sensing operation and channel access method for DL transmission of the LAA cell and the channel sensing operation for UL transmission and channel access method of the LAA UE from the viewpoint of the UE  510  which is receiving data from the LAA cells  505  and  510  with reference to  FIG. 5 . The LAA cell  505  performs channel sensing operation  520  to occupy the channel in the idle state. At this time, the channel sensing operation  520  may be performed using at least one of the operations at operations  403 ,  408 , and  413  of  FIG. 4 . At this time, the maximum occupancy time allowed for the channel is denoted by reference number  530 . The signal  525  may be transmitted until the start point of the subframe  515  according to the channel occupancy start time. Afterward, the LAA cell  505  may transmit data scheduled to the UEs in the channel occupancy duration  530 . From the viewpoint of UE 1   510 , it receives data in subframes n, n+1, n+3, and n+4 in the channel occupancy time  530 . At this time, the LAA cell  505  may transmit data to UE 1   510  and UE 2   511  simultaneously using different frequency resources (e.g. different resource blocks). In the FDD system, the UE 1   510  and UE 2   511  receives data in subframe n and transmits data reception results  550  and  555  respectively to the eNB at subframe n+4. The eNB may retransmit the data according to the data reception results. That is, after transmitting the data to the UE 1   510  and UE 2   511  in the channel occupancy time  530 , the eNB may receive the data reception results from the UEs during the period from subframe n+4 to subframe n+8. At this time, the eNB may configure a contention period for channel sensing operation depending on the data reception results from the UEs. 
     Although the description is made based on the transmission relationship between data transmission time of the LAA cell and the data reception result transmission time of the UEs assuming the LAA cell and UE operating in the FDD mode for explanation convenience, the methods according to the above embodiment can be applied to the LAA cell and UE operating in the TDD mode identically according to the predefined time (e.g. HARQ feedback timing specified in TS36.213). 
     In the case of the LAA cell operating in the FDD mode, the UE 1   510  and UE 2   515  which receive data at subframe n transmit corresponding data reception results  550  and  555  to the eNB at subframe n+4 respectively. The eNB may receive the data reception results from the UEs in the duration [n+4˜n+8]. At this time, the period for receiving A/N from the UEs for contention period configuration (hereinafter, referred to as contention period configuration reference time) is configured as follows. 
     Method A-1: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to all data transmitted within the channel occupancy time of the LAA cell. 
     Method A-2: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to the data transmitted at the last data transmission timing within the channel occupancy time of the LAA cell. 
     Method A-3: Time at which the LAA cell receives data perception ACKs from part or all of the UEs in correspondence to the data transmitted at the first data transmission timing within the channel occupancy time of the LAA cell. 
     Method A-4: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to the data transmitted at a specific timing within the channel occupancy time of the LAA cell. 
     Method A-5: Time at which the LAA cell receives data reception results from part or all of the UEs in correspondence to the data transmitted before or after a specific time within the channel occupancy time of the LAA cell. 
     Method A-6: Predefined or configured time 
     Method A-7: Time at which the LAA cell receives data reception results or the latest data reception result from part or all of the UEs before a new channel sensing operation start time 
     Although the channel sensing operation start time has been described under the assumption that a random variable is selected, right before the channel sensing operation start time, to configure the channel sensing period necessary for perform new channel sensing operation, if the real channel sensing operation start time and the time at which the random variable is selected in the contention period for the channel sensing period configuration mismatch, the above described method may be applied based on the time of selecting the random variable within the contention period for configuring the channel sensing period configuration. 
     For the case where the LAA cell receives no data reception results corresponding to data which it has transmitted in the channel occupancy period before the new channel sensing operation execution time point or the UE receives no data reception result corresponding to the UL data or the LAA cell receives all data reception results corresponding to the DL data transmitted in the channel occupancy period before the new channel sensing operation execution time point, it may be possible to apply different contention period configuration methods. A description is made with a DL example hereinafter. If the LAA cell receives all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may configure the contention period for a new channel occupancy period based on the all or part of the data reception results in correspondence to the DL data transmitted in the channel occupancy period (e.g. Method A-1). If the LAA cell does not receive all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may configure the contention period for a new channel occupancy period based on part of the data reception results from the UEs in correspondence to the DL data transmitted in the previous channel occupancy period like Method A-7. 
     It is also possible to configure such that the new channel sensing operation start time of the LAA cell arrives after receiving all of the ACKs corresponding to the DL data transmitted in the previous channel occupancy period. 
     A description is made of Method A-1 hereinafter with reference to  FIG. 5 . The LAA cell  505  receives the data reception result (ACK/NACK) corresponding to the data transmitted by the LAA in the channel occupancy duration  530  cell from the UEs in the duration [n+4˜n+8]. Accordingly, the period in which the LAA cell receives the data reception result corresponding to the data transmitted in a specific channel occupancy period can be configured as the contention period configuration time. At this time, the DL data reception result corresponding to the DL data transmitted during a period shorter than a normal LTE subframe (1 ms) may not be included in the contention period configuration reference time for configuring the contention period. If there is at least one NACK among the DL data reception results corresponding to the data transmitted in the channel occupancy period of the LAA cell, the LAA cell may increase or maintain the contention period. If there is no NACK among the DL data reception results corresponding to the data transmitted in the channel occupancy period of the LAA cell, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     A description is made of Method A-2 hereinafter with reference to  FIG. 5 . The LAA cell  505  receives data reception results corresponding to the data transmitted by the LAA in the channel occupancy duration  530  from the UEs in the duration [n+4˜n+8]. At this time, the LAA cell may configure the last data transmission time requiring A/N transmission in the channel occupancy period  530  or the subframe n+8 at which the data reception result corresponding to the data transmitted at last subframe n+4 having the length of 1 ms as the contention period configuration reference time for contention period configuration. At this time, the reception result corresponding to the data transmitted in a time period shorter than a normal LTE subframe (1 ms) is not included in the contention period configuration reference time for configuring the contention period. If at least one NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may increase or maintain the contention period. If no NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     Referring to  FIG. 5 , a description is made of Method A-3. The LAA  505  receives data reception results corresponding to the data transmitted by the LAA in the channel occupancy period  530  from the UEs in the duration [n+4˜n+8]. At this time, the LAA cell configures the first data transmission time required for transmitting A/N in the channel occupancy period  530  or a time point when the data reception result corresponding to the data transmitted at the first subframe n is received as the contention period configuration reference time for configuring the contention period. At this time, the reception result corresponding to the data transmitted in a time period shorter than a normal LTE subframe (1 ms) is not included in the contention period configuration reference time for configuring the contention period. If at least one NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may increase or maintain the contention period. If no NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may maintain or decreases the contention period. At this time, it may be possible to use the initial contention period. 
     A description is made of Method A-4 and method A-5 hereinafter with reference to  FIG. 5 . The LAA cell  505  receives the data reception results corresponding to the data transmitted in the channel occupancy period  530  from the UEs in the duration [n+4˜n+8]. At this time, the LAA cell may configure a duration for receiving the data reception results corresponding to the data transmitted in a specific subframe at the data transmission time requiring A/N transmission in the channel occupancy duration  530  or transmitted before or after the specific subframe as the contention period configuration reference time. For example, the contention period configuration reference time may be configured according to the A/N transmission time relationship of the UE which is predefined between the LAA cell and the UE. That is, in the case of FDD, the A/N transmission time of the UE which is predefined between the LAA cell and UE is 4 ms after data transmission time of the LAA cell. Accordingly, the LAA cell may configure the contention period configuration reference time using the A/N transmission time relationship of the UE based on the channel occupancy period  530 . For example, a subframe before the A/N transmission time relationship of the UE which is defined based on the last data transmission time (or the last subframe having the length of 1 ms) of the channel occupancy period  530  may be configured as the contention period configuration reference time. That is, the subframe before the A/N transmission relationship (4 m) corresponding to the last channel occupancy time n+4 as denoted by reference number  530  of  FIG. 5 , i.e. the A/N transmission time n+4 corresponding to subframe n, may be configured as the contention period configure reference time. That is, if the LAA cell occupies the channel for 10 ms (n˜n+10), it is possible to configure the contention period using the reception result corresponding to the data transmitted to the UE at n+6. If at least one NACK is received in the contention period configuration reference time for configuring the contention period, the LAA cell may increase or maintain the contention period. If no NACK is received at the contention period configuration reference time for configuring the contention period, the LAA cell may maintain or decrease the contention period. At this time, it may be possible to use the initial contention period. 
     Referring to  FIG. 5 , a description is made of Method A-6. The LAA cell  505  may configure the time A preconfigured based on the time n when data transmission starts in the channel occupancy period  530  or the time n+4 when the data reception results are transmitted by the UEs or configured by the LAA as the contention period configuration reference time. For example, if A=100 ms, the LAA cell may configure the duration (n+100 or n+104) in the range of 100 ms based on the time n when the LAA transmits data of the time n+4 when the data reception result corresponding to the transmitted data is received as the contention period configuration reference time. 
     A description is made of the Method of A-7 with reference to  FIG. 5 . The LAA  505  operating in the FDD mode receives the channel reception results in the duration [n+4˜n+8] in correspondence to the data transmitted by the LAA  505  to the UEs  510  and  511  in the channel occupancy time  530 . 
     At this time, with reference to  FIG. 5 , the LAA cell  505  may configure the contention period for a new channel sensing operation  540  based on the reception results received from the UEs in the duration starting from the previous channel sensing operation start time till right before the new channel sensing operation  540  in correspondence to the DL data transmitted by the LAA cell in the unlicensed band, or the latest data reception results  550  and  555 . That is, the LAA cell may configure time n+4 when the reception results corresponding to the DL data transmitted by the LAA cell in the unlicensed band are received as the contention period configuration reference time for configuring the contention period. In other words, the LAA cell  505  may configure the contention period for the new channel sensing operation  540  based on the data reception results received from the UEs before the new channel sensing operation  540  is started for reoccupying the unlicensed band (e.g. before the random variable N necessary for performing the new channel sensing operation  540  at operation  407  shown in  FIG. 4  is selected) or the latest one of the data reception results. That is, if at least one NACK  555  is included in the data reception results  550  and  555  or in the latest one of the reception results  550  and  555  received between the previous channel sensing period  520  and the new channel sensing period  540 , the LAA cell may configure the contention period to be longer than or equal to the previous contention period. If no NACK is included in the data reception results  550  and  555  or in the latest one of the reception results  550  and  555  received between the previous channel sensing period  520  and the new channel sensing period  540 , the LAA cell may configure the contention period to be equal to or less than the previous contention period. At this time, the LAA cell may configure the contention period for the new channel sensing period  540  as the initial contention period. 
     At this time, the LAA cell may use the A/N information received from part or all of the UEs at the configured contention period configuration reference time for configuration of the contention period. For example, it is possible to configure all UEs transmitting the data reception results in the contention configuration time as the contention period change reference UEs. In another example, it is possible to configure UEs selected, based on the channel quality information (or allocated MCS value), among all UEs transmitting the data reception results in the contention period configuration reference time as the contention period change reference UEs. For example, it may be possible to configure the UEs configured with the lowest MCS or preconfigured or allocated an MCS in the range selected by the LAA cell as the contention period change reference UE. That is, the UE having a low MCS may be regarded as a victim of significant interferences from the neighboring devices so as to be used as the contention period change reference. It may be also possible to configure the UE which transmits measured channel information to the LAA cell most recently or which transmits a signal predefined for reporting the UE channel condition (e.g. RSSI) among the UEs transmitting data reception results in the contention period configuration reference time as the contention period change reference UE. The contention period configuration reference UE may be configured based on one or any combination of the above methods. 
     Also, the contention period configuration reference time may be configured in such a way of combining or extending the aforementioned method as well as applying each method without modification. For example, the contention period configuration reference time may be configured based on the channel occupancy time of at least one LAA cells in Method A-1. For example, a period corresponding to two channel occupancy periods  530  and  535  may be configured as the contention period configuration reference time. The aforementioned methods and cases are just examples but the present disclosure is not limited thereto. That is, although Methods A-1 to A-6 are directed to the cases of configuring the contention period configuration reference time for contention period configuration based on one channel occupancy time, it may be possible to configure the contention period configuration reference time corresponding to at least one channel occupancy period as the contention period configuration reference time for the contention period configuration. 
     The LAA cell may configure the contention period for the second channel sensing operation based on part or all of the data reception results transmitted by the UEs using at least one or any combination of the above method. At this time, the contention period change method is described in association with the application to the channel sensing operation based on the ACK/NACK received from the UEs in association with the second channel sensing operation, but it may also be used to change at least one of the first and third channel sensing operation criteria using the contention period change method or at least one of the criteria. For example, it is determined that it is necessary to change the contention period using the methods to be described hereinafter, it may be possible to change at least one of the first or third channel sensing operation criteria (e g channel sensing reference time) according to a predetermined method or set to other values. The LAA cell applies a contention period change method to the second channel sensing operation using the contention period configuration reference time and ACK/NACK received from the contention period configuration reference UE as follows. 
     Method B-1: If at least one NACK is received from the contention period configuration reference UE during the contention period configuration reference time, then it maintains or changes the contention period applied to the next second channel sensing operation. 
     Method B-2: Maintain or change the contention period applied to the next second channel sensing operation based on the number or ratio of NACKs (or ACKs) received from the contention period configuration reference UE during the contention period configuration reference time. 
     A description is made of Method B-1 in detail hereinafter with reference to  FIG. 5 . In the case that the contention period configuration reference time is configured to [n+4˜n+8] corresponding to the channel occupancy time  530  as in Method A-1 and all UEs which transmit data reception results in the contention period configuration reference time as the contention period configuration reference UEs, the LAA cell receives the NACK  555  from the UE 2   511  during the configured contention period configuration reference time and thus may change (e.g. increase) the contention period. At this time, the contention period may be changed exponentially (i.e., 16→32→64→128, . . . , →1024). That is, the contention period may increase exponentially from the minimum contention period value (or initial value) of 16 to the maximum contention period value of 1024. The exponential increment method is just an example, and it is possible to use a linear increment method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. If neither ACK nor NACK is received from the contention period configuration reference UEs in the configured contention period configuration reference time, the LAA cell may assume receipt of NACK and increase the contention period. If neither ACK nor NACK is received from the contention period configuration reference UEs in the contention period configuration reference time configured as above, the LAA cell may reuse the previously configured contention period without change or initialize the content period to the initial length. If one or more NACK is not received from the contention period configuration reference UEs in the configured contention period configuration reference time or if all of the received are ACKs received, the LAA cell may change (e.g. decrease) the contention period. At this time, the contention period may be maintained as a predetermined length or decreased exponentially (e.g., 1024→512→ . . . →32→16). At this time, the exponential decrement method is just an example, and it may be possible to use a linear decrement method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. The aforementioned contention period value denotes the maximum value of the contention period. However, the minimum contention period value may also be configured in the aforementioned method. For example, a description thereof is made under the assumption of the initial contention period configuration period of [1, 16]. If at least one NACK is received from the contention period configuration reference UE during the contention period configuration reference time, the LAA may change the contention period to be applied to the subsequent channel sensing operation to [1, 32]. Alternatively, it may be possible to change the maximum value of the previous contention period with the maximum value of the contention period. In the above example, the contention period may be set to [16, 32]. Changing from the maximum value of the contention period to the minimum value of the contention period is just an example, and it may also be possible to set the contention period to the minimum value in various ways. For example, it is possible to set the contention period to a fixed minimum value of 0 or 1 which is predefined. 
     Referring to  FIG. 5 , a description is made of Method B-2 in detail. If the contention period configuration reference time of the LAA cell is set to (n+4˜n+8) corresponding to the channel occupancy time  530  of the LAA cell and if all UEs which transmit data reception results in the contention period configuration reference time is configured as the contention period configuration reference UEs, the eNB receives 1 NACK  555  from the UE 2   511  during the contention period configuration reference time configured as shown in  FIG. 5 . If it is determined that when the number of NACKs is equal to or greater than K (e.g. K=2) or when the percentage of NACKs is equal to or greater than P % (e.g. 10%) the contention period change reference is configured in Method B-2, the contention period may not be changed but be maintained or may be decreased or initialized to the initial length in the example of  FIG. 5 . If the number or percentage of the NACKs received from the contention period configuration reference UEs during the contention period configuration reference time configured as above is equal to or K or P %, the LAA cell may maintain or increase the contention period. The contention period may be increased exponentially (e.g., 16→32→64→128→ . . . 4 1024) or decreased exponentially (e.g., 1024→512→ . . . →32→16). The exponential increment and decrement methods are just examples, and it may be possible to use a linear increment or decrement method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. If neither ACK nor NACK is received from the contention period configuration UEs during the contention period configuration reference time as configured above, the LAA cell may assume the receipt of NACK to increase the contention period or reuse the preconfigured contention period. At this time, the LAA cell may change the contention period even when the condition of Method B-2 is not fulfilled. That is, although the contention period change condition is configured to be fulfilled when the number of NACKs received from the contention period configuration reference UE during the contention period configuration reference time is equal to or greater than 2 in Method B-2, if neither ACK nor NACK is received from the contention period configuration reference UEs during the contention period configuration reference time, the LAA may change the contention period according to the configured threshold or regardless of the configured threshold. Also, if it is configured that when the number or percentage of the received NACKs is equal to or greater than K or P % the contention period is changed in Method B-2, the contention period may be changed according to the number or percentage of the received NACKs. For example, if the percentage of NACKs among the data reception results received from the UEs during the previous data transmission period is P % or if the percentage of the NACKs among the data reception results received during the period starting from the previous channel sensing period start time and ending right before the new channel sensing period start time or starting from the time when generating a random variable (e.g. random backoff number) for the previous channel sensing period and ending right before the time when generating a random variable (e.g., random backoff number) of the new channel sensing period operation is P %, the contention period may increase as much as P % from the previous contention period. In more detail, if an exponential increment method (e.g., 16→32→64→128→ . . . →1024) or an exponential decrement method (e.g., 1024→512→ . . . →32→16) is used for the contention period and if the percentage of NACKs among the data reception results received from part or all of the UEs during the period from the previous channel sensing operation start time to the new channel sensing operation start time is 40% (e.g., 4 of 10 A/N values are NACKs) as in Method A-7, it may be possible to increase the contention period for the new channel sensing operation as much as 40% (22.4, 22, or 23) in comparison to the previous channel contention period (e.g., 16). At this time, the increased contention period may be rounded off to an integer. If the percentage of NACKs of the data reception results received from part or all of the UEs during a period the previous channel sensing operation start time to the new channel sensing operation start time is higher than a predetermined threshold value (e.g. 50%), it may be possible to increase the contention period (16→32) regardless of the percentage of NACKs. At this time, if the number of NACKs is equal to or less than (or less than) K or the percentage of the NACKs is equal to or less than (or less than) P % (K and P may be 0), the contention period for the new channel sensing operation is set to the same value as the previous contention period or initialized to the initial length. 
     As described above, if neither ACK nor NACK is received from the contention period configuration reference UEs during the contention period configuration reference time (DTX), the LAA cell assumes receipt of NACK to increase the contention period according to the contention period determination criteria or reuse the previously configured contention period. A description is mad of the case where the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the configured contention period configuration reference time in more detail hereinafter. The LAA cell performs DL data transmission to the UE in the channel occupancy period, and the UE which receives the data transmits to the LAA cell the data reception result through one of the licensed band or the unlicensed band through which the data has been received or another unlicensed band. Typically, the LAA cell sends the UE a DL control channel (PDCCH) carrying DL data scheduling information for transmitting DL data (PDSCH). The UE checks the scheduling information included in the UL control channel and performs DL data reception operation as scheduled. However, if the UE does not receive the control channel carrying the scheduling information through the DL control channel, it determines that it has not been scheduled by the LAA cell and thus neither perform the DL data reception operation nor transmit any data reception result. Also, it may be possible that the LAA cell has transmitted the scheduling information to the UE through the DL control channel and the UE checks the scheduling information received through the DL control channel and transmits the data reception result to the LAA cell through a licensed band or an unlicensed band but the LAA cell does not receive the data reception result. At this time, if the UE is configured to transmit the data reception result through the licensed band and if the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX), it may be possible not to increase the contention period for unlicensed band channel sensing but to maintain the previous contention period or use the initial value. Also, if the LAA cell receives the DTX or NACK/DTX from the contention period configuration reference UEs during the contention period configuration reference time, it regards the receipt of the DTX or NACK/DTX as the receipt of NACK or distinguishes between the NACK/DTX and DTX. For example, since LAA cell cannot identify whether the NACK/DTX information is NACK or DTX transmitted by the LAA UE, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as not to be included in determining the contention period. That is, the DTX may be excluded in determining the contention period. For more details on ACK, NACK, NACK/DTX, and DTX configuration and transmission method, see TS36.211, TS36.212, and TS36.213. For example, a UE configured to transmit ACK/NACK information to the LAA with 2 antenna ports and PUCCH format 1b with channel selection can transmit to the LAA cell four types of reception status information of ACK, NACK, DTX, and NACK/DTX. At this time, the LAA cell may interpret the DTX and NACK/DTX information as NACK or distinguish NACK/DTX and DTX among the ACK/NACK status information types. For example, since the LAA cell cannot identify whether the NACK/DTX information transmitted by the LAA UE is NACK or DTX, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. At this time, it may be possible to interpret both the NACK/DTX and DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. 
     If the LAA UE which is configured with the ACK/NACK information transmission scheme for cell 1 and cell 2 with the PUCCH format 1b with channel selection does not receive PDCCH scheduling PDSCH to the cell 1 and cell 2 (DTX) or if the combination of ACK/NACK information for cell 1 and cell 2 includes only DTX or NACK/DTX without ACK and NACK, the LAA cell does not transmit any ACK/NACK information to the LAA cell (no transmission). The LAA UE configured with the PUCCH format 1b with channel selection does not transmit ACK/NACK information, and the LAA cell which has not received the ACK/NACK information from the LAA UE interprets the DTX or NACK/DTX as not receiving ACK/NACK so as not to be included in determining the contention period. At this time, it may be possible to interpret both the DTX and NACK/DTX for the LAA cell as NACK or distinguish between NACK/DTX and DTX. For example, since the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX transmitted by the LAA UE, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as not to be included in determining the contention period. At this time, assuming that cell 1 is a licensed band cell (licensed carrier and cell 2 as an LAA cell, if the LAA UE configured with the PUCCH format 1b with channel selection determines cell 1 as DTX and cell 2 as NACK/DTX, the LAA UE may transmit no ACK/NACK information to the LAA cell. In this case, the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX for cell 2 of the LAA UE, it interprets the NACK/DTX as NACK so as to be included in determining the contention period. 
     The contention period may be changed according to the above described criteria and schemes in such a way of comparing the data reception results received from the contention period configuration reference UEs during the configuration period configuration reference time with the contention period channel threshold set by the LAA cell to determine whether to change the contention period and changing the contention period based on the determination result and contention period change scheme. However, since a new channel sensing operation  630  may be executed before the change of the contention period, there is a need of a contention period configuration method for the channel sensing operation  630  executed before the contention period change time point. 
     Referring to  FIG. 6 , the LAA cell  600  performs the channel sensing operation  610  during the preconfigured contention period to determine whether the channel is occupied, for data transmission to the UE  605 . If it is determined that the channel is idle, the LAA cell  600  occupies the channel during the configured channel occupancy period  620  or predetermined or preconfigured maximum channel occupancy period to transmit data to the UE  605 . At this time, it is assumed that the contention period configuration reference time of the LAA cell is configured with Method A-2, the contention period configuration reference UEs are all UEs which transmit data reception results during the contention period configuration reference time, and the contention period change method is Method B-1. That is, the LAA cell  600  changes the contention period based on the A/N  674  corresponding to the last subframe among the A/N&#39;s corresponding to data transmitted during the channel occupancy period  620 . The LAA cell  600  may perform the channel sensing operation  630  to secure further channel occupancy period  640  before receipt of the A/N  674  for the channel sensing period  620 , i.e. before the changed contention period application timing. In the typical Wi-Fi operation, the contention period is changed for reoccupying the channel. That is, the contention period may increase or decrease according to the data reception results of the UEs during the channel occupancy period. However, since the LAA cell  600  may perform the channel sensing operation  630  before the contention period is changed, it is necessary to configure the contention period for the channel sensing operation  630  which is performed before the changed contention period application timing. 
     The contention period may be configured for the channel sensing operation  630 , which is performed before the contention period change timing, as follows. 
     Method C-1: Reuse contention period configured for previous channel occupancy 
     Method C-2: Use initial value of contention period 
     Method C-3: Change according to UE reception performance received before channel sensing operation 
     Method C-4: Use predefined contention period 
     Method C-1 is described hereinafter in more detail. As shown in  FIG. 6 , the contention period for the channel sensing operation  630  executed before the changed contention period application timing may be configured to be identical with the contention period used for the latest channel sensing operation  610 . Alternatively, the contention period for the channel sensing operation  630  which is performed before the changed contention period application timing is configured to use the initial value of the contention period as in Method C-2 or to use a predetermined period as in Method C-4. Alternatively, the contention period may be changed based on the A/N information received from the UEs before the channel sensing operation  630  which is performed before the changed contention period application timing. At this time, it may be possible to change the contention period based on the A/N information received from the UEs by excluding at least one of the configured contention period configuration reference time and the contention period configuration reference UE or changing the method. 
     With reference to  FIG. 6 , the LAA cell  600  may perform the channel sensing operation  650  to further occupy the channel after the channel occupancy period  640 . At this time, the channel sensing operation  650  may be performed based on the previously changed contention period. 
     In addition, the LAA cell  600  may use another contention period, other than the previously changed contention period, for the channel sensing operation after the contention period change time point. That is, it may be possible to perform the channel sensing operation by not using the previously changed contention period or using a separately defined contention period according to the type of the signal which the LAA  600  wants to transmit on the occupied channel. 
     For example, it may be possible to perform the channel sensing operation with another contention period configured for the channel occupancy to transmit a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without common data transmission (PDSCH/PUSCH) as denoted by reference numbers  620 ,  640 , and  660  and UE-specific data transmission. For example, in the case of attempting channel occupancy for transmitting a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission, it may be possible to use another contention period  680  different from the previously change contention period. For example, it may be possible to perform the channel sensing operation using the initially configured contention period or the contention period configured separately for transmitting control information for transmitting a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission. At this time, it may be possible to occupy the channel to transmit the control channel without extra channel sensing operation. It may also be possible to occupy the channel without at least one of the first to third channel sensing operations. It may also be possible to perform the channel sensing operation for transmitting control information (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission by setting at least one of the first to third channel sensing operation thresholds to the minimum value. If it is necessary to occupy the channel for normal data transmission  695  after the channel occupancy for control channel transmission, the LAA cell  600  may perform the channel sensing operation with the previously configured contention period (e.g., contention period changed through the channel occupancy period  660 ), the contention period  650  used for channel occupancy for previous data transmission, or the initial contention period. 
     Method C-3 is described hereinafter in more detail with reference to  FIG. 6 . The LAA cell  600  operating in the FDD mode may receive the data reception results  670 , 671 , 672 , and  673  corresponding to the data, which have transmitted to the UE  605  during the channel occupancy period  620 , within the period [n+4˜n+8]. Although the description is made with a relationship between the data transmission time of the LAA cell and the data reception result transmission time of the UE under the assumption of the LAA cell and LAA UE, it may be possible to apply all of the methods proposed in the above embodiments to the communication between the LAA cell and UEs operating in the TDD mode according to a predefined timing (e.g., HARQ feedback timing defined in TS36.213). 
     At this time, it may be possible to configure the contention period for the channel sensing operation  630  based on the data reception results received from the UEs till before performing the new channel sensing operation  630  including the previous channel sensing operation  610 . At this time, since the LAA cell  600  does not receive the data reception result from the UEs in the duration from the previous channel sensing operation  610  to the new channel sensing operation  630 , it may be possible to reuse the contention period configured for the previous channel sensing operation  610 , the initial contention period, or a predetermined contention period as the contention period for the new channel sensing operation  630 . A description is made of a case where the LAA cell  600  performs the channel sensing operation  630  to occupy the unlicensed band as denoted by reference number  640 . The LAA cell  600  receives the data reception results  670 ,  671 ,  672 , and  673  from the UE  605  in the channel sensing and occupancy periods  630  and  640 . If it is necessary to reoccupy the unlicensed band after the channel occupancy period, the LAA cell may perform the new channel sensing operation  650  to reoccupy the unlicensed and. At this time, according to Method C-3, the contention period for the channel sensing operation  650  may be configured based on the data reception results  670 ,  671 ,  672 , and  673  received in the duration from the start time of the previous channel sensing operation  630  to the start time of the new channel sensing operation  650  or the most recently received data reception result  673 . For example, if the data reception result  673  received most recently from the UE in the duration from the start time of the previous channel sensing operation  630  to the start time of the new channel sensing operation  650  is NACK, it may be possible to configure the contention period for the new channel sensing operation  650  by maintaining or increasing the contention period of the previous channel sensing operation exponentially or linearly or selecting one of the preconfigured contention period candidate values. If the data reception result  673  transmitted by the UE is ACK, it may be possible to configure the contention period by maintaining or decreasing the contention period of the previous channel sensing operation  630  exponentially or linearly or selecting one of the preconfigured contention period candidate values. 
       FIG. 7  is a diagram illustrating a contention period configuration method according to various embodiments of the present disclosure. 
     Referring to  FIG. 7 , a description is made of the method for using the unlicensed band for both the UL and between the LAA cell and UE hereinafter with reference to  FIG. 7 . At this time, this method may be applicable to the case of using the unlicensed band for one of UL and DL between the LAA cell and UE. 
     The LAA cell  700  configures the cell  702  operating in the licensed band as a PCell and the cell  705  operating in the unlicensed band as a SCell to communicate with the LAA UEs  710  and  712  on the licensed and unlicensed bands. Although the description is directed to the case of a CA aggregating one licensed band and one unlicensed band, it may also be possible to aggregate one or more licensed bands and one or more unlicensed bands according to the capabilities of the LAA cell and LAA UE and minimum values of the LAA cell and UE capabilities. Although  FIG. 7  is directed to the case where the PCell is operating in the TDD mode (UL/DL configuration 1, Table 4.2-2 in TS36.211), this embodiment may also be applicable to the cell operating in the FDD mode. The TDD licensed band cell  702  may be comprised of the DL subframe  717 , UL subframe  720 , and special subframe  725 . The structure of the special subframe is out of the scope of the disclosure, detailed description thereof is omitted herein. For detailed description of the special subframe  725  of the TDD cell operating in the licensed band, see the LTE standard TS36.211 and TS36.213. In order to use the SCell  705  operating in the unlicensed band, the LAA cell  700  may perform the channel sensing operation  730  and transmit a channel occupancy signal or initial signal to occupy the channel in a predefined data or control signal transmission available period as denoted by reference number  735 . Although it is assumed that the data or control signal transmission available time is used as start time point of every subframe (or first OFDM symbol of the subframe) in  FIG. 7 , the data or control signal transmission available time may be predefined or configured through higher layer signaling to include at least the start time of the subframe. After performing the channel sensing operation  730  in the unlicensed band, the LAA SCell  705  occupied the unlicensed band may transmit at least one signal such as DL control channel, DL control signal, DL data, and UL control information in the unlicensed band. For example, the LAA SCell  705  may transmit the DL control channel, DL control signal, and DL data to the UE  710  and the unlicensed band UL control information to the UE  710  in the unlicensed band. At this time, the unlicensed band UL control information of the UEs  710  and  712  may be transmitted to the UE through the licensed band LAA PCell  702  or the unlicensed LAA SCell  705  depending on the LAA cell configuration. If the DL data are received from the LAA SCell  705  in the unlicensed band DL subframe  720 , the UE  712  transmits the data reception result (e.g. ACK and NACK) to the LAA PCell  702  using the licensed UL resource predefined or configured by received UL control signal as denoted by reference number  790 . If the DL data  779  is received from the LAA SCell  705  in the unlicensed band DL subframe  740 , the UE  712  may transmit the data reception result (e.g. ACK and NACK) to the LAA SCell  705  using the unlicensed UL resource predefined or configured by a received UL control signal. If the unlicensed band UL control information is received from the LAA SCell  705  in the DL subframe  740 , the UE  710  may perform UL transmission in the unlicensed band using the unlicensed resource and transmission time configured by the LAA SCell  705  (e.g. UL signal transmission timing defined based on the UL control signal reception timing) as denoted by reference number  774 . At this time, the UE  710  may perform the unlicensed band channel sensing operation  772  before the UL transmission  774 . At this time, the channel sensing operation for unlicensed band UL transmission of the UE may differ from the channel sensing operation  730  for unlicensed band DL transmission of the SCell. For example, the UE may perform the unlicensed band UL transmission without channel sensing operation  772  for the unlicensed band UL transmission. Alternatively, the channel sensing operation for unlicensed band UL transmission of the UE may be performed during a fixed channel sensing time period (e.g. fixed CCA length and predefined number of (E)CCA slots). Alternatively, with reference to  FIG. 7 , the channel sensing operation  772  for the unlicensed band UL transmission of the UE may be performed with a fixed contention period (e.g. set to minimum value of the contention period) or a period set to a value shorter than the maximum value of the contention period for the channel sensing operation  730  for the unlicensed band DL transmission of the SCell. 
     At this time, the LAA cell may notify the UE of the reception result corresponding to the UL data through the DL control channel. That is, if the UL data are received from the UE, the LAA cell may send the UE the data reception result along with the UL scheduling information through the DL control channel. At this time, the UE may determine the data reception result corresponding to the UL data based on at least one of new data indicator (NDI), HARQ Process number, and redundancy version (RV) carried in the UL scheduling information transmitted by the LAA cell. For example, if the UL data transmitted by the UE is not received correctly, the LAA cell transmits the scheduling information including the NDI bit which is not toggled to be identical with that of the previous UL scheduling information. That is, since the NDI is toggled when new data is transmitted, the UE may determine the UL data reception result at the LAA cell based on the NDI value. If a RV configuration order for retransmission is predefined, the UE may determine the UL data reception result at the LAA cell according to the RV value included in the UL scheduling information. For example, if the current NDI and RV match the NDI and RV carried in the previous scheduling information, it may be possible to determine that the data reception result is NACK. At this time, the UL data reception result determination based on the NDI and RV may be applied to the same HARQ process number. 
     At this time, referring to  FIG. 7 , if the UE needs to perform the channel sensing operation  772  for unlicensed band UL transmission, the LAA SCell  705  may not perform the unlicensed band DL signal or channel transmission during the period of the channel sensing operation  772  for UL transmission of the UE. For example, the LAA SCell  705  may not transmit any DL signal or channel in the unlicensed band for the duration including the period of channel sensing operation  772  for the unlicensed band UL transmission  774 . At this time, the period  745  during which the LAA SCell  705  does not transmit any DL signal or channel for unlicensed band channel sensing operation of the UE may be configured to include at least one of channel sensing time of the UE, a period for the UE to transmit a UL signal in advance to compensate for signal transmission/reception delay caused by distance between the LAA cell and the UE (e.g. Timing Advance (TA)), and the time required for the UE to switch from DL reception to UL transmission. 
     The UE  710  may transmit the UL data or control signal to the LAA cell  700  using the unlicensed UL depending on the result of the unlicensed band channel sensing operation  772  performed before the time of the unlicensed band UL transmission  774  configured by the LAA SCell  705  through the licensed DL  717  or unlicensed DL  740 . At this time, the time periods  780 ,  785  and  796  during the UL transmission  774  of the UE  710  (e.g. last SC-OFDM symbol duration of the UL subframe) may be configured not to transmit the UL signal in order for the UE or other UEs or the LAA cells to perform the channel sensing operation correctly. For example, it may be possible to puncture the last SC-OFDMA symbol or transmit the data or control signal using the UL SC-OFDM symbols with the exception of the last SC-OFDM symbol. At this time, the period in which the UL signal is not transmitted may be predefined or configured by the LAA cell through higher layer signaling. In the case that the LAA cell schedules the UE to perform the UL transmission consecutively in one or more UL subframes, e.g. the UE is scheduled to perform UL transmission in the UL subframes  774  and  776 , the UE may transmit UL signals without the period  774  having no UL transmission for channel sensing operation in the consecutive UL subframes. At this time, although the LAA cell schedule the UE to perform UL transmission consecutively in one or more UL subframes, it may always be possible to use the period in which the UL signal for channel sensing operation is not transmitted. In the case that the UE is scheduled to perform the UL transmission consecutively in one or more UL subframe and configured to transmit the UL signal without the period  774  having no UL transmission for channel sensing operation in the consecutive UL subframes, the UE may apply the period  785  having no UL signal transmission for the channel sensing operation in the last UL subframe among the consecutive UL subframes. Also, the UE may receive the information on the UL subframes which can be used with extra channel sensing operation or the information on the available time from the LAA cell through a separate licensed or unlicensed signal or may recognize the information according to the predetermined UL use time of the LAA SCell. In this case, the UE may transmit the UL signal with or without applying the period  796  having no UL signal transmission for channel sensing operation in the last UL subframe among the UL subframe which the LAA SCell can use without channel sensing period. 
     The LAA SCell  705  which occupies the unlicensed band during the predefined or configured channel occupancy period after performing channel sensing operation  730  has to perform the channel sensing operation  750  for reoccupy the unlicensed band. At this time, the LAA SCell  705  may configure the contention period required for the channel sensing operation  750  in the method B-1 or B-2 or another method based on the DL signal transmission results which the LAA cell  700  receives from the UE through the licensed or unlicensed band or the UL signal transmission result which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-1: Configure the contention period based on both the DL signal transmission results which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-2: Configure the contention period based on the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-3: Configure the contention period based on the most recently received or transmitted one of the DL transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-4: Configure the contention period of the channel sensing operation  750  based on the signal transmission result corresponding to the data carried a specific DL subframe (e.g. first or last DL subframe) of the LAA SCell which the LAA cell  700  receives through the licensed or unlicensed band and the signal transmission result corresponding to the data carried in a specific UL subframe of the UE which the LAA cell transmits through the licensed or licensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Although the channel sensing operation start time has been described under the assumption that a random value for configuring the channel sensing period necessary for performing the channel sensing operation is selected right before starting the channel sensing operation, if the channel sensing operation start time and the time of selecting the random value in the contention period for the channel sensing period configuration mismatch, Method D-1, D-2, D-3, or D4 may be applied based on the timing of selecting the random value in the contention period for the channel sensing period configuration. 
     Although the DL and UL channel sensing operations and contention period configuration methods have been described from the viewpoint of the LAA cell for explanation convenience, it may be possible to interpret and apply from the viewpoint of the UE because the time point when the LAA cell transmits the UL signal transmission result to the UE through the licensed or unlicensed band matches the time point when the UE receives the signal. For example, it may be possible to change or configure the contention period for a new channel sensing operation at the UE based on the UL signal transmission result which the UE receives from the LAA cell through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     A description is made of the example of using Method D-1 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on both the DL signal transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . That is, if the DL signal transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  so as to be longer than the contention period configured in the previous channel sensing operation  730  or configure or maintain the contention period for the new channel sensing period  750  to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  so as to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial contention period or a predefined value. 
     A description is made of the example of using Method D-2 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . At this time, if the LAA SCell  705  receives more than one DL signal transmission result or transmits more than one UL signal transmission result to the UE, it may configure the contention period of the channel sensing operation  750  based on all of the transmission results. At this time, it may be possible to configure the contention period of the channel sensing operation  750  based on part of the transmission results carried in a specific DL subframe of a specific UE or LAA SCell (e.g. the first or last DL subframe) or a specific UL subframe of the UE (e.g. the first or last UL subframe). That is, if the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  to be longer than the contention period configured in the channel sensing operation  730  or configure or maintain the contention period to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial period or a predetermined value. 
     A description is made of the example of using Method D-3 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on the most recently received or transmitted one of the DL transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . At this time, if the LAA SCell  705  receives more than one DL signal transmission result from the UE or transmits more than one UL signal transmission result in the same subframe, it may be possible to configure the contention period of the channel sensing operation  750  based on all of the signal transmission results. At this time, it may be possible to configure the contention period of the channel sensing operation  750  based on part of the transmission results carried in a specific DL subframe (e.g. the first or last DL subframe) of the LAA SCell or the transmission results carried in a specific UL subframe (e.g. the first or last UL subframe) of the UE. That is, if the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  to be longer than the contention period configured in the channel sensing operation  730  or configure or maintain the contention period to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial period or a predetermined value. 
     A description is made of the example of using Method D-4 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on the signal transmission result corresponding to the data carried in a specific DL subframe (e.g. first or last DL subframe) of the LAA SCell which the LAA cell  700  receives through the licensed or unlicensed band and the signal transmission result corresponding to the data carried in a specific UL subframe of the UE which the LAA cell transmits through the licensed or licensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . At this time, if the LAA SCell  705  receives more than one DL signal transmission result from the UE or transmits more than one UL signal transmission result in the same subframe, it may be possible to configure the contention period of the channel sensing operation  750  based on all of the signal transmission results. At this time, it may be possible to configure the contention period of the channel sensing operation  750  based on the transmission result related to a specific UE among all of the transmission results. That is, if the DL signal transmission result which the LAA cell  700  receives from the UE in correspondence to the data carried in a specific DL subframe through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE in correspondence to the data carried in a specific UL subframe through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  to be longer than the contention period configured in the channel sensing operation  730  or configure or maintain the contention period to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives from the UE in correspondence to the data carried in a specific DL subframe through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE in correspondence to the data carried in a specific UL subframe through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial period or a predetermined value. 
     With reference to  FIG. 7 , if the LAA cell  705  neither receives any DL signal transmission result from the UE nor transmits any UL signal transmission result to the UE through the licensed or unlicensed band in the duration from the end of the previous channel sensing operation  730  to the start of the channel sensing operation  750 , the contention period of the channel sensing operation  750  may be configured to maintain or reuse the contention period used for the previous channel sensing operation  730  or set to the initial value or a predefined value of the contention period. The contention period configuration method may change depending on whether the DL signal transmission result or UL signal transmission result is transmitted in the licensed or unlicensed band, if there is neither any DL signal transmission result transmitted by the UE nor UL signal transmission result transmitted by the LAA SCell in the licensed or unlicensed band. In more detail, in the case that the LAA cell  700  is configured to receive the DL signal transmission result of the LAA SCell from the UE in the licensed band and to transmit the UL signal transmission result to the UE in the licensed band in the duration from the end of the channel sensing operation  730  to the start of the channel sensing operation  750 , if there is neither the DL signal transmission result received from the UE nor the UL signal transmission result transmitted to the UE, the contention period of the channel sensing operation  750  may be configured to maintain or reuse the contention period used for the previous channel sensing operation  730 . In the case that the LAA cell  700  is configured to receive the DL signal transmission result of the LAA SCell from the UE in the licensed band and to transmit the UL signal transmission result to the UE in the unlicensed band in the duration from the end of the channel sensing operation  730  to the start of the channel sensing operation  750 , if there is neither the DL signal transmission result received from the UE nor the UL signal transmission result transmitted to the UE, the signal transmission result is regarded as NACK and thus the contention period of the channel sensing operation  750  is configured to be longer than the contention period used for the previous channel sensing operation  730 . 
     If the LAA cell  700  neither receive any DL signal transmission result of the LAA SCell from the UE nor transmit any UL signal transmission result to the UE through the licensed or unlicensed band in the duration from the end of the previous channel sensing operation  730  to the start of the channel sensing operation  750 , the contention period of the channel sensing operation  750  may be configured to maintain or reuse the contention period used for the previous channel sensing operation  730  or set to the initial value or a predefined value of the contention period. 
     Also, if there is no data to transmit in the LAA SCell  705  or if the contention period is not changed during a predefined or configured time period, the contention period for the channel sensing operation may be set to the initial value. 
     At this time, the method for configuring the contention period required for the channel sensing operation  750  based on at least one of the DL signal transmission result received from the UE and the UL signal transmission result transmitted to the UE through the licensed or unlicensed band in the duration from the start of the previous channel sensing operation  730  to the start of the channel sensing operation  750  may be applied for the UE to configure the contention period and update the contention period based on the signal transmission/reception result in the unlicensed band as described above. At this time, the UE may configure the contention period based on the DL signal reception result corresponding to the signal received in the unlicensed band and the UL signal reception result transmitted by the LAA cell using one of the above methods. The UE may also configure the contention period based on the UL signal transmission result corresponding to the signal which it has transmitted in the unlicensed band, i.e. the UL signal transmission result received from the LAA cell using one of the above method. The UE may also configure the contention period based on the DL signal reception result corresponding to the signal received in the unlicensed band. A description is made of the contention period configuration method of the eNB for the channel sensing operation according to an embodiment of the present disclosure hereinafter with reference to  FIG. 8 . The eNB configures the contention period configuration criteria at operation  801 . A specific time point configured based on whole or part of the A/N transmission period of the UE in the channel occupancy period or an A/N transmission relationship may be configured as the contention period configuration reference time. It may also possible to configure at least one of the DL signal transmission result of the LAA SCell which the LAA cell receives from the UE and the UL signal transmission result of the UE which the LAA cell transmits to the UE in the licensed or unlicensed band during the period between channel sensing operations of the LAA SCell as the contention period reference. 
       FIG. 8  is a flowchart illustrating a contention period configuration method of an eNB for channel sensing operation according to various embodiments of the present disclosure. 
     Referring to  FIG. 8 , the LAA cell may configure at least one of the minimum and maximum values of the contention period differently depending on the service quality requirements (Quality of Service (QoS)) of the data to be transmitted in the channel occupancy period. In more detail, in the case of providing a real time service such as VoIP and streaming service of which service quality drops due to the data transmission delay, it may be possible to configure the minimum and maximum values of the contention period so as to minimize the contention period for channel sensing required for data transmission. For example, it may be possible to set the minimum and maximum contention period values for VoIP service to 7 and 15 respectively. In the case of providing an FTP service or HTTP service of which service quality is less influenced by data transmission delay, it may be possible to set the minimum and maximum contention periods to values greater than those for the VoIP service. For example, it may be possible to set the minimum and maximum values of the contention period for the FTP service to 15 and 1023 respectively. At this time, if the data transmitted from the LAA cell to the UE in the channel occupancy period include two or more different QoSs, the contention period for the channel sensing operation of the LAA cell may be configured differently depending on the QoS. That is, if the LAA cell transmits multiple data having different QoSs to multiple UEs in the channel occupancy period, the contention period for the channel occupancy operation of the LAA cell may be configured differently as follows. 
     Method G-1: Configure contention period according to QoS having the least contention period configuration value among multiple QoSs 
     Method G-2: Configure contention period according to QoS having maximum contention period configuration value among multiple QoSs 
     Method G-3: Configure contention period using contention period configuration values of part or all of multiple QoSs 
     For example, it is assumed that the LAA cell transmits data to at least two UEs (e.g., UE 1  and UE 2 ). At this time, UE 1  is receiving data for a VoIP service, and UE 2  is receiving data for a FTP service. The minimum and maximum contention periods for VoIP service are 7 and 15 respectively, and the minimum and maximum contention periods for the FTP service are 15 and 1023 respectively according to QoS. That is, the maximum and minimum contention periods are determined based on the QoS of the service. In the case of transmitting the data for VoIP and FTP services to UE 1  and UE 2  during the channel occupancy period of the LAA cell, Method G-1 determines the contention period for the channel sensing operation necessary for the channel occupancy period based on the QoS having the least contention period configuration values among the contentions period configuration values for the VoIP and FTP services (e.g. minimum and maximum contention period values of 7 and 15). That is, the contention period may be configured with the minimum contention period value of 7 and maximum contention period value of 15. Method G-2 determines the contention period for the channel sensing operation necessary for the channel sensing period based on the QoS having the greatest contention period configuration value among the contention period configuration values for the VoIP and FTP services (e.g. minimum and maxim contention periods of 15 and 1023). That is, the contention period may be configured with the minimum contention period value of 15 and maximum contention period value of 1023. Method G-3 determines the contention period by averaging the contention period configuration values for the VoIP and FTP services. That is, it may be possible to determine the contention period for the channel sensing operation necessary for the channel occupancy period based on the contention period configuration values for the VoIP and FTP services, e.g. values of (11, 519) obtained by averaging the minimum contention period value (7, 15) and the maximum contention period value (15, 1023). 
     At this time, the QoS of the data transmitted in the previous channel occupancy period and the QoS of the data to be transmitted in the new channel occupancy period may differ from each other. At this time, it may be possible to configure the contention period for the channel sensing operation performed to occupy the channel newly based on the QoS of the data to be transmitted in the new channel occupancy period. At this time, the contention period for the new channel sensing operation may be configured differently depending on the data reception results transmitted in the previous channel occupancy period. A description is made thereof with reference to  FIG. 5 . It is assumed that the FTP service data are transmitted to UE 1  and UE 2  in the previous channel occupancy period  530 . At this time, if the contention period for the case of performing the channel sensing operation  520  for the channel occupancy period  530  is configured according to Method G-1 among Methods G-1, G-2, and G-3, the minimum and maximum values of the contention period may be set to 15 and 1023 respectively. If the contention period for the channel sensing operation  520  is set to the minimum value of the contention period, the LAA cell  505  selects a random value in the contention period [0, 15] or [1, 15] and senses the channel during the channel sensing operation period corresponding to the selected value. In the case that the LAA cell transmits data to UE 1  and UE 2  in the channel occupancy period  530  and then wants to occupy the channel again during the channel occupancy period  535 , the LAA cell has to perform the channel sensing operation  540 . However, if the VoIP service data are transmitted to the UE 3  and UE 4  in the channel occupancy period  535 , the contention period for the channel sensing operation  540  may be configured with the minimum value of 7 and the maximum value of 15 according to Method G-1. That is, the LAA cell  505  selects a random value in the contention period [0, 7] or [1, 7] and senses the channel during the channel sensing operation period. In the case of using Method A-7, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  to the new channel sensing period  540  or the mostly recently received data reception result is NACK, the new channel contention period  540  may increase according to Method B-1 or B-2. That is, if the new channel contention period is configured with the exponential increment scheme, it has to increase [0, 15] or [1, 15] to [1, 15] or [0, 31]. In the case of transmitting data having different QoSs, however, the contention period configuration value may vary according to QoS of the data to be transmitted in the new channel occupancy period and the contention period configuration method (e.g. contention period is configured as [0, 7] or [1, 7] with Method G-1) but the number of contention period increments may be maintained. That is, if the VoIP service data are transmitted in the new channel occupancy period, the contention period for the new channel sensing operation may be configured according to the channel sensing period configuration for the VoIP service. In the case of using Method A-7, however, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, the new channel contention period  540  should be increased and thus the contention period for the new channel sensing operation increases from [0, 7] or [1, 7] to [0, 15] or [1, 15]. That is, the contention period for the channel sensing operation may be configured differently depending on QoS, but the number of contention period increments may be configured regardless of QoS. For example, if the contention period is increased from [0, 15] to [0, 31] and then to [0, 63], the number of contention period increments is 2. If the contention period is initialized from [0, 63] to the initial value, the number of contention period increments is 0. That is, in the case that there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, if it is necessary to increase the contention period according to Method B-1 or B-2, it may be possible to configure the new channel contention period  540  by applying the number of contention period increments which is increased by 1 in comparison to the number of previous contention period increments. That is, if the number of contention period increments in the previous channel sensing period  520  is 2, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, and if it is necessary to increase the contention period according to Method B-1 or B-2, it may be possible to configure the new channel contention period  540  by applying the number of contention period increments of 3. That is, the contention period becomes [0, 127]. 
     Embodiment 3 
     With reference to  FIGS. 4 and 5 , descriptions are made of the DL channel sensing operation and channel access method for DL transmission of the LAA cell and the UL channel sensing operation for UL transmission and channel access method of the LAA UE from the viewpoint of the UE 1   510  which is receiving data from the LAA cells  505  and  510  with reference to  FIG. 5 . The LAA cell  505  attempts to occupy the unlicensed band channel for DL data transmission to UE 1   510 . That is, the LAA cell  505  performs the channel sensing operation  520  and, if it is determined that the channel is idle, performs DL data transmission during the maximum allowed occupancy time in the unlicensed band. At this time, the channel sensing operation  520  may be performed using at least one of operations  403 ,  408 , and  413  of  FIG. 4  as described in embodiment 1. From the viewpoint of UE 1   510 , the UE 1   510  may receive the DL data from the LAA cell  505  in subframes n, n+1, n+3, and n+4 of the channel occupancy time  530 . At this time, the LAA cell  505  may perform the DL data transmission to multiple UEs using different frequency resources (e.g. Resource Block) or different spatial resources (spatial domain) through MU-MIMO. In the case of the FDD system, UE 1   510  and UE 2   511  which receive data in subframe n may transmit DL data reception results  550  and  555  to the LAA cell  505  at subframe n+4. The LAA cell  505  may retransmit the DL data depending on the received DL data reception results. That is, the LAA cell  505  which has transmitted the DL data to UE 1   510  and UE 2   511  during the channel occupancy time  530  may receive the DL data reception results from the UEs in the subframes n+4 to n+8. At this time, the eNB may change the contention period for channel sensing operation based on the received data reception results. Although the description is made based on the transmission relationship between data transmission time of the LAA cell and the data reception result transmission time of the UEs assuming the LAA cell and UE operating in the FDD mode for explanation convenience, the methods according to the above embodiment can be applied to the LAA cell and UE operating in the TDD mode identically according to the predefined time (e.g. HARQ feedback timing specified in TS36.213). 
     At this time, the contention period for channel sensing operation may be changed (or configured) based on the DL data reception result from the UE as follows. 
     Method A-1: Use DL data reception results from part or all of the UEs in correspondence to all data transmitted during the channel occupancy time of the LAA cell 
     Method A-2: Use DL data reception results from part or all of the UEs in correspondence to the data transmitted at specific data transmission time during the channel occupancy time of the LAA cell 
     Method A-3: Use data reception results received from part or all of the UEs before the new channel sensing operation start time or the most recently received one of the data reception results 
     Although the channel sensing operation start time has been described under the assumption that a random variable is selected, right before the channel sensing operation start time, to configure the channel sensing period necessary for perform new channel sensing operation, if the real channel sensing operation start time and the time at which the random variable is selected in the contention period for the channel sensing period configuration mismatch, the above described method may be applied based on the time of selecting the random variable within the contention period for configuring the channel sensing period configuration. 
     For the case where the LAA cell receives no data reception results corresponding to data which it has transmitted in the channel occupancy period before the new channel sensing operation execution time point or the UE receives no data reception result corresponding to the UL data or the LAA cell receives all data reception results corresponding to the DL data transmitted in the channel occupancy period before the new channel sensing operation execution time point, it may be possible to apply different contention period configuration methods. A description is made with a DL example hereinafter. If the LAA cell receives all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may configure the contention period for a new channel occupancy period based on the all or part of the data reception results in correspondence to the DL data transmitted in the channel occupancy period (e.g. Method A-1). If the LAA cell does not receive all data reception results from the UEs in correspondence to the DL data transmitted in the channel occupancy duration before the new channel sensing operation start time, it may configure the contention period for a new channel occupancy period based on part of the data reception results from the UEs in correspondence to the DL data transmitted in the previous channel occupancy period like Method A-7. 
     It is also possible to configure such that the new channel sensing operation start time of the LAA cell arrives after receiving all of the ACKs corresponding to the DL data transmitted in the previous channel occupancy period. 
     A description is made of Method A-1 hereinafter with reference to  FIG. 5 . The LAA cell  505  receives the data reception result (ACK/NACK) corresponding to the data transmitted by the LAA in the channel occupancy duration  530  from the UEs in the duration [n+4˜n+8]. Accordingly, it may be possible to change (or configure) the contention period for new channel sensing operation based on the data reception results corresponding to all data transmitted from the LAA cell to one or more UEs during the channel occupancy period. At this time, the data reception results transmitted in the time duration shorter than a normal LTE subframe (1 ms) may not be included in determining the contention period change reference. If the data reception results corresponding to all data transmitted from the LAA cell to one or more UEs during the channel occupancy period include at least one NACK, the LAA cell may increase the length of the contention period for new channel sensing operation or maintain the previous length of the contention period to be equal to the previous contention period. If the data reception results corresponding to all data transmitted from the LAA cell to one or more UEs during the channel occupancy period include n NACK, the LAA cell maintains the contention period for new channel sensing operation to be equal to the previous contention period or decreases the length of the contention period. At this time, the contention period may be set to the initial value. 
     Referring to  FIG. 5 , a description is made of Method A-2. The LAA cell  505  receives data reception results corresponding to the data transmitted by the LAA in the channel occupancy duration  530  from the UEs in the duration [n+4˜n+8]. At this time, the LAA may change (or configure) the contention period for use in performing the new channel sensing operation based on the data reception result corresponding to the data transmitted at a specific data transmission timing e.g. the data reception result corresponding to the data transmitted at the last subframe n+4 having the length of 1 ms during the channel occupancy period  530 , data reception result corresponding to the data transmitted at the first subframe n having the length of 1 sm during the channel occupancy period  530  of the LAA cell, or the data reception result corresponding to the data transmitted at a subframe predefined or configured according to the A/N transmission time relationship predefined between the LAA cell and UE and channel occupancy period. For example, in the case of FDD mode, the A/N transmission time relationship of the UE predefined between the LAA cell and UE is transmitting A/N 4 ms after the data transmission timing of the LAA cell. Accordingly, if the channel occupancy period of the LAA cell is subframes n to n+10, it may be possible to use the data reception results corresponding to the data transmitted by the LAA cell at a specific subframe n+6 or specific subframe set (e.g. subframe n˜n+6). At this time, if the data reception results corresponding to the data transmitted to at least one UE in a predefined or configured specific subframe during the channel occupancy period include at least one NACK, the LAA cell may increase the contention period for performing new channel sensing operation or maintain the contention period to be equal to the previous contention period. If the data reception results corresponding to the data transmitted to at least one UE in a predefined or configured specific subframe during the channel occupancy period include no NACK, the LAA cell may maintain the contention period for performing new channel sensing operation to be equal to the previous contention period or decrease the contention period. At this time, the contention period may be set to the initial contention period value. 
     With reference to  FIG. 5 , a description is made of the Method of A-3. The LAA  505  operating in the FDD mode receives the channel reception results in the duration [n+4˜n+8] in correspondence to the data transmitted by the LAA  505  to the UEs  510  and  511  in the channel occupancy time  530 . 
     At this time, the LAA cell  505  may configure the contention period for a new channel sensing operation  540  based on the reception results received from the UEs in the duration starting from the previous channel sensing operation start time till right before the new channel sensing operation  540  in correspondence to the DL data transmitted by the LAA in the unlicensed band or the data reception results  550  and  555  that are most recently received from the UEs. That is, it may be possible to change or configure the contention period for the new channel sensing operation based on the data reception result corresponding to the DL data which are received from the UE before the start of the new channel sensing operation or before selecting the random variable N for use in performing the new channel sensing operation  540  or the most recently received one of the data reception results. That is, if at least one NACK  555  is included in the data reception results received in the duration from the previous channel sensing period  520  to the new channel sensing period  540  or in the most recent data reception results  550  and  555  received between the previous channel sensing period  520  and the new channel sensing period  540 , the LAA cell may configure the contention period to be longer than or equal to the previous contention period. If no NACK is included in the data reception results or in the most recent data reception results  550  and  555  received between the previous channel sensing period  520  and the new channel sensing period  540 , the LAA cell may configure the contention period to be equal to or less than the previous contention period. At this time, the LAA cell may initialize the contention period for the new channel sensing period  540  to the initial contention period. 
     At this time, although the above descriptions have been made to the cases where the LAA cell changes or configures the contention period for channel sensing operation based on the DL data reception results corresponding to the data transmitted to the UE during one occupancy period, the above methods may be applied in the cases where the LAA cell changes or configures the contention period for the channel sensing operation based on the DL data reception results corresponding to the data transmitted to the UE during a plurality of channel occupancy periods. For example, Method-1 may be used for changing or reconfiguring the contention period based on the DL data reception results corresponding to the data transmitted to the UE during the two channel occupancy periods  530  and  535 . 
     At this time, the LAA cell may uses the A/N information received from part or all of the UEs as the contention period change reference. For example, some UEs selected from all of the UEs which have transmitted the data reception results based on the channel quality information or allocated MCS value may be configured as the contention period change reference UEs according to the contention period change method. For example, the UEs allocated lowest MCS or the MCSs in a range selected by the LAA cell may be configured as the contention period change reference UEs according to the contention period change method. That is, the UEs with low MCSs may be regarded as victims of interference from neighboring devices so as to be used as the contention period change reference UEs. It may also be possible to configure the UE which transmits the measured channel information to the LAA cell most recently among the UEs transmitting the data reception results or the UE which transmits a signal (e.g. RSSI) predefined separately for conveying the channel condition of the UE as the contention period change reference UE according to the contention period change method. The contention period configuration reference UE may be configured based on one or any combination of the above methods. 
     The LAA cell may configure the contention period for the second channel sensing operation based on part or all of the data reception results transmitted by the UEs using at least one or any combination of the above method. At this time, the contention period change method is described in association with the application to the channel sensing operation based on the ACK/NACK received from the UEs in association with the second channel sensing operation, but it may also be used to change at least one of the first and third channel sensing operation criteria using the contention period change method or at least one of the criteria. For example, it is determined that it is necessary to change the contention period using the methods to be described hereinafter, it may be possible to change at least one of the first or third channel sensing operation criteria (e g channel sensing reference time) according to a predetermined method or set to other values. 
     The LAA cell applies a contention period change method to the second channel sensing operation using the contention period configuration reference time and ACK/NACK received from the contention period configuration reference UE as follows. 
     Method B-1: If at least one NACK is received from the contention period configuration reference UE during the contention period change reference time, then it maintains or changes the contention period applied to the next second channel sensing operation. 
     Method B-2: Maintain or change the contention period applied to the next second channel sensing operation based on the number or ratio of NACKs (or ACKs) received from the contention period configuration reference UE during the contention period configuration reference time. 
     With reference to  FIG. 5 , a description is made of Method B-1 in detail. In the case that the contention period configuration reference time is configured to [n+4 n+8] corresponding to the channel occupancy time  530  as in Method A-1 and all UEs which transmit data reception results in the contention period configuration reference time as the contention period configuration reference UEs, the LAA cell receives the NACK  555  from the UE 2   511  during the configured contention period configuration reference time and thus may change (e.g. increase) the contention period. At this time, the contention period may be changed exponentially (i.e., 16→32→64→128, . . . , →1024). That is, the contention period may increase exponentially from the minimum contention period value (or initial value) of 16 to the maximum contention period value of 1024. The exponential increment method is just an example, and it is possible to use a linear increment method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. If neither ACK nor NACK is received from the contention period configuration reference UEs in the configured contention period configuration reference time, the LAA cell may assume receipt of NACK and increase the contention period. If neither ACK nor NACK is received from the contention period configuration reference UEs in the contention period configuration reference time configured as above, the LAA cell may reuse the previously configured contention period without change or initialize the content period to the initial length. If no ACK is received from the contention period configuration reference UEs in the configured contention period configuration reference time or if all of the received are ACK, the LAA cell may change (e.g. decrease) the contention period. At this time, the contention period may be maintained as a predetermined length or decreased exponentially (e.g., 1024→512→. . . →32→16). At this time, the exponential decrement method is just an example, and it may be possible to use a linear decrement method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. The aforementioned contention period value denotes the maximum value of the contention period. However, the minimum contention period value may also be configured in the aforementioned method. For example, a description thereof is made under the assumption of the initial contention period configuration period of [1, 16]. If at least one NACK is received from the contention period configuration reference UE during the contention period configuration reference time, the LAA may change the contention period to be applied to the subsequent channel sensing operation to [1, 32]. Alternatively, it may be possible to change the maximum value of the previous contention period with the maximum value of the contention period. In the above example, the contention period may be set to [16, 32]. Changing from the maximum value of the contention period to the minimum value of the contention period is just an example, and it may also be possible to configure the contention period to the minimum value in various ways. For example, it is possible to set the contention period to a fixed minimum value of 0 or 1 which is predefined. 
     With reference to  FIG. 5 , a description is made of Method B-2 in detail. If the contention period change threshold of the LAA cell is set to (n+4˜n+8) corresponding to the channel occupancy time  530  of the LAA cell and if all UEs which transmit data reception results in the contention period change threshold as the contention period configuration reference UEs, the eNB receives 1 NACK  555  from the UE 2   511  during the configured contention period configuration change threshold as shown in  FIG. 5 . If it is determined that when the number of NACKs is equal to or greater than K (e.g. K=2) or when the percentage of NACKs is equal to or greater than P % (e.g. 10%) the contention period is changed in Method B-2, the contention period may not be changed but be maintained or may be decreased or initialized to the initial length in the example of  FIG. 5 . If the number or percentage of the NACKs received from the contention period configuration reference UEs during the contention period configuration reference time configured as above is equal to or K or P %, the LAA cell may maintain or increase the contention period. The contention period may be increased exponentially (e.g., 16→32→64→128→ . . . →1024) or decreased exponentially (e.g., 1024→512→ . . . →32→16). The exponential increment and decrement methods are just examples, and it may be possible to use a linear increment or decrement method or a method of selecting one of the preconfigured contention period candidate values (or a set of values {16, 32, 64, 256, 1024}) sequentially or randomly. If neither ACK nor NACK is received from the contention period configuration UEs during the contention period configuration reference time as configured above, the LAA cell may assume the receipt of NACK to increase the contention period or reuse the preconfigured contention period. At this time, the LAA cell may change the contention period even when the condition of Method B-2 is not fulfilled. That is, although the contention period change condition is configured to be fulfilled when the number of NACKs received from the contention period configuration reference UE during the contention period configuration reference time is equal to or greater than 2 in Method B-2, if neither ACK nor NACK is received from the contention period configuration reference UEs during the contention period configuration reference time, the LAA may change the contention period according to the configured threshold or regardless of the configured threshold. Also, if it is configured that when the number or percentage of the received NACKs is equal to or greater than K or P % the contention period is changed in Method B-2, the contention period may be changed according to the number or percentage of the received NACKs. For example, if the percentage of NACKs among the data reception results received from the UEs during the previous data transmission period is P % or if the percentage of the NACKs among the data reception results received during the period starting from the previous channel sensing period start time and ending right before the new channel sensing period start time or starting from the time when generating a random variable (e.g. random backoff number) for the previous channel sensing period and ending right before the time when generating a random variable (e.g., random backoff number of the new channel sensing period operation is P %, the contention period may increase as much as P % from the previous contention period. In more detail, if an exponential increment method (e.g., 16→32→64→128→. . . →1024) or an exponential decrement method (e.g., 1024→512→ . . . →32→16) is used for the contention period and if the percentage of NACKs among the data reception results received from part or all of the UEs during the period from the previous channel sensing operation start time to the new channel sensing operation start time is 40% (e.g., 4 of 10 A/N values are NACKs) as in Method A-7, it may be possible to increase the contention period for the new channel sensing operation as much as 40% (22.4, 22, or 23) in comparison to the previous channel contention period (e.g., 16). At this time, the increased contention period may be rounded off to an integer. If the percentage of NACKs of the data reception results received from part or all of the UEs during a period the previous channel sensing operation start time to the new channel sensing operation start time is higher than a predetermined threshold value (e.g. 50%), it may be possible to increase the contention period (16→32) regardless of the percentage of NACKs. At this time, if the number of NACKs is equal to or less than (or less than) K or the percentage of the NACKs is equal to or less than (or less than) P % (K and P may be 0), the contention period for the new channel sensing operation is set to the same value as the previous contention period or initialized to the initial length. 
     As described above, if neither ACK nor NACK is received from the contention period configuration reference UEs during the contention period configuration reference time (DTX) the LAA cell assumes receipt of NACK to increase the contention period according to the contention period determination criteria or reuse the previously configured contention period. A description is mad of the case where the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the configured contention period configuration reference time in more detail hereinafter. The LAA cell performs DL data transmission to the UE in the channel occupancy period, and the UE which receives the data transmits to the LAA cell the data reception result through one of the licensed band or the unlicensed band through which the data have been received or another unlicensed band. Typically, the LAA cell sends the UE a DL control channel (PDCCH) carrying DL data scheduling information for transmitting DL data (PDSCH). The UE checks the scheduling information included in the UL control channel and performs DL data reception operation as scheduled. However, if the UE does not receive the control channel carrying the scheduling information through the DL control channel, it determines that it has not been scheduled by the LAA cell and thus neither perform the DL data reception operation nor transmit any data reception result. Also, it may be possible that the LAA cell has transmitted the scheduling information to the UE through the DL control channel and the UE checks the scheduling information received through the DL control channel and transmits the data reception result to the LAA cell through a licensed band or an unlicensed band but the LAA cell does not receive the data reception result. At this time, if the UE is configured to transmit the data reception result through the licensed band and if the LAA cell receives neither ACK nor NACK from the contention period configuration reference UEs during the contention period configuration reference time (DTX), it may be possible to do not increase the contention period for unlicensed band channel sensing but to maintain the previous contention period or use the initial value. Also, if the LAA cell receives the DTX or NACK/DTX from the contention period configuration reference UEs during the contention period configuration reference time, it determines regards the receipt of the DTX or NACK/DTX as the receipt of NACK or distinguishes between the NACK/DTX and DTX. For example, since LAA cell cannot identify whether the NACK/DTX information is NACK or DTX transmitted by the LAA UE, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as not to be included in determining the contention period. That is, the DTX may be excluded in determining the contention period. For more details on ACK, NACK, NACK/DTX, and DTX configuration and transmission method, see TS36.211, TS36.212, and TS36.213. For example, a UE configured to transmit ACK/NACK information to the LAA with 2 antenna ports and PUCCH format 1b with channel selection can transmit to the LAA cell four types of reception status information of ACK, NACK, DTX, and NACK/DTX. At this time, the LAA cell may interpret the DTX and NACK/DTX information as NACK or distinguish NACK/DTX and DTX among the ACK/NACK status information types. For example, since the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. At this time, it may be possible to interpret both the NACK/DTX and DTX as not receiving ACK/NACK so as to be excluded in determining the contention period. 
     If the LAA UE which is configured with the ACK/NACK information transmission scheme for cell 1 and cell 2 with the PUCCH format 1b with channel selection does not receive PDCCH scheduling PDSCH to the cell 1 and cell 2 (DTX) or if the combination of ACK/NACK information for cell 1 and cell 2 includes only DTX or NACK/DTX without ACK and NACK, the LAA cell does not transmit any ACK/NACK information to the LAA cell (not transmission). The LAA UE configured with the PUCCH format 1b with channel selection does not transmit ACK/NACK information, and the LAA cell which has not received the ACK/NACK information from the LAA UE interprets the DTX or NACK/DTX as not receiving ACK/NACK so as not to be included in determining the contention period. At this time, it may be possible to interpret both the DTX and NACK/DTX for the LAA cell as NACK or distinguish between NACK/DTX and DTX. For example, since the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX transmitted by the LAA UE, it interprets the NACK/DTX as NACK and the DTX as not receiving ACK/NACK so as not to be included in determining the contention period. At this time, assuming that cell 1 is a licensed band cell (licensed carrier) and cell 2 as an LAA cell, if the LAA UE configured with the PUCCH format 1b with channel selection determines cell 1 as DTX and cell 2 as NACK/DTX, the LAA UE may transmit no ACK/NACK information to the LAA cell. In this case, the LAA cell cannot identify whether the NACK/DTX information is NACK or DTX for cell 2 of the LAA UE, it interprets the NACK/DTX as NACK so as to be included in determining the contention period. 
     The contention period may be changed according to the above described criteria and schemes in such a way of using the data reception results received from the contention period configuration reference UEs during the configuration period configuration reference time with the contention period channel threshold set by the LAA cell to determine whether to change the contention period and changing the contention period based on the determination result and contention period change scheme. However, since a new channel sensing operation  630  may be executed before the change of the contention period, there is a need of a contention period configuration method for the channel sensing operation  630  executed before the contention period change time point. 
     With reference to  FIG. 6 , the LAA cell  600  may perform the channel sensing operation  610  during the preconfigured contention period to determine whether the channel is occupied, for data transmission to the UE  605 . If it is determined that the channel is idle, the LAA cell  600  occupies the channel during the configured channel occupancy period  620  or predetermined or preconfigured maximum channel occupancy period to transmit data to the UE  605 . At this time, it is assumed that the contention period change threshold of the LAA cell is configured with Method A-2 using the data reception result corresponding to the data transmitted in the last subframe of 1 ms in the channel occupancy period of the LAA cell, the contention period configuration reference UEs are all UEs which transmit data reception results during the contention period configuration reference time, and the contention period change method is Method B-1. That is, the LAA cell  600  changes the contention period based on the A/N  674  corresponding to the last subframe among the A/N&#39;s corresponding to data transmitted during the channel occupancy period  620 . However, it may be necessary for the LAA cell  600  to perform the channel sensing operation  630  for additional channel occupancy before the receipt of the A/N  674  corresponding to the channel sensing period  620 , i.e. before applying the changed contention period as shown in  FIG. 6 . 
     Since the LAA cell  600  may perform the channel sensing operation  630  before the contention period is changed, it is necessary to configure the contention period for the channel sensing operation  630  which is performed before the changed contention period application timing. 
     The contention period may be configured for the channel sensing operation  630 , which is performed before the contention period change timing, as follows. 
     Method C-1: Reuse contention period configured for previous channel occupancy 
     Method C-2: Use initial value of contention period 
     Method C-3: Change according to UE reception performance received before channel sensing operation 
     Method C-4: Use predefined contention period 
     Method C-1 is described hereinafter in more detail. Referring to  FIG. 6 , the contention period for the channel sensing operation  630  executed before the changed contention period application timing may be configured to be identical with the contention period used for the latest channel sensing operation  610 . Alternatively, the contention period for the channel sensing operation  630  which is performed before the changed contention period application timing is configured to use the initial value of the contention period as in Method C-2 or to use a predetermined period as in Method C-4. Alternatively, the contention period may be changed based on the DL data reception result, i.e. the A/N information received from the UEs, before the channel sensing operation  630  which is performed before the changed contention period application timing. For example, it may be possible to change the contention period for the channel sensing operation based on the DL data reception results, or most recent A/N information, received in the duration from time of the previous channel sensing operation  610  to the new channel sensing operation  630 . At this time, it may be possible to change the contention period using the DL data reception results from the UE by excluding at least one of the configured contention period change threshold and contention period configuration reference UE. 
     Method C-3 is described hereinafter in more detail with reference to  FIG. 6 . The LAA cell  600  operating in the FDD mode may receive the data reception results  670 , 671 , 672 , and  673  corresponding to the data, which have transmitted to the UE  605  during the channel occupancy period  620 , within the period [n+4˜n+8]. Although the description is made with a relationship between the data transmission time of the LAA cell and the data reception result transmission time of the UE under the assumption of the LAA cell and LAA UE, it may be possible to apply all of the methods proposed in the above embodiments to the communication between the LAA cell and UEs operating in the TDD mode according to a predefined timing (e.g., HARQ feedback timing defined in TS36.213). 
     At this time, it may be possible to configure the contention period for the channel sensing operation  630  based on the data reception results received from the UEs till before performing the new channel sensing operation  630  including the previous channel sensing operation  610  as in Method C-3. At this time, if the LAA cell  600  does not receive the data reception result from the UEs in the duration from the start time of the previous channel sensing operation  610  till just before the start time of the new channel sensing operation  630 , it may be possible to reuse the contention period configured for the previous channel sensing operation  610 , the initial contention period, or a predetermined contention period as the contention period for the new channel sensing operation  630 . A description is made of a case where the LAA cell  600  performs the channel sensing operation  630  to occupy the unlicensed band as denoted by reference number  640 . The LAA cell  600  receives the data reception results  670 ,  671 ,  672 , and  673  from the UE  605  in the channel sensing and occupancy periods  630  and  640 . If it is necessary to reoccupy the unlicensed band after the channel occupancy period, the LAA cell may perform the new channel sensing operation  650  to reoccupy the unlicensed and. At this time, according to Method C-3, the contention period for the channel sensing operation  650  may be configured based on the data reception results  670 ,  671 ,  672 , and  673  received in the duration from the start time of the previous channel sensing operation  630  to the start time of the new channel sensing operation  650  or the most recently received data reception result  673 . For example, if the data reception result  673  received most recently from the UE in the duration from the start time of the previous channel sensing operation  630  to the start time of the new channel sensing operation  650  is NACK, it may be possible to configure the contention period for the new channel sensing operation  650  by maintaining or increasing the contention period of the previous channel sensing operation exponentially or linearly or selecting one of the preconfigured contention period candidate values assuming that the LAA cell is configured to change or configure the contention period based on the data reception results received most recently from the UE. If the data reception result  673  transmitted by the UE is ACK, it may be possible to configure the contention period by maintaining or decreasing the contention period of the previous channel sensing operation  630  exponentially or linearly or selecting one of the preconfigured contention period candidate values. 
     It may be possible not to use the previously changed contention period or to perform the channel sensing operation by applying a separately defined contention period. For example, it may be possible to perform the channel sensing operation with another contention period configured for the channel occupancy to transmit a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without common data transmission (PDSCH/PUSCH) as denoted by reference numbers  620 ,  640 , and  660  and UE-specific data transmission. For example, in the case of attempting channel occupancy for transmitting a control channel (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission, it may be possible to use another contention period  680  different from the previously changed contention period. For example, it may be possible to perform the channel sensing operation using the initially configured contention period or the contention period configured separately for transmitting control information (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission. In the case of transmitting a signal, it may also be possible to perform the channel sensing operation during a fixe contention period or a predefined or preconfigured period to occupy the channel according to the channel sensing operation result. At this time, it may also be possible to occupy the channel to transmit the control information without extra channel sensing operation. It may also be possible to occupy the channel without at least one of the first to third channel sensing operations. It may also be possible to perform the channel sensing operation for transmitting control information (e.g. (E)PDCCH, Discovery RS, SRS, and CSI-RS as denoted by reference number  685  of  FIG. 6 ) without UE-specific data transmission by setting at least one of the first to third channel sensing operation thresholds to the minimum value. If it is necessary to occupy the channel for normal data transmission  695  after the channel occupancy for control channel transmission, the LAA cell  600  may perform the channel sensing operation with the previously configured contention period (e.g., contention period changed through the channel occupancy period  660 ), the contention period  650  used for channel occupancy for previous data transmission, or the initial contention period. 
     A description is made of the method for using the unlicensed band for both the UL and DL between the LAA cell and UE hereinafter with reference to  FIG. 7 . At this time, this method may be applicable to the case of using the unlicensed band for one of UL and DL between the LAA cell and UE. 
     Referring to  FIG. 7 , the LAA cell  700  configures the cell  702  operating in the licensed band as a PCell and the cell  705  operating in the unlicensed band as a SCell to communicate with the LAA UEs  710  and  712  on the licensed and unlicensed bands. Although the description is directed to the case of a CA aggregating one licensed band and one unlicensed band, it may also be possible to aggregate one or more licensed bands and one or more unlicensed bands according to the capabilities of the LAA cell and LAA UE and minimum values of the LAA cell and UE capabilities. Although  FIG. 7  is directed to the case where the PCell is operating in the TDD mode (UL/DL configuration 1, Table 4.2-2 in TS36.211), this embodiment may also be applicable to the cells operating in the FDD mode or in other TDD configuration modes. The TDD licensed band cell  702  is comprised of the DL subframe  717 , UL subframe  720 , and special subframe  725 . The structure of the special subframe is out of the scope of the disclosure, detailed description thereof is omitted herein. For detailed description of the special subframe  725  of the TDD cell operating in the licensed band, see the LTE standard TS36.211 and TS36.213. In order to use the SCell  705  operating in the unlicensed band, the LAA cell  700  may perform the channel sensing operation  730  and transmit a channel occupancy signal (reservation signal) or initial signal to occupy the channel and/or acquire synchronization of the UE in a predefined data or control signal transmission available period as denoted by reference number  735 . Although it is assumed that the data or control signal transmission available time is used as start time point of every subframe (or first OFDM symbol of the subframe) in  FIG. 7 , the data or control signal transmission available time may be predefined or configured as one or more OFDM symbols including at least the start time of the subframe through higher layer signaling. After performing the channel sensing operation  730  in the unlicensed band, the LAA SCell  705  occupied the unlicensed band may transmit at least one signal such as DL control channel, DL control signal, DL data, and UL control information in the unlicensed band. For example, the LAA SCell  705  may transmit the DL control channel, DL control signal, and DL data to the UE  710  and the unlicensed band UL control information to the UE  710  in the unlicensed band. At this time, the unlicensed band UL control information of the UEs  710  and  712  may be transmitted to the UE through the licensed band LAA PCell  702  or the unlicensed LAA SCell  705  depending on the LAA cell configuration. If the DL data are received from the LAA SCell  705  in the unlicensed band DL subframe  740 , the UE  712  transmits the data reception result (e.g. ACK and NACK) to the LAA PCell  702  using the licensed UL resource predefined or configured by received UL control signal as denoted by reference number  790 . If the DL data  779  is received from the LAA SCell  705  in the unlicensed band DL subframe  740 , the UE  712  may transmit the data reception result (e.g. ACK and NACK) to the LAA SCell  705  using the unlicensed UL resource predefined or configured by a received UL control signal. The UE which has received the unlicensed band UL control information from the LAA PCell  702  or LAA SCell  705  in the DL subframe  740  may perform UL band UL transmission  774  in the UL signal transmission timing (subframe n+4) configured by the LAA PCell  702  or the LAA SCell  705 . At this time, the UE  710  may perform unlicensed band channel sensing operation  772  before the unlicensed band UL transmission  774  timing configured by the LAA SCell  705 . 
     The channel sensing operation  772  for the unlicensed band UL transmission of the UE may be different from the channel sensing operation  730  for the unlicensed band DL transmission of the SCell. For example, the channel sensing operation  730  for the unlicensed band DL transmission is performed with the changed contention period or fixed contention period or fixed channel sensing period, and the channel sensing operation  772  for the unlicensed band UL transmission of the LAA UE is performed in such a way of transmitting the unlicensed band UL transmission without extra channel sensing operation. Alternatively, the channel sensing operation for the unlicensed band DL transmission of the LAA SCell is configured to change the contention period, and the channel sensing operation  772  for the unlicensed band UL transmission of the LAA UE may be configured to fix the contention period or to be performed during a fixed channel sensing period (e.g. fixed CCA length or predefined number of (E)CCA slots). Alternatively, although both the channel sensing operation  730  for the unlicensed band DL transmission of the LAA SCell and the channel sensing operation  772  for the unlicensed band UL transmission of the LAA UE are configured to change the contention period, the minimum and maximum values of the contention period for the channel sensing operation  772  for the unlicensed band UL transmission of the LAA UE may be set to values less than the minimum and maximum values of the contention period used in the channel sensing operation  730  for the unlicensed band DL transmission of the SCell. 
     The channel sensing operation  772  for unlicensed band UL transmission of the UE may differ from the channel sensing operation  730  for unlicensed band DL transmission of the SCell. For example, the channel sensing operation  730  for the unlicensed band DL transmission of the LAA SCell 
     At this time, the LAA cell may notify the UE of the reception result corresponding to the UL data through the DL control channel. That is, if the UL data are received from the UE, the LAA cell may send the UE the data reception result along with the UL scheduling information through the DL control channel. At this time, the UE may determine the data reception result corresponding to the UL data based on at least one of New Data Indicator (NDI), HARQ Process number, and Redundancy Version (RV) carried in the UL scheduling information transmitted by the LAA cell. For example, if the UL data transmitted by the UE is not received correctly, the LAA cell transmits the scheduling information including the NDI bit which is not toggled to be identical with that of the previous UL scheduling information. That is, since the NDI is toggled when new data is transmitted, the UE may determine the UL data reception result at the LAA cell based on the NDI value. If a RV configuration order for retransmission is predefined, the UE may determine the UL data reception result at the LAA cell according to the RV value included in the UL scheduling information. For example, if the current NDI and RV match the NDI and RV carried in the previous scheduling information, it may be possible to determine that the data reception result is NACK. At this time, the UL data reception result determination based on the NDI and RV may be applied to the same HARQ process number. 
     Also, the channel sensing operation for DL of UL transmissions may be configured depending on the type and mode of the cell configuring (or scheduling) the DL or UL transmission in the unlicensed band. That is, the channel sensing operation schemes of the LAA cell and LAA UE may be determined differently for: 
     (E-1) the case of configuring or scheduling DL reception and UL transmission in the LAA SCell or unlicensed band cell of the UE using the DL control channel of the LAA PCell or licensed band cell, 
     (E-2) the case of configuring or scheduling the UL transmission in the LAA SCell of the UE using the DL control channel of the LAA PCell and configuring or scheduling DL reception in the LAA SCell of the UE using the DL control channel of the LAA SCell, and 
     (E-3) the case of configuring or scheduling the DL reception and UL transmission in the LAA SCell of the UE using the DL control channel of the LAA SCell. 
     For example, with reference to  FIG. 7 , the channel sensing operation scheme for the case of configuring or scheduling UL transmission in the LAA SCell of the LAA UEs  710  and  712  using the DL control channel of the LAA SCell  750 , i.e. the case where the cell for scheduling the UL transmission of the UE is identical with the cell for use in UL transmission of the UE, may be configured to be different from the case of configuring or scheduling the UL transmission through the LAA SCell of the LAA UEs  710  and  712  using the DL control channel of the LAA PCell, i.e. the case where the cell for scheduling the UL transmission of the UE is different from the cell for use in UL transmission of the UE. For example, the contention period for channel sensing operation is configured to change for the case of configuring or scheduling the UL transmission through the LAA SCell of the LAA UEs  710  and  712  using the DL control channel of the LAA PCell  702  or to be fixed (e.g. to the minimum contention period), to perform the channel sensing operation with no contention period (e.g. maximum value of the contention period is set to 0 or the channel sensing operation is performed in the fixed CCA period), or to perform UL transmission without extra channel sensing operation for the case of configuring or scheduling the UL transmission through the LAA SCell of the LAA UEs  710  and  712 . Alternatively, it may be possible to configure the size of the contention period for the channel sensing operation for the case of configuring or scheduling the UL transmission through the LAA SCell of the LAA UEs  710  and  712  using the DL control channel of the LAA PCell  702  to be different from (less than) the size of the contention period for the channel sensing operation for the case of configuring or scheduling the UL transmission through the LAA SCell of the LAA UEs  710  and  712  using the DL control channel of the LAA SCell  701 . 
     At this time, if the UE needs to perform the channel sensing operation  772  for unlicensed band UL transmission, the LAA SCell  705  may not perform the unlicensed band DL signal or channel transmission during the period of the channel sensing operation  772  for UL transmission of the UE. For example, the LAA SCell  705  may not transmit any DL signal or channel in the unlicensed band for the duration including the period of channel sensing operation  772  for the unlicensed band UL transmission  774 . At this time, the period  745  during which the LAA SCell  705  does not transmit any DL signal or channel for unlicensed band channel sensing operation of the UE may be configured to include at least one of channel sensing time of the UE, a period for the UE to transmit a UL signal in advance to compensate for signal transmission/reception delay caused by distance between the LAA cell and the UE (e.g. Timing Advance (TA)), and the time required for the UE to switch from DL reception to UL transmission. 
     The UE  710  may transmit the UL data or control signal to the LAA cell  700  using the unlicensed UL depending on the result of the unlicensed band channel sensing operation  772  performed before the time of the unlicensed band UL transmission  774  configured by the LAA SCell  705  through the licensed DL  717  or unlicensed DL  740  (and similarly for unlicensed DL  741 ,  742 , and  743 ). At this time, the time periods  780 ,  785  and  796  during the UL transmission  774  of the UE  710  (e.g. last SC-OFDM symbol duration of the UL subframe) may be configured not to transmit the UL signal in order for the UE or other UEs or the LAA cells to perform the channel sensing operation correctly. For example, it may be possible to puncture the last SC-OFDMA symbol or transmit the data or control signal using the UL SC-OFDM symbols with the exception of the last SC-OFDM symbol. At this time, the period in which the UL signal is not transmitted may be predefined or configured by the LAA cell through higher layer signaling. In the case that the LAA cell schedules the UE to perform the UL transmission consecutively in one or more UL subframes, e.g. the UE is scheduled to perform UL transmission in the UL subframes  774  and  776 , the UE may transmit UL signals without the period  774  having no UL transmission for channel sensing operation in the consecutive UL subframes. That is, the UE configured to transmit consecutive UL subframes may not perform the channel sensing operation in the configured consecutive UL subframes. At this time, although the LAA cell schedules the UE to perform UL transmission consecutively in one or more UL subframes, it may a be possible to transmit the UL data assuming the period in which a UL signal for channel sensing operation is not transmitted. In the case that the UE is scheduled to perform the UL transmission consecutively in one or more UL subframe and configured to transmit the UL signal without the period  774  having no UL transmission for channel sensing operation in the consecutive UL subframes, the UE may apply the period  785  having no UL signal transmission for other UEs with the exception of the corresponding UE or LAA cells to perform the channel sensing operation correctly in the last UL subframe among the consecutive UL subframes. Also, the UE may receive the information on the UL subframes which can be used with extra channel sensing operation or the information on the available time from the LAA cell through a separate licensed or unlicensed signal or may recognize the information according to the predetermined UL use time of the LAA SCell. In this case, the UE may transmit the UL signal with or without applying the period  796  having no UL signal transmission for channel sensing operation in the last UL subframe among the UL subframe which the LAA SCell can use without channel sensing period. In addition, the UE  712  may perform the unlicensed band channel sensing operation  792  before the UL transmission  794 , and the UE  712  may transmit the data reception result using the licensed UL resource predefined or configured by the received UL control signal as denoted by reference number  770 , as shown in  FIG. 7 . 
     The LAA SCell  705  which occupies the unlicensed band during the predefined or configured channel occupancy period after performing channel sensing operation  730  has to perform the channel sensing operation  750  for reoccupying the unlicensed band. At this time, the LAA SCell  705  may configure the contention period required for the channel sensing operation  750  in the method B-1 or B-2 or another method based on the DL signal transmission results which the LAA cell  700  receives from the UE through the licensed or unlicensed or the UL signal transmission result which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-1: Change or configure the contention period for the new channel sensing operation  750  of the LAA cell based on both the DL signal transmission results which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-2: Change or configure the contention period for the new channel sensing operation  750  (shown in  FIG. 7 ) of the LAA cell based on the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-3: Change or configure the contention period for the new channel sensing operation  750  of the LAA cell based on the most recently received or transmitted one of the DL transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Method D-4: Change or configure the contention period for the new channel sensing operation  750  of the LAA cell based on the signal transmission result corresponding to the data carried a specific DL subframe (e.g. first or last DL subframe) of the LAA SCell which the LAA cell  700  receives through the licensed or unlicensed band and the signal transmission result corresponding to the data carried in a specific UL subframe of the UE which the LAA cell transmits through the licensed or licensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     Although the channel sensing operation start time has been described under the assumption that the a random value for configuring the channel sensing period necessary for performing the channel sensing operation is selected right before starting the channel sensing operation, if the channel sensing operation start time and the time of selecting the random value in the contention period for the channel sensing period configuration mismatch, Method D-1, D-2, D-3, or D4 may be applied based on the timing of selecting the random value in the contention period for the channel sensing period configuration. 
     Although the DL and UL channel sensing operations and contention period configuration methods have been described from the viewpoint of the LAA cell for explanation convenience, it may be possible to interpret and apply from the viewpoint of the UE because the time point when the LAA cell transmits the UL signal transmission result to the UE through the licensed or unlicensed band matches the time point when the UE receives the signal. For example, it may be possible to change or configure the contention period for a new channel sensing operation at the UE based on the UL signal transmission result which the UE receives from the LAA cell through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . 
     A description is made of the example of using Method D-1 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on both the DL signal transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . That is, if the DL signal transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  so as to be longer than the contention period configured in the previous channel sensing operation  730  or configure or maintain the contention period for the new channel sensing period  750  to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission which the LAA cell transmits to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  so as to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial contention period or a predefined value. 
     A description is made of the example of using Method D-2 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . At this time, if the LAA SCell  705  receives more than one DL signal transmission result or transmits more than one UL signal transmission result to the UE, it may configure the contention period of the channel sensing operation  750  based on all of the transmission results. At this time, it may be possible to configure the contention period of the channel sensing operation  750  based on part of the transmission results carried in a specific DL subframe of a specific UE or LAA SCell (e.g. the first or last DL subframe) or a specific UL subframe of the UE (e.g. the first or last UL subframe). That is, if the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  to be longer than the contention period configured in the channel sensing operation  730  or configure or maintain the contention period to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial period or a predetermined value. 
     A description is made of the example of using Method D-3 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on the most recently received or transmitted one of the DL transmission result which the LAA cell  700  receives from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . At this time, if the LAA SCell  705  receives more than one DL signal transmission result from the UE or transmits more than one UL signal transmission result in the same subframe, it may be possible to configured the contention period of the channel sensing operation  750  based on all of the signal transmission results. At this time, it may be possible to configure the contention period of the channel sensing operation  750  based on part of the transmission results carried in a specific DL subframe (e.g. the first or last DL subframe) of the LAA SCell or the transmission results carried in a specific UL subframe (e.g. the first or last UL subframe) of the UE. That is, if the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  to be longer than the contention period configured in the channel sensing operation  730  or configure or maintain the contention period to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives most recently from the UE through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits most recently to the UE through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial period or a predetermined value. 
     A description is made of the example of using Method D-4 hereinafter with reference to  FIG. 7 . The LAA SCell  705  may configure the contention period of the channel sensing operation  750  based on the signal transmission result corresponding to the data carried a specific DL subframe (e.g. first or last DL subframe) of the LAA SCell which the LAA cell  700  receives through the licensed or unlicensed band and the signal transmission result corresponding to the data carried in a specific UL subframe of the UE which the LAA cell transmits through the licensed or licensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750 . At this time, if the LAA SCell  705  receives more than one DL signal transmission result from the UE or transmits more than one UL signal transmission result in the same subframe, it may be possible to configure the contention period of the channel sensing operation  750  based on all of the signal transmission results. At this time, it may be possible to configure the contention period of the channel sensing operation  750  based on the transmission result related to a specific UE among all of the transmission results. That is, if the DL signal transmission result which the LAA cell  700  receives from the UE in correspondence to the data carried in a specific DL subframe through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE in correspondence to the data carried in a specific UL subframe through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes at least one NACK, the LAA SCell  705  may increase the contention period for the new channel sensing period  750  to be longer than the contention period configured in the channel sensing operation  730  or configure or maintain the contention period to be equal to the previously configured contention period. If the DL signal transmission result which the LAA cell  700  receives from the UE in correspondence to the data carried in a specific DL subframe through the licensed or unlicensed band and the UL signal transmission result which the LAA cell transmits to the UE in correspondence to the data carried in a specific UL subframe through the licensed or unlicensed band in the duration from the start of the previous sensing operation  730  to the start of the channel sensing operation  750  includes no NACK, the LAA SCell  705  may configure or maintain the contention period for the new channel sensing period  750  to be equal to the contention period configured in the previous channel sensing operation  730  or decrease the contention period for the new channel sensing period  750  to be shorter than the previously configured contention period. At this time, the LAA cell may set the contention period for the new channel sensing period  750  to the value of the initial period or a predetermined value. 
     If the LAA cell  700  neither receives any DL signal transmission result from the UE nor transmit any UL signal transmission result to the UE through the licensed or unlicensed band in the duration from the end of the previous channel sensing operation  730  to the start of the channel sensing operation  750 , the contention period of the channel sensing operation  750  may be configured to maintain or reuse the contention period used for the previous channel sensing operation  730  or set to the initial value or a predefined value of the contention period. The contention period configuration method may change depending on whether the DL signal transmission result or UL signal transmission result is transmitted in the licensed or unlicensed band, if there is neither any DL signal transmission result transmitted by the UE nor UL signal transmission result transmitted by the LAA SCell in the licensed or unlicensed band. In more detail, in the case that the LAA cell  700  is configured to receive the DL signal transmission result of the LAA SCell from the UE in the licensed band and to transmit the UL signal transmission result to the UE in the licensed band in the duration from the end of the channel sensing operation  730  to the start of the channel sensing operation  750 , if there is neither the DL signal transmission result received from the UE nor the UL signal transmission result transmitted to the UE, the contention period of the channel sensing operation  750  may be configured to maintain or reuse the contention period used for the previous channel sensing operation  730 . In the case that the LAA cell  700  is configured to receive the DL signal transmission result of the LAA SCell from the UE in the licensed band and to transmit the UL signal transmission result to the UE in the unlicensed band in the duration from the end of the channel sensing operation  730  to the start of the channel sensing operation  750 , if there is neither the DL signal transmission result received from the UE nor the UL signal transmission result transmitted to the UE, the signal transmission result is regarded as NACK and thus the contention period of the channel sensing operation  750  is configured to be longer than the contention period used for the previous channel sensing operation  730 . 
     If the LAA cell  700  neither receive any DL signal transmission result of the LAA SCell from the UE nor transmit any UL signal transmission result to the UE through the licensed or unlicensed band in the duration from the end of the previous channel sensing operation  730  to the start of the channel sensing operation  750 , the contention period of the channel sensing operation  750  may be configured to maintain or reuse the contention period used for the previous channel sensing operation  730  or set to the initial value or a predefined value of the contention period. Also, if there is no data to transmit in the LAA SCell  705  or if the contention period is not changed during a predefined or configured time period, the contention period for the channel sensing operation may be set to the initial value. 
     Also, the channel sensing operation for UL transmission may be configured differently depending on the type of the cell configuring (or scheduling) the unlicensed band UL transmission. That is, the channel sensing operation for the case of configuring or scheduling the DL reception and/or UL transmission through the LAA SCell  705  (or unlicensed band cell) of the UE using the DL control channel of the LAA PCell  702  (licensed band cell) (i.e. the case where the cell for transmitting the scheduling information for the DL reception and/or UL transmission differs from the cell in which the UE transmits UL data) may differ from the channel sensing operation for the case of configuring or scheduling the DL reception and/or UL transmission through the LAA SCell  705  of the UE using the DL control channel of the LAA SCell  705 . Also, the channel sensing operation for the case of configuring or scheduling the DL reception and UL transmission through the LAA SCell  705  of the UE using the DL control channel of the LAA SCell  705  may differ from the channel sensing operation for the case of configuring or scheduling the DL reception through the LAA SCell  705  of the UE but not configuring or scheduling the UL transmission (or configuring or scheduling the UL transmission through the LAA SCell of the UE at the LAA PCell), i.e. the case where the cells configuring or scheduling the DL reception and UL transmission differ from each other. That is, the channel sensing operation of the LAA cell and LAA UE and the contention period change method for the channel sensing operation may be determined differently for: 
     (F-1) the case of configuring or scheduling DL reception and UL transmission in the LAA SCell or unlicensed band cell of the UE using the DL control channel of the LAA PCell or licensed band cell, 
     (F-2) the case of configuring or scheduling the UL transmission in the LAA SCell of the UE using the DL control channel of the LAA PCell and configuring or scheduling DL reception in the LAA SCell of the UE using the DL control channel of the LAA SCell, and 
     (F-3) the case of configuring or scheduling the DL reception and UL transmission in the LAA SCell of the UE using the DL control channel of the LAA SCell. 
     For example, the LAA cell and/or LAA UE configured to configure or schedule the DL reception and UL transmission through the LAA SCell  705  of the UE using the DL control channel of the LAA PCell  702  as in (F-1) (typically, referred to as cross-carrier scheduling) and to change the contention periods of all channel sensing operations and may change or configure the contention period for the channel sensing operation of the LAA SCell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission results corresponding to the DL transmission of the LAA SCell through the licensed or unlicensed band from the UE, during the period from the start time of the previous channel sensing operation  730  to the start of the channel sensing operation  750 , and may change or configure the contention period for the channel sensing operation of the LAA UE using at least one of methods D-1, D-2, D-3, and D-4 based on the UL signal transmission results corresponding to the UL transmission of the UE through the licensed or unlicensed band which is transmitted from the LAA cell to the UE. At this time, it may be possible to change or configure the contention period of the channel sensing operation of the LAA SCell, using at least one of methods D-1, D-2, D-3, and D-4, based on the DL signal transmission results (DL A/N) of the LAA SCell which the LAA cell  700  receives from the UE through the licensed or unlicensed band during the period from the start time of the previous channel sensing operation  730  to the start of the channel sensing operation  750  and the UL signal transmission results (UL A/N) which the LAA cell transmits to the UE through the licensed or unlicensed band. 
     Alternatively, with reference to  FIG. 7 , in the case of configuring or scheduling the UL transmission through the LAA SCell  705  of the UE using the DL control channel of the LAA PCell  702  and configuring or scheduling the DL reception through the LAA SCell  705  and LAA SCell  705  of the UE using the DL control channel of the LAA SCell  705  as in (F-2), the LAA cell and/or LAA UE configured to change the contention period for all channel sensing operations may change or configure the contention period for channel sensing operation of the last SCell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission results of the LAA SCell which the LAA cell  7000  receives from the UE through the licensed or unlicensed band in the duration from the start of the previous channel sensing operation  730  to the start of the channel sensing operation  750  and may change or configured the contention period for channel sensing operation of the LAA UE using at least one of methods D-1, D-2, D-3, and D-4 based on the UL signal transmission result of the UE which the LAA cell transmits to the UE. At this time, it may be possible to change or configure the contention period for the channel sensing operation of the LAA cell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission result (DL A/N) of the LAA SCell which the LAA SCell  700  received from the UE in the licensed or unlicensed band the UL signal transmission result (UL A/N) of the UE which the LAA cell transmits to the UE through in the licensed or unlicensed band in the duration from the start time of the previous channel sensing operation  730  to the start of the channel sensing operation  750 . 
     Alternatively, in the case that the LAA cell is configured to configure or schedule the DL reception and UL transmission through the LAA SCell  705  of the UE using the DL control channel of the LAA SCell  705  as in (F-3) (typically, referred to as self-scheduling), to change the contention period for the channel sensing operation in the LAA cell, and to use no channel sensing operation of the LAA UE, fixed channel sensing period, or fixed contention period, it may be possible to change or configure the contention period for the channel sensing operation of the LAA SCell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission result of the LAA SCell which the LAA cell  700  receives from the UE in the licensed or unlicensed band in the duration from the start time of the previous channel sensing operation  730  to the start time of the channel sensing operation  750  or to change or configure the contention period for the channel sensing operation of the LAA SCell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission result of the LAA SCell which the LAA cell  700  receives from the UE in the licensed or unlicensed band in the duration from the start time of the previous channel sensing operation  730  to the start time of the channel sensing operation  750 . At this time, if it is configured to change the contention period for all channel sensing operations of the LAA cell and LAA UE, it may be possible to change or configure the contention period for the channel sensing operation of the LAA SCell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission result of the LAA SCell which the LAA cell  700  receives from the UE in the licensed or unlicensed band in the duration from the start time of the previous channel sensing operation  730  to the start time of the channel sensing operation  750  or to change or configure the contention period for the channel sensing operation of the LAA UE using at least one of method D-1, D-2, D-3, and D-4 based on UL signal transmission result of the UE which the LAA cell transmits to the UE in the licensed or unlicensed band. At this time, it may be possible to change or configure the contention period for the channel sensing operation of the LAA SCell using at least one of methods D-1, D-2, D-3, and D-4 based on the DL signal transmission results (DL A/N) of the LAA SCell which the LAA cell  700  receives from the UE in the licensed or unlicensed band and the UL signal transmission results (UL A/N) of the UE which the LAA cell transmits to the UE in the licensed or unlicensed band, in the duration from the start time of the previous channel sensing operation  730  to the start time of the channel sensing operation  750  shown in  FIG. 7 . 
     At this time, the UL signal transmission results of the UE which the LAA cell sends to the UE in the licensed or unlicensed band for use in changing or configuring the contention period for the channel sensing operation of the LAA UE may be only the reception results corresponding to the UL signals transmitted from the UE to the LAA cell. 
     Also, the LAA cell may configure at least one of the minimum and maximum values of the contention period differently depending on the service quality requirements QoS) of the data to be transmitted in the channel occupancy period. In more detail, in the case of providing a real time service such as VoIP and streaming service of which service quality drops due to the data transmission delay, it may be possible to configure the minimum and maximum values of the contention period so as to minimize the contention period for channel sensing required for data transmission. For example, it may be possible to set the minimum and maximum contention period values for VoIP service to 7 and 15 respectively. In the case of providing an FTP service or HTTP service of which service quality is less influenced by data transmission delay, it may be possible to set the minimum and maximum contention periods to values greater than those for the VoIP service. For example, it may be possible to set the minimum and maximum values of the contention period for the FTP service to 12 and 1023 respectively. At this time, if the data transmitted from the LAA cell to the UE in the channel occupancy period include two or more different QoSs, the contention period for the channel sensing operation of the LAA cell may be configured differently depending on the QoS. That is, if the LAA cell transmits multiple data having different QoSs to multiple UEs in the channel occupancy period, the contention period for the channel occupancy operation of the LAA cell may be configured differently as follows. 
     Method G-1: Configure contention period according to QoS having the least contention period configuration value among multiple QoSs 
     Method G-2: Configure contention period according to QoS having maximum contention period configuration value among multiple QoSs 
     Method G-3: Configure contention period using contention period configuration values of part or all of multiple QoSs 
     For example, it is assumed that the LAA cell transmit data to two UEs. At this time, UE 1  is receiving data for a VoIP service, and UE 2  is receiving data for a FTP service. The minimum and maximum contention periods for VoIP service are 7 and 15 respectively, and the minimum and maximum contention periods for the FTP service are 15 and 1023 respectively according to QoS. That is, the maximum and minimum contention periods are determined based on the QoS of the service. In the case of transmitting the data for VoIP and FTP services to UE 1  and UE 2  during the channel occupancy period of the LAA cell, Method G-1 determines the contention period for the channel sensing operation necessary for the channel occupancy period based on the QoS having the least contention period configuration values among the contentions period configuration values for the VoIP and FTP services (e g minimum and maximum contention period values of 7 and 15). That is, the contention period may be configured with the minimum contention period value of 7 and maximum contention period value of 15. Method G-2 determines the contention period for the channel sensing operation necessary for the channel sensing period based on the QoS having the greatest contention period configuration value among the contention period configuration values for the VoIP and FTP services (e g minimum and maxim contention periods of 15 and 1023). That is, the contention period may be configured with the minimum contention period value of 15 and maximum contention period value of 1023. Method G-3 determines the contention period by averaging the contention period configuration values for the VoIP and FTP services. That is, it may be possible to determine the contention period for the channel sensing operation necessary for the channel occupancy period based on the contention period configuration values for the VoIP and FTP services, e.g. values of (11, 519) obtained by averaging the minimum contention period value (7, 15) and the maximum contention period value (15, 1023). 
     At this time, the QoS of the data transmitted in the previous channel occupancy period and the QoS of the data to be transmitted in the new channel occupancy period may differ from each other. At this time, it may be possible to configure the contention period for the channel sensing operation performed to occupy the channel newly based on the QoS of the data to be transmitted in the new channel occupancy period. At this time, the contention period for the new channel sensing operation may be configured differently depending on the data reception results transmitted in the previous channel occupancy period. A description is made thereof with reference to  FIG. 5 . It is assumed that the FTP service data are transmitted to UE 1  and UE 2  in the previous channel occupancy period  530  (shown in  FIG. 5 ). At this time, if the contention period for the case of performing the channel sensing operation  520  for the channel occupancy period  530  is configured according to Method G-1 among Methods G-1, G-2, and G-3, the minimum and maximum values of the contention period may be set to 15 and 1023 respectively. If the contention period for the channel sensing operation  520  is set to the minimum value of the contention period, the LAA cell  505  selects a random value in the contention period [0, 15] or [1, 15] and senses the channel during the channel sensing operation period corresponding to the selected value. In the case that the LAA cell transmits data to UE 1  and UE 2  in the channel occupancy period  530  and then wants to occupy the channel again during the channel occupancy period  535 , the LAA cell has to perform the channel sensing operation  540 . However, if the VoIP service data are transmitted to the UE 3  and UE 4  in the channel occupancy period  535 , the contention period for the channel sensing operation  540  may be configured with the minimum value of 7 and the maximum value of 15 according to Method G-1. That is, the LAA cell  505  selects a random value in the contention period [0, 7] or [1, 7] and senses the channel during the channel sensing operation period. In the case of using Method A-7, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  to the new channel sensing period  540  or the mostly recently received data reception result is NACK, the new channel contention period  540  may increase according to Method B-1 or B-2. That is, if the new channel contention period is configured with the exponential increment scheme, it has to increase [0, 15] or [1, 15] to [1, 15] or [0, 31]. In the case of transmitting data having different QoSs, however, the contention period configuration value may vary according to QoS of the data to be transmitted in the new channel occupancy period and the contention period configuration method (e.g. contention period is configured as [0, 7] or [1, 7] with Method G-1) but the number of contention period increments may be maintained. That is, if the VoIP service data are transmitted in the new channel occupancy period, the contention period for the new channel sensing operation may be configured according to the channel sensing period configuration for the VoIP service. In the case of using Method A-7, however, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, the new channel contention period  540  should be increased and thus the contention period for the new channel sensing operation increases from [0, 7] or [1, 7] to [0, 15] or [1, 15]. That is, the contention period for the channel sensing operation may be configured differently depending on QoS, but the number of contention period increments may be configured regardless of QoS. For example, if the contention period is increased from [0, 15] to [0, 31] and then to [0, 63], the number of contention period increments is 2. If the contention period is initialized from [0, 63] to the initial value, the number of contention period increments is 0. That is, in the case that there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, if it is necessary to increase the contention period according to Method B-1 or B-2, it may be possible to configure the new channel contention period  540  by applying the number of contention period increments which is increase by 1 in comparison to the number of previous contention period increments. That is, if the number of contention period increments in the previous channel sensing period  520  is 2, if there is a NACK among the data reception results received from the UEs in the duration from the previous channel sensing period  520  till right before the new channel sensing period  540  or the most recently received data reception result is NACK, and if it is necessary to increase the contention period according to Method B-1 or B-2, it may be possible to configure the new channel contention period  540  by applying the number of contention period increments of 3. That is, the contention period becomes [0, 127]. 
     Referring to  FIG. 8 , a description is made of the contention period configuration method of an eNB for channel sensing operation according to embodiments 1, 2, and 3 of the present disclosure. Specific time configured based on whole or part of the A/N transmission period of the UE in a channel occupancy period is configured as the contention period configuration reference time. It may also be possible to configure at least one of the DL signal transmission result of the LAA SCell which the LAA cell receives from the UE and the UL signal transmission result of the UE which the LAA cell transmits to the UE in the licensed or unlicensed between channel sensing operations of the LAA SCell as the contention period determination criterion. At this time, the contention period configuration criteria may be predefined. 
     The eNB configures the contention period configuration criteria at operation  801 . The eNB configures a contention period configuration reference UE at operation  802 . It may be possible to configure all or part of the UEs transmitting or receiving A/N as the contention period configuration reference UEs. The eNB configures the contention period change scheme at operation  803 . That is, the eNB may determine whether to change the contention period exponentially or linearly or based on a predetermined value at operation  803 . The eNB receives the reception result corresponding to the DL data at operation  804 , transmits the reception result corresponding to the UL data at operation  805 . At operation  806 , the eNB determines whether to change the contention period based on the contention period change scheme configured through steps  801 ,  802 , and  803  and the signal transmission results received at operation  804  and transmitted at operation  805 . If it is determined that there is no need to increase the contention period, the eNB may decrease or maintain the contention period or initialize the convention period to the initial value at operation  808  according to the contention period decrement scheme configured at operation  803 . If it is determined that there is a need to increase the contention period at operation  806 , the eNB may increase or maintain the contention period at operation  807  according to the contention period increment scheme configured at operation  803 . 
       FIG. 9  is a flowchart illustrating a contention period configuration method of a UE for channel sensing operation according to various embodiments of the present disclosure. 
     Referring to  FIG. 9 , a description is made of the contention period configuration method of a UE for channel sensing operation according to embodiments 1, 2, and 3 of the present disclosure with reference to  FIG. 9 . The UE receives the contention period configuration criteria configured by the eNB at operation  901 . Specific time configured based on whole or part of the A/N transmission period of the UE in a channel occupancy period is configured as the contention period configuration reference time. It may also be possible to receive at least one of the DL signal transmission result of the LAA SCell which the LAA cell receives from the UE and the UL signal transmission result of the UE which the LAA cell transmits to the UE in the licensed or unlicensed band between channel sensing operations of the LAA SCell as the contention period determination criterion. At this time, the contention period configuration criteria may be predefined. 
     The UE receives the configuration about the contention period configuration reference UE at operation  902 . The UE may configure all or part of the UEs transmitting or receiving A/N as contention period configuration reference UE. The UE receives the contention period change scheme at operation  903 . That is, it may be possible to change the contention period based on the number or percentage of the ACK/NACKs among the signal transmission results received at operation  904  and transmitted at operation  905  according to the contention period configuration criteria received at operation  901  and the contention period configuration reference UE information received at operation  902 . It may also be possible to change the contention period exponentially or linearly or based on a predetermined value according to the contention period change scheme received at operation  903 . At this time, operations  901 ,  902 , and  903  may be preconfigured. The UE determines whether to change the contention period based on the contention period change scheme received through operations  801 ,  802 , and  803  and the signal transmission results received at operation  904  and transmitted at operation  905 . If it is determined that there is no need to increase the contention period at operation  906 , the UE may decrease or maintain the contention period or initialize the convention period to the initial value at operation  908  according to the contention period decrement scheme configured at operation  903 . If it is determined that there is a need to increase the contention period at operation  906 , the UE may increase the contention period or maintain the previous contention period at operation  907  according to the contention period increment scheme configured at operation  906 . 
       FIG. 10  is a diagram illustrating a configuration of the eNB for configuring the contention period for channel occupancy operation of the UE using the unlicensed band according to various embodiments of the present disclosure (particularly embodiments 1, 2, and 3). The UE may increase or maintain the contention period at operation  907  according to the contention period increment scheme configured at operation  903 . 
     The receiver  1020  of the eNB receives signals from other eNBs or UEs, measures channels from the other eNBs and UEs, and senses an unlicensed band channel based on the channel measurement operation configuration values configured by the controller  1000 . The controller  1000  determines whether the unlicensed band is idle based on the information of the unlicensed band sensed by the receiver  1020 . If it is determined that the unlicensed band is idle, the controller  1000  of the eNB may transmit a signal for channel occupancy and control and data channel information to a specific UE by means of the transmitter  1010 . If it is determined that the unlicensed band is not idle, the controller  1000  controls the receiver  1020  to continue the channel sensing operation. 
     The controller  1000  may control configuring control channels such as PDCCH/EPDCCH and various reference signal transmission parameters, scheduling PDSCH/EPDSCH, and sets parameters and variables for use in channel sensing operation such as contention period. The controller may transmit the parameters necessary for communication between the eNB and the UE to the UE by means of the transmitter. 
     Meanwhile, the controller  1000  may control the receiver  1020  to receive feedback corresponding to the data transmitted in the first subframe arriving first after the sensing period in the unlicensed band from at least one UE. 
     The controller  1000  may assume the data reception state of the at least one UE as one of ACK, NACK, and Discontinuous Transmission (DTX) based on the feedback. The controller  1000  may also determine the length of the second sensing period based on the assumed data reception state of the UE. 
     The controller  1000  may transmit to the UE a control message for configuring the PUCCH format 1b. The controller  1000  may also control the receiver  1020  to receive the feedback predetermined in accordance with the data reception state from the UE. 
     The controller  1000  may also check the state predetermined in association with no receipt of the feedback when no feedback is received from the UE. If the feedback state is NACK/DTX, the controller  1000  may assume the UE&#39;s data reception state as NACK. 
     If it is assumed that the data reception state is DTX, the controller  1000  may not use the UE&#39;s data reception assumed as DTX in determining the length of the second sensing period. 
     In the case that the PUCCH format 1b is not configured to the UE, if it is assumed that the UE&#39;s data reception state is DTX, the controller  100  may interpret the DTX as NACK or may not use the DTX in determining the length of the second sensing period. 
     The DTX may mean that the feedback corresponding to the transmitted data is not received from the at least one UE. 
     The controller  100  may determine the ratio of NACK to all data reception states of the at least one UE and, if the ratio of the NACK to all data reception states is greater than a predetermined threshold, increase the range of the contention window (CW) value. The controller  1000  may determine the length of the second sensing period using a CW value in the range of the increased CW value range. 
     The transmitter  1010  and the receiver  1020  of the eNB may be implemented as a single component rather than two separate components. 
       FIG. 11  is a diagram illustrating a configuration of a UE considering channel occupancy state determination of the eNB operating on the unlicensed band according various embodiments 1, 2, and 3 of the present disclosure (particularly embodiments 1, 2, and 3). 
     Referring to  FIG. 11 , the controller  1100  of the UE receives the eNB-UE configuration information for use in receiving signals from the eNB using the licensed band and unlicensed band and operates on the unlicensed band based on the received configuration value. The controller  1100  may acquire the unlicensed band state information based on at least one of the configuration value for use in determining whether the eNB can schedule data in the subframe in which configured channel sensing operation is performed and the unlicensed band state information which the eNB transmits to the UE on the licensed or unlicensed band. The controller  1100  also determines reception result of the data signal from the eNB and transmits the data reception result to the eNB by means of the transmitter  1110 . 
     The controller  1100  may check the PDSCH/EPDSCH scheduling information based on the control information received by the receiver  1120 . The controller  1100  receives the PDSCH/EPDSCH by means of the receiver and decodes the PDSCH/EPDSCH by means of a decoder. 
     The transmitter  1110  and the receiver  1120  of the UE may be implemented as a single component rather than two separate components. 
       FIG. 12  is a flowchart illustrating a method of an eNB according to various embodiments of the present disclosure. Referring to  FIG. 12 , the eNB may configure a physical uplink control channel (PUCCH) format 1b to a UE. For example, the eNB may send the UE a control message for configuring PUCCH format 1b. 
     Referring to  FIG. 12 , if it is determined that the PUCCH format 1b is configured at operation S 1200 , the eNB may determine whether the feedback corresponding to the data transmitted to the UE is indicative of “No Transmission” at operation S 1210 . 
     For example, the data reception state of the UE may be predetermined according to the resource position of the feedback signal transmitted by the UE based on the PUCCH format 1b. In detail, if the feedback signal is received from the UE at a predetermined resource position, the eNB may assume that the UE&#39;s data reception state indicated by the feedback signal is ACK for cell 1 and NACK/DTX for cell 2. 
     If no feedback signal is received from the UE at the predetermined resource position, the eNB may assume that the UE&#39;s data reception state indicated by the feedback signal is DTX for cell 1 and NACK/DTX for cell 2. 
     It is determined that the feedback corresponding to the data transmitted to the UE is indicative of “No Transmission”, the eNB may determine whether the UE&#39;s data reception state which corresponds to the “No Transmission” is NACK/DTX at operation S 1220 . 
     If the UE&#39;s data reception state predetermined for “No Transmission” is NACK/DTX, the eNB assumes the UE&#39;s data reception state is NACK at operation S 1230 . For example, the eNB may check whether the ratio of the NACK to all feedbacks is equal to or greater than P % to determine the length of the next LBT. Accordingly, the eNB may reflect the reception state determined as NACK by eNB to check the ratio of NACKs. 
     If the UE&#39;s data reception state predetermined for “No Transmission” is NACK/DTX, the eNB may determine whether the predetermined UE&#39;s data reception state is DTX at operation S 1240 . 
     If the predetermined data reception state of the UE is DTX, the eNB may ignore the UE&#39;s data reception state at operation S 1250 . As described above, the eNB may check whether the ratio of the NACKs to all feedbacks received from the UEs is equal to or greater than P % to determine the length of the next LBT. At this time, in the case of ignoring the UE&#39;s data reception state, the eNB may not reflect the ignored data reception state to the total feedbacks. In detail, the eNB may exclude the number of ignored feedbacks from the total number of received feedbacks. 
     If it is determined that the PUCCH format 1b is configured to the UE at operation S 1200  and that the feedback corresponding to the data transmitted to the UE is not “No transmission” but DTX at operation  1210 , or if it is determined that the feedback corresponding to the data transmitted to the UE is DTX, the eNB may ignore the DTX or regards the DTX as NACK at operation S 1260 . 
     The description is directed to the case where the first one of two cells is a cell operating on a licensed carrier and the second one is a cell operating on an LAA carrier. At this time, the eNB may transmit the scheduling information for the second cell through the first cell. Transmitting the scheduling information for one cell through another cell in this way is referred to as cross carrier scheduling. In the cross carrier scheduling mode, if the feedback corresponding to the data transmitted to the UE is DTX, the eNB may ignore the DTX. 
     In the case of self-carrier scheduling mode in which the scheduling information for the first cell is transmitted through the first cell and the scheduling information for the second cell is transmitted through the second cell, if the feedback corresponding to the data transmitted to the UE is DTX, the eNB may assume the data reception state of the UE as NACK. 
     However, ignoring the DTX feedback or assuming the DTX feedback as NACK received from the UE according to whether the scheduling mode is the cross carrier scheduling or self-carrier scheduling is just an example. For example, the eNB may ignore the DTX feedback or assume the DTX feedback as NACK according to the configuration between the eNB and the UE even in the same scheduling method. 
     As described above, the present disclosure is advantageous in terms of improving coexistence performance of the devices sharing an unlicensed band by performing the channel occupancy operation efficiently and configuring the channel occupancy operation criteria unambiguously. 
     According to various embodiments of the present disclosure, the method of the eNB may be stored in a non-transitory readable medium after being coded in software. The non-transitory readable medium may be installed for use in various devices. 
     The non-transitory readable medium is a medium capable of storing data semi-persistently, unlike a medium storing data temporarily such as a register and a cache memory, and readable by devices. Examples of the non-transitory readable medium include compact disc (CD), digital versatile disc (DVD), hard disc, Blu-ray disc, universal serial bus (USB), memory card, and read only memory (ROM). 
     While the present disclosure has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims and their equivalents.