Patent Publication Number: US-9854463-B2

Title: Method and apparatus for transmitting split availability of cell in wireless communication system

Description:
This application is a 35 USC §371 National Stage entry of International Application No. PCT/KR2014/005354 filed on Jun. 18, 2014, and claims priority to U.S. Provisional Application No. 61/838,352 filed on Jun. 24, 2013, all of which are hereby incorporated by reference in their entireties as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting split availability of a cell in a wireless communication system. 
     Related Art 
     Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS. 
     The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement. 
       FIG. 1  shows LTE system architecture. The communication network is widely deployed to provide a variety of communication services such as voice over internet protocol (VoIP) through IMS and packet data. 
     Referring to  FIG. 1 , the LTE system architecture includes one or more user equipment (UE;  10 ), an evolved-UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC). The UE  10  refers to a communication equipment carried by a user. The UE  10  may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc. 
     The E-UTRAN includes one or more evolved node-B (eNB)  20 , and a plurality of UEs may be located in one cell. The eNB  20  provides an end point of a control plane and a user plane to the UE  10 . The eNB  20  is generally a fixed station that communicates with the UE  10  and may be referred to as another terminology, such as a base station (BS), a base transceiver system (BTS), an access point, etc. One eNB  20  may be deployed per cell. There are one or more cells within the coverage of the eNB  20 . A single cell is configured to have one of bandwidths selected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlink or uplink transmission services to several UEs. In this case, different cells can be configured to provide different bandwidths. 
     Hereinafter, a downlink (DL) denotes communication from the eNB  20  to the UE  10 , and an uplink (UL) denotes communication from the UE  10  to the eNB  20 . In the DL, a transmitter may be a part of the eNB  20 , and a receiver may be a part of the UE  10 . In the UL, the transmitter may be a part of the UE  10 , and the receiver may be a part of the eNB  20 . 
     The EPC includes a mobility management entity (MME) which is in charge of control plane functions, and a system architecture evolution (SAE) gateway (S-GW) which is in charge of user plane functions. The MME/S-GW  30  may be positioned at the end of the network and connected to an external network. The MME has UE access information or UE capability information, and such information may be primarily used in UE mobility management. The S-GW is a gateway of which an endpoint is an E-UTRAN. The MME/S-GW  30  provides an end point of a session and mobility management function for the UE  10 . The EPC may further include a packet data network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which an endpoint is a PDN. 
     The MME provides various functions including non-access stratum (NAS) signaling to eNBs  20 , NAS signaling security, access stratum (AS) security control, Inter core network (CN) node signaling for mobility between 3GPP access networks, idle mode UE reachability (including control and execution of paging retransmission), tracking area list management (for UE in idle and active mode), P-GW and S-GW selection, MME selection for handovers with MME change, serving GPRS support node (SGSN) selection for handovers to 2G or 3G 3GPP access networks, roaming, authentication, bearer management functions including dedicated bearer establishment, support for public warning system (PWS) (which includes earthquake and tsunami warning system (ETWS) and commercial mobile alert system (CMAS)) message transmission. The S-GW host provides assorted functions including per-user based packet filtering (by e.g., deep packet inspection), lawful interception, UE Internet protocol (IP) address allocation, transport level packet marking in the DL, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/S-GW  30  will be referred to herein simply as a “gateway,” but it is understood that this entity includes both the MME and S-GW. 
     Interfaces for transmitting user traffic or control traffic may be used. The UE  10  and the eNB  20  are connected by means of a Uu interface. The eNBs  20  are interconnected by means of an X2 interface. Neighboring eNBs may have a meshed network structure that has the X2 interface. The eNBs  20  are connected to the EPC by means of an S1 interface. The eNBs  20  are connected to the MME by means of an S1-MME interface, and are connected to the S-GW by means of S1-U interface. The S1 interface supports a many-to-many relation between the eNB  20  and the MME/S-GW. 
       FIG. 2  shows a block diagram of architecture of a typical E-UTRAN and a typical EPC. Referring to  FIG. 2 , the eNB  20  may perform functions of selection for gateway  30 , routing toward the gateway  30  during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCH) information, dynamic allocation of resources to the UEs  10  in both UL and DL, configuration and provisioning of eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway  30  may perform functions of paging origination, LTE_IDLE state management, ciphering of the user plane, SAE bearer control, and ciphering and integrity protection of NAS signaling. 
       FIG. 3  shows a block diagram of a user plane protocol stack and a control plane protocol stack of an LTE system.  FIG. 3 -( a ) shows a block diagram of a user plane protocol stack of an LTE system, and  FIG. 3 -( b ) shows a block diagram of a control plane protocol stack of an LTE system. 
     Layers of a radio interface protocol between the UE and the E-UTRAN may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. The radio interface protocol between the UE and the E-UTRAN may be horizontally divided into a physical layer, a data link layer, and a network layer, and may be vertically divided into a control plane (C-plane) which is a protocol stack for control signal transmission and a user plane (U-plane) which is a protocol stack for data information transmission. The layers of the radio interface protocol exist in pairs at the UE and the E-UTRAN, and are in charge of data transmission of the Uu interface. 
     A physical (PHY) layer belongs to the L1. The PHY layer provides a higher layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer, which is a higher layer of the PHY layer, through a transport channel. A physical channel is mapped to the transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. Between different PHY layers, i.e., a PHY layer of a transmitter and a PHY layer of a receiver, data is transferred through the physical channel using radio resources. The physical channel is modulated using an orthogonal frequency division multiplexing (OFDM) scheme, and utilizes time and frequency as a radio resource. 
     The PHY layer uses several physical control channels. A physical downlink control channel (PDCCH) reports to a UE about resource allocation of a paging channel (PCH) and a downlink shared channel (DL-SCH), and hybrid automatic repeat request (HARQ) information related to the DL-SCH. The PDCCH may carry a UL grant for reporting to the UE about resource allocation of UL transmission. A physical control format indicator channel (PCFICH) reports the number of OFDM symbols used for PDCCHs to the UE, and is transmitted in every subframe. A physical hybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement (ACK)/non-acknowledgement (NACK) signal in response to UL transmission. A physical uplink control channel (PUCCH) carries UL control information such as HARQ ACK/NACK for DL transmission, scheduling request, and CQI. A physical uplink shared channel (PUSCH) carries a UL-uplink shared channel (SCH). 
       FIG. 4  shows an example of a physical channel structure. 
     A physical channel consists of a plurality of subframes in time domain and a plurality of subcarriers in frequency domain. One subframe consists of a plurality of symbols in the time domain. One subframe consists of a plurality of resource blocks (RBs). One RB consists of a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use specific subcarriers of specific symbols of a corresponding subframe for a PDCCH. For example, a first symbol of the subframe may be used for the PDCCH. The PDCCH carries dynamic allocated resources, such as a physical resource block (PRB) and modulation and coding scheme (MCS). A transmission time interval (TTI) which is a unit time for data transmission may be equal to a length of one subframe. The length of one subframe may be 1 ms. 
     The transport channel is classified into a common transport channel and a dedicated transport channel according to whether the channel is shared or not. A DL transport channel for transmitting data from the network to the UE includes a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting a paging message, a DL-SCH for transmitting user traffic or control signals, etc. The DL-SCH supports HARQ, dynamic link adaptation by varying the modulation, coding and transmit power, and both dynamic and semi-static resource allocation. The DL-SCH also may enable broadcast in the entire cell and the use of beamforming. The system information carries one or more system information blocks. All system information blocks may be transmitted with the same periodicity. Traffic or control signals of a multimedia broadcast/multicast service (MBMS) may be transmitted through the DL-SCH or a multicast channel (MCH). 
     A UL transport channel for transmitting data from the UE to the network includes a random access channel (RACH) for transmitting an initial control message, a UL-SCH for transmitting user traffic or control signals, etc. The UL-SCH supports HARQ and dynamic link adaptation by varying the transmit power and potentially modulation and coding. The UL-SCH also may enable the use of beamforming. The RACH is normally used for initial access to a cell. 
     A MAC layer belongs to the L2. The MAC layer provides services to a radio link control (RLC) layer, which is a higher layer of the MAC layer, via a logical channel. The MAC layer provides a function of mapping multiple logical channels to multiple transport channels. The MAC layer also provides a function of logical channel multiplexing by mapping multiple logical channels to a single transport channel. A MAC sublayer provides data transfer services on logical channels. 
     The logical channels are classified into control channels for transferring control plane information and traffic channels for transferring user plane information, according to a type of transmitted information. That is, a set of logical channel types is defined for different data transfer services offered by the MAC layer. The logical channels are located above the transport channel, and are mapped to the transport channels. 
     The control channels are used for transfer of control plane information only. The control channels provided by the MAC layer include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH) and a dedicated control channel (DCCH). The BCCH is a downlink channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging information and is used when the network does not know the location cell of a UE. The CCCH is used by UEs having no RRC connection with the network. The MCCH is a point-to-multipoint downlink channel used for transmitting MBMS control information from the network to a UE. The DCCH is a point-to-point bi-directional channel used by UEs having an RRC connection that transmits dedicated control information between a UE and the network. 
     Traffic channels are used for the transfer of user plane information only. The traffic channels provided by the MAC layer include a dedicated traffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCH is a point-to-point channel, dedicated to one UE for the transfer of user information and can exist in both uplink and downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE. 
     Uplink connections between logical channels and transport channels include the DCCH that can be mapped to the UL-SCH, the DTCH that can be mapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH. Downlink connections between logical channels and transport channels include the BCCH that can be mapped to the BCH or DL-SCH, the PCCH that can be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, and the DTCH that can be mapped to the DL-SCH, the MCCH that can be mapped to the MCH, and the MTCH that can be mapped to the MCH. 
     An RLC layer belongs to the L2. The RLC layer provides a function of adjusting a size of data, so as to be suitable for a lower layer to transmit the data, by concatenating and segmenting the data received from a higher layer in a radio section. In addition, to ensure a variety of quality of service (QoS) required by a radio bearer (RB), the RLC layer provides three operation modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The AM RLC provides a retransmission function through an automatic repeat request (ARQ) for reliable data transmission. Meanwhile, a function of the RLC layer may be implemented with a functional block inside the MAC layer. In this case, the RLC layer may not exist. 
     A packet data convergence protocol (PDCP) layer belongs to the L2. The PDCP layer provides a function of header compression function that reduces unnecessary control information such that data being transmitted by employing IP packets, such as IPv4 or IPv6, can be efficiently transmitted over a radio interface that has a relatively small bandwidth. The header compression increases transmission efficiency in the radio section by transmitting only necessary information in a header of the data. In addition, the PDCP layer provides a function of security. The function of security includes ciphering which prevents inspection of third parties, and integrity protection which prevents data manipulation of third parties. 
     A radio resource control (RRC) layer belongs to the L3. The RLC layer is located at the lowest portion of the L3, and is only defined in the control plane. The RRC layer takes a role of controlling a radio resource between the UE and the network. For this, the UE and the network exchange an RRC message through the RRC layer. The RRC layer controls logical channels, transport channels, and physical channels in relation to the configuration, reconfiguration, and release of RBs. An RB is a logical path provided by the L1 and L2 for data delivery between the UE and the network. That is, the RB signifies a service provided the L2 for data transmission between the UE and E-UTRAN. The configuration of the RB implies a process for specifying a radio protocol layer and channel properties to provide a particular service and for determining respective detailed parameters and operations. The RB is classified into two types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmitting an RRC message in the control plane. The DRB is used as a path for transmitting user data in the user plane. 
     Referring to  FIG. 2 , the RLC and MAC layers (terminated in the eNB on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARM). The RRC layer (terminated in the eNB on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE. 
     Referring to  FIG. 3 , the RLC and MAC layers (terminated in the eNB on the network side) may perform the same functions for the control plane. The PDCP layer (terminated in the eNB on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering. 
     An RRC state indicates whether an RRC layer of the UE is logically connected to an RRC layer of the E-UTRAN. The RRC state may be divided into two different states such as an RRC connected state and an RRC idle state. When an RRC connection is established between the RRC layer of the UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, and otherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has the RRC connection established with the E-UTRAN, the E-UTRAN may recognize the existence of the UE in RRC_CONNECTED and may effectively control the UE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN, and a CN manages the UE in unit of a TA which is a larger area than a cell. That is, only the existence of the UE in RRC_IDLE is recognized in unit of a large area, and the UE must transition to RRC_CONNECTED to receive a typical mobile communication service such as voice or data communication. 
     In RRC_IDLE state, the UE may receive broadcasts of system information and paging information while the UE specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area and may perform public land mobile network (PLMN) selection and cell re-selection. Also, in RRC_IDLE state, no RRC context is stored in the eNB. 
     In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the eNB becomes possible. Also, the UE can report channel quality information and feedback information to the eNB. In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs. Therefore, the network can transmit and/or receive data to/from UE, the network can control mobility (handover and inter-radio access technologies (RAT) cell change order to GSM EDGE radio access network (GERAN) with network assisted cell change (NACC)) of the UE, and the network can perform cell measurements for a neighboring cell. 
     In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically, the UE monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle. The paging occasion is a time interval during which a paging signal is transmitted. The UE has its own paging occasion. 
     A paging message is transmitted over all cells belonging to the same tracking area. If the UE moves from one TA to another TA, the UE will send a tracking area update (TAU) message to the network to update its location. 
     When the user initially powers on the UE, the UE first searches for a proper cell and then remains in RRC_IDLE in the cell. When there is a need to establish an RRC connection, the UE which remains in RRC_IDLE establishes the RRC connection with the RRC of the E-UTRAN through an RRC connection procedure and then may transition to RRC_CONNECTED. The UE which remains in RRC_IDLE may need to establish the RRC connection with the E-UTRAN when uplink data transmission is necessary due to a user&#39;s call attempt or the like or when there is a need to transmit a response message upon receiving a paging message from the E-UTRAN. 
     It is known that different cause values may be mapped o the signature sequence used to transmit messages between a UE and eNB and that either channel quality indicator (CQI) or path loss and cause or message size are candidates for inclusion in the initial preamble. 
     When a UE wishes to access the network and determines a message to be transmitted, the message may be linked to a purpose and a cause value may be determined. The size of the ideal message may be also be determined by identifying all optional information and different alternative sizes, such as by removing optional information, or an alternative scheduling request message may be used. 
     The UE acquires necessary information for the transmission of the preamble, UL interference, pilot transmit power and required signal-to-noise ratio (SNR) for the preamble detection at the receiver or combinations thereof. This information must allow the calculation of the initial transmit power of the preamble. It is beneficial to transmit the UL message in the vicinity of the preamble from a frequency point of view in order to ensure that the same channel is used for the transmission of the message. 
     The UE should take into account the UL interference and the UL path loss in order to ensure that the network receives the preamble with a minimum SNR. The UL interference can be determined only in the eNB, and therefore, must be broadcast by the eNB and received by the UE prior to the transmission of the preamble. The UL path loss can be considered to be similar to the DL path loss and can be estimated by the UE from the received RX signal strength when the transmit power of some pilot sequence of the cell is known to the UE. 
     The required UL SNR for the detection of the preamble would typically depend on the eNB configuration, such as a number of Rx antennas and receiver performance. There may be advantages to transmit the rather static transmit power of the pilot and the necessary UL SNR separately from the varying UL interference and possibly the power offset required between the preamble and the message. 
     The initial transmission power of the preamble can be roughly calculated according to the following formula:
 
Transmit power=TransmitPilot−RxPilot+ULInterference+Offset+SNRRequired
 
     Therefore, any combination of SNRRequired, ULInterference, TransmitPilot and Offset can be broadcast. In principle, only one value must be broadcast. This is essentially in current UMTS systems, although the UL interference in 3GPP LTE will mainly be neighboring cell interference that is probably more constant than in UMTS system. 
     The UE determines the initial UL transit power for the transmission of the preamble as explained above. The receiver in the eNB is able to estimate the absolute received power as well as the relative received power compared to the interference in the cell. The eNB will consider a preamble detected if the received signal power compared to the interference is above an eNB known threshold. 
     The UE performs power ramping in order to ensure that a UE can be detected even if the initially estimated transmission power of the preamble is not adequate. Another preamble will most likely be transmitted if no ACK or NACK is received by the UE before the next random access attempt. The transmit power of the preamble can be increased, and/or the preamble can be transmitted on a different UL frequency in order to increase the probability of detection. Therefore, the actual transmit power of the preamble that will be detected does not necessarily correspond to the initial transmit power of the preamble as initially calculated by the UE. 
     The UE must determine the possible UL transport format. The transport format, which may include MCS and a number of resource blocks that should be used by the UE, depends mainly on two parameters, specifically the SNR at the eNB and the required size of the message to be transmitted. 
     In practice, a maximum UE message size, or payload, and a required minimum SNR correspond to each transport format. In UMTS, the UE determines before the transmission of the preamble whether a transport format can be chosen for the transmission according to the estimated initial preamble transmit power, the required offset between preamble and the transport block, the maximum allowed or available UE transmit power, a fixed offset and additional margin. The preamble in UMTS need not contain any information regarding the transport format selected by the EU since the network does not need to reserve time and frequency resources and, therefore, the transport format is indicated together with the transmitted message. 
     The eNB must be aware of the size of the message that the UE intends to transmit and the SNR achievable by the UE in order to select the correct transport format upon reception of the preamble and then reserve the necessary time and frequency resources. Therefore, the eNB cannot estimate the SNR achievable by the EU according to the received preamble because the UE transmit power compared to the maximum allowed or possible UE transmit power is not known to the eNB, given that the UE will most likely consider the measured path loss in the DL or some equivalent measure for the determination of the initial preamble transmission power. 
     The eNB could calculate a difference between the path loss estimated in the DL compared and the path loss of the UL. However, this calculation is not possible if power ramping is used and the UE transmit power for the preamble does not correspond to the initially calculated UE transmit power. Furthermore, the precision of the actual UE transmit power and the transmit power at which the UE is intended to transmit is very low. Therefore, it has been proposed to code the path loss or CQI estimation of the downlink and the message size or the cause value in the UL in the signature. 
     Self-organizing networks (SON) enhancements are necessary for the interoperability of the existing features as well as for the new features and new deployments considered in 3GP LTE rel-12. In 3GPP LTE rel-11, mobility robustness optimization (MRO) has been enhanced to identify for which UE type the failure has occurred. Other SON use cases might require similar enhancements. For example, mobility load balancing (MLB) is not able to distinguish between UEs that support cell range expansion (CRE) and non-CRE UEs. 
     Active antennas allow the creation of multiple vertical and horizontal beams making the deployment dynamic. That enables dynamic cell splitting/merging to handle changing load conditions. For example, beams may be steered to distribute capacity precisely according to actual traffic mix, traffic location and user demands. That makes active antennas particularly good for suburban and rural areas, where fixed deployment of pico cells is expensive, but the network may face congestion situations nonetheless. SON can automate the network deployment based on active antennas. 
     SON enhancements and new features needed for the deployments based on active antennas have been identified. Possible deployment scenarios of an active antenna system (AAS), and additionally required SON features for the AAS needs to be discussed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for transmitting split availability of a cell in a wireless communication system. The present invention provides a method for transmitting a request to split a neighbor cell. 
     In an aspect, a method for transmitting, by a first eNodeB (eNB), split availability of a cell in a wireless communication system is provided. The method includes transmitting a split availability of a first cell, managed by the first eNB, to neighbor eNBs. 
     The split availability of the first cell may indicate whether the first cell supports cell split operation or not. 
     The split availability of the first cell may indicate whether the first eNB supports an active antenna system (AAS). 
     The split availability of the first cell may indicate whether splitting the first cell reduce traffic load of other cell or not. 
     The method may further include receiving a request to spilt the first cell from a second eNB. A second cell, managed by the second eNB, may be in heavy traffic load status. 
     In another aspect, a method for transmitting, by a second eNodeB (eNB), a request to split a cell in a wireless communication system is provided. The method includes transmitting a request to split a first cell, managed by a first eNB, to the first eNB when a second cell, managed by the second eNB, is in heavy traffic load status. 
     The method may further include receiving a split availability of the first cell from the first eNB. 
     Active antenna system (AAS) operations can be supported efficiently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows LTE system architecture. 
         FIG. 2  shows a block diagram of architecture of a typical E-UTRAN and a typical EPC. 
         FIG. 3  shows a block diagram of a user plane protocol stack and a control plane protocol stack of an LTE system. 
         FIG. 4  shows an example of a physical channel structure. 
         FIG. 5  shows options for AAS-based deployment. 
         FIG. 6  shows an example of cell split operations among eNBs. 
         FIG. 7  shows another example of cell split operations among eNBs. 
         FIG. 8  shows an example of a method for transmitting split availability of a cell according to an embodiment of the present invention. 
         FIG. 9  shows an example of a method for transmitting split availability of a cell according to another embodiment of the present invention. 
         FIG. 10  shows a wireless communication system to implement an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The technology described below can be used in various wireless communication systems such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), etc. The CDMA can be implemented with a radio technology such as universal terrestrial radio access (UTRA) or CDMA-2000. The TDMA can be implemented with a radio technology such as global system for mobile communications (GSM)/general packet ratio service (GPRS)/enhanced data rate for GSM evolution (EDGE). The OFDMA can be implemented with a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc. IEEE 802.16m is an evolution of IEEE 802.16e, and provides backward compatibility with an IEEE 802.16-based system. The UTRA is a part of a universal mobile telecommunication system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in downlink and uses the SC-FDMA in uplink. LTE-advance (LTE-A) is an evolution of the 3GPP LTE. 
     For clarity, the following description will focus on the LTE-A. However, technical features of the present invention are not limited thereto. 
     An active antenna system (AAS) refers to a base station (BS) equipped with an antenna array system, the radiation pattern of which may be dynamically adjustable. The AAS introduces an alternative antenna system from the one installed in the conventional BS. The interactions between the antenna array system and the transmitters and receivers within the AAS might be different from the conventional BS and the conventional antenna system. 
     AAS deployment scenarios are described. It may be referred to Section 5 of 3GPP TR 37.840 V12.0.0 (2013-03). 
     1) Tilt and Radiation Pattern Control 
     Antennas are usually manufactured with a fixed beamwidth, and antenna manufacturers typically offer a limited number of beamwidth variations within their conventional product lines. Conventional BS installations often introduce physical tilt to the antenna in order to orient the main lobe of the antenna response towards the ground. Antenna tilt is selected to optimize desired cell coverage and to minimize interference to and from adjacent cells. Some installations employ remote electrical tilt (RET) devices which allow adjustment of the phase shift to facilitate remote control of the antenna tilt angle. 
     An AAS may dynamically control the elevation and azimuth angles, as well as the beamwidth of its radiation pattern via electronic means. Electronic control may be used along with mechanical control. The AAS radiation pattern may be adapted to the specific deployment scenario and possibly to changing traffic patterns. The AAS radiation pattern may also be independently optimized for different links such as independently for uplink and downlink, for coverage and beam forming gain purposes. 
     The concepts of tilt and beamwidth control can be extended by a technique known as cell partitioning in which the cell is subdivided in vertical or horizontal directions by adjustment of the antenna pattern. For example, one cell partition is located close to the BS and the other cell partition is located farther away from the BS. 
     2) Multiple-Input Multiple-Output (MIMO) 
     MIMO is a general terminology that includes the various spatial processing techniques such as beamforming, diversity, and spatial multiplexing. Brief description of each is provided below.
         Beamforming: The use of a dedicated beam formed towards the UE when data demodulation using a dedicated reference signal is supported by the UE.   Diversity: The use of diversity techniques to jointly optimize in the spatial and frequency domain through the use of, for example, spatial-frequency block code (SFBC) or frequency switching transmit diversity (FSTD), or combinations of both.   Spatial multiplexing: The transmission of multiple signal streams to one (SU-MIMO) or more (MU-MIMO) UEs using multiple spatial layers created by combinations of the available antennas.
 
3) Differentiated Antenna Behaviors at Different Carrier Frequencies
       

     AAS supports the use of different antennas at different carrier frequencies and for different radio access technologies (RATs). For example, an AAS may create 4 virtual antennas for an LTE carrier and 2 antennas for a GSM or high-speed packet access (HSPA) carrier. 
     4) Per RB (or UE) Transmission and Reception 
     Each UE may get its own beam that tracks the movement of the UE. The current specification support for spatial multiplexing, beamforming and transmit diversity includes the ability to schedule transmission and reception to one UE within one resource block. This allows beamforming to individual UEs with adaptation to mobility, as an example. 
     The AAS BS can be deployed for wide area, medium range, and local area coverage.
         The wide area coverage deployment scenario is typically found in outdoor macro environments, where the BS antennas are located on masts, roof tops or high above street level. An AAS BS designed for wide area coverage is called a macro AAS.   The medium range coverage deployment scenario is typically found in outdoor micro environments, where the AAS BSs are located below roof tops. An AAS BS designed for medium range coverage is called a micro AAS.   The local area BS deployment scenario is typically found indoors (offices, subway stations, etc.) where antennas are located on ceilings or walls. Deployment scenarios for local area coverage can also be found outdoors in hot spot areas like marketplaces, high streets or railway stations. An AAS BS designed for local area coverage is called a pico AAS.       

     The radiation pattern for an AAS BS can be dynamically adjustable, while a fixed beam pattern is assumed for the conventional BS. Coexistence of an AAS BS with a conventional BS based on an un-coordinated deployment shall be considered. Analytical approaches can be used to study the coexistence requirements based on existing results, supplemented with additional simulations when necessary. The following initial scenarios are identified for the purpose of studying the spatial characteristics for AAS BS:
         E-UTRA macro AAS BS co-located with another E-UTRA macro AAS BS   E-UTRA macro AAS BS co-located with E-UTRA macro legacy BS       

     Based on the deployment/coexistence scenarios described above, the deployments scenarios of AAS may be categorized on three aspects as described in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Amount of 
               
               
                   
                   
                 development 
               
               
                   
                   
                 required 
               
               
                   
                 Possible scenarios 
                 further 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Cell 
                 split to two parts (inner area and outer area) 
                 medium 
               
               
                 partitioning 
                 track the movement of each UE 
                 high 
               
               
                 Frequency 
                 identical frequency for all partition in a cell 
                 medium 
               
               
                 and RAT 
                 different frequency for each partition 
                 medium 
               
               
                 assignment 
                 different RAT for each partition 
                 high 
               
               
                 Coexistence 
                 macro AAS BS co-located with another 
                 medium 
               
               
                 scenarios 
                 macro AAS BS 
               
               
                   
                 macro AAS BS co-located with macro 
                 medium 
               
               
                   
                 legacy BS 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, related to the cell partitioning, splitting a cell to inner part and outer part is considered as a basic deployment scenario. If more accurate and adaptive beam steering is assumed, per UE transmission and reception by tracking each UE&#39;s movement would be possible scenario. 
     Related to frequency and RAT assignment for each cell partition, there are three possible options: 1) assigning an identical frequency for all partition, 2) different frequency for each partition, and 3) different RAT for each partition. Among three options, assigning different RAT for each partition may require more efforts in the network point of view than other options, e.g., inter-RAT handover, cell identifier (ID) allocation, and need more discussion about use cases. 
     Related to the coexistence scenarios, if the interworking SON mechanisms between AAS BSs co-located with each other are developed, then it can be used for the case of AAS BS co-located with legacy BS. However in this case the compatibility problems should be considered. 
     Therefore, it is preferable to focus on the following AAS deployment scenarios first.
         splitting cell to inner area and outer area,   assigning identical frequency for all partition in a cell or different frequency for each partition, and   macro AAS BS co-located with another macro AAS BS.       

     Regarding AAS-based deployment, there are three possible options as follows based on whether a cell is split or not, and if a cell is split, then whether a new physical cell identity (PCI) is assigned to the split cell or not.
         Option 1: Cell shaping/beamforming without splitting cell   Option 2: Cell splitting without PCI allocation   Option 3: Cell splitting with PCI allocation       

       FIG. 5  shows options for AAS-based deployment. 
       FIG. 5 -( a ) shows a case of option 1 described above. In option 1, the eNB uses beamforming to cover additional area. It has an effect to extend the cell coverage. Option 1 is not helpful to release the traffic load in the existing cell area. 
       FIG. 5 -( b ) shows a case of option 2 described above. Option 2 makes the eNB reuse resources at each split cell. Accordingly, the traffic load can be reduced by half ideally. However, option 2 has the problem that the UE at the border of two split cells cannot distinguish the signal from one split cell and the signal from the other split cell. 
       FIG. 5 -( c ) shows a case of option 3 described above. Option 3 assigns an independent PCI to the split cell. Even though it increases the system operational complexity, option 3 has an advantage of reducing the cell traffic load without any confusion on UE side. Accordingly, if considering the aspects of high UE density environment and UE service stability, option 3 would be the most reasonable deployment scenario. However, the present invention described below according to embodiments of the present invention does not limited to option 3. That is, all of option 1, 2, and 3 may be considered for the present invention. 
       FIG. 6  shows an example of cell split operations among eNBs.  FIG. 6  shows cell split operations among two eNBs, i.e., eNB 1  and eNB 2 . Referring to  FIG. 6 , cell  1  managed by eNB 1  is split into cell  1   a  and cell  1   b . By splitting cell, the service capacity of eNB can be improved. Thus, if the amount of the required traffic (or, the number of UEs) in cell  1  of eNB 1  is high, eNB 1  can escape from the heavy load status by splitting cell  1  into cell  1   a  and cell  1   b . Moreover, if cell  2  is in the heavy load status, it is possible that eNB 1  splits cell  1  and accepts UEs of cell  2 . 
       FIG. 7  shows another example of cell split operations among eNBs.  FIG. 7  shows cell split operations among three eNBs, i.e., eNB 1 , eNB 2  and eNB 3 . It is assumed that eNB 2  and eNB 3  have no ability to split their cells. That is, eNB 2  and eNB 3  do not support the AAS operations. 
     Referring to  FIG. 7 , before splitting cell  1 , cell  1  and cell  2  are in the medium load status in which each cell cannot accept inbound UE handover (HO) from neighbor cells anymore, and cell  3  is in the high load status in which it needs to reduce traffic load. Then, cell  1  is split into cell  1   a  and cell  1   b . After cell splitting and before load-balancing, cell  1   a  and cell  1   b  are in the low load status in which they can accept inbound UE HOs from neighbor cells. Then, in order to reduce traffic load of cell  3 , load balancing is performed. By the load-balancing operation among three eNBs, cell  2  hands over its UEs to cell  1   b , and cell  3  hands over its UEs to cell  2 . After the load-balancing, cell  3  can escape from the high load status. 
     Hereinafter, a method for transmitting split availability of a cell according to an embodiment of the present invention is described. 
     To support AAS operations described above, the following information need to be exchanged among eNBs.
         Split availability: the eNB may inform the neighbor eNBs whether the eNB supports the AAS. That is, the eNB may inform the neighbor eNBs whether the eNB supports cell split operation. Further, the eNB may inform the neighbor eNBs whether a cell managed by the eNB is in the environment where splitting the cell is advantageous. For example, if UEs are densely deployed at the border between split cells (e.g. at the cell border between cell  1   a  and cell  1   b  in  FIG. 6  described above), the effect of load relief would be low because of interferences between split cells after splitting a cell. Accordingly, the cell should be split only when splitting the cell can reduce the traffic load of other cell effectively.   Request to split the cell: when a cell is in the heavy traffic load status, the eNB managing the cell requests its neighbor eNB to split neighbor eNB&#39;s cell and to accept UEs of the cell in the heavy traffic load status. For example, referring to  FIG. 6 , when cell  2  is in the heavy traffic load status, eNB 2  may request eNB 1  to split cell  1  and to accept UEs served by cell  2 .       

       FIG. 8  shows an example of a method for transmitting split availability of a cell according to an embodiment of the present invention. 
     Referring to  FIG. 8 , in step S 100 , the first eNB transmits a split availability of its cell, i.e., first cell, to neighbor eNBs. The split availability of the first cell may indicate whether the first cell supports cell split operation or not. That is, the split availability of the first cell may indicate whether the first eNB supports the AAS. Further, the split availability of the first cell may indicate whether splitting the first cell reduce traffic load of other cell or not. That is, the split availability of the first cell may indicate whether the first cell is in the environment where splitting the first cell is advantageous or not. 
       FIG. 9  shows an example of a method for transmitting split availability of a cell according to another embodiment of the present invention. 
     Referring to  FIG. 9 , in step S 200 , the second eNB transmits a request to split a first cell, managed by a first eNB, to the first eNB when a second cell, managed by the second eNB, is in heavy traffic load status. In step S 210 , the first eNB transmits a split availability of the first cell to the second eNB. The split availability of the first cell may indicate whether the first cell supports cell split operation or not. That is, the split availability of the first cell may indicate whether the first eNB supports the AAS. Further, the split availability of the first cell may indicate whether splitting the first cell reduce traffic load of other cell or not. That is, the split availability of the first cell may indicate whether the first cell is in the environment where splitting the first cell is advantageous or not. 
       FIG. 10  shows a wireless communication system to implement an embodiment of the present invention. 
     A first eNB  800  includes a processor  810 , a memory  820 , and a radio frequency (RF) unit  830 . The processor  810  may be configured to implement proposed functions, procedures, and/or methods in this description. Layers of the radio interface protocol may be implemented in the processor  810 . The memory  820  is operatively coupled with the processor  810  and stores a variety of information to operate the processor  810 . The RF unit  830  is operatively coupled with the processor  810 , and transmits and/or receives a radio signal. 
     A second eNB  900  includes a processor  910 , a memory  920  and an RF unit  930 . The processor  910  may be configured to implement proposed functions, procedures and/or methods described in this description. Layers of the radio interface protocol may be implemented in the processor  910 . The memory  920  is operatively coupled with the processor  910  and stores a variety of information to operate the processor  910 . The RF unit  930  is operatively coupled with the processor  910 , and transmits and/or receives a radio signal. 
     The processors  810 ,  910  may include application-specific integrated circuit (ASIC), other chipset, logic circuit and/or data processing device. The memories  820 ,  920  may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and/or other storage device. The RF units  830 ,  930  may include baseband circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The modules can be stored in memories  820 ,  920  and executed by processors  810 ,  910 . The memories  820 ,  920  can be implemented within the processors  810 ,  910  or external to the processors  810 ,  910  in which case those can be communicatively coupled to the processors  810 ,  910  via various means as is known in the art. 
     In view of the exemplary systems described herein, methodologies that may be implemented in accordance with the disclosed subject matter have been described with reference to several flow diagrams. While for purposed of simplicity, the methodologies are shown and described as a series of steps or blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the order of the steps or blocks, as some steps may occur in different orders or concurrently with other steps from what is depicted and described herein. Moreover, one skilled in the art would understand that the steps illustrated in the flow diagram are not exclusive and other steps may be included or one or more of the steps in the example flow diagram may be deleted without affecting the scope and spirit of the present disclosure.