Patent Publication Number: US-9838182-B2

Title: Method for transmitting HARQ ACK/NACK and wireless device using same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the National Phase of PCT International Application No. PCT/KR2013/004559, filed on May 24, 2013, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application Nos. 61/651,029 filed on May 24, 2012, 61/678,631 filed on Aug. 2, 2012, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to wireless communications, and more particularly, to a method of transmitting positive-acknowledgement (ACK)/negative-acknowledgement (NACK) for hybrid automatic repeat request (HARQ) in a wireless communication system, and a wireless device using the method. 
     Related Art 
     Long term evolution (LTE) based on 3 rd  generation partnership project (3GPP) technical specification (TS) release 8 is a promising next-generation mobile communication standard. Recently, LTE-advanced (LTE-A) based on 3GPP TS release 10 supporting multiple carriers is under standardization. 
     As disclosed in 3GPP TS 36.211 V10.2.0 (2011-06) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)”, a physical channel of 3GPP LTE/LTE-A can be classified into a downlink channel, i.e., a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH), and an uplink channel, i.e., a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH). 
     Recently, an enhanced PDCCH (EPDCCH) for more flexible scheduling of a control channel has been introduced. The EPDCCH supports various schemes such as a search space in a PDSCH region, multiple antenna transmission, etc. 
     For an HARQ operation, a PDCCH-PUCCH resource linkage is established in 3GPP LTE/LTE-A. Upon receiving the PDCCH, a user equipment sends HARQ ACK/NACK through a PUCCH corresponding to the PDCCH by using a resource linkage. This is to implicitly exchange information about to which transport block the HARQ ACK/NACK belongs between a base station and the user equipment. 
     The EPDCCH supports a coordinated multi-point (CoMP). In the CoMP operation, the UE receives the EPDCCH from a first base station, and may transmit a corresponding PUCCH to a second base station. That is, this is a case where a downlink base station and an uplink base station are different from each other. Since the base stations are different, there is a need to modify the PDCCH-PUCCH resource linkage. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of transmitting positive-acknowledgement (ACK)/negative-acknowledgement (NACK) for hybrid automatic repeat request (HARQ) in a wireless communication system, and a wireless device using the method. 
     In an aspect, a method for transmitting hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in a wireless communication system is provided. The method includes receiving, by a wireless device, downlink control information from a first base station on a downlink control channel, receiving, by the wireless device, a downlink transport block from the first base station on a downlink shared channel according to the downlink control information, and transmitting, by the wireless device, ACK/NACK for the downlink transport block to a second base station on an uplink control channel. Information regarding a cell identity for the uplink control channel for the second base station is received from the first base station, and the downlink control information includes an indicator used to determine a radio resource for the uplink control channel. 
     The method may further include receiving, by the wireless device, information regarding a plurality of uplink channel resource candidates from the first base station. The indicator may indicate one of the plurality of uplink channel resource candidates. 
     The transmitting of the ACK/NACK on the uplink control channel may include modulating the ACK/NACK to generate a modulation symbol, spreading the modulation symbol to a sequence which is cyclically shifted by a cyclic shift amount, and transmitting the spread sequence. 
     The cyclic shift amount may be determined based on the cell identity. 
     In another aspect, a wireless device for transmitting hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in a wireless communication system is provided. The wireless device includes a radio frequency (RF) unit configured to transmit and receive a radio signal, and a processor operatively coupled to the RF unit and configured to receive downlink control information from a first base station on a downlink control channel, receive a downlink transport block from the first base station on a downlink shared channel according to the downlink control information, and transmit ACK/NACK for the downlink transport block to a second base station on an uplink control channel. Information regarding a cell identity for the uplink control channel for the second base station is received from the first base station, and the downlink control information includes an indicator used to determine a radio resource for the uplink control channel. 
     A radio resource for an uplink control channel which is linked to a resource of an enhanced physical downlink control channel (EPDCCH) can be ensured. The radio resource for the uplink control channel can be ensured in various coordinated multi-point (CoMP) environments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a structure of a downlink (DL) radio frame in 3 rd  generation partnership project (3GPP) long term evolution (LTE). 
         FIG. 2  shows a structure of an uplink (UL) subframe in 3GPP LTE. 
         FIG. 3  shows a diagram showing a configuration of a physical downlink control channel (PDCCH). 
         FIG. 4  shows an example of monitoring a PDCCH. 
         FIG. 5  shows an example of arranging a reference signal and a control channel in a DL subframe of 3GPP LTE. 
         FIG. 6  shows an example of a subframe having an enhanced PDCCH (EPDCCH). 
         FIG. 7  shows an example of a physical resource block (PRB) pair. 
         FIG. 8  shows a DL hybrid automatic repeat request (HARQ) operation in 3GPP LTE. 
         FIG. 9  shows transmission of a positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal when a physical uplink control channel (PUCCH) format 1b is used in a normal cyclic prefix (CP) case in 3GPP LTE. 
         FIG. 10  shows a structure of a PUCCH format 3 in a normal CP case. 
         FIG. 11  shows an example of a CoMP scenario. 
         FIG. 12  shows a PUCCH resource allocation according to an embodiment of the present invention. 
         FIG. 13  shows a PUCCH resource allocation according to another embodiment of the present invention. 
         FIG. 14  is another example of a CoMP scenario. 
         FIG. 15  shows a PUCCH resource allocation according to another embodiment of the present invention. 
         FIG. 16  is a flowchart showing an ACK/NACK transmission method according to an embodiment of the present invention. 
         FIG. 17  is a block diagram showing a wireless communication system according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A wireless device may be fixed or mobile, and may be referred to as another terminology, such as a user equipment (UE), a mobile station (MS), a mobile terminal (MT), a user terminal (UT), a subscriber station (SS), a personal digital assistant (PDA), a wireless modem, a handheld device, etc. The wireless device may also be a device supporting only data communication such as a machine-type communication (MTC) device. 
     A base station (BS) is generally a fixed station that communicates with the wireless device, and may be referred to as another terminology, such as an evolved-NodeB (eNB), a base transceiver system (BTS), an access point, etc. 
     Hereinafter, it is described that the present invention is applied according to a 3rd generation partnership project (3GPP) long term evolution (LTE) based on 3GPP technical specification (TS) release 8 or 3GPP LTE-advanced (LTE-A) based on 3GPP TS release 10. However, this is for exemplary purposes only, and thus the present invention is also applicable to various wireless communication networks. In the following description, LTE and/or LTE-A are collectively referred to as LTE. 
     The wireless device may be served by a plurality of serving cells. Each serving cell may be defined with a downlink (DL) component carrier (CC) or a pair of a DL CC and an uplink (UL) CC. 
     The serving cell may be classified into a primary cell and a secondary cell. The primary cell operates at a primary frequency, and is a cell designated as the primary cell when an initial network entry process is performed or when a network re-entry process starts or in a handover process. The primary cell is also called a reference cell. The secondary cell operates at a secondary frequency. The secondary cell may be configured after an RRC connection is established, and may be used to provide an additional radio resource. At least one primary cell is configured always. The secondary cell may be added/modified/released by using higher-layer signaling (e.g., a radio resource control (RRC) message). 
     A cell index (CI) of the primary cell may be fixed. For example, a lowest CI may be designated as the CI of the primary cell. It is assumed hereinafter that the CI of the primary cell is 0 and a CI of the secondary cell is allocated sequentially starting from 1. 
       FIG. 1  shows a structure of a DL radio frame in 3GPP LTE-A. The section 6 of 3GPP TS 36.211 V10.2.0 (2011-06) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)” may be incorporated herein by reference. 
     A radio frame includes 10 subframes indexed with 0 to 9. One subframe includes 2 consecutive slots. A time required for transmitting one subframe is defined as a transmission time interval (TTI). For example, one subframe may have a length of 1 millisecond (ms), and one slot may have a length of 0.5 ms. 
     One slot may include a plurality of orthogonal frequency division multiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink (DL), the OFDM symbol is only for expressing one symbol period in the time domain, and there is no limitation in multiple access schemes or terminologies. For example, the OFDM symbol may also be referred to as another terminology such as a single carrier frequency division multiple access (SC-FDMA) symbol, a symbol period, etc. 
     Although it is described that one slot includes 7 OFDM symbols for example, the number of OFDM symbols included in one slot may vary depending on a length of a cyclic prefix (CP). According to 3GPP TS 36.211 V10.2.0, in case of a normal CP, one slot includes 7 OFDM symbols, and in case of an extended CP, one slot includes 6 OFDM symbols. 
     A resource block (RB) is a resource allocation unit, and includes a plurality of subcarriers in one slot. For example, if one slot includes 7 OFDM symbols in a time domain and the RB includes 12 subcarriers in a frequency domain, one RB can include 7×12 resource elements (REs). 
     A DL subframe is divided into a control region and a data region in the time domain. The control region includes up to first four OFDM symbols of a first slot in the subframe. However, the number of OFDM symbols included in the control region may vary. A physical downlink control channel (PDCCH) and other control channels are allocated to the control region, and a physical downlink shared channel (PDSCH) is allocated to the data region. 
     As disclosed in 3GPP TS 36.211 V10.2.0, examples of a physical control channel in 3GPP LTE/LTE-A include a physical downlink control channel (PDCCH), a physical control format indicator channel (PCFICH), and a physical hybrid-ARQ indicator channel (PHICH). 
     The PCFICH transmitted in a first OFDM symbol of the subframe carries a control format indicator (CFI) regarding the number of OFDM symbols (i.e., a size of the control region) used for transmission of control channels in the subframe. A wireless device first receives the CFI on the PCFICH, and thereafter monitors the PDCCH. 
     Unlike the PDCCH, the PCFICH does not use blind decoding, and is transmitted by using a fixed PCFICH resource of the subframe. 
     The PHICH carries a positive-acknowledgement (ACK)/negative-acknowledgement (NACK) signal for an uplink hybrid automatic repeat request (HARQ). The ACK/NACK signal for uplink (UL) data on a PUSCH transmitted by the wireless device is transmitted on the PHICH. 
     A physical broadcast channel (PBCH) is transmitted in first four OFDM symbols in a second slot of a first subframe of a radio frame. The PBCH carries system information necessary for communication between the wireless device and a BS. The system information transmitted through the PBCH is referred to as a master information block (MIB). In comparison thereto, system information transmitted on the PDCCH is referred to as a system information block (SIB). 
       FIG. 2  shows a structure of a UL subframe in 3GPP LTE. 
     A UL subframe can be divided into a control region and a data region in frequency domain. The control region is a region to which a physical uplink control channel (PUCCH) is allocated. The data region is a region to which a physical uplink shared channel (PUSCH) is allocated. 
     The PUCCH is allocated in an RB pair in a subframe. RBs belonging to the RB pair occupy different subcarriers in each of a 1st slot and a 2nd slot. m is a location index indicating a logical frequency-domain location of the RB pair allocated to the PUCCH in the subframe. 
     It shows that RBs having the same value m occupy different subcarriers in the two slots. 
     The PUSCH is allocated by a UL grant on a PDCCH. Although not shown, a 4th OFDM symbol of each slot of a normal CP is used in transmission of a demodulation reference signal (DM RS). 
     Uplink control information (UCI) includes at least any one of HARQ ACK/NACK, channel state information (CSI), and scheduling request (SR). Hereinafter, as an index of indicating a state of a DL channel, the CSI may include at least any one of a channel quality indicator (CQI) and a precoding matrix indicator (PMI). 
     PUCCH formats are defined as a combination of UCI and PUCCH in order to transmit various UCI on PUCCH. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 PUCCH format 
                 Transmitted UCI 
               
               
                   
               
             
            
               
                   
                 PUCCH format 1 
                 Positive SR 
               
               
                   
                 PUCCH format 1a/1b 
                 1 bit or 2 bits HARQ ACCK/NACK 
               
               
                   
                 PUCCH format 2 
                 CSI report 
               
               
                   
                 PUCCH format 2a/2b 
                 CSI report and 1 bit or 2 bits 
               
               
                   
                   
                 HARQ ACCK/NACK 
               
               
                   
                 PUCCH format 3 
                 HARQ ACCK/NACK, SR, CSI 
               
               
                   
               
            
           
         
       
     
     A PUCCH format 1a/1b is used to carry 1 bit or 2 bits HARQ ACK/NACK by using binary phase shift keying (BPSK) modulation or quadrature phase shift keying (QPSK) modulation. 
     A PUCCH format 3 can be used to transmit 48 bits encoded UCI. The PUCCH format 3 can carry HARQ ACK/NACK for a plurality of serving cells and a CSI report for a single serving cell. 
     Hereinafter, transmissions of PDCCH and reference signals are described. 
     Control information transmitted through the PDCCH is referred to as downlink control information (DCI). The DCI may include resource allocation of the PDSCH (this is referred to as a downlink (DL) grant), resource allocation of a PUSCH (this is referred to as an uplink (UL) grant), a set of transmit power control commands for individual UEs in any UE group, and/or activation of a voice over Internet protocol (VoIP). 
     In 3GPP LTE/LTE-A, transmission of a DL transport block is performed in a pair of the PDCCH and the PDSCH. Transmission of a UL transport block is performed in a pair of the PDCCH and the PUSCH. For example, the wireless device receives the DL transport block on a PDSCH indicated by the PDCCH. The wireless device receives a DL resource assignment on the PDCCH by monitoring the PDCCH in a DL subframe. The wireless device receives the DL transport block on a PDSCH indicated by the DL resource assignment. 
       FIG. 3  is a block diagram showing a structure of a PDCCH. 
     The 3GPP LTE/LTE-A uses blind decoding for PDCCH detection. The blind decoding is a scheme in which a desired identifier is de-masked from a cyclic redundancy check (CRC) of a received PDCCH (referred to as a candidate PDCCH) to determine whether the PDCCH is its own control channel by performing CRC error checking. 
     A BS determines a PDCCH format according to DCI to be transmitted to a wireless device, attaches a CRC to control information, and masks a unique identifier (referred to as a radio network temporary identifier (RNTI)) to the CRC according to an owner or usage of the PDCCH (block  210 ). 
     If the PDCCH is for a specific wireless device, a unique identifier (e.g., cell-RNTI (C-RNTI)) of the wireless device may be masked to the CRC. Alternatively, if the PDCCH is for a paging message, a paging indication identifier (e.g., paging-RNTI (P-RNTI)) may be masked to the CRC. If the PDCCH is for system information, a system information identifier (e.g., system information-RNTI (SI-RNTI)) may be masked to the CRC. To indicate a random access response that is a response for transmission of a random access preamble of the wireless device, a random access-RNTI (RA-RNTI) may be masked to the CRC. To indicate a transmit power control (TPC) command for a plurality of wireless devices, a TPC-RNTI may be masked to the CRC. 
     When the C-RNTI is used, the PDCCH carries control information for a specific wireless device (such information is called UE-specific control information), and when other RNTIs are used, the PDCCH carries common control information received by all or a plurality of wireless devices in a cell. 
     The CRC-attached DCI is encoded to generate coded data (block  220 ). Encoding includes channel encoding and rate matching. 
     The coded data is modulated to generate modulation symbols (block  230 ). 
     The modulation symbols are mapped to physical resource elements (REs) (block  240 ). The modulation symbols are respectively mapped to the REs. 
     A control region in a subframe includes a plurality of control channel elements (CCEs). The CCE is a logical allocation unit used to provide the PDCCH with a coding rate depending on a radio channel state, and corresponds to a plurality of resource element groups (REGs). The REG includes a plurality of REs. According to an association relation of the number of CCEs and the coding rate provided by the CCEs, a PDCCH format and a possible number of bits of the PDCCH are determined. 
     One REG includes 4 REs. One CCE includes 9 REGs. The number of CCEs used to configure one PDCCH may be selected from a set {1, 2, 4, 8}. Each element of the set {1, 2, 4, 8} is referred to as a CCE aggregation level. 
     The BS determines the number of CCEs used in transmission of the PDCCH according to a channel state. For example, a wireless device having a good DL channel state can use one CCE in PDCCH transmission. A wireless device having a poor DL channel state can use 8 CCEs in PDCCH transmission. 
     A control channel consisting of one or more CCEs performs interleaving on an REG basis, and is mapped to a physical resource after performing cyclic shift based on a cell identifier (ID). 
       FIG. 4  shows an example of monitoring a PDCCH. The section 9 of 3GPP TS 36.213 V10.2.0 (2011-06) can be incorporated herein by reference. 
     The 3GPP LTE uses blind decoding for PDCCH detection. The blind decoding is a scheme in which a desired identifier is de-masked from a CRC of a received PDCCH (referred to as a candidate PDCCH) to determine whether the PDCCH is its own control channel by performing CRC error checking. A wireless device cannot know about a specific position in a control region in which its PDCCH is transmitted and about a specific CCE aggregation or DCI format used for PDCCH transmission. 
     A plurality of PDCCHs can be transmitted in one subframe. The wireless device monitors the plurality of PDCCHs in every subframe. Monitoring is an operation of attempting PDCCH decoding by the wireless device according to a format of the monitored PDCCH. 
     The 3GPP LTE uses a search space to reduce a load of blind decoding. The search space can also be called a monitoring set of a CCE for the PDCCH. The wireless device monitors the PDCCH in the search space. 
     The search space is classified into a common search space and a UE-specific search space. The common search space is a space for searching for a PDCCH having common control information and consists of 16 CCEs indexed with 0 to 15. The common search space supports a PDCCH having a CCE aggregation level of {4, 8}. However, a PDCCH (e.g., DCI formats 0, 1A) for carrying UE-specific information can also be transmitted in the common search space. The UE-specific search space supports a PDCCH having a CCE aggregation level of {1, 2, 4, 8}. 
     Table 2 shows the number of PDCCH candidates monitored by the wireless device. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Search 
                 Aggregation 
                 Size 
                 Number of PDCCH 
                 DCI 
               
               
                 Space Type 
                 level L 
                 [in CCEs] 
                 candidates 
                 formats 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 UE-specific 
                 1 
                 6 
                 6 
                 0, 1, 1A, 
               
               
                   
                 2 
                 12 
                 6 
                 1B, 1D, 
               
               
                   
                 4 
                 8 
                 2 
                 2, 2A 
               
               
                   
                 8 
                 16 
                 2 
                   
               
               
                 Common 
                 4 
                 16 
                 4 
                 0, 1A, 1C, 
               
               
                   
                 8 
                 16 
                 2 
                 3/3A 
               
               
                   
               
            
           
         
       
     
     A size of the search space is determined by Table 2 above, and a start point of the search space is defined differently in the common search space and the UE-specific search space. Although a start point of the common search space is fixed irrespective of a subframe, a start point of the UE-specific search space may vary in every subframe according to a UE identifier (e.g., C-RNTI), a CCE aggregation level, and/or a slot number in a radio frame. If the start point of the UE-specific search space exists in the common search space, the UE-specific search space and the common search space may overlap with each other. 
     In a CCE aggregation level Lε{1,2,3,4}, a search space S (L)   k  is defined as a set of PDCCH candidates. A CCE corresponding to a PDCCH candidate m of the search space S (L)   k  is given by Equation 1 below.
 
 L ·{( Y   k   +m ′)mod └ N   CCE,k   /L┘}+i   [Equation 1]
 
     Herein, i=0,1, . . . ,L−1, m=0, . . . ,M (L) −1, and N CCE,k  denotes the total number of CCEs that can be used for PDCCH transmission in a control region of a subframe k. The control region includes a set of CCEs numbered from 0 to N CCE,k-1 . M (L)  denotes the number of PDCCH candidates in a CCE aggregation level L of a given search space. 
     If a carrier indicator field (CIF) is set to the wireless device, m′=m+M (L) n cif . Herein, n cif  is a value of the CIF. If the CIF is not set to the wireless device, m′=m. 
     In a common search space, Y k  is set to 0 with respect to two aggregation levels L=4 and L=8. 
     In a UE-specific search space of the aggregation level L, a variable Y k  is defined by Equation 2 below.
 
 Y   k =( A·Y   k-1 )mod  D   [Equation 2]
 
     Herein, Y −1 =n RNTI ≠0, A=39827, D=65537, k=floor(n s /2), and n s  denotes a slot number in a radio frame. 
     When the wireless device monitors the PDCCH by using the C-RNTI, a search space and a DCI format used in monitoring are determined according to a transmission mode of the PDSCH. Table 3 below shows an example of PDCCH monitoring in which the C-RNTI is set. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Transmis- 
                   
                   
                 Transmission mode of 
               
               
                 sion mode 
                 DCI format 
                 search space 
                 PDSCH based on PDCCH 
               
               
                   
               
             
            
               
                 Mode 1 
                 DCI format 1A 
                 common and 
                 Single-antenna port, port 0 
               
               
                   
                   
                 UE specific 
                   
               
               
                   
                 DCI format 1 
                 UE specific 
                 Single-antenna port, port 0 
               
               
                 Mode 2 
                 DCI format 1A 
                 common and 
                 Transmit diversity 
               
               
                   
                   
                 UE specific 
                   
               
               
                   
                 DCI format 1 
                 UE specific 
                 Transmit diversity 
               
               
                 Mode 3 
                 DCI format 1A 
                 common and 
                 Transmit diversity 
               
               
                   
                   
                 UE specific 
                   
               
               
                   
                 DCI format 2A 
                 UE specific 
                 CDD(Cyclic Delay Diversity) 
               
               
                   
                   
                   
                 or Transmit diversity 
               
               
                 Mode 4 
                 DCI format 1A 
                 common and 
                 Transmit diversity 
               
               
                   
                   
                 UE specific 
                   
               
               
                   
                 DCI format 2 
                 UE-specific 
                 Closed-loop spatial multi- 
               
               
                   
                   
                   
                 plexing 
               
               
                 Mode 5 
                 DCI format 1A 
                 common and 
                 Transmit diversity 
               
               
                   
                   
                 UE specific 
                   
               
               
                   
                 DCI format 1D 
                 UE specific 
                 MU-MIMO(Multi-User 
               
               
                   
                   
                   
                 Multiple Input Multiple 
               
               
                   
                   
                   
                 Output) 
               
               
                 Mode 6 
                 DCI format 1A 
                 common and 
                 Transmit diversity 
               
               
                   
                   
                 UE specific 
                   
               
               
                   
                 DCI format 1B 
                 UE specific 
                 Closed-loop spatial multi- 
               
               
                   
                   
                   
                 plexing 
               
               
                 Mode 7 
                 DCI format 1A 
                 common and 
                 If the number of PBCH 
               
               
                   
                   
                 UE specific 
                 transmission ports is 1, 
               
               
                   
                   
                   
                 single antenna port, port 0, 
               
               
                   
                   
                   
                 and if not, Transmit diversity 
               
               
                   
                 DCI format 1 
                 UE specific 
                 Single antenna port, port 5 
               
               
                 Mode 8 
                 DCI format 1A 
                 common and 
                 If the number of PBCH 
               
               
                   
                   
                 UE specific 
                 transmission ports is 1, 
               
               
                   
                   
                   
                 single antenna port, port 0, 
               
               
                   
                   
                   
                 and if not, Transmit diversity 
               
               
                   
                 DCI format 2B 
                 UE specific 
                 Dual layer transmission 
               
               
                   
                   
                   
                 (port 7 or 8), or a single 
               
               
                   
                   
                   
                 antenna port, port 7 or 8 
               
               
                   
               
            
           
         
       
     
       FIG. 5  shows an example of arranging a reference signal and a control channel in a DL subframe of 3GPP LTE. 
     A control region (or a PDCCH region) includes first three OFDM symbols, and a data region in which a PDSCH is transmitted includes the remaining OFDM symbols. 
     A PCFICH, a PHICH, and/or a PDCCH are transmitted in the control region. A control format indictor (CFI) of the PCFICH indicates three OFDM symbols. A region excluding a resource in which the PCFICH and/or the PHICH are transmitted in the control region is a PDCCH region which monitors the PDCCH. 
     Various reference signals are transmitted in the subframe. 
     A cell-specific reference signal (CRS) may be received by all wireless devices in a cell, and is transmitted across a full downlink frequency band. In  FIG. 4 , ‘R 0 ’ indicates a resource element (RE) used to transmit a CRS for a first antenna port, ‘R 1 ’ indicates an RE used to transmit a CRS for a second antenna port, ‘R 2 ’ indicates an RE used to transmit a CRS for a third antenna port, and ‘R 3 ’ indicates an RE used to transmit a CRS for a fourth antenna port. 
     An RS sequence r 1,ns (m) for a CRS is defined as follows. 
     
       
         
           
             
               
                 
                   
                     
                       r 
                       
                         l 
                         , 
                         
                           n 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           s 
                         
                       
                     
                     ⁡ 
                     
                       ( 
                       m 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         1 
                         
                           2 
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           1 
                           - 
                           
                             2 
                             · 
                             
                               c 
                               ⁡ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   m 
                                 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       j 
                       ⁢ 
                       
                         1 
                         
                           2 
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           1 
                           - 
                           
                             2 
                             · 
                             
                               c 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     m 
                                   
                                   + 
                                   1 
                                 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Herein, m=0,1, . . . ,2N maxRB −1. N maxRB  is the maximum number of RBs. ns is a slot number in a radio frame. l is an OFDM symbol index in a slot. 
     A pseudo-random sequence c(i) is defined by a length- 31  gold sequence as follows.
 
 c ( n )=( x   1 ( n+Nc )+ x   2 ( n+Nc )mod 2
 
 x   1 ( n+ 31)=( x   1 ( n+ 3)+ x   1 ( n ))mod 2
 
 x   2 ( n+ 31)=( x   2 ( n+ 3)+ x   2 ( n+ 2)+ x   2 ( n+ 1)+ x   2 ( n ))mod 2  [Equation 4]
 
     Herein, Nc=1600, and a first m-sequence is initialized as x 1 (0)=1, x 1 (n)=0, m=1,2, . . . ,30. 
     A second m-sequence is initialized as c init =2 10 (7(ns+1)+1+1)(2N cell   ID +1)+2 N cell   ID +N CP  at a start of each OFDM symbol. N cell   ID  is a physical cell identifier (PCI). N cp =1 in a normal CP case, and N CP =0 in an extended CP case. 
     A UE-specific reference signal (URS) is transmitted in the subframe. Whereas the CRS is transmitted in the entire region of the subframe, the URS is transmitted in a data region of the subframe and is used to demodulate the PDSCH. In  FIG. 4 , ‘R 5 ’ indicates an RE used to transmit the URS. The URS is also called a dedicated reference signal (DRS) or a demodulation reference signal (DM-RS). 
     The URS is transmitted only in an RB to which a corresponding PDSCH is mapped. Although R 5  is indicated in  FIG. 4  in addition to a region in which the PDSCH is transmitted, this is for indicating a location of an RE to which the URS is mapped. 
     The URS is used only by a wireless device which receives a corresponding PDSCH. A reference signal (RS) sequence r ns (m) for the URS is equivalent to Equation 3. In this case, m=0,1, . . . ,12N PDSCH,RB −1, and N PDSCH,RB  is the number of RBs used for transmission of a corresponding PDSCH. A pseudo-random sequence generator is initialized as c init =(floor(ns/2)+1)(2 N cell   ID +1)2 16 +n RNTI  at a start of each subframe. n RNTI  is an identifier of the wireless device. 
     The aforementioned initialization method is for a case where the URS is transmitted through the single antenna, and when the URS is transmitted through multiple antennas, the pseudo-random sequence generator is initialized as c init =(floor(ns/2)+1)(2 N cell   ID +1)2 16 +n SCID  at a start of each subframe. n SCID  is a parameter acquired from a DL grant (e.g., a DCI format 2B or 2C) related to PDSCH transmission. 
     Meanwhile, the PDCCH is monitored in an area restricted to the control region in the subframe, and a CRS transmitted in a full band is used to demodulate the PDCCH. As a type of control data is diversified and an amount of control data is increased, scheduling flexibility is decreased when using only the existing PDCCH. In addition, in order to decrease an overhead caused by CRS transmission, an enhanced PDCCH (EPDCCH) is introduced. 
       FIG. 6  is an example of a subframe having an EPDCCH. 
     The subframe may include zero or one PDCCH region  410  and zero or more EPDCCH regions  420  and  430 . 
     The EPDCCH regions  420  and  430  are regions in which a wireless device monitors the EPDCCH. The PDCCH region  410  is located in up to first four OFDM symbols of the subframe, whereas the EPDCCH regions  420  and  430  may be flexibly scheduled in an OFDM symbol located after the PDCCH region  410 . 
     One or more EPDCCH regions  420  and  430  may be assigned to the wireless device. The wireless device may monitor EPDDCH data in the assigned EPDCCH regions  420  and  430 . 
     The number/location/size of the EPDCCH regions  420  and  430  and/or information regarding a subframe for monitoring the EPDCCH may be reported by a BS to the wireless device by using a radio resource control (RRC) message or the like. 
     In the PDCCH region  410 , a PDCCH may be demodulated on the basis of a CRS. In the EPDCCH regions  420  and  430 , instead of the CRS, a DM-RS may be defined for demodulation of the EPDCCH. An associated DM-RS may be transmitted in the EPDCCH regions  420  and  430 . 
     An RS sequence for the associated DM-RS is equivalent to Equation 3. In this case, m=0, 1, . . . ,12N RB −1, and N RB  is a maximum number of RBs. A pseudo-random sequence generator may be initialized as c init =(floor(ns/2)+1)(2 N EPDCCH,ID +1)2 16 +n EPDCCH,SCID  at a start of each subframe. ns is a slot number of a radio frame. N EPDCCH,ID  is a cell index related to a corresponding EPDCCH region. n EPDCCH,SCID  is a parameter given from higher layer signaling. 
     Each of the EPDCCH regions  420  and  430  may be used to schedule a different cell. For example, an EPDCCH in the EPDCCH region  420  may carry scheduling information for a primary cell, and an EPDCCH in the EPDCCH region  430  may carry scheduling information for a secondary cell. 
     When the EPDCCH is transmitted through multiple antennas in the EPDCCH regions  420  and  430 , the same precoding as that used in the EPDCCH may be applied to a DM-RS in the EPDCCH regions  420  and  430 . 
     Comparing with a case where the PDCCH uses a CCE as a transmission resource unit, a transmission resource unit for the EPDCCH is called an enhanced control channel element (ECCE). An aggregation level may be defined as a resource unit for monitoring the EPDCCH. For example, when 1 ECCE is a minimum resource for the EPDCCH, it may be defined as an aggregation level L={1, 2, 4, 8, 16}. 
     An EPDCCH search space may correspond to an EPDCCH region. One or more EPDCCH candidates may be monitored at one or more aggregation levels in the EPDCCH search space. 
     Now, resource allocation for an EPDCCH will be described. 
     The EPDCCH is transmitted by using one or more ECCEs. The ECCE includes a plurality of enhanced resource element groups (EREGs). According to a CP and a subframe type based on a time division duplex (TDD) DL-UL configuration, the ECCE may include 4 EREGs or 8 EREGs. For example, the ECCE may include 4 EREGs in a normal CP case, and may include 8 EREGs in an extended CP case. 
     A physical resource block (PRB) pair is 2 PRBs having the same RB number in one subframe. The PRB pair is a first PRB of a first slot and a second PRB of a second slot in the same frequency domain. In the normal CP case, the PRB pair includes 12 subcarriers and 14 OFDM symbols, and thus includes 168 resource elements (REs). 
       FIG. 7  shows an example of a PRB pair. Although it is assumed hereinafter that a subframe includes 2 slots and a PRB pair in one slot includes 7 OFDM symbols and 12 subcarriers, the number of OFDM symbols and the number of subcarriers are for exemplary purposes only. 
     In one subframe, the PRB pair includes 168 REs in total. 16 EREGs are configured from 144 REs, except for 24 REs for a DM RS. Therefore, 1 EREG may include 9 REs. However, a CRS-RS or a CRS may be placed to one PRB pair, in addition to the DM RS. In this case, the number of available REs may be decreased, and the number of REs included in 1 EREG may be decreased. The number of REs included in the EREG may be changed, whereas there is no change in the number (i.e., 16) of EREGs included in one PRB pair. 
     In this case, as shown in  FIG. 7 , an RE index may be assigned sequentially starting from a first subcarrier of a first OFDM symbol (1=0). Assume that 16 EREGs are indexed from 0 to 15. In this case, 9 REs having an RE index  0  are assigned to an EREG  0 . Likewise, 9 REs corresponding to an RE index k (k=0, . . . , 15) are assigned to an EREG k. 
     An EREG group is defined by aggregating a plurality of EREGs. For example, if an EREG group having 4 EREGs is defined, it may be defined as an EREG group # 0 ={EREG  0 , EREG  4 , EREG  8 , EREG  12 }, an EREG group # 1 ={EREG  1 , EREG  5 , EREG  9 , EREG  3 }, an EREG group # 2 ={EREG  2 , EREG  6 , EREG  10 , EREG  14 }, and an EREG group # 3 ={EREG  3 , EREG  7 , EREG  11 , EREG  15 }. If an EREG group having 8 EREGs is defined, it may be defined as an EREG group # 0 ={EREG  0 , EREG  2 , EREG  4 , EREG  6 , EREG  8 , EREG  10 , EREG  12 , EREG  14 } and an EREG group # 1 ={EREG  1 , EREG  3 , EREG  5 , EREG  7 , EREG  9 , EREG  11 , EREG  13 , EREG  15 }. 
     As described above, the ECCE may include 4 EREGs. In an extended CP case, the ECCE may include 8 EREGs. The ECCE is defined by the EREG group. For example, it is exemplified in  FIG. 7  that an ECCE # 0  includes an EREG group # 0 , an ECCE # 1  includes an EREG group # 1 , an ECCE # 2  includes an EREG group # 2 , and an ECCE # 3  includes an EREG group # 3 . 
     ECCE-to-EREG mapping has two types of transmission, i.e., localized transmission and distributed transmission. In the localized transmission, an EREG group constituting one ECCE is selected from EREGs of one PRB pair. In the distributed transmission, an EREG constituting one ECCE is selected from EREGs of different PRB pairs. 
     Now, a DL HARQ operation and a PUCCH structure will be described. 
       FIG. 8  shows a DL HARQ operation in 3GPP LTE. 
     A wireless device monitors a PDCCH, and receives a DL grant including a DL resource allocation on a PDCCH  501  (or EPDDCH) in an n th  DL subframe. The wireless device receives a DL transport block through a PDSCH  502  indicated by the DL resource allocation. 
     The wireless device transmits an ACK/NACK signal for the DL transport block on a PUCCH  511  in an (n+4) th  UL subframe. The ACK/NACK signal corresponds to an ACK signal when the DL transport block is successfully decoded, and corresponds to a NACK signal when the DL transport block fails in decoding. Upon receiving the NACK signal, a BS may retransmit the DL transport block until the ACK signal is received or until the number of retransmission attempts reaches its maximum number. 
     In 3GPP LTE, PUCCH formats 1 a/1b/3 are used to carry an ACK/NACK signal which is a reception acknowledgement for HARQ. 
     All PUCCH formats use a cyclic shift (CS) of a sequence in each OFDM symbol. The cyclically shifted sequence is generated by cyclically shifting a base sequence by a specific CS amount. The specific CS amount is indicated by a CS index. 
     An example of a base sequence r u (n) is defined by the following equation.
 
 r   u ( n )= e   jb(n)π/4   [Equation 5]
 
     Herein, u denotes a root index, and n denotes a component index in the range of 0≦n≦N−1, where N is a length of the base sequence. b(n) is defined in the section 5.5 of 3GPP TS 36.211 V10.2.0. 
     A length of a sequence is equal to the number of elements included in the sequence. u can be determined by a cell identifier (ID), a slot number in a radio frame, etc. When it is assumed that the base sequence is mapped to one resource block in a frequency domain, the length N of the base sequence is 12 since one resource block includes 12 subcarriers. A different base sequence is defined according to a different root index. 
     The base sequence r(n) can be cyclically shifted by the following equation to generate a cyclically shifted sequence r(n, I cs ). 
     
       
         
           
             
               
                 
                   
                     
                       r 
                       ⁡ 
                       
                         ( 
                         
                           n 
                           , 
                           
                             I 
                             cs 
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         r 
                         ⁡ 
                         
                           ( 
                           n 
                           ) 
                         
                       
                       · 
                       
                         exp 
                         ⁡ 
                         
                           ( 
                           
                             
                               j 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               π 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 I 
                                 cs 
                               
                               ⁢ 
                               n 
                             
                             N 
                           
                           ) 
                         
                       
                     
                   
                   , 
                   
                     0 
                     ≤ 
                     
                       I 
                       cs 
                     
                     ≤ 
                     
                       N 
                       - 
                       1 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     Herein, I cs  denotes a CS index indicating a CS amount (0≦I cs ≦N−1). 
     Hereinafter, the available CS of the base sequence denotes a CS index that can be derived from the base sequence according to a CS interval. For example, if the base sequence has a length of 12 and the CS interval is 1, the total number of available CS indices of the base sequence is 12. Alternatively, if the base sequence has a length of 12 and the CS interval is 2, the total number of available CS indices of the base sequence is 6. 
       FIG. 9  shows transmission of an ACK/NACK signal when a PUCCH format 1b is used in a normal CP case in 3GPP LTE. 
     One slot includes 7 OFDM symbols. Three OFDM symbols are reference signal (RS) OFDM symbols for an RS. Four OFDM symbols are data OFDM symbols for an ACK/NACK signal. 
     In the PUCCH format 1b, a modulation symbol d(0) is generated by modulating a 2-bit ACK/NACK signal based on quadrature phase shift keying (QPSK). 
     A CS index I cs  may vary depending on a slot number n s  in a radio frame and/or a symbol index 1 in a slot. 
     In the normal CP case, there are four data OFDM symbols for transmission of the ACK/NACK signal in one slot. Therefore, assume that CS indices corresponding to the respective data OFDM symbols are denoted by I cs0 , I cs1 , I cs2 , and I cs3 . 
     The modulation symbol d(0) is spread to a cyclically shifted sequence r(n,I cs ). When a one-dimensional spreading sequence corresponding to an (i+1) th  OFDM symbol in a slot is denoted by m(i), it can be expressed as follows.
 
{ m (0), m (1), m (2), m (3)}={ d (0) r ( n,I   cs0 ), d (0) r ( n,I   cs1 ), d (0) r ( n,I   cs2 ), d (0) r ( n,I   cs3 )}
 
     In order to increase UE capacity, the one-dimensional spreading sequence can be spread by using an orthogonal sequence. An orthogonal sequence w i (k) (where i is a sequence index, 0≦k≦K−1) based on a spread factor K is as follows. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Index 
                 K = 4 
                 K = 3 
               
               
                 (i) 
                 [w i (0), w i (1), w i (2), w i (3)] 
                 [w i (0), w i (1), w i (2)] 
               
               
                   
               
             
            
               
                 0 
                 [+1, +1, +1, +1] 
                 [+1, +1, +1] 
               
               
                 1 
                 [+1, −1, +1, −1] 
                 [+1, e j2π/3 , e j4π/3 ] 
               
               
                 2 
                 [+1, −1, −1, +1] 
                 [+1, e j4π/3 , e j2π/3 ] 
               
               
                   
               
            
           
         
       
     
     A different spread factor can be used for each slot. 
     Therefore, when any orthogonal sequence index i is given, two-dimensional spreading sequences {s(0), s(1), s(2), s(3)} can be expressed as follows.
 
{ s (0), s (1), s (2), s (3)}={ w   i (0) m (0), w   i (1) m (1), w   i (2) m (2), w   i (3) m (3)}
 
     The two-dimensional spreading sequences {s(0), s(1), s(2), s(3)} are subjected to inverse fast Fourier transform (IFFT) and thereafter are transmitted in corresponding OFDM symbols. Accordingly, the ACK/NACK signal is transmitted on a PUCCH. 
     An RS of the PUCCH format 1b is also transmitted by cyclically shifting the base sequence r(n) and then by spreading it by the use of an orthogonal sequence. When CS indices corresponding to three RS OFDM symbols are denoted by I cs4 , I cs5 , and I cs6 , three cyclically shifted sequences r(n,I cs4 ), r(n,I cs5 ), and r(n,I cs6 ) can be obtained. The three cyclically shifted sequences are spread by the use of an orthogonal sequence w RS   i (k) having a spreading factor K=3. 
     An orthogonal sequence index i, a CS index I cs , and a resource block index m are parameters required to construct the PUCCH, and are also resources used to identify the PUCCH (or UE). If the number of available cyclic shifts is 12 and the number of available orthogonal sequence indices is 3, PUCCHs for 36 UEs in total can be multiplexed to one resource block. 
     In the 3GPP LTE, a resource index n (1)   PUCCH  is defined in order for the UE to obtain the three parameters for constructing the PUCCH. The resource index n (1)   PUCCH  is defined to n CCE +N (1)   PUCCH , where n CCE  is a number indicating a first CCE used for transmission of a corresponding DCI (i.e., a DL resource allocation used to receive DL data corresponding to an ACK/NACK signal), and N (1)   PUCCH  is a parameter reported by a BS to the UE by using a higher-layer message. 
     To configure a resource index for the PUCCH, the UE uses a resource allocation of a PDCCH. That is, a lowest CCE index (or an index of a first CCE) used for transmission of the PDCCH is n CCE , and the resource index is determined as n (1)   PUCCH =n CCE +N (1)   PUCCH . 
     Time, frequency, and code resources used for transmission of the ACK/NACK signal are referred to as ACK/NACK resources or PUCCH resources. As described above, an index of the ACK/NACK resource required to transmit the ACK/NACK signal on the PUCCH (also referred to as an ACK/NACK resource index or a PUCCH index) can be expressed with at least any one of an orthogonal sequence index i, a CS index I cs , a resource block index m, and a PUCCH index n (1)   PUCCH  for obtaining the three indices. The ACK/NACK resource may include at least any one of an orthogonal sequence, a cyclic shift, a resource block, and a combination thereof. 
       FIG. 10  shows a structure of a PUCCH format 3 in a normal CP case. 
     One slot includes 7 OFDM symbols. l denotes an OFDM symbol number in the slot, and has a value in the range of 0 to 6. Two OFDM symbols with l=1, 5 are used as RS OFDM symbols for a reference signal, and the remaining OFDM symbols are used as data OFDM symbols for a UCI signal. 
     A symbol sequence d={d(0), d(1), d(23)} is generated by performing quadrature phase-shift keying (QPSK) modulation on 48-bit encoded UCI (e.g., encoded ACK/NACK). d(n)(n=0,1, . . . ,23) is a complex-valued modulation symbol. The symbol sequence d can be regarded as a set of modulation symbols. The number of bits of the UCI or a modulation scheme is for exemplary purposes only, and thus the present invention is not limited thereto. 
     One PUCCH uses one resource block (RB), and one subframe includes a first slot and a second slot. A symbol sequence d={d(0), d(1), d(23)} is divided into two sequences d1={d(0), . . . , d(11)} and d2={d(12), . . . ,d(23)}, each having a length of 12. The first sequence d1 is transmitted in the first slot, and the second sequence d2 is transmitted in the second slot. In  FIG. 3 , the first sequence d1 is transmitted in the first slot. 
     The symbol sequence is spread with an orthogonal sequence w i . Symbol sequences correspond to respective data OFDM symbols. An orthogonal sequence is used to identify a PUCCH (or wireless device) by spreading the symbol sequence across the data OFDM symbols. 
     An orthogonal sequence w i (k) (i is a sequence index, 0≦k≦K−1) based on a spreading factor K is as follows. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Index 
                 K = 5 
                 K = 4 
               
               
                 (i) 
                 [w i (0), w i (1), w i (2), w i (3), w i (3)] 
                 [w i (0), w i (1), w i (2), w i (2)] 
               
               
                   
               
             
            
               
                 0 
                 [+1, +1, +1, +1] 
                 [+1, +1, +1, +1] 
               
               
                 1 
                 [+1, e j2π/5 , e j4π/5 , e j6π/5 , e j8π/5 ] 
                 [+1, −1, +1, −1] 
               
               
                 2 
                 [+1, e j4π/5 , e j8π/5 , e j2π/5 , e j6π/5 ] 
                 [+1, +1, −1, −1] 
               
               
                 3 
                 [+1, e j6π/5 , e j2π/5 , e j8π/5 , e j4π/5 ] 
                 [+1, −1, −1, +1] 
               
               
                 4 
                 [+1, e j8π/5 , e j6π/5 , e j4π/5 , e j2π/5 ] 
                 — 
               
               
                   
               
            
           
         
       
     
     In two RS OFDM symbols, an RS sequence used in UCI demodulation is mapped and transmitted. 
     In the PUCCH format 3, only the orthogonal sequence is used to distinguish wireless devices. To allocate the orthogonal sequence for the PUCCH format 3, a resource index may be defined similarly to the PUCCH format 1. A BS allocates a resource index set for the PUCCH format 3 in advance to the wireless device through an RRC message. In addition, a resource index to be used in the resource index set is directly indicated by the DL grant. 
     Now, a PUCCH resource allocation in multiple cells is described according to an embodiment of the present invention. 
     A coordinated multi-point (CoMP) is a technique in which a wireless device receives a signal from several BSs or transmits the signal to the several BSs, and is generally used to increase a data throughput at a boundary of a serving BS. 
     First, the following terms are defined. 
     Macro base station (M-BS): A BS to which a serving cell of a macro-wireless device (M-WD) belongs. It is also called a serving BS. 
     Pico BS (P-BS): A BS having a coverage partially or entirely overlapping with the M-BS. The M-WD also belongs to the coverage of the P-BS. The coverage of the P-BS is smaller than a coverage of the M-BS, but the present invention is not limited thereto. The P-BS may also be called in various terms such as a femto-BS, a home eNB, a closed subscriber group (CSG) cell, a reception point (RP), a transmission point (TP), etc. 
     Physical cell identity (PCID): A physical identity for identifying a corresponding BS. It may be directly acquired by the wireless device by receiving a synchronization signal (e.g., a primary synchronization signal (PSS) and a secondary synchronization signal (SSS)) of the corresponding BS. 
     Virtual cell identity (VCID): An identity allocated from a current serving BS. The M-WD does not need to acquire the PCID of the P-BS even if it belongs to the coverage of the P-BS since the M-BS is the serving BS. In order for the M-WD to perform transmission to the P-BS, the cell identity of the P-BS may be required. An identity to be used as the cell identity of the P-BS is reported by the M-BS, and this is called the VCID. 
       FIG. 11  shows an example of a CoMP scenario. 
     An M-WD 1  and an M-WD 2  receive a service from an M-BS. The M-WD 1  belongs to a coverage of a P-BS 1 , and the M-WD 2  belongs to a coverage of a P-BS 2 . A PCID of the M-BS is a PCID 0 , a PCID of the P-BS 1  is a PCID 1 , and a PCID of the P-BS 2  is a PCID 2 . That is, this is a case where PCIDs of all BSs are different. 
     The M-WD 1  receives an EPDCCH/PDSCH from the M-BS. In addition, the M-WD  1  transmits ACK/NACK corresponding to the PDSCH to the ‘P-BS 1 ’ on a PUCCH. The M-WD 2  receives the EPDCCH/PDSCH from the M-BS. In addition, the M-WD 2  transmits ACK/NACK corresponding to the PDSCH to the ‘P-BS 2 ’ on the PUCCH. 
     Since a BS for transmitting the EPDCCH and a BS for receiving the PUCCH are different from each other, it may be difficult to directly use the existing PDCCH-PUCCH resource linkage. This is because a PUCCH resource acquired from the PDCCH can be used by another BS. 
     The PUCCH resource region linked to the EPDCCH may be separated from a PUCCH resource region linked to the PDCCH. In addition, instead of the PCID, a VCID may be used in the PUCCH resource region linked to the EPDCCH. The PUCCH resource region implies a set of PUCCH resources that can be allocated for a corresponding PUCCH. 
       FIG. 12  shows a PUCCH resource allocation according to an embodiment of the present invention. 
     A PUCCH resource region linked to a PDCCH is separated in all of an M-BS, a P-BS 1 , and a P-BS 2 . That is, a PUCCH resource region linked to a PDCCH for the M-BS and a PDCCH-PUCCH resource region for the P-BS 1  and the P-BS 2  are different from each other. This is a case where a high interference is expected between the M-BS and the P-BS 1 /P-BS 2 . 
     The PUCCH resource region linked to the EPDCCH is shared in all of the M-BS, the P-BS 1 , and the P-BS 2 . That is, the PUCCH resource region linked to the EPDCCH (this is called a shared PUCCH resource region) is identical in all of the M-BS, the P-BS 1 , and the P-BS 2 . A VCID is used to identify a shared PUCCH resource region and the PUCCH resource region linked to the PDCCH. 
     In the shared PUCCH resource region, instead of the PCID, the VCID may be used to configure the PUCCH. For example, I cs  of Equation 6 is a cyclic shift index indicating a CS amount. I cs  is defined by a pseudo-random sequence. A pseudo-random sequence generator may be initialized based on the VCID. 
       FIG. 13  shows a PUCCH resource allocation according to another embodiment of the present invention. 
     A PUCCH resource region linked to a PDCCH is identical in all of an M-BS, a P-BS 1 , and a P-BS 2 . A PUCCH resource region linked to a PDCCH for the M-BS, a PDCCH-PUCCH resource region for the P-BS  1 , and a PDCCH-PUCCH resource region for the P-BS 2  are different from one another. This a case where a low interference is expected among the M-BS, the P-BS 1 , and the P-BS 2 . 
     The PUCCH resource region linked to the EPDCCH is shared in all of the M-BS, the P-BS 1 , and the P-BS 2 . The PUCCH resource region linked to the EPDCCH is identical in all of the M-BS, the P-BS 1 , and the P-BS 2 . A VCID is used to identify a shared PUCCH resource region and the PUCCH resource region linked to the PDCCH. 
       FIG. 14  is another example of a CoMP scenario. 
     In comparison with the example of  FIG. 11 , a PCID of an M-BS, a PCID of a P-BS 1 , and a PCID of a P-BS 2  are all equal to PCID 0 . Wireless devices belonging to the P-BS 1  and the P-BS 2  may be all scheduled by an EPDCCH based on a VCID. 
       FIG. 15  shows a PUCCH resource allocation according to another embodiment of the present invention. 
     Since a PDCCH is used only by an M-BS, only a PUCCH resource region linked to a PDCCH for the M-BS is present. 
     The PUCCH resource region linked to the EPDCCH is shared in all of the M-BS, the P-BS 1 , and the P-BS 2 . That is, the PUCCH resource region linked to the EPDCCH is identical in all of the M-BS, the P-BS 1 , and the P-BS 2 . A VCID is used to identify a shared PUCCH resource region and the PUCCH resource region linked to the PDCCH. 
     If a BS for receiving an EPDCCH/PDSCH differs from a BS for transmitting a PUCCH in a CoMP environment, an orthogonality may be damaged between PUCCH resources which are multiplexed in the same UL subframe due to a difference of PUCCH transmission power. Therefore, a PUCCH resource for CoMP and a PUCCH resource for non-CoMP are separated. 
       FIG. 16  is a flowchart showing an ACK/NACK transmission method according to an embodiment of the present invention. 
     In step S 910 , a wireless device receives an EPDCCH configuration from a first BS. The first BS may be an M-BS. 
     The EPDCCH configuration may include information regarding one or more EPDCCH sets. The EPDDCH set may correspond to one search space in which the EPDCCH is monitored, and may include one or more PRB pairs (or PRBs). For example, the EPDDC set may include at least any one of the following fields. 
     
       
         
           
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Field 
                 Content 
               
               
                   
               
             
            
               
                 Identity 
                 Identity of EPDCCH set (or also referred to as an 
               
               
                   
                 EPDCCH set index) 
               
               
                 Transport type 
                 To indicate distributed transmission or local 
               
               
                   
                 transmission 
               
               
                 RB allocation 
                 PRB pair for EPDCCH set 
               
               
                 PUCCH offset 
                 Offset for PUCCH resource 
               
               
                 RS scramble identity 
                 Scrambling sequence initialization parameter of 
               
               
                   
                 DM RS for EPDCCH 
               
               
                   
               
            
           
         
       
     
     Together with or independently from the EPDCCH configuration, the information regarding the VCID may be transmitted from the first BS to the wireless device. 
     In step S 920 , the wireless device monitors a configured EPDCCH set, and receives a DL grant on an EPDCCH. 
     In step S 930 , the wireless device receives a DL transport block on a PDSCH indicated by the DL grant. 
     In step S 940 , the wireless device transmits ACK/NACK for the DL transport block to a second BS on a PUCCH on the basis of a PUCCH resource. The second BS may be a P-BS. The PUCCH may be configured on the basis of a PUCCH resource and a VCID. 
     A PUCCH resource for a PDSCH scheduled by the EPDCCH may be determined as follows. 
     In a first embodiment, an additional offset may be defined in a resource index for a PUCCH. 
     For example, a resource index n (1)   PUCCH  is defined as follows.
 
 n   PUCCH   (1)   =n   ECCE,q   +N   PUCCH,q   (1)   +No   [Equation 7]
 
     Herein, n ECCE,q  is a number indicating a first ECCE in which a corresponding EPDCCH is detected in an EPDCCH set q, N (1)   PUCCH  is a value reported by a higher layer for the EPDCCH set q, No is an offset added due to CoMP. These values may be reported on a higher layer message or on a corresponding EPDCCH. 
     In a second embodiment, information regarding a position (e.g., a start point) of a PUCCH resource region for an EPDDCH may be reported by a BS. The information may be cell specific or a wireless device specific or wireless device group specific. 
     In a third embodiment, a DL grant on an EPDCCH may include PUCCH resource allocation information. 
     For example, the DL grant may include information regarding a reference value used to determine a PUCCH resource. 
     For another example, the BS reports information regarding a plurality of PUCCH resource candidates in advance to the wireless device through an RRC message, etc. In addition, the DL grant may include an indicator regarding a PUCCH resource to be used among the plurality of PUCCH resource candidates. 
     For example, if the indicator is 2 bits, it may be expressed as follows. 
     
       
         
           
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Bit 
                 Contents 
               
               
                   
               
             
            
               
                 ‘00’ 
                 1 st  PUCCH resource 
               
               
                 ‘01’ 
                 2 nd  PUCCH resource 
               
               
                 ‘10’ 
                 3 rd  PUCCH resource 
               
               
                 ‘11’ 
                 4 th  PUCCH resource 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Bit 
                 Contents 
               
               
                   
               
             
            
               
                 ‘00’ 
                 Use of existing PDCCH-PUCCH linkage 
               
               
                 ‘01’ 
                 1 st  PUCCH resource 
               
               
                 ‘10’ 
                 2 nd  PUCCH resource 
               
               
                 ‘11’ 
                 3 rd  PUCCH resource 
               
               
                   
               
            
           
         
       
     
     The reference value may be determined by the RRC message, and the indicator may indicate an offset from the reference value. 
     
       
         
           
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Bit 
                 Contents 
               
               
                   
               
             
            
               
                 ‘00’ 
                 Offset 0 
               
               
                 ‘01’ 
                 Offset −1 
               
               
                 ‘10’ 
                 Offset −2 
               
               
                 ‘11’ 
                 Offset −3 
               
               
                   
               
            
           
         
       
     
     In Table 7 to Table 8 above, the number of bits of the indicator and the content of a corresponding value are for exemplary purposes only. 
     In a fourth embodiment, the wireless device may determine a PUCCH resource region for an EPDCCH according to a size of a control region for a PDCCH. For example, a resource index n (1)   PUCCH  may be defined as follows.
 
 n   PUCCH   (1)   =n   ECCE,q   +N   PUCCH,q   (1)   +Fo   [Equation 8]
 
     Herein, Fo is a value acquired based on a size of a control region. 
     A PUCCH resource region may be determined according to a DCI format, a search space type, an aggregation level, a wireless device-specific parameter, etc. 
       FIG. 17  is a block diagram showing a wireless communication system according to an embodiment of the present invention. 
     A BS  50  includes a processor  51 , a memory  52 , and a radio frequency (RF) unit  53 . The memory  52  is coupled to the processor  51 , and stores a variety of information for driving the processor  51 . The RF unit  53  is coupled to the processor  51 , and transmits and/or receives a radio signal. The processor  51  implements the proposed functions, procedures, and/or methods. In the aforementioned embodiment, an operation of the BS may be implemented by the processor  51 . The processor  51  may configure an EPDCCH, and may transmit the EPDCCH and/or a PDCCH. The processor  51  may support an HARQ operation, and may receive HARQ ACK/NACK. 
     A wireless device  60  includes a processor  61 , a memory  62 , and an RF unit  63 . The memory  62  is coupled to the processor  61 , and stores a variety of information for driving the processor  61 . The RF unit  63  is coupled to the processor  61 , and transmits and/or receives a radio signal. The processor  61  implements the proposed functions, procedures, and/or methods. In the aforementioned embodiment, an operation of the wireless device may be implemented by the processor  60 . The processor  61  may monitor an EPDDCH, and may transmit HARQ ACK/NACK. 
     The processor may include an application-specific integrated circuit (ASIC), a separate chipset, a logic circuit, and/or a data processing unit. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and/or other equivalent storage devices. The RF unit may include a base-band circuit for processing a radio signal. When the embodiment of the present invention is implemented in software, the aforementioned methods can be implemented with a module (i.e., process, function, etc.) for performing the aforementioned functions. The module may be stored in the memory and may be performed by the processor. The memory may be located inside or outside the processor, and may be coupled to the processor by using various well-known means. 
     Although the aforementioned exemplary system has been described on the basis of a flowchart in which steps or blocks are listed in sequence, the steps of the present invention are not limited to a certain order. Therefore, a certain step may be performed in a different step or in a different order or concurrently with respect to that described above. Further, it will be understood by those ordinary skilled in the art that the steps of the flowcharts are not exclusive. Rather, another step may be included therein or one or more steps may be deleted within the scope of the present invention.