Abstract:
Disclosed is a base station which can prevent degradation in the reception characteristics of nearby cells when carrier aggregation and channel selection are applied. In accordance with the resistance to interference of a plurality of uplink component carriers of a femtocell, a response protocol controller ( 101 ) in a base station ( 100 ) changes combination rules for the uplink component carriers and signal points which a terminal ( 200 ) in a microcell covered by the base station ( 100 ) uses in the feedback of a response signal. Information in relation to the combination rules is notified to the terminal ( 200 ). A controller ( 217 ) in the terminal ( 200 ) changes the combination rules for the uplink component carriers and the signal points of the microcell covered by the base station ( 100 ) in accordance with the information notified from the base station ( 100 ), and controls transmission of the response signal in accordance with said rules.

Description:
TECHNICAL FIELD 
     The present invention relates to base stations, terminals, retransmission control methods, and response methods. 
     BACKGROUND ART 
     In LTE-Advanced under standardization developed by the 3GPP, carrier aggregation is now being studied in which a transmission band is divided into component carriers (CCs), uplink or downlink packets are transmitted every CC, and some coordination is made among CCs. A downlink component carrier may be defined as a hand obtained by division of a frequency band based on downlink frequency band information in a BCH broadcasted from a base station or as a band defined by a discrete width in the case where a downlink control channel (PDCCH) is discretely allocated in the frequency domain. An uplink component carrier may be defined as a band divided by uplink frequency band information in a BCH broadcasted from a base station or as a base unit of a communication band of 20 MHz or less having a physical uplink shared channel (PUSCH) domain in the center and having pieces of a PUCCH for LTE at the both ends. 
     The component carrier may be defined by a physical cell number or a carrier frequency number, and is also called a cell. 
     LTE and LTE-Advanced employ single carrier (SC)-FDMA as an uplink transmission scheme. In SC-FDMA, a transmission waveform by an individual user has single-carrier characteristics. For this reason, SC-FDMA can maintain cubic metric (CM)/PARP low. Furthermore, SC-FDMA has characteristics of orthogonal frequency division multiplexing. SC-FDMA thus can employ frequency division multiplexing (FDM) as a multiplexing method to frequency-division multiplex pieces of data in adjacent subcarriers at the same time, the data being to be transmitted to a plurality of users. 
     LTE and LTE-Advanced employ hybrid automatic repeat request (H-ARQ) to improve the efficiency of the transmission rate of downlink packets. When H-ARQ is employed in transmission of downlink packets, an uplink control channel (PUCCH) is used as a response channel for transmitting from a terminal to a base station an ACK or a NACK that indicates whether a downlink packet is successfully received or not received as a feedback. 
     In LTE in which communication is made using CC pairs each formed of an uplink CC and a downlink CC, downlink packets are transmitted in one downlink CC and a response signal is transmitted in a PUCCH of an uplink CC corresponding to the downlink CC. 
     When this scheme for LTE is applied to LTE-Advanced in which downlink packets are concurrently transmitted using a plurality of downlink CCs, response signals are transmitted using a plurality of uplink CCs. Transmitting a response signal by SC-FDMA in this situation causes the single-carrier characteristics of SC-FDMA to be lost. For this reason, cubic metric (CM)/peak-to-average power ratio (PAPR) increases (see  FIG. 1 ), consequently causing the transmission waveform to be distorted. To prevent this distortion, it may be possible to cut off an amount of transmission power to be amplified, which, however impairs required reception quality. 
     To overcome this problem, i.e. to prevent the response signals from being transmitted concurrently in PUCCHs of a plurality of CCs, a channel selection technique has been suggested. The channel selection is a technique in which one response signal is transmitted by changing a combination of an uplink CC and constellation points (points on the constellation) according to a pattern about successful reception of each of downlink packets that have been transmitted using a plurality of downlink CCs, the uplink CCs and constellation points being used to transmit a response signal (see Non-Patent Literature 1). That is, information about ACKs or NACKs for a plurality of transmitted downlink packets are combined and one response signal is transmitted as a feedback in a PUCCH of one CC (see  FIGS. 2 and 3 ). Here, “CC# 1  DTX” in  FIG. 3  refers to the case where a terminal fails to receive downlink control information for reporting to the terminal that DL# 1  has been transmitted (i.e. downlink control information required for the terminal to receive DL# 1  and is transmitted prior to DL# 0 ). 
     CITATION LIST 
     Non-Patent Literature 
     NPL 1 
     
         
         3GPP TS 36.213 V8.7.0 10.1 UE procedure for determining physical uplink control channel assignment 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Meanwhile, a heterogeneous network is being studied as a system in which a plurality of cells having different cell radiuses and accommodating a different number of UEs are flexibly arranged. In the heterogeneous network, a macrocell having a large cell radius (macrocell) and a femtocell having a small cell radius are provided (see  FIG. 4 ). The above LTE and LTE-Advanced systems are applied to both of the macrocells and femtocells. 
     To adaptively control interference power toward uplink channels of the femtocell, it may be possible to control transmission power of terminals (UEs) covered by the macrocell for every CC. That is, by taking into consideration the situation of femtocell, it may be possible to control the transmission power of terminals (UEs) covered by the macrocell so that interference power from the macrocell to the femtocell does not become a predetermined value or more in a certain uplink CC. By this means, by suppressing transmission power for some CCs among a plurality of CCs, interference power toward the femtocell can be reduced while reception characteristics of other terminals covered by the macrocell can be maintained. As a result, CCs in the femtocell receive a different amount of interference. In this case, a communication scheme having lower interference robustness can be employed in CCs receiving smaller interference than in CCs receiving greater interference (see  FIG. 4 ). That is, in the case where BPSK is used in the modulation scheme for CCs receiving large interference, QPSK, which has low interference robustness but providing a large transmission rate, can be used in CCs receiving small interference. 
     Here, regarding the constellations used in the channel selection shown in  FIG. 3 , the first CC employs a constellation similar to QPSK (corresponding to three constellation points out of four constellation points in the QPSK constellation) and the second CC employs a constellation of BPSK. 
     Accordingly, in the case where two CCs, CC# 0  and CC# 1 , are used in the channel selection, the following two patterns of combination of a CC and a constellation may be employed: the first pattern for associating the constellation similar to QPSK and the constellation of BPSK with CC# 0  and CC# 1 , respectively, and the second pattern for associating the constellation of BPSK and the constellation similar to QPSK with CC# 0  and CC# 1 , respectively. Generally, when transmission power of a signal transmitted in a CC associated with the constellation similar to QPSK is compared with that of a signal transmitted using a CC associated with the constellation of BPSK, the transmission power of the signal transmitted using the CC associated with the constellation similar to QPSK is higher than that of the signal transmitted using the CC associated with the constellation of BPSK to meet the required quality. 
     For this reason, depending on the combination of a communication scheme used in an uplink CC of a femtocell and a communication scheme used in an uplink CC of a macrocell that corresponds to the uplink CC of the femtocell, interference from the macrocell to the femtocell becomes too large to be ignored, thus causing deterioration of reception characteristics in the femtocell. 
     It is therefore an object of the present invention to provide a base station, a terminal, a retransmission control method, and a response method that can prevent deterioration of reception characteristics of neighboring cells in the case where carrier aggregation and channel selection are employed. 
     Solution to Problem 
     One aspect of a base station of the present invention includes: a control section that changes a rule for combination of a component carrier and constellation points based on a parameter related to interference robustness of a plurality of component carriers in a plurality of cells, the component carrier and the constellation points being to be used by a terminal to transmit a response signal as a feedback to downlink data that has been transmitted using the plurality of component carriers; a reception section that performs a reception process using a combination of a candidate component carrier and candidate constellation points and outputs a result of the reception process, the candidate component carrier and the candidate constellation points corresponding to information about the changed combination rule; and an identification section that identifies the combination of the candidate component carrier and the candidate constellation points based on the result of the reception process, the candidate component carrier and the candidate constellation points having been used to transmit the response signal as a feedback. 
     One aspect of a terminal of the present invention includes: a reception section that receives downlink data using a plurality of component carriers; a control section that changes a rule for combination of a component. carrier and constellation points that are to be used to transmit a response signal as a feedback to the downlink data that has been transmitted using the component carriers; and a transmission section that transmits the response signal to a base station based on the changed combination rule; wherein the control section changes the combination rule based on information indicating the combination rule or based on a parameter related to interference robustness of the component carriers in a plurality of cells, the information indicating the combination rule being determined by the base station based on the interference robustness of the component carriers. 
     One aspect of a retransmission control method of the present invention includes: changing a rule for combination of a component carrier and constellation points based on a parameter related to interference robustness of a plurality of component carriers in a plurality of cells, the component carrier and the constellation points being to be used by a terminal to transmit a response signal as a feedback to downlink data that has been transmitted using the component carriers; performing a reception process using a combination of a candidate component carrier and candidate constellation points, the candidate component carrier and the candidate constellation points corresponding to information about the changed combination rule; and identifying the combination of the candidate component carrier and the candidate constellation points based on the result of the reception process, the candidate component carrier and the candidate constellation points having been used to transmit the response signal as a feedback. 
     One aspect of a response method of the present invention includes: changing a rule for combination of a component carrier and constellation points that are to be used to transmit a response signal as a feedback to downlink data that has been transmitted using a plurality of component carriers; and transmitting the response signal to a base station based on the changed combination rule; wherein the combination rule is changed based on information indicating the combination rule or based on a parameter related to interference robustness of the component carriers in a plurality of cells, the information indicating the combination rule being determined by the base station based on the interference robustness of the component carriers. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a base station, a terminal, a retransmission control method, and a response method that can prevent deterioration of reception characteristics of neighboring cells in the case where carrier aggregation and channel selection are employed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a problem in a conventional system; 
         FIG. 2  shows a channel selection technique; 
         FIG. 3  shows a channel selection technique; 
         FIG. 4  shows a heterogeneous network; 
         FIG. 5  is a block diagram showing a configuration of a base station according to Embodiment 1 of the present invention; 
         FIG. 6  is a block diagram showing a configuration of a terminal according to Embodiment 1 of the present invention; 
         FIG. 7  shows operations of a base station and a terminal; 
         FIG. 8  shows operations of a base station and a terminal.; 
         FIG. 9  shows operations of a base station and a terminal according to Embodiment 2 of the present invention; 
         FIG. 10  shows operations of a base station and a terminal according to Embodiment 2 of the present invention; 
         FIG. 11  shows operations of a base station and a terminal according to Embodiment 3 of the present invention; 
         FIG. 12  shows operations of a base station and a terminal according to Embodiment 3 of the present invention; 
         FIG. 13  is a block diagram showing a configuration of a base station according to Embodiment 4 of the present invention; 
         FIG. 14  is a block diagram showing a configuration of a terminal according to Embodiment 4 of the present invention; 
         FIG. 15  shows operations of a base station and a terminal; 
         FIG. 16  shows operations of a base station and a terminal; 
         FIG. 17  shows operations of a base station and a terminal; 
         FIG. 18  shows operations of a base station and a terminal; and 
         FIG. 19  shows operations of a base station and a terminal according to Embodiment 5 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, components having the same functions in the embodiments will be assigned the same reference numerals, and overlapping explanations will be omitted. 
     [Embodiment 1] 
     [Overview of System] 
     In a communication system including base station  100  and terminals  200 , which will be described later, communication employing carrier aggregation is performed in which a plurality of uplink component carriers and a plurality of component carriers associated with the uplink component carriers are used. 
     When communication is performed using the carrier aggregation in this communication system, the channel selection is employed in ARQ. That is, this communication system is an LTE-A system, base station  100  is an LTE-A base station, and terminal  200  is an LTE-A terminal, for example. 
     Hereinafter, a case will be described as an example, in which two component carriers are used (i.e. a case where component carriers # 1  and # 2  are used). 
     [Configuration of Base Station] 
       FIG. 5  is a block diagram showing a configuration of base station  100  according to Embodiment 1 of the present invention. With reference to  FIG. 5 , base station  100  includes response scheme control section  101 , encoding sections  102 ,  103 , and  104 , data transmission control sections  105  and  106 , modulation sections  107 ,  108 , and  109 , serial-to-parallel conversion sections (S/Ps)  110 ,  111 , and  112 , multiplexing section  113 , IFFT section  114 , CP addition section  115 , radio transmission section  116 , radio reception section  117 , CP removal section  118 , DFT section.  119 , demultiplexing section  120 , channel compensation sections  121  and  122 , IDFT sections  123  and  124 , demodulation sections  125  and  126 , and channel selection determination section  127 . 
     Response scheme control section  101  determines a rule for combination of an uplink component carrier and constellation points, and outputs information indicating the determined rule to encoding section  102 . Here, the uplink component carrier and the constellation points are used by terminal  200  to transmit a response signal as a feedback to downlink data that has been transmitted from base station  100  using a plurality of downlink component carriers. Response scheme control section  101  determines the above combination rule based on femtocell information to be input, which contains information indicating interference robustness of a plurality of uplink component carriers used by a femtocell. The femtocell is a cell adjacent to a macrocell covered by base station  100  or a cell that is smaller than that macrocell and is contained in the macrocell. The above combination rule will be described in detail later. 
     Encoding section  102  receives as input downlink control data including information indicating the above combination rule, and performs an encoding process (e.g. turbo encoding or convolutional encoding) on and adds a CRC bit for retransmission control to the received data. The data that has undergone the encoding process and the addition of the CRC bit is modulated in modulation section  107 . 
     Encoding section  103  receives as input transmission data (a packet) to be transmitted using component carrier # 0 , and performs an encoding process (e.g. turbo encoding or convolutional encoding) on and adds a CRC bit for retransmission control to the transmission data. The data that has undergone the encoding process and the addition of the CRC bit is output to data transmission control section  105 . 
     Encoding section  104  receives as input transmission data (a packet) to be transmitted using component carrier # 1 , and performs an encoding process (e.g. turbo encoding or convolutional encoding) on and adds a CRC bit for retransmission control to the transmission data. The data that has undergone the encoding process and the addition of the CRC bit is output to data transmission control section  106 . 
     Data transmission control section  105  maintains and outputs the encoded transmission data for CC# 0  to modulation section  108 , at the initial transmission. The encoded transmission data is maintained for every destination terminal  200 . 
     Upon receiving from channel selection determination section  127  a NACK or a DTX for downlink data that has been transmitted using CC# 0 , data transmission control section  105  outputs the maintained data corresponding to this CC# 0  to modulation section  108 . Upon receiving from channel selection determination section  127  an ACK for the downlink data that has been transmitted using CC# 0 , data transmission control section  105  deletes the maintained data corresponding to this CC# 0 . 
     Data transmission control section  106  performs the same process as data transmission control section  103  on the data to be transmitted using CC# 1 . 
     The process in data transmission control section  105  and data transmission control section  106  makes it possible to control. retransmission not only for the entire data that has been transmitted to destination terminal  200  but also for data that has been transmitted using every component carrier. 
     Modulation sections  108  and  109  modulate transmission data received from transmission control sections  105  and  106 , respectively. By so doing, modulation signals to be transmitted using CC# 0  and CC# 1  are generated. 
     As described above, because the modulation signals generated in modulation sections  107 ,  108 , and  109  will be transmitted as OFDM signals, the modulation signals undergo processes in serial-to-parallel conversion sections  110 ,  111 , and  112 , multiplexing section  113 , IFFT section  114 , and CP addition section  115 , as will be described later. 
     Serial-to-parallel conversion section  110  performs serial-to-parallel conversion on the modulation signal generated in modulation section  107  and outputs a group of the obtained parallel signals to multiplexing section  113 . Serial-to-parallel conversion sections  111  and  112  perform the same process as serial-to-parallel conversion section  110  on the modulation signals generated in modulation sections  108  and  109 . 
     Multiplexing section  113  multiplexes signals received from serial-to-parallel conversion sections  110 ,  111 , and  112  (i.e. multiplexing section  113  multiplexes a packet to be transmitted using downlink CC# 0 , a packet to be transmitted using downlink CC# 1 , and the control channel). 
     IFFT section  114  converts the signals from frequency domain signals into time domain signals by performing an IFFT process on the signals multiplexed in multiplexing section  113 , to generate an OFDM symbol. 
     CP addition section  115  adds the end section of the OFDM symbol obtained in IFFT section  114  as a CP to the heading of the OFDM symbol. 
     Radio transmission section  116  performs a radio transmission process (e.g. D/A conversion and up-conversion) on the OFDM signal received from CP addition section  115 , and transmits the radio transmission-processed OFDM signal via an antenna. 
     Radio reception section  117  performs a radio reception process (e.g. down-conversion and A/D conversion) on the signal received via the antenna. 
     CP removal section  118  removes a CP from a reception signal that has undergone the radio reception process. 
     DFT section  119  performs a DFT process on the signal without the CP to convert the reception signal into a frequency domain signal. 
     Demultiplexing section  120  extracts a signal corresponding to a PUCCH resource domain of every uplink CC from the reception signal received from DFT section  119 . Here, because uplink CC# 0  and uplink CC# 1  are used in this embodiment, demultiplexing section  120  extracts a signal corresponding to the PUCCH resource domain of uplink CC# 0  from the reception signal received from DFT section  119  and outputs the extracted signal to channel compensation section  121 , and demultiplexing section  120  extracts a signal corresponding to the PUCCH resource domain of uplink CC# 1  from the reception signal received from DFT section  119  and outputs the extracted signal to channel compensation section  122 . 
     Channel compensation sections  121  and  122  extract a pilot symbol from the signal received from demultiplexing section  120  and perform a frequency domain equalization process using the pilot symbol. 
     IDFT sections  123  and  124  convert the signal that has undergone the channel compensation from the frequency domain signal into the time domain signal. 
     Demodulation sections  125  and  126  perform a demodulation process on the input signal based on the combination rule determined in response scheme control section  101 . 
     Channel selection determination section  127  determines results of error detection (i.e. ACK or NACK) for pieces of downlink data that have been transmitted using downlink CC# 0  and downlink CC# 1  based on the demodulated signals corresponding to the respective uplink CCs that are obtained in demodulation section  125  and demodulation section  126  and based on the combination rule determined in response scheme control section  101 . 
     [Configuration of Terminal] 
       FIG. 6  is a block diagram showing a configuration of terminal  200  according to Embodiment 1 of the present invention. With reference to  FIG. 6 , terminal  200  includes radio reception section  201 , CP removal section  202 , FFT section  203 , channel compensation section  204 , demultiplexing section  205 , parallel-to-serial conversion sections (P/Ss)  206 ,  207 , and  208 , demodulation sections  209 ,  210 , and  211 , decoding sections  212 ,  213 , and  214 , error determination sections  215  and  216 , control section  217 , channel selection section  218 , modulation sections  219  and  220 , DFT sections  221  and  222 , multiplexing section  223 , IDFT section  224 , CP addition section  225 , and radio transmission section  226 . 
     Radio reception section  201  performs a radio reception process (e.g. down-conversion and A/D conversion) on a signal received via an antenna. 
     CP removal section  202  removes a CP from the reception signal that has undergone the radio reception process. 
     FFT section  203  performs an FFT process on the signal without the CP to convert the reception signal into a frequency domain signal. 
     Channel compensation section  204  extracts a pilot symbol from the signal received from FFT section  203  and performs a frequency domain equalization process using the pilot symbol. 
     Demultiplexing section  205  extracts a downlink control channel and reception symbols of the respective downlink CCs from the reception signal received from channel compensation section  204 . Here, because downlink CC# 0  and downlink CC# 1  are used in this embodiment, demultiplexing section  205  extracts a reception symbol of downlink CC# 0  from the reception signal received from channel compensation section  204  and outputs the extracted reception symbol to parallel-to-serial conversion section  207 , and demultiplexing section  205  extracts a reception symbol of downlink CC# 1  from the reception signal received from channel compensation section  204  and outputs the extracted reception symbol to parallel-to-serial conversion section  208 . Furthermore, demultiplexing section.  205  extracts the downlink control channel from the reception signal received from channel compensation section  204  and outputs the extracted downlink control channel to parallel-to-serial conversion section  206 . 
     Parallel-to-serial conversion section  206  performs parallel-to-serial conversion on the downlink control channel received from demultiplexing section  205  and outputs the obtained serial signal to demodulation section  209 . Parallel-to-serial conversion sections  207  and  208  perform the same process as parallel-to-serial conversion section  206  on the reception signals of downlink CC# 0  and downlink CC# 1  that are received from demultiplexing section  205 . 
     Demodulation sections  209 ,  210 , and  211  perform a demodulation process on the signals received from parallel-to-serial conversion sections  206 ,  207 , and  208 , respectively. 
     Decoding section  212  performs error correction decoding (e.g. turbo encoding or Viterbi encoding) on the likelihood of reception bit obtained from the downlink control channel, which has been received from demodulation section  209 . Decoding section  213  performs error correction decoding (e.g. turbo encoding or Viterbi encoding) on the reception bits of a downlink packet of downlink CC# 0  before decoding, which has been received from demodulation section  210 . Decoding section  214  performs error correction decoding (e.g. turbo encoding or Viterbi encoding) on the reception bits of a downlink packet of downlink CC# 1 . before decoding, which has been received from demodulation section  211 . 
     Error determination section  215  determines whether or not the decoding result (i.e. decoding result of a downlink packet of downlink CC# 0 ) obtained in decoding section  213  has errors. Error determination section  216  determines whether or not the decoding result (i.e. decoding result of a downlink packet of downlink CC# 1 ) obtained in decoding section  214  has errors. 
     Control section  217  controls transmission of a response signal based on the error detection results for the downlink packet of downlink CC# 0  and the downlink packet of downlink CC# 1 , and based on information about the combination rule contained in the downlink control channel which has been received from decoding section  212 . 
     Specifically, control section  217  first selects a transmission rule for the response signal based on the information about the combination rule. 
     Next, control section  217  determines a combination of an uplink component carrier and constellation points that is to be used to transmit a response signal based on the selected transmission rule and based on the error detection result for the downlink packet of downlink CC# 0  and the downlink packet of downlink CC# 1  (i.e. pattern about whether or not downlink packets transmitted using downlink CC# 0  and CC# 1  are successfully received (pattern about successful reception)). Thus determined information about an uplink component carrier to be used to, transmit a response signal (uplink component carrier information) is output to channel selection section  218  while information about constellation points is output to modulation sections  219  and  220 . 
     The process in this control section  217  will be described in detail later. 
     Channel selection section  218  outputs a response signal to either modulation section  219  or  220  that corresponds to the uplink component carrier indicated in the uplink component carrier information received from control section  217 : upon receiving from control section  217  the uplink component carrier information indicating uplink CC# 0 , channel selection section  218  outputs a response signal to modulation section  219  while upon receiving from control section  217  the uplink component carrier information indicating uplink CC# 1 , channel selection section  218  outputs a response signal to modulation section  220 . 
     Modulation sections  219  and  220  modulate the response signal using the constellation points indicated in the information received from control section  217 . 
     DFT sections  221  and  222  perform a DFT process on the respective modulation signals received from modulation sections  219  and  220  to convert the respective modulation signals into frequency domain signals. 
     Multiplexing section  223  is configured to multiplex the signals received from DFT sections  221  and  222 . However, multiplexing section  223  does not multiplex the signals, practically, because response signals will not be output from channel selection section  218  to both modulation sections  219  and  220  at the same time. 
     IDFT section  224  converts the signal from the frequency domain signal into the time domain signal by performing an IDFT process on the signal received from multiplexing section  223 , to generate an OFDM symbol. 
     CP addition section  225  adds the end section of the OFDM symbol obtained in IDFT section  224  as a CP to the heading of the OFDM symbol. 
     Radio transmission section  226  performs a radio transmission process (e.g. D/A conversion and up-conversion) on the OFDM signal received from CP addition section  225 , and transmits the radio transmission-processed OFDM signal via an antenna. 
     [Operation of Base Station and Terminal] 
     Next, a method of determining the above rule for combination of an uplink component carrier and constellation points in base station  100 , and a method of setting the above transmission rule for a response signal in terminal  200  will be described. 
     As described above, in base station  100 , response scheme control section  101 . determines the rule for combination of an uplink component carrier and constellation points based on the femtocell information, the uplink component carrier and the constellation points being to be used by terminal  200  to transmit a response signal as a feedback. 
     Specifically, according to this combination rule, constellations having the same number as that of uplink component carriers to be used to transmit a response signal are provided. The above respective patterns about successful reception determined in terminal  200  are associated with any one of the constellation points of every constellation. Here, because uplink CC# 0  and uplink CC# 1  are used to transmit a response signal, two constellations are provided. The number of constellation points to be used to transmit a response signal in the first constellation is greater than the number of constellation points to be used to transmit a response signal in the second constellation, i.e., the M-ary modulation value corresponding to the first constellation is greater than the M-ary modulation value corresponding to the second constellation. Furthermore, the pattern about successful reception indicating that downlink packets using all component carriers have been successfully received is associated with any one of the constellation points contained in the first constellation. 
     Response scheme control section  101  assigns the first constellation to uplink CC# 0  and assigns the second constellation to uplink CC# 1  when the femtocell information indicates that uplink CC# 0  has higher interference robustness than uplink. CC# 1  in a femtocell. That is, response scheme control section  101  assigns the first constellation to the component carrier of the femtocell with high interference robustness. With reference to  FIG. 7 , the constellation similar to QPSK is used for the first constellation and BPSK is used for the second constellation. 
     Base station  100  transmits to terminal  200  the information about the above combination rule determined in response scheme control section  101 . Here, base station  100  and terminal  200  share in advance information about combination pattern of a component carrier and a constellation in which either CC# 0  or CC# 1  is associated with either the first constellation or the second constellation (see  FIG. 8 ). Therefore, transmission of flag information corresponding to the combination pattern of a component carrier and a constellation to terminal  200  from base station  100  enables terminal  200  to transmit a response signal according to the rule indicated by base station  100 . 
     Upon receiving the information about the above combination rule, terminal  200  sets a transmission rule corresponding to that combination rule, and transmits a response signal based on the transmission rule and the pattern about successful reception. This transmission rule is the same as the above combination rule. 
     As described above, according to the present embodiment, in base station  100 , response scheme control section  101  changes the combination rule of an uplink component carrier and constellation points that are to be used by terminal  200  to transmit a response signal as a feedback, based on interference robustness of a plurality of uplink component carriers of femtocell, the terminal  200  being located in a macrocell covered by base station  100 . Information about this combination rule is reported to terminal  200 . 
     By this means, a constellation that can suppress a transmission power to a low level can be assigned to an uplink component carrier of the macrocell that corresponds to an uplink component carrier with low interference robustness in a femtocell, accordingly making it possible to reduce the interference to the femtocell. 
     Specifically, according to the above changed combination rule, the number of candidate constellation points contained in the first constellation associated with the first component carrier of the macrocell is greater than that in the second constellation associated with the second component carrier of the macrocell in the following case. Downlink data is transmitted using two downlink component carriers of the macrocell, and among two uplink component carriers of a femtocell corresponding to the two downlink component carriers of that macrocell, the first component carrier of the femtocell has higher interference robustness than the second component carrier of the femtocell. 
     Furthermore, the pattern about successful reception indicating that downlink packets of all component carriers have been successfully received is associated with any one of the constellation points contained in the first constellation. Here, ACK and NACK occur with an approximately ratio of ACK to NACK being 9:1. That is, the pattern about successful reception indicating that downlink packets of all component carriers have been successfully received is more likely to occur than other patterns about successful reception. For this reason, the second constellation is less likely to be used to transmit a response signal than the first constellation. That is, it is possible to reduce the probability of using an uplink component carrier of the macrocell corresponding to the uplink component carrier having low interference robustness in the femtocell, making it possible to reduce interference to the femtocell. 
     Furthermore, according to the present embodiment, in terminal  200 , control section  217  changes the combination rule of an uplink component carrier of the macrocell covered by base station  100  and constellation points, based on the information reported from base station  100 . In addition, according to the above changed rule, channel selection section  218  and modulation sections  219  and  220  transmit a response signal to base station  100  using one combination of an uplink component carrier of the macrocell and constellation points, the one combination corresponding to the pattern about successful reception of downlink data transmitted using a plurality of downlink component carriers. 
     Specifically, the above information reported from base station  100  is information about the combination rule of an uplink component carrier of the macrocell and constellation points, the information being determined by base station  100  based on the interference robustness of uplink component carriers of the femtocell. 
     [Embodiment 2] 
     Embodiment 2 employs the same configuration as Embodiment 1 in that a terminal changes the transmission rule of a response signal based on information reported from a base station. According to Embodiment 1, terminal  200  receives information about the combination rule of an uplink component carrier of the macrocell and constellation points and, based on that information, changes the transmission rule of a response signal, the information about the combination rule being determined by base station  100  based on the interference robustness of uplink component carriers of a femtocell. In contrast, according to Embodiment 2, a terminal changes the transmission rule of a response signal based on information about the maximum transmission power for every uplink component carrier of a macrocell, the information being transmitted from the base station. That is, according to Embodiment 2, the base station does not need to transmit information about the above combination rule to a terminal. For this reason, it is not necessary to set additional signaling in addition to the signaling set in the standard for the LTE system. 
     The basic configurations of a base station and terminals according to Embodiment 2, which are identical to those of Embodiment 1, will be described with reference to  FIGS. 5 and 6 . 
     Base station  100  of Embodiment 2 reports to terminal  200  the information about the maximum transmission power for every CC. This maximum transmission power is controlled by base station  100  according to the allowable interference power set for neighboring cells. That is, as shown in  FIG. 9 , the maximum transmission power for an uplink component carrier of the macrocell that corresponds to the uplink component carrier of the femtocell with high interference robustness is set greater than the maximum transmission power for an uplink component carrier of the macrocell that corresponds to the uplink component carrier of that femtocell with low interference robustness, the femtocell neighboring to the macrocell. 
     Therefore, comparison between the maximum transmission powers for respective uplink component carriers that are reported from base station  100  enables terminal  200  to recognize the relative relationships about interference robustness among uplink component carriers in the femtocell. 
     Accordingly, in terminal  200  of Embodiment 2, control section  217  obtains the information about the maximum transmission power for every uplink component carrier that is contained in an downlink control channel. Here, the values of the maximum transmission powers for respective uplink component carriers indicate which one of uplink component carriers has lower or higher interference robustness than other uplink component carriers in a femtocell. Thus, control section  217  changes the combination rule of an uplink component carrier and constellation points (i.e. transmission rule of the response signal) based on the maximum transmission power for every uplink component carrier, the uplink component carrier and the constellation points being used to transmit a response signal as a feedback. 
     Specifically, as shown in  FIG. 10 , in the case where the maximum transmission power for CC# 0  is greater than that for CC# 1  (i.e. interference robustness is higher in uplink CC# 0  than in uplink CC# 1  in femtocell), control section  217  assigns the first constellation to uplink CC# 0  and the second constellation to uplink CC# 1 . 
     On the other hand, in the ease where the maximum transmission power for CC# 0  is smaller than that for CC# 1  (i.e. interference robustness is lower in uplink CC# 0  than in uplink CC# 1  in femtocell), control section  217  assigns the first constellation to uplink CC# 1  and the second constellation to uplink CC# 0 . Here, the first constellation and the second constellation are identical to those described in Embodiment 1. 
     [Embodiment 3] 
     Embodiment 3 employs the same configuration as Embodiment 1 in that a terminal changes the transmission rule of a response signal based on information reported from a base station. According to Embodiment 3, a base station changes the above combination rule based on information about power head room (PHR) for every uplink component carrier of a macrocell, and a terminal changes the transmission rule of a response signal based on the information about power head room (PHR) for every uplink component carrier of the macrocell, the information about power head room being known by the terminal. That is, according to Embodiment 3, the base station does not need to transmit information about the above combination rule unlike Embodiment 1, For this reason, it is not necessary to transmit additional signaling to the signaling required for the LTE system. 
     The basic configurations of a base station and terminals according to Embodiment 3, which are identical to those of Embodiment 1, will be described with reference to  FIGS. 5 and 6 . 
     Base station  200  of Embodiment 3 reports to terminal  100  the information about power head room (PHR) for every CC. This information about power head room indicates a value obtained by subtracting current transmission power from the maximum transmission power for every CC that has been reported from base station  100 , and is used to report the allowable amount of transmission power to be amplified. That is, as shown in  FIG. 11 , the power head room for an uplink component carrier of the macrocell that overlaps the uplink component carrier of the femtocell with high interference robustness, is set greater than the power head room for an uplink component carrier of the macrocell that overlaps the uplink component carrier of the femtocell with low interference robustness, the femtocell neighboring to the macrocell. 
     Specifically, as shown in  FIG. 12 , in the case where the power head room for CC# 0  is greater than that for CC# 1  (i.e. interference robustness is higher in uplink CC# 0  than in uplink CC# 1  in a femtocell), response scheme control section  101  in base station  100  assigns the first constellation to uplink CC# 0  and the second constellation to uplink CC# 1 . 
     On the other hand, as shown in  FIG. 12 , in the ease where the power head room for CC# 0  is smaller than that for CC# 1  (i.e. interference robustness is lower in uplink CC# 0  than in uplink CC# 1 . in a femtocell), response scheme control section  101  assigns the first constellation to uplink CC# 1  and assigns the second constellation to uplink CC# 0 . Here, the first constellation and the second constellation are identical to those described in Embodiment 1. 
     Terminal  200  of Embodiment 3 may change the transmission rule of a response signal based on the information about the maximum transmission power for every uplink component carrier of a macrocell as with Embodiment 2, and may change the transmission rule of a response signal based on the information about the power head room (PHR) for every CC that is to be transmitted to base station  100  as shown in  FIG. 12 . 
     [Embodiment 4] 
     According to Embodiment 4, in the transmission rule of a response signal that is used by a terminal to transmit a response signal, the probability that constellations corresponding to respective uplink component carriers are used is equalized. By this means, it is possible to prevent a problem from arising in the case where the probability that a constellation corresponding to certain uplink component carrier is used extremely increases so that the uplink component carrier corresponds to an uplink component carrier of a femtocell with low interference robustness. 
       FIG. 13  is a block diagram showing a configuration of base station  300  according to Embodiment  4 . With reference to  FIG. 13 , base station  300  includes control information determination sections  301  and  302  and response scheme control section  303 . 
     Control information determination sections  301  and  302  determine control information to be transmit in a downlink control channel. The control information includes information related to the error rate of a downlink data channel for every downlink component carrier that is transmitted from base station  300 . The information related to the error rate of a downlink data channel for every downlink component carrier of base station  300  includes the error rate of a downlink data channel for every downlink component carrier of base station  300 , the number of retransmissions of downlink data that is transmitted using a plurality of downlink component carriers of base station  300 , and the modulation scheme for a downlink control channel for every downlink component carrier of base station  300 . 
     Control information determination sections  301  and  302  output the determined control information to encoding sections  102 - 1  and  102 - 2  and response scheme control section  303 . 
     Control information determination sections  301  and  302  determine control information for CC# 0  and CC# 1 , respectively. 
     Response scheme control section  303  determines a rule for combination of an uplink component carrier and constellation points that are to be used by terminal  400  (described later) to transmit a response signal as a feedback to downlink data that has been transmitted from base station  300  using a plurality of downlink component carriers. Response scheme control section  303  determines the above combination rule based on the control information received from control information determination sections  301  and  302 . 
       FIG. 14  is a block diagram showing a configuration of terminal  400  according to Embodiment 4. With reference to  FIG. 14 , terminal  400  includes control section  401 . 
     Control section  401  controls transmission of a response signal based on the information related to the error rate of a downlink data channel for every downlink component carrier that is transmitted from base station  300 . That is, control section  401  changes the combination rule of an uplink component carrier and constellation points that are used to transmit a response signal as a feedback based on the information related to the error rate of a downlink data channel for every downlink component carrier that is transmitted from base station  300 . 
     According to the above changed combination rule, with reference to  FIG. 15 , in the case where downlink data is transmitted using two downlink component carriers of the macrocell, and among two uplink component carriers of the macrocell, the error rate for the first component carrier (CC# 0 ) of the macrocell is greater than that for the second component carrier (CC# 1 ) of the macrocell, the number of candidate constellation points contained in the first constellation associated with the first component carrier of the macrocell is greater than that contained in the second constellation associated with the second component carrier of the macrocell. 
     Furthermore, the pattern about successful reception indicating that downlink packets of all component carriers have been successfully received is associated with any one of the constellation points contained in the first constellation. Here, the following assumption is made: the pattern about successful reception indicating that downlink data transmitted using the first component carrier has been successfully received and downlink data transmitted using the second component carrier has not been received is associated with any one of the constellation points contained in a constellation corresponding to the first component carrier, and on the other hand, the pattern about successful reception indicating that downlink data transmitted using the first component carrier has not been received and downlink data transmitted using the second component carrier has been successfully received is associated with any one of the constellation points contained in a constellation corresponding to the second component carrier. 
     As described above, the information related to the error rate of a downlink data channel for every downlink component carrier that is transmitted from base station  300  includes the error rate of a downlink data channel for every downlink component carrier of base station  300 , the number of retransmissions of downlink data transmitted using a plurality of downlink component carriers of base station  300 , and the modulation scheme for a downlink control channel for every downlink component carrier of base station  300 . 
     By using the information about these items, terminal  400  can recognize which one of downlink component carriers has a smaller or greater error rate than other downlink component carriers. 
     &lt;Number of Retransmissions&gt; 
     When HARQ is employed, generally, the modulation scheme and the coding rate of a data channel at the initial transmission is set in such a way that the packet error rate is approximately 0.1 (ACK=0.9, NACK=0.1). The packet error rate of the data channel at the retransmission improves compared to the previous transmission because the reception quality improves as packets are retransmitted. Hence, it is possible to judge which one of downlink component carriers has a smaller or greater error rate than other downlink component carriers based on the number of retransmissions for every downlink component carrier. 
     Hence, as shown in, for example,  FIG. 16 , control section  401  associates a constellation having a greater number of constellation points with CC# 0  than the constellation to be associated with CC# 1  in the case where the number of retransmissions using CC# 0  is smaller than that using CC# 1 . On the other hand, control section  401  associates a constellation having a greater number of constellation points with CC# 1  than the constellation to be associated with CC# 0  in the case where the number of retransmissions using CC# 0  is greater than that using CC# 1 . 
     In the above explanation, which one of downlink component carriers has a smaller or greater error rate than other downlink component carriers is determined based on the number of retransmissions for every downlink component carrier, but may be determined based on the total number of transmissions (the initial transmission+the number of retransmissions). 
     Furthermore, in the system in which data is retransmitted once at the greatest, it is possible to determine whether or not the transmission is the initial transmission or retransmission. 
     &lt;MCS Level&gt; 
     In the downlink control channel, control information for demodulating the downlink data channel is reported. In the case where the interference power among cells is large, the MCS for the control channel is set to the modulation scheme (or the coding rate) with a low transmission rate. in the case where the interference power among cells is large, the packet error rate tends to increase. Hence, it is possible to judge which one of downlink component carriers has a smaller or greater error rate than other downlink component carriers based on the modulation and coding scheme (MCS) level of control channel for every downlink component carrier (i.e. the modulation scheme and the encoding scheme indicated by downlink control channel for every downlink component carrier). 
     For this reason, as shown in, for example,  FIG. 17 , control section  401  associates a constellation having a greater number of constellation points with CC# 0  than the constellation to be associated with CC# 1 . in the case where the .MCS level for CC# 0  is lower than the MCS level for CC# 1 . On the other hand, control section  401  associates a constellation having a greater number of constellation points with CC# 1  than the constellation to be associated with CC# 0  in the case where the MCS level for CC# 0  is greater than the MCS level for CC# 1 . 
     In the LTE specification, information about the number of domains in which control channels are multiplexed (i.e. the number of CCEs) is used for information about the MCS level for a control channel (see  FIG. 18 ). This indicates that data is transmitted using the modulation scheme or the coding rate having a lower MCS level as the number of CCEs is greater. 
     [Embodiment 5] 
     According to Embodiment 5, in the transmission rule of a response signal that is used by a terminal to transmit a response signal, constellation A and constellation B containing a smaller number of constellation points than constellation A are provided. In addition, the error rate for the component carrier associated with constellation B is smaller than that for the component carrier associated with constellation A. Furthermore, the total of the probabilities that constellation points contained in constellation B are used is greater than the total of the probabilities that constellation points contained in constellation A are used. The basic configurations of a base station and terminals according to Embodiment 5 are identical to those of Embodiment 4. 
     In terminal  400  of Embodiment 5, control section  401  controls transmission of a response signal based on information related to the error rate of a downlink data channel for every downlink component carrier that is transmitted from base station  300 . That is, control section  401  changes the combination rule of an uplink component carrier and constellation points based on the information related to the error rate of a downlink data channel for every downlink component carrier that is transmitted from base station  300 , the uplink component carrier and the constellation points being used to transmit a response signal as a feedback. 
     According to the above changed combination rule, as shown in  FIG. 19 , the number of candidate constellation points contained in the first constellation associated with the first component carrier of the macrocell is greater than that contained in the second constellation associated with the second component carrier of the macrocell in the case where downlink data is transmitted using two downlink component carriers of the macrocell, and among two downlink component carriers of the macrocell, the error rate for the first component carrier (CM) of the macrocell is greater than that for the second component carrier (CC# 1 ) of the macrocell. 
     Furthermore, a constellation point contained in the second constellation is associated with a pattern about successful reception in order of the pattern that is more highly likely to occur. That is, with reference to  FIG. 19 , the patterns about successful reception are highly likely to occur in which downlink data is successfully received using CC# 0  and CC# 1  and in which downlink data is not received using CC# 0  but downlink data is successfully received using CC# 1 . For this reason, these two patterns are associated with any one of the constellation points of the second constellation (constellation of BPSK in  FIG. 19 ). With reference to  FIG. 19 , for easier calculation, the probability of failure in receiving a PDCCH (i.e. DTX) is 0. Accordingly, assignment of the pattern about successful reception that is highly likely to occur to the constellation of BPSK increases the probability of BPSK being used. By so doing, the amount of interference to neighboring cells can be reduced. 
     [Other Embodiments] 
     (1) According to Embodiment 1, base station  100  reports to terminal  200  information about the combination pattern of a CC and a constellation, but the present invention is not limited to this configuration. Identification information of CC that is used for the pattern about successful reception in which downlink data is successfully received using all CCs may be reported. This is because only the pattern about successful reception in which downlink data is successfully received using all CCs can be transmitted using any CC. 
     Alternatively, the number of constellation points for each of the constellations may be reported. 
     Furthermore, base station  100  may report these pieces of information to every terminal  200  or terminals  200  in the entire cell. 
     Furthermore, only terminals  200  located near a femtocell may follow the order of the above reporting, while terminals  200  other than such terminals  200  may transmit a response signal in a predetermined random CC using the constellation corresponding to QPSK. This prevents all terminals  200  subject to channel selection from transmitting a response signal in the same CC using the constellation corresponding to QPSK, thus preventing congestion of the PUCCH of that component carrier in the case where a base station reports information to terminals  200  in the entire cell. 
     Furthermore, using bundling (logical product of ACK/NACK) instead of channel selection also reduces interference to the femtocell by reporting a CC for transmitting a PUCCH for bundling. 
     (2) According to Embodiment 1, two component carriers are used, but the present invention is not limited to this configuration. Even when three or more component carriers are used, the same embodiment as Embodiment 1 can be performed by reporting the number of constellation points for each of the constellations. 
     (3) According to Embodiment 4, both base station  300  and terminal  400  independently judge which component carrier has a smaller or greater error rate of a downlink data channel than other component carriers. The present invention, however, is not limited to this configuration. Base station  300  may make this judgment and report the result of the judgment to terminal  400  by including the result in control information. 
     (4) According to Embodiment 4, for example, even in the case where an ACK and a NACK occur at the same probability, the combination of constellation points of ACK/ACK in the constellation similar to QPSK is highly likely to occur. For this reason, to assign each terminal  400  a different CC to be used for transmission using the constellation corresponding to the constellation similar to QPSK, it is effective to make base station  300  report to terminal  400  the CC to be used for transmission using the constellation corresponding to the constellation similar to QPSK. 
     (5) The maximum transmission power (Pmax) in Embodiment 2 is a maximum value at which terminal  200  can perform transmission. The present invention, however, is not limited to this configuration. Instead, base station  100  and terminal  200  share information on transmission power for every downlink CC of base station  100 , and then, terminal  200  may transmit a response signal using the constellation corresponding to the constellation similar to QPSK in an uplink CC corresponding to a downlink CC of large transmission power. 
     (6) According to Embodiments 1 to 3, the constellation used for channel selection of a PUCCH for a terminal communicating with a macrocell base station neighboring a femtocell has been described. The present invention, however, is not limited to this configuration. Specifically, every base station may compare the relative cell sizes of neighboring cells and may take into consideration the relative relationship about the significance of interference of the neighboring cells to judge the constellation for channel selection of a PUCCH. That is, this configuration makes it possible to select the optimum constellation by taking into consideration interference robustness against interference power among cells even in the ease where a femtocell and a macrocell are not clearly defined. Furthermore, regarding a terminal communicating with a femtocell base station neighboring a macrocell, the reception characteristics for channel selection can be improved by making the femtocell base station report to such a terminal the constellation for channel selection of a PUCCH. The number of candidate constellation points contained in the constellation in this case is greater for the constellation associated with a component. carrier having high interference robustness in the femtocell than for the second constellation associated with the second component carrier having low interference robustness in the femtocell. 
     (7) Also, although cases have been described with the above embodiment as examples where the present invention is configured by hardware, the present invention can also be realized by software. 
     Each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
     Furthermore, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible. After LSI manufacture, utilization of a programmable FPGA (Field Programmable Gate Array) or a reconfigurable processor where connections and settings of circuit cells within an LSI can be reconfigured is also possible. 
     Furthermore, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
     The disclosure of Japanese Patent Application No. 2010-018715, filed on Jan. 29, 2010, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The base station, terminal, retransmission control method, and response method according to the present invention are effective for preventing deterioration of reception characteristics of neighboring cells in the case where carrier aggregation and channel selection are employed. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  300  base station 
           101 ,  303  response scheme control section 
           102 ,  103 ,  104  encoding section 
           105 ,  106  data transmission control section 
           107 ,  108 ,  109 ,  219 ,  220  modulation section 
           110 ,  111 ,  112  serial-to-parallel conversion section 
           113 ,  223  multiplexing section 
           114  IFFT section 
           115 ,  225  CP addition section 
           116 ,  226  radio transmission section 
           117 ,  201  radio reception section 
           118 ,  202  CP removal section 
           119 ,  221 ,  222  DFT section 
           120 ,  205  demultiplexing section 
           121 ,  122 ,  204  channel compensation section 
           125 ,  126 ,  209 ,  210 ,  211  demodulation section 
           127  channel selection determination section 
           200 ,  400  terminal 
           203  FFT section 
           206 ,  207 ,  208  parallel-to-serial conversion section 
           212 ,  213 ,  214  decoding section 
           215  error determination section 
           216  determination section 
           217 ,  401  control section 
           218  channel selection section 
           224  IDFT section 
           301 ,  302  control information determination section