Abstract:
A relay station and a wireless communication system wherein novel retransmission control is achieved in cases when a TTI-bundling technique and a relay technique are used in communication between a terminal and a base station. A relay station ( 300 ) relays wireless communication between a terminal that transmits a wireless signal in which code words obtained by encoding a single set of transmission data have been mapped to a TTI bundle consisting of a plurality of TTIs, and a base station that receives the wireless signal and transmits error detection information related to the code word transmitted in the tail TTI of the TTI bundle. At the relay station ( 300 ), a control information generating unit ( 309 ) transmits error detection information related to the code word transmitted in the front TTI of the TTI bundle.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a relay apparatus and a radio communication system. 
       BACKGROUND ART  
       [0002]    Third-generation mobile communication services have been launched, and multimedia communication such as data communication or video communication is increasingly popular. It is expected that demand for communication in all circumstances increases, and therefore coverage area is expanded in future. 
         [0003]    Therefore, with 3GPP-LTE (Long Term Evolution), introduction of a technique referred to as “TTI-bundling” has been agreed in order to expand coverage for uplink transmission from terminals (UEs: User Equipments) to a base station (eNB: enhanced Node B). With TTI-bundling, a terminal residing near a cell edge bundles a plurality of TTIs in uplink transmission, and this is regarded as one HARQ process. Then, small data such as VoIP data is encoded with a low coding rate, a resultant codeword is mapped to a plurality of TTIs and transmitted to improve the uplink reception quality in a base station (see Non-Patent Literature 1). Hereinafter, a plurality of bundled TTIs may be referred to as “TTI bundle.” 
         [0004]      FIG. 1  explains a retransmission process in a communication system adopting the TTI-bundling technique.  FIG. 1  shows a case in which three TTIs are bundled. 
         [0005]    In  FIG. 1 , a terminal bundles TTIs  0  to  2  to transmit data to a base station. Here, the terminal transmits a codeword mapped to at least TTI  0 , adding CRC to the codeword. The base station receives and decodes this data. The base station performs error detection on only data transmitted in the first TTI using CRC check. Then, upon detecting an error, the base station transmits NACK to the terminal. Upon receiving the NACK, the terminal retransmits data in a retransmission-scheduled period. 
         [0006]    Here, when TTIs are assigned to uplink data from the terminal at the first transmission, retransmission-scheduled periods are determined at this time. In  FIG. 1 , the retransmission-scheduled periods corresponding to TTIs  0  to  2  are TTIs  8  to  10 . Therefore, the terminal performs retransmission in TTIs  8  to  10 . Here, an interval (here, eight TTIs) between transmission-scheduled periods (including the first transmission period and a retransmission-scheduled period) is determined based on the round trip time of a HARQ process (HARQ-RTT) between a terminal residing near a cell edge and a base station. A HARQ-RTT is determined based on the time to propagate transmission signals (the first transmission signal and NACK) between a terminal and a base station, and the time to perform processing, including transmission signal generation processing, in the terminal and the base station. 
         [0007]    However, the above-described conventional communication system has a problem that unnecessary retransmission is performed. That is, as shown in  FIG. 2 , a base station performs error detection on only data transmitted in the first in a group of bundled TTIs, and transmits ACK/HACK to a terminal, based on this result of the detection. Therefore, even if an error is corrected in process of decoding subsequent TTIs in a group of bundled TTIs (that is, in a state in which ACK should be transmitted), when a base station has already transmitted NACK to a terminal, the terminal will perform retransmission processing. This causes a problem that the system throughput decreases. 
         [0008]    By the way, standardization of 3GPP LTE-Advanced that realizes faster speed of communication than by 3GPP LTE, has been launched (see Non-Patent Literature 2). With this 3GPP LTE-Advanced, studies are underway to place a relay station (RN: relay node) between each terminal and a base station in order to expand uplink transmission coverage. 
       CITATION LIST  
     Non-Patent Literature 
       [0000]    
       
         [NPL 1] R1-081103, RAN1, “Reply LS on Uplink Coverage for LTE,” 3GPP TSG RAN WG1 #52, Sorrento, Feb. 11-15, 2008 
         [NPL 2] R1-081722, Samsung, “Future 3GPP Radio Technologies for LTE-Advanced,” 3GPP TSG RAN WG1 #53, Kansas City, May. 5-9, 2008 
       
     
       SUMMARY OF INVENTION  
     Technical Problem 
       [0011]    However, with 3GPP LTE-Advanced, automatic retransmission control has not been studied yet. As described above, with 3GPP LTE, effective retransmission control cannot be performed in communication between terminals and a base station in a radio communication system so far, and if relay stations are added to this, innovation is required to efficiently perform automatic retransmission control. 
         [0012]    It is therefore an object of the present invention to provide a relay apparatus and a radio communication system to realize new retransmission control when a TTI-bundling technique and a relay technique are adopted for communication between terminals and a base station. 
       Solution to Problem 
       [0013]    The relay apparatus according to the present invention adopts a configuration to include: 
         [0014]    The radio communication system according to the present invention adopts a configuration to include: a relay apparatus that relays radio communication between a terminal and a base station, the terminal transmitting a radio signal in which a codeword obtained by encoding one transmission data is mapped to a transmission time interval (TTI) bundle composed of a plurality of transmission time intervals, and the base station receiving the radio signal and transmitting error detection information about the codeword transmitted in a first transmission time interval in the transmission time interval bundle. The relay apparatus includes a decoding section that decodes, per transmission time interval, the codeword mapped to the transmission time interval bundle contained in a received radio signal; an error detecting section that performs error detection on each decoding result; and a transmission section that transmits error detection result information about the codeword transmitted in a second transmission time interval before at least the first transmission time interval in the transmission time interval bundle. 
       Advantageous Effects of Invention 
       [0015]    According to the present invention, it is possible to provide a relay apparatus and a radio communication system to realize new retransmission control when a TTI-bundling technique and a relay technique are adopted for communication between terminals and a base station. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  explains a retransmission process in a communication system adopting the TTI-bundling technique; 
           [0017]      FIG. 2  explains a retransmission process in a communication system adopting the TTI-bundling technique; 
           [0018]      FIG. 3  is a block diagram showing a configuration of a radio communication system according to Embodiment 1 of the present invention; 
           [0019]      FIG. 4  is a block diagram showing a configuration of a terminal according to Embodiment 1 of the present invention; 
           [0020]      FIG. 5  is a block diagram showing a configuration of a base station according to Embodiment 1 of the present invention; 
           [0021]      FIG. 6  is a block diagram showing a configuration of a relay station according to Embodiment 1 of the present invention; 
           [0022]      FIG. 7  explains operations of a terminal, a base station and a relay station according to Embodiment 1 of the present invention; 
           [0023]      FIG. 8  explains a state in which a codeword is stored in a circular buffer, and a method of reading the codeword from the circular buffer (at the time of the first transmission); 
           [0024]      FIG. 9  explains a state in which a codeword is stored in a circular buffer, and a method of reading the codeword from the circular buffer (at the time of retransmission); 
           [0025]      FIG. 10  explains a comparative technique; 
           [0026]      FIG. 11  explains operations of a terminal, a base station and a relay station according to Embodiment 2 of the present invention; 
           [0027]      FIG. 12  explains operations of a terminal, a base station and a relay station according to Embodiment 3 of the present invention; 
           [0028]      FIG. 13  explains operations of a terminal, a base station and a relay station according to Embodiment 4 of the present invention; and 
           [0029]      FIG. 14  explains operations of a terminal, a base station and a relay station according to Embodiment 4 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Here, in the embodiments, the same components are assigned the same reference numerals and overlapping descriptions will be omitted. 
       Embodiment 1  
     [The Configuration of a Radio Communication System] 
       [0031]      FIG. 3  explains the configuration of a radio communication system according to Embodiment 1 of the present invention. In  FIG. 3 , radio communication system  10  has terminal  100 , base station  200  and relay station  300 . Although  FIG. 3  only shows one terminal  100 , one base station  200  and one relay station  300  for ease of explanation, actually, a plurality of relay stations  300  are distributed and arranged in the cell of one base station  200 . Therefore, in radio communication system  10 , an environment is realized in which the separation distance between terminal  100  and relay station  300  and the separation distance between base station  200  and relay station  300  are likely to be shorter than the separation distance between terminal  100  and base station  200 . Therefore, in radio communication system  10 , it is possible to consider that the communication quality between terminal  100  and relay station  300  and the communication quality between base station  200  and relay station  300  are higher than the communication quality between terminal  100  and base station  200 , regardless of the position of terminal  100 . 
         [0032]    [The configuration of Terminal  100 ] 
         [0033]      FIG. 4  is a block diagram showing the configuration of terminal  100  according to Embodiment 1 of the present invention. In  FIG. 4 , terminal  100  has CRC section  101 , encoding section  102 , modulation section  103 , multiplexing section  104 , transmission RF section  105 , antenna  106 , reception RF section  107 , demodulation section  108 , decoding section  109  and control section  110 . 
         [0034]    CRC section  101  performs error detection (CRC: cyclic redundancy check) and encoding on an information bit stream, and outputs a resultant information bit stream to which CRC parity bits have been added, to encoding section  102 . 
         [0035]    Encoding section  102  has a circular buffer (not shown). Encoding section  102  performs turbo encoding on the information bit stream with CRC parity bits, with a mother coding rate and stores a resultant codeword in the circular buffer. Encoding section  102  extracts an output codeword matching control information received from control section  110 , from codewords stored in the circular buffer, and outputs it to modulation section  103 . The control information received from control section  110  includes transmission type information (including a coding rate) indicating transmission by TTI-bundling, a new transmission command, a retransmission preparation command, a retransmission execution command, M-ary modulation number information, or assigned frequency resource information. 
         [0036]    At the time of new (the first) transmission, encoding section  102  extracts an output codeword matching the coding rate contained in control information received from control section  110 , from codewords stored in the circular buffer, and outputs it to modulation section  103 . Encoding section  102  performs processing associated with preparation for retransmission, retransmission and transmission of new data (including processing to delete the codeword for the data transmitted last time from the circular buffer), based on control information received from control section  110 . Processing in encoding section  102  will be described in detail later. 
         [0037]    Modulation section  103  generates a data symbol by modulating the codeword received from encoding section  102  with the M-ary modulation number contained in the control signal received from control section  110 , and outputs the resultant data symbol to multiplexing section  104 . 
         [0038]    Multiplexing section  104  multiplexes the data symbol received from modulation section  103 , the control information received from control section  110  and a pilot signal, and forms a multiplexed signal, which is a baseband signal. At this time, the data symbol is placed in the assigned frequency indicated by assigned frequency resource information contained in the control information received from control section  110 . 
         [0039]    Transmission RF section  105  transforms the multiplexed signal to a frequency domain signal, and transmits a resultant RF signal via antenna  106 . 
         [0040]    Reception RF section  107  receives a control signal (including assignment information or an ACK/NACK signal) transmitted from after-mentioned base station  200 , and an ACK/NACK signal transmitted from after-mentioned relay station  300  via antenna  106 , and transforms a received signal to a frequency domain signal to obtain a baseband signal. This baseband signal is outputted to demodulation section  108 . 
         [0041]    Demodulation section  108  demodulates a control signal contained in the baseband signal received from reception RF section  107  and an ACK/NACK signal from relay station  300 , and outputs a demodulated control signal and a modulated ACK/NACK signal to decoding section  109 . 
         [0042]    Decoding section  109  decodes the demodulated control signal and ACK/NACK signal, and outputs resultant control information and ACK/NACK information to control section  110 . 
         [0043]    Control section  110  specifies a coding rate, an M-ary modulation number, assigned frequency resources and ACK/NACK information contained in the control information received from decoding section  109 . In addition, control section  110  determines whether or not to perform processing, including preparation for retransmission, decision to perform retransmission, retransmission and transmission of new data, based on the specified ACK/NACK information from base station  200  and the specified ACK/NACK information from relay station  300 , and outputs control information according to the result of the determination to encoding section  102 . In addition, among the specified control information, the coding rate is outputted to encoding section  102 , the M-ary modulation number is outputted to modulation section  103 , and the assigned frequency resources are outputted to multiplexing section  104 . 
         [0044]    [The Configuration of Base Station  200 ] 
         [0045]      FIG. 5  is a block diagram showing the configuration of base station  200  according to Embodiment 1 of the present invention. In  FIG. 5 , base station  200  has antenna  201 , reception RF section  202 , demultiplexing section  203 , demodulation section  204 , decoding section  205 , error detecting section  206 , channel quality estimating section  207 , scheduler  208 , control information generating section  209 , encoding section  210 , modulation section  211 , transmission RF section  212  and ACK/NACK processing section  213 . 
         [0046]    Reception RF section  202  receives a data signal transmitted from terminal  100  and a data signal transmitted from relay station  300  via antenna  201 , and transforms each of the relieved data signals to a frequency domain signal to obtain a baseband signal. This baseband signal is outputted to demultiplexing section  203 . 
         [0047]    Demultiplexing section  203  demultiplexes the baseband signal received from reception RF section  202  into a data symbol, a received pilot signal and an ACK/NACK signal transmitted from relay station  300 . Moreover, demultiplexing section  203  outputs a data symbol matching assigned frequency resource information contained in assignment information received from scheduler  208 , to demodulation section  204 , outputs the received pilot signal to channel quality estimating section  207  and outputs an ACK/NACK signal transmitted from relay station  300 , to ACK/NACK processing section  213 . 
         [0048]    Demodulation section  204  demodulates the data symbol received from demodulation section  203 , according to M-ary modulation number information contained in the assignment information received from scheduler  208 . 
         [0049]    Decoding section  205  performs error correction decoding on the result of the demodulation received from demodulation section  204 , based on coding rate information contained in the assignment information received from scheduler  208  to obtain a decoded bit stream. This obtained decoded bit stream (received data) is stored in a memory (not shown) provided in decoding section  205 , and is outputted to error detecting section  206 . The result of decoding of the TTI targeted for this decoding in a TTI bundle is used to decode the codeword transmitted in the next TTI. Therefore, in a TTI bundle, the error rate of a codeword transmitted in a later TTI is lower. In addition, only when receiving an ACK signal from error detecting section  206 , decoding section  205  discards received data having already been stored in a memory. 
         [0050]    Error detecting section  206  performs error detection (CRC) per TTI on the decoded bit stream received from decoding section  205 . 
         [0051]    When there is an error in the decoded bit stream as a result of error detection, error detecting section  206  generates a NACK signal as a response signal, and, on the other hand, when there is no error in the decoded bit stream, generates an ACK signal as a response signal. This generated ACK/NACK signal is outputted to decoding section  205 , scheduler  208  and control information generating section  209 . In addition, when there is no error in the decoded bit stream, error detecting section  206  outputs the decoded bit stream as a received bit stream. 
         [0052]    Channel quality estimating section  207  estimates channel quality (SINR: signal-to-interference and noise power ratio) from the received pilot signal. The SINR estimation value is outputted to scheduler  208 . 
         [0053]    ACK/NACK processing section  213  performs reception processing on an ACK/NACK signal transmitted from relay station  300 , and outputs an ACK/NACK signal after reception processing to scheduler  208 . 
         [0054]    Scheduler  208  generates assignment information, based on the SINR estimation value received from channel quality estimating section  207 , the ACK/NACK signal received from error detecting section  206  and the ACK/NACK signal from relay station  300 , This assignment information includes M-ary modulation number information, coding rate information and assigned resource information. This assignment information is outputted to control information generating section  209 , demultiplexing section  203 , demodulation section  204  and decoding section  205 . Scheduling of retransmission data in scheduler  208  will be described later. 
         [0055]    Control information generating section  209  receives an ACK/NACK signal from error detecting section  206 . Then, when data transmission is performed using the TTI-bundling technique, control information generating section  209  transmits an ACK/NACK signal for the codeword transmitted in the last TTI in a TTI bundle, according to the detecting timing. Control information generating section  209  generates a control signal frame by combining an ACK/NACK signal and assignment information received from scheduler  208 , and transmits this frame via encoding section  210 , modulation section  211  and transmission RF section  212 . 
         [0056]    The control signal frame generated in control information generating section  209  is encoded in encoding section  210 , modulated in modulation section  211 , transformed to a frequency domain signal in transmission RF section  212 , and then transmitted via antenna  201 . 
         [0057]    [The Configuration of Relay Station  300 ] 
         [0058]      FIG. 6  shows a block diagram showing the configuration of relay station  300  according to Embodiment 1 of the present invention. In  FIG. 6 , relay station  300  has antenna  301 , reception RF section  302 , demultiplexing section  303 , demodulation section  304 , decoding section  305 , error correction section  306 , control signal processing section  307 , ACK/NACK processing section  308 , control information generating section  309 , relay signal processing section  310 , CRC section  311 , coding section  312 , modulation section  313  and transmission RF section  314 . 
         [0059]    Reception RF section  302  receives a data signal transmitted from terminal  100  and a control signal (including assignment information and an ACK/NACK signal) transmitted from base station  200  via antenna  301 , and transforms each of the received signals to a frequency domain signal to obtain a baseband signal. This baseband signal is outputted to demultiplexing section  303 . 
         [0060]    Demultiplexing section  303  demultiplexes the base band signal received from reception RF section  302  into a data symbol transmitted from terminal  100  and a control signal transmitted from base station  200 . Moreover, demultiplexing section  303  outputs the data symbol to demodulation section  304 , and outputs the control signal transmitted from base station  200  to control signal processing section  307  and ACK/NACK processing section  308 . 
         [0061]    Demodulation section  304  demodulates the data symbol received from demultiplexing section  303 , according to M-ary modulation number information contained in assignment information received from control signal processing section  307 . 
         [0062]    Decoding section  305  obtains a decoded bit stream by performing error correction decoding on the result of the demodulation received from demodulation section  304 , based on coding rate information contained in the assignment information received from control signal processing section  307 . This obtained bit stream (received data) after decoding is stored in a memory (not shown) provided in decoding section  305 , and outputted to error detecting section  306 . The result of decoding of the TTI targeted for this decoding in a TTI bundle is used to decode the codeword transmitted in the next TTI. Therefore, in a TTI bundle, the error rate of a codeword transmitted in a later TTI is lower. In addition, only when receiving an ACK signal from error detecting section  306 , decoding section  305  discards received data having already been stored in a memory. 
         [0063]    Error detecting section  306  performs error detection (CRC) per TTI on the decoded bit stream received from decoding section  305 . 
         [0064]    When there is an error in the decoded bit stream as a result of error detection, error detecting section  306  generates a NACK signal as a response signal, and, on the other hand, when there is no error in the decoded bit stream, generates an ACK signal as a response signal. This generated ACK/NACK signal is outputted to decoding section  305 , control information generating section  309  and relay signal processing section  310 . In addition, when there is no error in the decoded bit stream, error detecting section  306  outputs the decoded bit stream as a received bit stream. 
         [0065]    Control signal processing section  307  demodulates and decodes a control signal received from demultiplexing section  303 , and specifies assignment information contained in the control signal. The assignment information contains a coding rate, an M-ary modulation number and assigned frequency resources. Then, the assignment information is outputted to demodulation section  304 , decoding section  305 , encoding section  312  and modulation section  313 . 
         [0066]    ACK/NACK processing section  308  performs reception processing on an ACK/NACK signal contained in the control signal received from demultiplexing section  303 , and outputs resultant ACK/NACK information to relay signal processing section  310 . 
         [0067]    Control information generating section  309  receives an 
         [0068]    ACK/NACK signal from error detecting section  306 . Then, when data transmission is performed using the TTI-bundling technique, control information generating section  309  transmits an ACK/NACK signal for the codeword transmitted in the first TTI in a TTI bundle, according to the detecting timing. 
         [0069]    Relay signal processing section  310  receives an ACK/NACK signal from error detecting section  306  and receives ACK/NACK information from ACK/NACK processing section  308 . Then, relay signal processing section  310  determines whether or not to perform relay processing, based on the ACK/NACK signal from error detecting section  306  and the ACK/NACK information from ACK/NACK processing section  308  (that is, ACK/NACK information from base station  200 ). When it is determined to perform relay processing, relay signal processing section  310  transmits relay information. This relay information is retransmission data to be transmitted by relay signal processing section  310 , in the retransmission-scheduled period corresponding to the last TTI in a TTI bundle, instead of terminal  100 . Relay processing section  310  will be described in detail later. 
         [0070]    CRC section  311  performs error detection coding on relay information, and outputs resultant relay data to which CRC parity hits have been added, to encoding section  312 . 
         [0071]    Encoding section  312  has a buffer (not shown). Encoding section  312  performs turbo coding on an information bit stream with CRC parity bits, with a mother coding rate, and stores a resultant codeword in the buffer. Encoding section  312  extracts an output codeword matching a relay signal coding rate contained in assignment information received from control signal processing section  307 , and outputs it to modulation section  313 . 
         [0072]    Modulation section  313  generates a data symbol by modulating the codeword received from encoding section  312  with the M-ary modulation number contained in assignment information received from control signal processing section  307 , and outputs the obtained data symbol to transmission RE section  314 . 
         [0073]    Transmission RF section  314  transforms an ACK/NACK signal received from control information generating section  309  to a frequency domain signal, transforms the data symbol received from modulation section  313  to a frequency domain data symbol, and transmits resultant RE signals via antenna  301 . 
         [0074]    [Descriptions of Operations of Terminal  100 , Base Station  200  and Relay Station  300 ] 
       (The First Transmission by Terminal  100 ) 
       [0075]    As shown in  FIG. 7 , terminal  100  bundles TTIs  0  to  2  to transmit data. That is, in terminal  100 , encoding section  102  extracts an output codeword matching the coding rate contained in control information received from control section  110 , from codewords stored in a circular buffer, and outputs it to modulation section  103 . 
         [0076]      FIG. 8  explains a case in which a codeword is stored in a circular buffer, and a method of reading the codeword from the circular buffer (at the time of the first transmission). 
         [0077]    As shown in  FIG. 8 , the circular buffer is composed of ninety-six columns and stores a codeword. S (composed of thirty-two columns) in the left part is formed with information bits to which CRC parity bits have been added (that is, systematic bits), and P 1  and P 2  (composed of sixty-four columns) in the right part is formed with parity bits generated by turbo coding. Here, the systematic bit side is defined as the front, and the parity bit side is defined as the back. 
         [0078]    Encoding section  102  reads the codeword of a predetermined length from a predetermined reading start position toward the back, as data  1  transmitted in TTI  0 , and outputs the codeword to modulation section  103 . Here, the predetermined reading start position (RV  0 ) is the third column from the left in the circular buffer ( FIG. 8 ). In addition, the predetermined length corresponds to sixty-four columns in the circular buffer. Therefore, data  1  is equivalent to part of the circular buffer from the third column to the sixty-sixth column. 
         [0079]    Next, encoding section  102  also reads the codeword of a predetermined length (equivalent to data  2  in  FIG. 8 ) from the column following the last column read in data  1 , as a reading start position, toward the back, and outputs it to demodulation section  103 . Here, when the last column is arrived at in the circular buffer before completion of reading of the codeword of a predetermined length, reading is continued from the first column in the circular buffer Therefore, data  2  is equivalent to the part from the sixty-seventh column to the ninety-sixth column and the part from the first column to the thirty-fourth column in the circular buffer. 
         [0080]    Next, encoding section  102  also reads the codeword of a predetermined length (equivalent to data  3  in  FIG. 8 ) from the column following the last column read in data  2 , as a reading start position, toward the back, and outputs it to demodulation section  103 . Data  3  is equivalent to the part from the thirty-fifth column to the ninety-sixth column and the part from the first column to the second column in the circular buffer. Here, RV (redundancy version) is command information to specify the position in the circular buffer from which a codeword is read. 3GPP LTE defines that RV  0  corresponds to the third column, RV  1  corresponds to the twenty-seventh column, RV  2  corresponds to the fifty-first column and RV  3  corresponds to seventy-fifth column. Then, RV  0  is used at the time of the first transmission. 
         [0081]    As described above, as shown in  FIG. 7 , a plurality of codewords read from a circular buffer are transmitted in a TTI bundle composed of TTIs  0  to  2 , and received in base station  200  and relay station  300 . 
         [0082]    (ACK/NACK Signal Transmission in Base Station  200 ) 
         [0083]    In base station  200 , error detecting section  206  performs error detection on received data per TTI. 
         [0084]    Then, control information generating section  209  transmits the result of the detection (i.e. an ACK/NACK signal) for the codeword transmitted in the last TTI in a TTI bundle, according to the detecting timing. Here, an ACK/NACK signal for the codeword transmitted in TTI  2 , which is the last TTI, is transmitted. 
         [0085]    (ACK/NACK signal transmission in relay station  300 ) In relay station  300 , error detecting section  306  performs error detection on received data per TTI. 
         [0086]    Then, control information generating section  309  transmits the result of the detection (i.e. an ACK/NACK signal) for the codeword transmitted in the first TTI in a TTI bundle, according to the detecting timing. Here, an ACK/NACK signal for the codeword transmitted in TTI  0 , which is the first TTI, is transmitted. 
         [0087]    (Scheduling of Retransmission Data from Terminal  100  and Relay Information from Relay Station  300  in Base Station  200 ) 
         [0088]    As described later, in terminal  100 , an ACK/NACK signal for the first TTI, which is transmitted from relay station  300 , is used as a trigger for preparation for retransmission, and an ACK/NACK signal for the last TTI, which is transmitted from base station  200 , is used as a criterion for decision to perform retransmission. That is, as described later, only when NACK signals are transmitted in both the first TTI and the last TTI, terminal  100  retransmits the entire TTI bundle. Therefore, when NACK signals are transmitted in both the first TTI and the last TTI, scheduler  208  in base station  200  secures resources for retransmission using a TTI bundle from terminal  100  (that is, for example, frequency resources for retransmission-scheduled periods TTI  8  to TTI  10 ). 
         [0089]    In addition, as described later, relay station  300  uses the result of the error detection about the first TTI and an ACK/NACK signal for the last TTI, which is transmitted from base station  200 , as criteria for decision to perform retransmission processing (relay processing). That is, as described later, when no error is detected in the first TTI and base station  200  transmits NACK, relay station  300  transmits relay information in the retransmission-scheduled period corresponding to the last TTI. Therefore, when relay station  300  transmits an ACK signal for the first TTI and base station  200  transmits a NACK signal for the last TTI, scheduler  208  in base station  200  secures resources for retransmission (relay) using one TTI from relay station  300  (that is, for example, frequency resources for retransmission-scheduled period TTI  10 ). 
         [0090]    (Scheduling of Retransmission Data from Terminal  100  in Relay Station  300 ) 
         [0091]    When NACK signals are transmitted in both the first TTI and the last TTI, terminal  100  retransmits the entire TTI bundle. Therefore, when an error is detected in the first TTI and base station  200  transmits NACK, relay station  300  secures resources for retransmission using a TTI bundle from terminal  100  (that is, for example, frequency resources in retransmission-scheduled periods TTI  8  to TTI  10 ). 
         [0092]    (Processing Associated with Preparation for Retransmission and Decision to Perform Retransmission in Terminal  100 ) 
         [0093]    Terminal  100  determines whether or not to start preparation for retransmission of the entire TTI bundle, based on an ACK/NACK signal for TTI  0 , which is transmitted from relay station  300 , and determines whether or not to perform retransmission of the prepared codeword for the entire TTI bundle, based on an ACK/NACK signal for TTI  2  transmitted from base station  200 . 
         [0094]    To be more specific, in terminal  100 , control section  110  determines whether or not to command encoding section  102  to start preparation for retransmission of the entire TTI bundle, based on an ACK/NACK signal for TTI  0 . Then, when relay station  300  transmits a NACK signal for TTI  0 , control section  110  commands encoding section  102  to start preparation for retransmission of the entire TTI bundle. On the other hand, when relay station  300  transmits an ACK signal for TTI  0 , terminal  100  does not perform retransmission, so that control section  110  commands encoding section  102  to prepare for transmission of new data. 
         [0095]    In addition, in terminal  100 , control section  110  determines whether or not to command to encoding section  102  to retransmit the prepared codeword for the entire TTI bundle, based on an ACK/NACK signal for TTI  2 . Then, when base station  200  transmits an ACK signal for TTI  2 , control section  110  does not command encoding section  102  to retransmit the prepared codeword for the entire TTI bundle. On the other hand, when base station  200  transmits a NACK signal for TTI  2 , control section  110  commands encoding section  102  to retransmit the prepared codeword for the entire TTI bundle (see  FIG. 7 ). 
         [0096]    (Retransmission from Terminal  100 ) 
         [0097]      FIG. 9  explains a state in which a codeword is stored in a circular buffer, and a method of reading the codeword from a circular buffer (at the time of retransmission). 
         [0098]    At the time of retransmission, encoding section  102  reads a codeword from a different position from the position at the time of last transmission, as a start position, extracts the codeword, and outputs it to modulation section  103 . In  FIG. 9 , RV  2  is the reading start position at the time of the first retransmission. 
         [0099]    (Decision to Perform Relay, and Relay Processing in Relay Station  300 ) 
         [0100]    Relay station  300  determines whether or not to perform relay processing, based on the result of the error detection in TTI  0  and an ACK/NACK signal for TTI  2  transmitted from base station  200 . 
         [0101]    To be more specific, in relay station  300 , relay signal processing section  310  determines whether or not to perform relay processing, based on the result of the error detection in TTI  0  and an ACK/NACK signal for TTI  2  transmitted from base station  200 . When no error is detected in TTI  0 , and base station  200  transmits a NACK signal for TTI  2 , relay signal processing section  310  performs relay processing. At this time, relay signal processing section  310  transmits the result of the decoding in TTI  0  in the retransmission-scheduled period corresponding to TTI  2 . In this way, relay station  300  performs retransmission instead of terminal  100 . 
         [0102]    As described above, according to the present embodiment, relay station  300  relays radio communication between a terminal that transmits a radio signal in which a codeword obtained by coding one transmission data is mapped to a TTI bundle composed of a plurality of TTIs, and a base station that receives the radio signal and transmits error detection information about the codeword transmitted in the last TTI in the TTI bundle. Then, in relay station  300 , control information generating section  309  transmits error detection information about the codeword transmitted in the first TTI in the TTI bundle. 
         [0103]    [Comparative Technique] 
         [0104]    Here, an aspect is possible where relay station  300  transmits the result of the error detection about the codeword transmitted in the last TTI in the TTI bundle, to terminal  100  (see  FIG. 10 ). However, with this aspect, terminal  100  can perform retransmission only in the retransmission-scheduled period corresponding to the last TTI at the time of retransmission. That is, terminal  100  cannot perform retransmission using a TTI bundle. Because, even if preparation for retransmission in the retransmission-scheduled period corresponding to TTIs other than the last TTI starts after receiving a NACK signal in the last TTI, is too late for retransmission in this retransmission-scheduled period. Therefore, it is not possible to perform retransmission using the TTI-bundling technique, so that error characteristics deteriorate in the data receiving side. 
         [0105]    By contrast with this, with the present embodiment, relay station  300  transmits error detection information about the codeword transmitted in the first TTI in a TTI bundle, so that terminal  100  can use this error detection information from relay station  300  as a trigger for starting preparing for retransmission. Therefore, terminal  100  can perform retransmission using a TTI bundle. 
         [0106]    In addition, base station  200  transmits error detection information about the codeword transmitted in the last TTI in a TTI bundle, so that terminal  100  can use this error detection information from base station  200 , as a criterion for decision to perform retransmission. 
         [0107]    Moreover, when error detection result information transmitted from base station  200  is NACK, and error detection result information about the first TTI transmitted from relay station  300  is ACK, relay signal processing section  310  transmits relay information in the retransmission-scheduled period corresponding to the last TTI in relay station  300 . 
         [0108]    By this means, retransmission load is removed from terminal  100 . Moreover, communication is performed between relay station  300  and base station  200  with higher quality than between terminal  100  and base station  200 , so that it is possible to increase the possibility of successful retransmission. 
         [0109]    Here, a case has been explained where relay station  300  transmits error detection information about the codeword transmitted in the first TTI in a TTI bundle, and base station  200  transmits error detection information about the codeword transmitted in the last TTI. However, the present invention is not limited to this. The important thing is that, when base station  200  transmits the result of the error detection about the codeword transmitted in the first TTI, base station  300  transmits the result of the error detection about the codeword transmitted in the second TTI before the first TTI. Here, the second TTI is not the first TTI, terminal  100  does not perform retransmission for the entire TTI bundle, but performs retransmission in the retransmission-scheduled periods corresponding to TTIs from the second TTI to the last TTI. Here, a configuration is adopted in Embodiment 1 where relay station  300  transmits error detection information about the codeword transmitted in the first TTI, because the channel between terminal  100  and relay station  300  exhibits high quality in their communication environment, and therefore an error is not likely to be detected in the first TTI. In addition, in order to employ the principle that the error rate of the code word transmitted in a later TTI in a TTI bundle is lower, a configuration is adopted where base station  200  transmits error detection information in the last TTI. 
       Embodiment 2 
       [0110]    Although, with Embodiment 1, a configuration has been explained where the result of the error detection about the first TTI obtained in relay station  300  is used as a trigger for starting preparing for retransmission in terminal  100 , and criteria to determine which of terminal  100  and relay station  300  performs retransmission, terminal  100  may perform unnecessary transmission, with this configuration. That is, when relay station  300  detects an error in the first TTI, even if the error is, corrected in process of decoding subsequent TTIs (that is, in a state in which ACK should be transmitted in subsequent TTIs), relay station  300  does not perform relay processing. However, if relay station  300  performs error correction on a codeword, it is advantageous to perform retransmission by relay station  300  placed in a good environment for communication with terminal  200 . 
         [0111]    Therefore, with Embodiment 2, a relay station sequentially transmits not only error detection result information about the codeword transmitted in the first TTI in a TTI bundle, but also error detection result information about the codeword transmitted in the last TTI, according to the detecting timing. Then, the result of the error detection about the last TTI in a relay station is used as criteria to determine which of a terminal and the relay station performs retransmission. By this means, it is possible to perform retransmission in a more advantageous environment. 
         [0112]    Here, respective basic configurations of a terminal, a base station and a relay station according to the present embodiment are the same as those of the terminal, the base station and the relay station described in Embodiment 1. Therefore, the terminal, the base station and the relay station according to the present embodiment will be explained with reference to  FIG. 4  to  FIG. 6 . 
         [0113]    In terminal  100  according to Embodiment 2, like in Embodiment 1, control section  110  determines whether or not to perform processing, including preparation for retransmission, decision to perform retransmission, retransmission and transmission of new data, based on ACK/NACK information from base station  200  and ACK/NACK information from relay station  300 , and outputs control information according to the result of the determination to encoding section  102 . 
         [0114]    Here, with Embodiment 2, relay station  300  transmits the result of the error detection about the codeword mapped to the last TTI. With Embodiment 2, this result of error detection about the codeword mapped to the last TTI is used as an criterion to determine which of a terminal and a relay station performs retransmission. 
         [0115]    That is, when relay station  300  transmits a NACK signal for the first TTI, control section  110  commands encoding section  102  to start preparing for retransmission of the entire TTI bundle. 
         [0116]    When at least one of relay station  300  and base station  200  transmits an ACK signal for the last TTI, control section  110  commands encoding section  102  not to perform retransmission. Only when relay station  300  and base station  200  transmit NACK signals for the last TTI, control section  110  commands to perform retransmission. 
         [0117]    In addition, when no error is detected in the last TTI and base station  200  transmits a NACK signal for the last TTI, relay signal processing section  310  in relay station  300  according to Embodiment 2 performs relay processing. 
         [0118]      FIG. 11  explains operations of terminal  100 , base station  200  and relay station  300  according to Embodiment 2. 
         [0119]    As shown in  FIG. 11 , terminal  100  bundles TTIs  0  to  2  to transmit data. 
         [0120]    In relay station  300 , error detecting section  306  performs error detection on received data per TTI. Then, control information generating section  309  sequentially transmits ACK/NACK signals for the codeword transmitted in the first TTI and the last TTI in a TTI bundle, according to respective detecting timings. In  FIG. 11 , a NACK signal is transmitted in TTI  0 , and an ACK signal is transmitted in TTI  2 . 
         [0121]    In addition, in base station  200 , error detecting section  206  performs error detection on received data per TTI. Then, control information generating section  209  transmits the result of the error detection in the last TTI, according to the detecting timing. In  FIG. 11 , a NACK signal is transmitted in TTI  2 . 
         [0122]    In terminal  100 , control section  110  commands to encoding section  102  to start preparing for retransmission of the entire TTI bundle because relay station  300  has transmitted a NACK signal for TTI  0 . 
         [0123]    Then, control section  110  commands encoding section  102  to stop preparing for retransmission having already been started because relay station  300  has transmitted an ACK signal for TTI  2 . 
         [0124]    In relay station  300 , relay signal processing section  310  transmits retransmission data reencoded using the result of the decoding without an error in the retransmission-scheduled period corresponding to TTI  2 , because no error is detected in TTI  2  and base station  200  has transmitted a NACK signal for TTI  2 . 
         [0125]    As described above, according to the present embodiment, control information generating section  309  in relay station  300  transmits not only an ACK/NACK signal for the codeword transmitted in the first TTI in a TTI bundle, but also an ACK/NACK signal for the codeword transmitted in the last TTI. 
         [0126]    By using this ACK/NACK signal for the codeword transmitted in the last TTI, as an criterion to determine which of a terminal and a relay station performs retransmission, retransmission is performed in a more advantageous environment as described above. 
       Embodiment 3 
       [0127]    With Embodiment 3, a relay station transmits an ACK/NACK signal for only the last TTI in a TTI bundle. Then, after transmitting data using a TTI bundle, a terminal automatically starts preparing for retransmission and determines whether or not to perform retransmission, based on ACK/NACK signals for the last TTI transmitted from a base station and a relay station. Here, the basic configurations of a terminal, a base station and a relay station according to the present embodiment are the same as those of the terminal, the base station and the relay station described in Embodiment 1. Therefore, the terminal, the base station and the relay station according to the present embodiment will be explained, with reference to  FIG. 4  to  FIG. 6 . 
         [0128]    In terminal  100  according to Embodiment 3, like in Embodiment 1, control section  110  determines whether or not to perform processing, including decision to perform retransmission, retransmission and transmission of new data, based on ACK/NACK information from base station  200  and ACK/NACK information from relay station  300 , and outputs control information according to the result of the determination, to encoding section  102 . Here, regarding preparation for retransmission, control section  110  transmits data using a TTI bundle and commands encoding section  102  to start preparing for retransmission. Then, control section  110  determines whether or not to perform retransmission, based on the result of the error detection in the last TTI transmitted from each of base station  200  and relay station  300 . 
         [0129]    That is, when NACK signals are transmitted from base station  200  and relay station  300 , control section  110  commands encoding section  102  to retransmit the entire TTI bundle. If this is not the case, control section  110  commands encoding section  102  not to perform retransmission. 
         [0130]    In addition, in relay station  300  according to Embodiment 3, relay signal processing section  310  determines whether or not to perform relay processing, based on the result of the error detection about the last TTI, and the result of the error detection in the last TTI which is transmitted from base station  200 . When no error is detected in the last TTI and base station  200  transmits a NACK signal for the last TTI, relay signal processing section  310  performs relay processing. This relay processing is performed in the retransmission-scheduled period corresponding to the last TTI. 
         [0131]      FIG. 12  explains operations of terminal  100 , base station  200  and relay station  300  according to Embodiment 3. 
         [0132]    As shown in  FIG. 12 , terminal  100  bundles TTIs  0  to  2  to transmit data. At this time, in terminal  100 , control section  110  commands encoding section  102  to start preparing for retransmission of the entire TTI bundle. 
         [0133]    In base station  200 , error detecting section  206  performs error detection on received data per TTI. Then, control information generating section  209  transmits only the result of the error detection in the last TTI. 
         [0134]    In relay station  300 , error detecting section  306  performs error detection on received data per TTI. Then, error detecting section  306  transmits only the result of the error detection in the last TTI. 
         [0135]    Then, terminal  100  determines whether or not to perform retransmission, based on the result of the error detection in the last TTI transmitted from each of base station  200  and relay station  300 . In  FIG. 12 , control section  110  commands encoding section  102  to retransmit the entire TTI bundle because base station  200  and relay station  300  have transmitted NACK signals. Here, relay station  300  detects no error in the last TTI and base station  200  transmits a NACK signal for the last TTI, relay station  300  performs relay processing. 
         [0136]    By this means, it is possible to perform retransmission by deciding more advantageous one between retransmission of the entire TTI bundle by terminal  100  and relay using one TTI by relay station  300 , and it is possible to limit the number of times of transmissions of ACK/NACK signals for one TTI bundle from base station  200  and relay station  300 , to 1. 
       Embodiment 4 
       [0137]    With Embodiment 4, a relay station transmits an ACK/NACK signal only about the last TTI in a TTI bundle, like Embodiment 3. Here, although, with embodiment 3, a terminal starts preparing for retransmission immediately after transmitting a TTI bundle to retransmit the entire TTI bundle, one retransmission-scheduled period is skipped and retransmission for the entire TTI bundle is performed in the next retransmission-scheduled period, with embodiment 4. Here, the basic configurations of a terminal, a base station and a relay station according to the present embodiment are the same as those of the terminal, the base station and the relay station described in Embodiment 1. Therefore, the terminal, the base station and the relay station according to the present embodiment will be described with reference to  FIG. 4  to  FIG. 6 . 
         [0138]    In terminal  100  according to Embodiment 4, like Embodiment 1, control section  110  determines whether or not to perform processing, including preparation for retransmission, decision to perform retransmission, retransmission and transmission of new data, based on ACK/NACK information from base station  200  and ACK/NACK information from relay station  300 , and outputs control information according to the result of the determination, to encoding section  102 . 
         [0139]    To be more specific, when base station  200  and relay station  300  transmit NACK signals, control section  110  commands encoding section  102  to start preparing for retransmission and then perform retransmission of the entire TTI bundle. Here, this retransmission is performed not in the next first retransmission-scheduled period, but in the second retransmission-scheduled period following the first retransmission-scheduled period. 
         [0140]    In addition, in relay station  300  according to Embodiment 4, relay signal processing section  310  determines whether or not to perform relay processing, based on the result of the error detection in the last TTI, and the result of the error detection in the last TTI which is transmitted from base station  200 . When no error is detected in the last TTI and base station  200  transmits a NACK signal for the last TTI, relay signal processing section  310  performs relay processing. This relay processing is performed in the retransmission-scheduled period corresponding to the last TTI. 
         [0141]      FIG. 13  explains operations of terminal  100 , base station  200  and relay station  300  according to Embodiment 4. 
         [0142]    As shown in  FIG. 13 , terminal  100  bundles TTIs  0  to  2  to transmit data. 
         [0143]    In base station  200 , error detecting section  206  performs error detection on received data per TTI. Then, control information generating section  209  transmits only the result of the error detection in the last TTI. 
         [0144]    In relay station  300 , error detecting section  306  performs error detection on received data per TTI. Then, error detecting section  306  transmits only the result of the error detection in the last TTI. 
         [0145]    Then, terminal  100  determines whether or not to perform retransmission, based on the result of the error detection about the last TTI transmitted from each of base station  200  and relay station  300 . In  FIG. 13 , base station  200  and relay station  300  transmit NACK signals, so that control section  110  commands encoding section  102  to start preparing for retransmission of the entire TTI bundle and to perform retransmission in the above-described second retransmission-scheduled period (from TTI  16  to TTI  18 ). Here, relay station  300  detects no error in the last TTI and base station  200  transmits a NACK signal for the last TTI, relay station  300  performs relay processing (see  FIG. 14 ). 
         [0146]    By this means, it is possible to perform retransmission by deciding more advantageous one between retransmission of the entire TTI bundle by terminal  100  and relay using one TTI by relay station  300 , and it is possible to limit the number of times of transmissions of ACK/NACK signals for one TTI bundle from base station  200  and relay station  300 , to 1. 
       Another Embodiment 
       [0147]    (1) With embodiment 2, relay station  300  transmits the result of the error detection about the codeword transmitted in the first TTI in a TTI bundle and the result of the error detection about the codeword transmitted in the last TIT. By contrast with this, another embodiment is possible where relay station  300  transmits all the results of error detection obtained in TTIs, with respect to a codeword mapped to a TTI bundle. 
         [0148]    With this embodiment, terminal  100  uses the result of the error detection about the TTI other than the first TTI and the last TTI, which is transmitted from relay station  300 , as a trigger to stop preparation for retransmission having already been started. 
         [0149]    That is, when relay station  300  transmits a NACK signal for the first TTI, control section  110  commands encoding section  102  to start preparing for retransmission of the entire TTI bundle. Then, if relay station  300  transmits an ACK signal for the TTI other than the first TTI and the last TTI in a TTI bundle, control section  110  commands encoding section  102  to stop preparation for retransmission having already been started at this time, and to prepare for transmission of new data. 
         [0150]    By this means, terminal  100  can stop preparing for retransmission at the time of receipt of an ACK signal without needing to wait for an ACK/NACK signal for the last TTI transmitted from relay station  300 , and therefore can reduce power consumption for preparation for retransmission. In addition, a terminal can start preparing for transmission of new data at the time of receipt of an ACK signal, so that it is possible to release a buffer area secured to retransmit data in an early stage. 
         [0151]    (2) With Embodiment 3, relay station  300  transmits only an ACK/NACK signal for the codeword transmitted in the last TTI. However, if another embodiment is adopted where relay station  300  transmits an ACK signal for only a TTI in which no error is detected for the first time in a TTI bundle, instead of the last TTI, it is possible to produce the same effect as in Embodiment 3. 
         [0152]    In this embodiment, control information generating section  209  transmits an ACK signal for only a TTI in which no error is detected for the first time, with respect to one TTI bundle. Terminal  100  uses this ACK/NACK signal as a criterion to determine which of terminal  100  and relay station  300  performs retransmission. 
         [0153]    By this means, terminal  100  can receive an ACK signal from relay station  300  in an earlier stage than in Embodiment 3, and therefore can stop preparing for retransmission at an early stage. Therefore, it is possible to reduce power consumption for preparation for retransmission. In addition, terminal  100  can start preparing for transmission of new data at the time of receipt of an ACK signal from relay station  300 , so that it is possible to release a buffer area secured for retransmission data in an early stage. 
         [0154]    (3) Here, although, with Embodiment 1, a case has been explained where a predetermined length read from a circular buffer is sixty-four columns, a predetermined length varies depending on the amount of resources assigned by base station  200 . In addition, a case has been explained where the column numbers of RVs, which are the positions to read in a circular buffer, are that RV  0  is the third column, RV  1  is the twenty-seventh column, RV  2  is the fifty-first column and RV  3  is the seventy-fifth column, respectively, they may be derived according to other relational equations. 
         [0155]    (4) Here, with Embodiments 1 to 4, although cases have been explained where decoding and error detection are performed per TTI, processing to perform decoding and error detection only at the timing to transmit an ACK/NACK signal, is possible. 
         [0156]    (5) Here, with Embodiments 1 to 4, although cases have been explained where a TTI bundle is composed of three TTIs, a TTI bundle may be composed of two or more TTIs. 
         [0157]    (6) With Embodiments 1 to 4, although descriptions have been explained by assuming a FDD (frequency division duplex) system using varying frequencies between the uplink and downlink, the present invention is not limited to this and is practicable in a TDD (time division duplex) system. 
         [0158]    (7) Also, although cases have been described with Embodiments 1 to 4 as examples where the present invention is configured by hardware, the present invention can also be realized by software. 
         [0159]    Each function block employed in the description of each of Embodiments 1 to 4 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. 
         [0160]    Further, 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. 
         [0161]    Further, 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. 
         [0162]    The disclosure of Japanese Patent Application No. 2008-235357, filed on Sep. 12, 2008, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY  
       [0163]    The relay apparatus and the radio communication system according to the present invention are useful to realize new retransmission control when the TTI-bundling technique and the relay technique are adopted for communication between terminals and a base station.