Abstract:
In a transmission apparatus in a MIMO-OFDM communication system employing a cyclic diversity, a cyclic delay controller sets plural delay magnitudes different for respective antennas, in cyclic delayers for each predetermined timing. The cyclic delayers receive symbols subjected to orthogonal frequency division multiplexing, for the respective ones of plural allotted antennas. Besides, the cyclic delayers bestow cyclic delays on the individual symbols of the respective antennas in accordance with plural set delay magnitudes. The symbols cyclically delayed are outputted from the antennas. As the delay magnitudes, a first delay magnitude at a first transmission timing and a second delay magnitude at a second transmission timing are different for one antenna, and the delay magnitudes differ in the respective antennas for one transmission timing. Thus, in a MIMO-OFDM transmission scheme, a frequency diversity and a time diversity are enhanced to heighten a retransmission efficiency in a data retransmission mode.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2008-144125 filed on Jun. 2, 2008, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a transmission apparatus, an access point and a symbol transmission method. More particularly, it relates to a transmission apparatus, an access point and a symbol transmission method of high data retransmission efficiency in a MIMO-OFDM communication system which employs a cyclic delay diversity. 
         [0004]    2. Description of the Related Art 
         [0005]    An orthogonal frequency division multiplexing (OFDM) mobile communication system is a transmission scheme which has excellent characteristics in a broad bandwidth, a frequency utilization efficiency and a radio propagation withstand characteristic. Further, there is a multiple input and multiple output (MIMO)-OFDM transmission scheme wherein transmission signals are spatially multiplexed with transmission antennas and reception antennas in the OFDM, thereby to realize the enhancement of a transmission rate. 
         [0006]    Also, as a transmission diversity scheme for error-correction-coded OFDM signals, there is a scheme wherein OFDM signals after an inverse Fourier transform or after a cyclic delay diversity in which individual transmission symbols are transmitted by performing cyclic delays different between the respectively adjacent antennas are time-shifted and are thereafter transmitted by inserting cyclic prefixes (CPs). 
         [0007]    The signals which do not correlate between the respective antennas are transmitted by applying this scheme to the MIMO-OFDM transmission scheme, so that the transmission rate can be raised in proportion to the transmission antennas. 
         [0008]    On the other hand, in transmitting broadcast data, there have been disclosed methods wherein the data are efficiently transmitted by employing the cyclic delay diversity (refer to, for example, Patent Document 1 being JP-A-2005-354708). Incidentally, the MIMO-OFDM transmission scheme has been adopted as the transmission scheme of next-generation mobile communications. 
       SUMMARY OF THE INVENTION 
       [0009]    Usually, the unicast communications of portable telephones, etc. have a data retransmission function such as automatic repeat request (ARQ). In the prior-art schemes of Patent Document 1, etc., however, the retransmission efficiency of a retransmission mode is not considered. 
         [0010]    In view of the above point, the present invention has an object to provide a transmission apparatus, an access point and a symbol transmission method which raise a retransmission efficiency in a data retransmission mode by enhancing a frequency diversity and a time diversity in a MIMO-OFDM transmission scheme. 
         [0011]    One of the objects of the invention is, in the process of an H-ARQ in fast radio communications, to control cyclic delay magnitudes by a cyclic delay controller, and transmit OFDM signals cyclically delayed with delay magnitudes which are different in respective antennas and for respective H-ARQ retransmissions, and obtain the frequency diversity by respectively changing frequency characteristics of transmission signals. Beside, one of the objects of the invention is to relieve the influence of frequency-selective fading, decrease the number of times of H-ARQ retransmissions utilizing the time diversity, and raise the retransmission efficiency. One of the objects of the invention is to decrease the number of times of the H-ARQ retransmissions by preventing a data omission in a specified path by the frequency diversity, raise a temporal retransmission efficiency, and make possible fast radio communications of high reliability. 
         [0012]    In a MIMO-OFDM transmission scheme wherein an orthogonal frequency division multiplexing (OFDM) signal applying an H-ARQ (Hybrid-ARQ) in which channel encoding employing an error correction code (for example, turbo code) and an automatic repeat request (ARQ) are combined is spatially multiplexed, the present invention consists in a transmission apparatus which executes a cyclic delay diversity (CDD) for performing cyclic delays different at respective transmission timings and in respective transmission antennas so that respective transmission signals may become orthogonal when received, and in which cyclic delay magnitudes differing in the respective antennas and for respective H-ARQ retransmissions are afforded by a cyclic delay controller included in the transmission apparatus, thereby to generate OFDM signals whose phases are respectively different. Accordingly, the frequency characteristic of a signal waveform synthesized by respective reception antennas changes every transmission (for example, every H-ARQ retransmission). 
         [0013]    A method for transmitting symbols in the present invention is a method in which data are transmitted and retransmitted by employing a cyclic diversity in, for example, an OFDM communication system, and as one feature thereof, it includes: 
         [0014]    the step of setting delay values that are different for respective symbols or frames in individual antennas of a transmission apparatus; 
         [0015]    the step of generating OFDM signals that contain the data, in the transmission apparatus; 
         [0016]    the step of delaying the individual OFDM signals with delay values that are different in the respective antennas and for the respective symbols; and 
         [0017]    the step of transmitting the plurality of delayed OFDM signals by using the respective antennas. 
         [0018]    The transmission apparatus of the present invention includes a cyclic delay controller which has the function of allotting delay magnitudes different for the respective symbols or frames in the individual antennas, in an apparatus which transmits and retransmits data by employing a cyclic diversity in, for example, a MIMO-OFDM communication system. 
         [0019]    Besides, the transmission apparatus includes cyclic delayers which cyclically delay the respective symbols in the individual antennas with the allotted delay magnitudes. 
         [0020]    By way of example, values are fetched from a storage medium which retains random delay magnitudes or delay magnitudes having fixed patterns and are inputted to the cyclic delayers under the control of the cyclic delay controller. 
         [0021]    Alternatively, the delay magnitudes are altered in synchronism with, for example, the timing signal of each transmission and retransmission of an H-ARQ which is inputted from an H-ARQ controller. 
         [0022]    According to the first solving means of this invention, there is provided a transmission apparatus which transmits and retransmits data by employing a cyclic diversity in a communication system wherein data are spatially multiplexed using a plurality of antennas and wherein communications are performed by orthogonal frequency division multiplexing, comprising: 
         [0023]    a symbol generation portion which multiplexes data by orthogonal frequency division multiplexing, and assigning the plurality of antennas to the data to generate symbols of respective antennas; 
         [0024]    a cyclic delay controller which sets a plurality of delay magnitudes different for the respective antennas, for each predetermined timing; 
         [0025]    a cyclic delayer which bestows cyclic delays on the individual symbols of the respective antennas, in accordance with the plurality of delay magnitudes set by said cyclic delay controller; and 
         [0026]    a transmission portion which outputs the cyclically delayed symbols from the antennas; 
         [0027]    wherein as the delay magnitudes, a first delay magnitude at a first transmission timing and a second delay magnitude at a second transmission timing are different for one antenna, and the delay magnitudes are different in the respective antennas for one transmission timing. 
         [0028]    According to the second solving means of this invention, there is provided an access point comprising: 
         [0029]    a transmission apparatus which transmits and retransmits data by employing a cyclic diversity in a communication system wherein data are spatially multiplexed using a plurality of antennas and wherein communications are performed by orthogonal frequency division multiplexing, the transmission apparatus comprises 
         [0030]    a symbol generation portion which multiplexes data by orthogonal frequency division multiplexing, and assigning the plurality of antennas to the data to generate symbols of respective antennas, 
         [0031]    a cyclic delay controller which sets a plurality of delay magnitudes different for the respective antennas, for each predetermined timing, 
         [0032]    a cyclic delayer which bestows cyclic delays on the individual symbols of the respective antennas, in accordance with the plurality of delay magnitudes set by said cyclic delay controller; and 
         [0033]    a transmission portion which outputs the cyclically delayed symbols from the antennas, 
         [0034]    wherein as the delay magnitudes, a first delay magnitude at a first transmission timing and a second delay magnitude at a second transmission timing are different for one antenna, and the delay magnitudes are different in the respective antennas for one transmission timing; and 
         [0035]    a reception apparatus which receives an acknowledgment notification that indicates data transmitted by the transmission apparatus has been normally received by a terminal, and/or a retransmission request that makes a request for retransmission of the data due to failure of the normal reception of the data at the terminal, from the terminal through the antennas; 
         [0036]    wherein the data is retransmitted from the transmission apparatus to the terminal in a case where the acknowledgment notification is not received from the terminal within a predetermined time period, or in a case where the retransmission request has been received. 
         [0037]    According to the third solving means of this invention, there is provided a symbol transmission method which transmits and retransmits data by employing a cyclic diversity in a communication system wherein data are spatially multiplexed using a plurality of antennas and wherein communications are performed by orthogonal frequency division multiplexing, the symbol transmission method comprising the steps of: 
         [0038]    multiplexing data by orthogonal frequency division multiplexing, and assigning the plurality of antennas to the data to generate symbols of respective antennas; 
         [0039]    setting a plurality of delay magnitudes different for the respective antennas, for each predetermined timing; 
         [0040]    bestowing cyclic delays on the individual symbols of the respective antennas, in accordance with set plurality of delay magnitudes; and 
         [0041]    outputting the cyclically delayed symbols from the antennas; 
         [0042]    wherein as the delay magnitudes, a first delay magnitude at a first transmission timing and a second delay magnitude at a second transmission timing are different for one antenna, and the delay magnitudes are different in the respective antennas for one transmission timing. 
         [0043]    According to the present invention, a frequency diversity and a time diversity are enhanced in a MIMO-OFDM transmission scheme, whereby a transmission apparatus, an access point and a symbol transmission method which raise a retransmission efficiency in a data retransmission mode can be provided. 
         [0044]    According to the invention, in the process of an H-ARQ in fast radio communications, cyclic delay magnitudes are controlled by a cyclic delay controller, and OFDM signals cyclically delayed with delay magnitudes which are different in respective antennas and for respective H-ARQ retransmissions are transmitted, whereby the frequency characteristics of transmission signals can be respectively changed to obtain the frequency diversity. Thus, the influence of frequency-selective fading can be relieved, the number of times of H-ARQ retransmissions utilizing the time diversity is decreased, and the retransmission efficiency rises. That is, the time diversity can be raised. 
         [0045]    Unlike broadcast communications for broadcast data, unicast communications in which data are communicated with an unspecified opposite party in one-to-one correspondence are furnished with a retransmission process such as H-ARQ, for a case where the data has not been successfully decoded on a reception side. Owing to the invention, a data omission in a specified path is prevented by the frequency diversity, whereby the number of times of the H-ARQ retransmissions can be decreased, a temporal retransmission efficiency rises, and fast radio communications of high reliability become possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]      FIG. 1  is a block diagram showing the configuration of an access point in a MIMO-OFDM communication system; 
           [0047]      FIG. 2  is a block diagram (# 1 ) of a transmission/reception unit in the access point of an OFDM mobile communication system in an example; 
           [0048]      FIG. 3  is a block diagram (# 2 ) of a transmission/reception unit in the access point of an OFDM mobile communication system in another example; 
           [0049]      FIG. 4  is a diagram for explaining cyclic delays and cyclic prefixes (CPs); 
           [0050]      FIG. 5  is a flow chart of the data transmission/reception between the access point and a terminal; 
           [0051]      FIG. 6  is a sequence diagram in the case where an acknowledgment signal (ACK) is returned; 
           [0052]      FIG. 7  is a sequence diagram in the case where a negative acknowledgment signal (NAK) is returned; 
           [0053]      FIGS. 8A and 8B  are diagrams for explaining a hybrid-ARQ (H-ARQ) retransmission mode in the case where the same H-ARQ retransmission data are transmitted in a manner to be synchronized by individual antennas; and 
           [0054]      FIGS. 9A and 9B  are diagrams for explaining a hybrid-ARQ (H-ARQ) retransmission mode in the case where H-ARQ retransmission data are transmitted independently by the individual antennas. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0055]    Now, an embodiment of the present invention will be described. 
         [0056]      FIG. 1  shows a configurational example of an access point in a MIMO-OFDM communication system according to this embodiment. 
         [0057]    The access point of this system includes a line interface unit  102  which inputs/outputs data from/to a network  101 , a modulation unit  219  to which the transmission data are inputted from the interface unit  102  and in which the MIMO-OFDM modulation of the transmission data is performed, a front end unit (FEU)  221  to which a modulated signal is inputted from the modulation unit  219  and in which the power amplification of the modulated signal is performed, a plurality of antennas  213   a - 213   n  to which transmission signals are inputted from the FEU  221  and which radiates the signals into a space, and a demodulation unit  220  to which signals received from the space by the antennas  213   a - 213   n  and then amplified by the FEU  221  are inputted so as to be demodulated, and which outputs the demodulated signals to the line interface unit  102 . In this embodiment, the data are spatially multiplexed and are communicated by a plurality of paths between the access point  103  and a terminal  104  in, for example, the orthogonal frequency division multiplexing (OFDM) communication system. 
         [0058]    Each of  FIGS. 2 and 3  is a functional block diagram showing the configurations of the modulation unit  219 , demodulation unit  220  and FEU  221  of the access point. 
         [0059]    The modulation unit  219  includes, for example, a serial/parallel (S/P) converter  201 , a retransmission control portion (H-ARQ controller)  202 , encoders  203 , symbol mapping portions  204 , an antenna mapping portion  205 , inverse Fourier transform portions (IFFTs)  206 , cyclic delayers  207 , a memory  208   a,  a cyclic delay controller  209 , parallel/serial (P/S) converters  210 , and cyclic prefixers (CPs)  211 . The demodulation unit  220  includes, for example, a demodulation module  216 , an ACK/NAK reception portion  217  and a P/S converter  218 . The FEU  221  includes, for example, transmission portion  212  and reception portions  215 . Duplexers  214 , for example, are interposed between the FEU  221  and the antennas  213 . 
         [0060]    The configuration shown in  FIG. 3  includes a random number generator  208   b  instead of the memory  208   a  in  FIG. 2 . The remaining configuration is the same as in  FIG. 2 . 
         [0061]    The data inputted from the line interface unit  102  are encoded via the S/P converter  201  and the encoders  203   a - 203   n . By way of example, the data are encoded with turbo codes. On this occasion, the encoded data are stored in, for example, memories within the encoders  203   a - 203   n  for the purpose of retransmissions. Incidentally, any other appropriate memories may well be employed. Subsequently, in the symbol mapping portions  204   a - 204   n , the transmission data are mapped on a complex plane and are subjected to a subcarrier modulation (for example, QAM modulation). Thereafter, the antenna mapping portion  205  performs a mapping in which transmission symbols to be transmitted are associated with the antennas. Inverse fast Fourier transforms are executed by the IFFTs  206   a - 206   n , and the transmission symbols are transformed from the signals of a frequency region into those of a time region. By the way, in this embodiment, the encoders  203 —the IFFTs  206  will sometimes be collectively called the “symbol generation portions” or “OFDM signal generation portions”. 
         [0062]    A plurality of cyclic delay patterns which contain a plurality of delay magnitudes different from one another, are stored in the memory  208   a.  A different or dispersed delay magnitude string (pattern) is fetched from the memory  208   a  by the cyclic delay controller  209  every transmission timing. The patterns different from one another are outputted from the cyclic delay controller  209  to the respective cyclic delayers  207   a - 207   n  in synchronism with the transmission timing of, for example, the H-ARQ. 
         [0063]    The random number generator  208   b  generates a random or dispersed delay magnitude string, and outputs this string to the cyclic delay controller  209 . Apart from the fetch of the delay magnitude string from the memory  208   a  as stated above, the cyclic delay controller  209  may well set the delay magnitudes of the respective cyclic delayers  207  by inputting the delay magnitude string from the random number generator  208   b  every transmission timing as shown in  FIG. 3 . 
         [0064]    A timing for altering the delay magnitudes may well be regularly changed into, for example, every transmission timing of a symbol or every transmission timing of a frame containing a plurality of symbols. Besides, the delay magnitudes may well be altered every transmission timing of retransmission data. Alternatively, the delay magnitudes may well be altered at appropriate timings at which the delay magnitudes differ between at the timing for transmitting the original data and the timing for transmitting the retransmission data. 
         [0065]    When the pattern signals and the output signals (symbols) from the IFFTs  206   a - 206   n  are inputted to the respective cyclic delayers  207   a - 207   n , the cyclic delays of the individual symbols are made. Incidentally, the details of the cyclic delays will be described later. The output signals from the cyclic delayers  207   a - 207   n  are converted into serial signals by the P/S converters  210 , whereupon the serial signals are endowed with cyclic prefixes (CPs) by the cyclic prefixers  211   a - 211   n . The transmission OFDM symbols endowed with the CPs are respectively upconverted by the transmission portion  212   a - 212   n , and the resulting signals are respectively transmitted from the antennas  213   a - 213   n  via the duplexers  214   a - 214   n . 
         [0066]    Besides, in a case where data have been transmitted from the terminal, they are received by the antennas  213   a - 213   n . Reception signals are downconverted by the reception portions  215   a - 215   n  via the duplexers  214   a - 214   n . Thereafter, the resulting reception signals are subjected to a demodulation process by the demodulation module  216 , and the resulting demodulated signal is outputted to the external network  101  through the line interface unit  102  via the P/S converter  218 . 
         [0067]    The ACK/NAK reception portion  217  receives an ACK signal (acknowledgment notification, namely affirmation signal) and an NAK signal (retransmission request, namely negation signal) from the terminal  104 . The H-ARQ controller  202  controls the retransmission of the data. In a case, for example, where the NAK signal has been received from the terminal  104  or where the ACK signal has not been received within a predetermined time period since the transmission of the data to the terminal  104 , the H-ARQ controller  202  fetches and retransmits the transmission data stored in the encoder  203 . On the other hand, in a case where the ACK signal has been received from the terminal  104 , the H-ARQ controller  202  erases the transmission data stored in the encoder  203 . 
         [0068]      FIG. 4  is a diagram for explaining the cyclic delays and the cyclic prefixes (CPs). 
         [0069]    Referring to  FIG. 4 , a symbol  301  denotes that symbols outputted from the IFFTs  206   a - 206   n  contain a plurality of samples. Those samples of a symbol tail which correspond to the delay magnitude of a cyclic delay pattern controlled by the cyclic delay controller  209  are shifted to a symbol head by the cyclic delayers  207   a - 207   n  ( 302  and  303 ). The illustrated symbols  302  and  303  correspond to an example in which the delay magnitude is “ 2 ”. Further, when a plurality of samples at a symbol tail as correspond to a CP length stipulated by specifications beforehand are copied at a symbol head, a transmission OFDM symbol  305  which is endowed with a CP for protection against any inter-symbolic interference is obtained ( 304  and  305 ). 
         [0070]      FIGS. 8A and 8B  and  FIGS. 9A and 9B  are diagrams (# 1 ) and (# 2 ) for explaining H-ARQ retransmission modes. 
         [0071]    Each of  FIGS. 8A and 9A  shows a configurational example of the memory  208   a.    
         [0072]    By way of example, a plurality of delay patterns which contain a plurality of delay magnitudes different for the respective antennas are stored in the memory  208   a  every transmission timing. Regarding the delay magnitudes, by way of example, a delay magnitude  1   a  at a transmission timing “ 1 ”, a delay magnitude  2   a  at a transmission timing “ 2 ”, . . . , and a delay magnitude Ma at a transmission timing “M” are respectively different for one antenna a. Incidentally, the letter M denotes a plus integer, and the M delay patterns can be repeatedly used. Besides, the delay magnitudes  1   a ,  1   b , . . . , and  1 N of the individual antennas are respectively different for one transmission timing “ 1 ”. These delay magnitudes can be stored in the memory  208   a  beforehand. 
         [0073]    Each of  FIGS. 8B and 9B  shows a situation where the transmission data are outputted from the cyclic prefixers  211   a - 211   n.  The transmission OFDM symbol contained in the H-ARQ retransmission data is cyclically delayed with a delay magnitude which is different from that of the last transmission mode. On the other hand, all the cyclic delay values of the transmission OFDM symbols within the identical H-ARQ retransmission data have the same values. A method for transmitting the H-ARQ retransmission data which are transmitted from the respective antennas is, for example, a 2-pattern method. 
         [0074]      FIG. 8B  corresponds to a case where the identical H-ARQ retransmission data are transmitted from the respective antennas at the same timings. The identical H-ARQ retransmission data whose cyclic delay values are different, are synchronized by the individual antennas and are transmitted from the respective antennas at the same retransmission timing. In addition to an effect based on the cyclic delays, the reliability of communications can be further heightened in proportion to the number of the antennas  213 . The reliability of communications, for example, is higher than with patterns in  FIGS. 9A and 9B  to be stated later. For the realization of the H-ARQ retransmission in  FIGS. 8A and 8B , a synchronization process is required in each of processes before the retransmission of the H-ARQ retransmission data (as include the processes of, for example, the encoders  203 , the symbol mapping portions  204 , the antenna mapping portion  205 , the IFFTs  206 , the cyclic delayers  207 , the P/S portions  210 , the CP portions  211 , the transmission portion  212 , the duplexers  214 , and the antennas  213 ). 
         [0075]    On the other hand,  FIG. 9B  corresponds to a case where the individual H-ARQ retransmission data are independently transmitted by the respective antennas. For the realization of the H-ARQ retransmission in  FIGS. 9A and 9B , accordingly, the process may be executed in any of the portions  1 -N in each of the processes (as stated before) before the retransmission of the H-ARQ retransmission data. By way of example, the process may be executed in any of the encoders # 1 -#N, or in any of the symbol mapping portions # 1 -#N. In the example  FIG. 9B , the H-ARQ retransmission data ( 1 ) is processed by the encoder etc. corresponding to the antenna a and is outputted from the antenna a, and the H-ARQ retransmission data ( 2 ) is processed by the encoder etc. corresponding to the antenna b and is outputted from the antenna b. Besides, the transmission timings of the retransmission data ( 1 ) and ( 2 ) may well be different. With this method, in addition to an effect based on the cyclic delays, communications at a throughput higher than in the case of  FIGS. 8A and 8B  are expected. 
         [0076]    At the individual H-ARQ retransmissions, the patterns of cyclic delay values are different (independent) for the respective antennas. All of the delay values are plus integers, and they are the pattern values of the respective antennas accumulated in the memory  208   a  as shown in  FIG. 2  or the random values generated by the random number generator  208   b  as shown in  FIG. 3 . These values are invoked by the cyclic delay controller  209  in synchronism with, for example, the timings of the cyclic delays. 
         [0077]      FIG. 5  shows a flow chart of the data transmission/reception between the access point and the terminal. 
         [0078]    Signals upconverted by the transmission portion  212   a - 212   n  in  FIG. 2  or  FIG. 3  are transmitted from the access point  103  toward the terminal (step  501 ). Data transmitted from the access point  103  is received and modulated by the mobile terminal  104 . On that occasion, the mobile terminal  104  performs a CRC check so as to decide if a packet has been correctly decoded (step  502 ). When the CRC check is “OK” (that is, when the packet has been correctly decoded), the mobile terminal  104  returns an ACK signal to the access point  103 , whereas when the CRC check is “NG” (that is, when the packet has not been correctly decoded), the mobile terminal  104  returns an NAK signal to the access point  103 . When the packet has been correctly decoded, the ACK signal is transmitted from the terminal  104  toward the access point  103 , and the access point  103  receives this ACK signal (step  503 ). After the ACK signal has been received by the ACK/NAK reception portion  217 , it is notified to the H-ARQ controller  202  (step  504 ). Thereafter, the storage of the data having been retained in the encoders  203   a - 203   n  in the last encoding is released (step  505 ), and the transmission process for the data is ended. 
         [0079]    On the other hand, when the packet has not been correctly decoded on the terminal side, the NAK signal is transmitted from the terminal  104  toward the access point  103 , and the access point  103  receives this NAK signal (step  506 ). After the NAK signal has been received by the ACK/NAK reception portion  217  of the access point  103 , it is notified to the H-ARQ controller  202  (step  507 ). 
         [0080]    Here, if the number of times of retransmissions has reached a predetermined specification number (“Yes” at a step  508 ), the access point  103  shifts to the step  505 , at which the storage of the data having been retained in the encoders  203   a - 203   n  in the last encoding is released (step  505 ), and the transmission process for the data is ended. If the number of times of retransmissions has not reached the specification number (“No” at the step  508 ), the access point  103  invokes from the memories, the data having been retained in the encoders  203   a - 203   n  in the last encoding (step  509 ). The invoked encoded data are transformed into an OFDM symbol by the IFFTs  206   a - 206   n  via the above transmission process (step  510 ), and they are cyclically delayed for the respective H-ARQ transmissions and the respective antennas again by the cyclic delayers  207   a - 207   n  (step  511 ). Delay magnitudes here are different from the delay magnitudes with which the data have been transmitted to the terminal at the last transmission timing (for example, the delay magnitudes in the case of transmitting the data at the step  501 ). The data endowed with the cyclic delays are transmitted from the access point to the terminal again (step  501 ). 
         [0081]      FIG. 6  shows a sequence diagram of the flow of signals in the case where the ACK signal has been returned. 
         [0082]    The data are transmitted from the access point  103  (step  601 ), and are received by the terminal  104 . Subsequently, when the data are correctly decoded (step  602 ), the ACK signal is transmitted from the terminal  104  to the access point  103  (step  603 ). Thereafter, when the ACK signal is received by the ACK/NAK reception portion  217  (step  604 ), the ACK is notified to the H-ARQ controller  202  (step  605 ). Thereafter, the storage of the data having been retained in the encoders  203   a - 203   n  in the last encoding is released (step  606 ), and the transmission process for the data is ended. 
         [0083]      FIG. 7  shows a sequence diagram of the flow of signals in the case where the NAK signal has been returned. 
         [0084]    The data are transmitted from the access point  103  (step  701 ), and are received by the terminal  104 . Subsequently, when the decoding of the data fails (step  702 ), the NAK signal is transmitted from the terminal  104  to the access point  103  (step  703 ). Thereafter, when the NAK signal is received by the ACK/NAK reception portion  217  (step  704 ), the NAK is notified to the H-ARQ controller  202  (step  705 ). Thereafter, in a case where the number of times of retransmissions has not reached the specification number, the data having been retained in the encoders  203   a - 203   n  in the last encoding are invoked from the memories (step  706 ). The invoked encoded data are transformed into an OFDM symbol by the IFFTs  206   a - 206   n  via the above transmission process (step  707 ), the cyclic delay magnitudes of the OFDM symbol are altered for the respective H-ARQ transmissions and the respective antennas again by the cyclic delayers  207   a - 207   n  (step  708 ), and the data are transmitted from the access point to the terminal again (step  709 ). Such series of retransmission processes are repeated until the number of times of retransmissions reaches the specification number. 
         [0085]    In this embodiment, owing to the above processing, the cyclic delays of the delay magnitudes differing for the respective transmissions are bestowed on the respective antennas, so that a frequency characteristic changes to afford frequency and time diversities. In the MIMO-OFDM, accordingly, a fixed recession in a specified channel does not occur, and the number of times of data retransmissions can be decreased, so that the averaged throughput of the access point can be enhanced. 
         [0086]    The present invention is applicable to, for example, a MIMO-OFDM communication system.