Patent Publication Number: US-10785071-B2

Title: Communication apparatus and received signal processing method

Description:
FIELD 
     The present invention relates to a communication apparatus and a received signal processing method, for performing single carrier or multicarrier transmission in which a cyclic prefix (CP) is not used or a delay amount in a delay wave exceeds a CP length. 
     BACKGROUND 
     In digital wireless communication, a process of equalizing a received signal is performed on a receiving side so as to prevent inter symbol interference (ISI) caused by the spread of delay waves. When equalization processing is performed in a frequency domain, there is used a technique for adding, as a guard interval called CP, a copy of data at the end of each of divided data pieces of a predetermined length, to the top of each of the divided data pieces. Addition of the CP enables prevention of interference from adjacent blocks when a received signal is converted into a signal in the frequency domain. However, a CP length needs to be larger than a multipath delay amount. Thus, the CP length needs to be increased so as to avoid the influence of a large multipath delay. Accordingly, there is a problem of a decrease in transmission efficiency due to an increase in the CP length. 
     There is known a method of block transmission using no CP called overlap frequency domain equalization (FDE). According to this method, in order to improve transmission efficiency, a section including a detection target block is converted into the frequency domain, and equalized in the frequency domain. Then, the detection target block is extracted after the section is converted into a time domain. In the overlap frequency domain equalization, interference components of inter block interference (IBI) and inter symbol interference remain in a received signal after equalization in the frequency domain. Therefore, reception performance tends to be lower than that in block transmission using a CP. 
     Patent Literature 1 discloses a method for reducing interference components included in a received signal in block transmission using no CP. In this method, replicas of the interference components are generated by use of the received signal on which equalization processing has been performed. Then, the remaining interference components are canceled by use of the generated replicas. Cancellation of the interference components is repeated by use of the overlap frequency domain equalization and the replicas. As a result, the interference components included in the received signal are reduced and a decrease in reception performance is prevented. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2012-70196 
     SUMMARY 
     Technical Problem 
     However, in the technique described in Patent Literature 1 above, the replica is generated in frame units. Thus, accuracy in generating the replica is low. Therefore, the technique described in Patent Literature 1 has a problem in that the interference components still remaining in the received signal cannot be reduced in some cases. 
     The present invention has been made in view of the above. An object of the present invention is to provide a communication apparatus capable of increasing accuracy in generating a replica of an interference component to prevent a decrease in reception performance in single carrier or multicarrier transmission in which a CP is not used or a delay amount in a delay wave exceeds a CP length. 
     Solution to Problem 
     A communication apparatus according to an aspect of the present invention includes: an equalization unit that performs equalization processing on a part of a received signal corresponding to a first range and a part of the received signal corresponding to a second range, the first range including detection target data, the second range being outside the first range; a replica generation unit that generates a replica of an interference component for the part of the received signal corresponding to the first range by using the part of the received signal corresponding to the second range on which the equalization processing has been performed; an interference cancellation unit that cancels the interference component from the part of the received signal corresponding to the first range by using the generated replica; and a data extraction unit that extracts the detection target data from the received signal. The replica generation unit is characterized by decomposing the part of the received signal corresponding to the second range into a plurality of signal components, and reproducing the part of the received signal corresponding to the second range for each of the signal components to generate the replica. 
     Advantageous Effects of Invention 
     The communication apparatus according to the present invention can achieve an effect of generating a replica of inter block interference with higher accuracy to prevent a decrease in reception performance in single carrier or multicarrier transmission in which a CP is not used or a delay amount in a delay wave exceeds a CP length. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a reception configuration of a wireless communication apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating the concept of overlap frequency domain equalization to be performed by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 3  is a diagram illustrating an example of signal components to be used by a replica generation unit illustrated in  FIG. 1 , for generating a replica. 
         FIG. 4  is a diagram illustrating definitions of interference components to be canceled from a received signal, by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 5  is a diagram illustrating an example of implementing the function of the wireless communication apparatus illustrated in  FIG. 1  by use of a processing circuit that is dedicated hardware. 
         FIG. 6  is a diagram illustrating an example of implementing the function of the wireless communication apparatus illustrated in  FIG. 1  by use of a processing circuit that is a control circuit. 
         FIG. 7  is a flowchart for describing a receiving operation of the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 8  is a diagram illustrating a first example of a replica to be used in an interference cancellation process to be performed by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 9  is a diagram illustrating a second example of the replica to be used in the interference cancellation process to be performed by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 10  is a diagram illustrating a third example of the replica to be used in the interference cancellation process to be performed by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 11  is a diagram illustrating a fourth example of the replica to be used in the interference cancellation process to be performed by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 12  is a diagram illustrating a wireless communication apparatus according to a variation of the first embodiment of the present invention. 
         FIG. 13  is a diagram illustrating a first variation of the definitions of the interference components to be canceled from the received signal, by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 14  is a diagram illustrating a second variation of the definitions of the interference components to be canceled from the received signal, by the wireless communication apparatus illustrated in  FIG. 1 . 
         FIG. 15  is a diagram illustrating a configuration for multiple users to transmit signals in a wireless communication system according to a second embodiment of the present invention. 
         FIG. 16  is a diagram illustrating a configuration for transmitting signals by spatial multiplexing in the wireless communication system according to the second embodiment of the present invention. 
         FIG. 17  is a diagram illustrating the configuration of a wireless communication apparatus according to the second embodiment of the present invention. 
         FIG. 18  is a flowchart illustrating the overall operation of the wireless communication apparatus illustrated in  FIG. 17 . 
         FIG. 19  is a flowchart illustrating a detailed operation of step S 201  in  FIG. 18 . 
         FIG. 20  is a flowchart illustrating a detailed operation of step S 203  in  FIG. 18 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, communication apparatuses and received signal processing methods according to embodiments of the present invention will be described in detail based on the drawings. It should be noted that the present invention is not limited to the embodiments. 
     First Embodiment 
       FIG. 1  is a diagram illustrating an example of a reception configuration of a wireless communication apparatus according to a first embodiment of the present invention. A wireless communication apparatus  1  illustrated in  FIG. 1  includes an antenna  11 , an analog front end  12 , an analog-to-digital converter (ADC)  13 , a band limiting unit  14 , a decimation unit  15 , a timing detection unit  16 , a frequency deviation correction unit  17 , and a first fast Fourier transform (FFT) unit  18 . The wireless communication apparatus  1  further includes a data separation unit  19 , a transmission path estimation unit  20 , an equalization weight calculation unit  21 , an equalization unit  22 , an interference cancellation unit  23 , an inverse FFT (IFFT) unit  24 , a data extraction unit  25 , and a log likelihood ratio (LLR) calculation unit  26 . The wireless communication apparatus  1  further includes a replica generation unit  27 , a replica control unit  28 , a second FFT unit  29 , a deinterleaver  30 , and an error correction decoding unit  31 . The replica generation unit  27  includes a first generation unit  27   a , a second generation unit  27   b , and a known signal addition unit  27   c.    
     The antenna  11  receives a wireless signal from a transmitting-side wireless communication apparatus. The wireless signal received by the antenna  11  includes a known signal such as a known preamble added on the transmitting side. The antenna  11  inputs the received signal to the analog front end  12 . The analog front end  12  down-converts the signal input from the antenna  11 , and inputs the down-converted signal to the ADC  13 . The ADC  13  converts the input analog signal into a digital signal, and inputs the digital signal to the band limiting unit  14 . The band limiting unit  14  limits the band of the input digital signal, and inputs the digital signal with the limited band to the decimation unit  15 . The decimation unit  15  reduces the sampling rate of the input signal, and inputs the signal subjected to reduction of the sampling rate to the timing detection unit  16 . The timing detection unit  16  detects the symbol block timing of the input signal. Specifically, the timing detection unit  16  detects the symbol block timing by performing cross-correlation processing using a known preamble and the like, and inputs the signal to the frequency deviation correction unit  17 . 
     The frequency deviation correction unit  17  estimates the frequency deviation of the input signal by using the known preamble and the like, and performs frequency deviation correction on the data area of the input signal by using the estimated frequency deviation. Then, the first FFT unit  18  performs FFT processing on the input signal to convert the input signal into a signal in the frequency domain, and inputs the received signal in the frequency domain to the data separation unit  19 . The data separation unit  19  separates the input received signal into a known signal including the above-described known preamble and the like, and data. The data separation unit  19  inputs the known signal to the transmission path estimation unit  20 , and inputs a signal in the frequency domain including the data to the equalization unit  22 . The transmission path estimation unit  20  estimates a transmission path by using the input known signal. The equalization weight calculation unit  21  calculates an equalization weight for overlap frequency domain equalization for each frequency based on a transmission path estimation result and a replica to be described below, and inputs the calculated equalization weights to the equalization unit  22 . The equalization weight is a weight determined based on the least-squares-error criterion on the basis of the received signal on which interference cancellation has been performed. The equalization weight is calculated in consideration of the electric power of an interference component to be canceled after equalization processing. The equalization unit  22  performs equalization processing on a part of the received signal corresponding to a first range and a part of the received signal corresponding to a second range for each frequency by using the input equalization weights. The first range includes detection target data. The second range is outside the first range. 
     The interference cancellation unit  23  cancels an interference component for the part of the received signal corresponding to the first range from the received signal. The interference cancellation unit  23  cancels the interference component by using a replica generated by the replica generation unit  27  to be described below. The IFFT unit  24  performs IFFT processing on the received signal from which the interference component has been canceled, to convert the received signal into a signal in a time domain, and inputs the converted signal to the data extraction unit  25 . The data extraction unit  25  extracts the detection target data from the input received signal, and inputs the detection target data to the LLR calculation unit  26 . The data extraction unit  25  extracts the detection target data from the central part of the received signal in the time domain by using the fact that interference components are unevenly distributed at both ends of the received signal in the time domain. The LLR calculation unit  26  calculates LLRs of all the bits of each symbol, and inputs the calculated LLRs to the replica generation unit  27 . 
       FIG. 2  is a diagram illustrating the concept of overlap frequency domain equalization to be performed by the wireless communication apparatus  1  illustrated in  FIG. 1 . Assuming that detection target data corresponds to an n b -th block among a plurality of blocks included in a received signal  150 , the first FFT unit  18  converts a part of the received signal corresponding to a processing range  153  into a signal in the frequency domain. The processing range  153  includes a first range  151  and a second range  152 . The first range  151  is a detection target range including the detection target data. The second range  152  is a range outside the first range. Therefore, the equalization unit  22  performs equalization processing on the part of the received signal corresponding to the processing range  153 . The interference cancellation unit  23  cancels an interference component from the received signal on which equalization processing has been performed. The data extraction unit  25  extracts the detection target data from the received signal. Then, the LLR calculation unit  26  calculates an LLR  154 . Here, the n b -th block is assumed to be the detection target data. Meanwhile, it is possible to obtain an LLR series  155  by similarly performing extraction of detection target data and calculating LLRs also for the other blocks. 
     The description returns to  FIG. 1 . The replica generation unit  27  decomposes a part of the received signal corresponding to the second range  152  into a plurality of signal components, and reproduces the part of the received signal corresponding to the second range  152  for each signal component to generate a replica. The replica generation unit  27  generates a replica by using the signal component specified by the replica control unit  28 . The replica generated by the replica generation unit  27  is used for cancelling an interference component from the received signal in the interference cancellation unit  23 . 
       FIG. 3  is a diagram illustrating an example of signal components to be used by the replica generation unit  27  illustrated in  FIG. 1 , for generating a replica. In the present embodiment, a received signal subjected to multi-level modulation is regarded as a plurality of hierarchized sub-symbols. In this case, the sub-symbol is an example of a signal component. For example, a 16 quadrature amplitude modulation (QAM) symbol  103  can be regarded as a quadrature phase shift keying (QPSK) symbol hierarchized into two levels different in signal power. The two levels of the QPSK symbol are referred to as a 0-th level QPSK sub-symbol  101  and a first level QPSK sub-symbol  102  in descending order of amplitude. The hierarchized sub-symbols are characterized in that the distance between signal points is shorter in an upper level, so that accuracy in determining a signal of a QPSK sub-symbol is low and accuracy in generating a replica is also low. The replica generation unit  27  can generate a replica for each sub-symbol level. 
     Described below is an example of a method for generating a replica of a sub-symbol, to be performed by the replica generation unit  27 . Assuming that λ m′  (0) is an LLR of a 0-th bit of an m′-th level, a replica of the m′-th level sub-symbol is expressed by mathematical formula (1) below. Here, an amplitude a m  in each level is expressed by mathematical formula (2) below. 
     
       
         
           
             
               
                 
                   
                     
                       
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     The generated replica of the sub-symbol is used in the equalization weight calculation unit  21  and the interference cancellation unit  23 . An example of interference cancellation will be cited below. In the following description, characters may be shown in the following manner in some cases: when a character with a bar is shown, “(bar)” is put after the character; when a character with a hat is shown, “(hat)” is put after the character; and when a character with a tilde is shown, “(tilde)” is put after the character. 
     With regard to processing for an f-th frequency, let R(tilde) (f) be a received signal subjected to frequency domain equalization, H(bar) (f) be the equivalent channel gain of an interference component after frequency domain equalization, and D(hat) (f) be the replica of the m′-th level sub-symbol in the frequency domain. Then, the received signal R(hat) (f) subjected to cancellation of interference is expressed by mathematical formula (3) below. It should be noted that each of m f  and m b  denotes the range of target levels for replicas of reproduced sub-symbols. 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 4  is a diagram illustrating definitions of interference components to be canceled from a received signal, by the wireless communication apparatus  1  illustrated in  FIG. 1 . The part of the received signal corresponding to the second range  152  includes a front IBI  104  and a rear IBI  105 . The front IBI  104  is a front signal corresponding to a past signal received prior to the first range  151  with respect to a direction on a time axis along which equalization processing proceeds. The rear IBI  105  is a rear signal corresponding to a future signal to be received later than the front signal with respect to the direction on the time axis. Interference components for a part of the received signal corresponding to the first range  151  include an interference component generated by an influence of the front IBI  104  and an interference component generated by an influence from the rear IBI  105 . 
     The description returns to  FIG. 1 . The first generation unit  27   a  decomposes the front signal into a plurality of signal components, and reproduces the front signal for each signal component to generate a replica of the interference component. Specifically, the first generation unit  27   a  decomposes the front signal into symbols of a plurality of levels to generate a replica of the interference component for each level. The second generation unit  27   b  decomposes the rear signal or ISI into a plurality of signal components, and reproduces the rear signal or ISI for each signal component to generate a replica of the interference component. Specifically, the second generation unit  27   b  decomposes a second interference component into symbols of a plurality of levels to generate a replica for each level. As a response to a known signal multiplexed to the received signal according to a frame format, the known signal addition unit  27   c  obtains a corresponding known signal from among a plurality of known signals internally held in advance, and adds the obtained known signal to the replica. With the above-described function, the replica generation unit  27  inputs the generated replicas to the second FFT unit  29  and the equalization weight calculation unit  21 . 
     The second FFT unit  29  converts the input replicas into signal components in the frequency domain, and inputs the converted replicas to the interference cancellation unit  23 . The equalization unit  22 , the interference cancellation unit  23 , and the data extraction unit  25  repeatedly perform equalization processing, cancellation of interference components, and extraction of detection target data. The number of the signal components to be used by the replica generation unit  27  for reproducing the front signal is equal to or larger than the number of the signal components to be used by the replica generation unit  27  for reproducing the rear signal. The number of the signal components to be used by the replica generation unit  27  for reproducing the front signal is increased as the equalization processing, the cancellation of interference components, and the process of extracting detection target data are repeated. The data extraction unit  25  regards the extracted detection target data as a final detection result when, as a result of expansion of a replica generation range, the number of the signal components included in the replica generation range has reached a predetermined number such as the maximum number of the signal components obtained by decomposition of a first interference component. Then, the detection result is input from the LLR calculation unit  26  to the deinterleaver  30 . 
     The deinterleaver  30  interleaves, in blocks, the LLR series that is latest at the time point of input of the detection result and restores a permutation, and inputs a signal to the error correction decoding unit  31 . The error correction decoding unit  31  performs error correction decoding of the input signal to obtain a bit sequence. 
     Next, the hardware configuration of the wireless communication apparatus  1  will be described. Each constituent element of the wireless communication apparatus  1  can be implemented by hardware. The antenna  11  is an antenna for wireless communication. The other constituent elements are implemented by, for example, processing circuits. A plurality of constituent elements may be implemented by a single processing circuit. Alternatively, a single constituent element may be implemented by a plurality of processing circuits. 
     In addition, the processing circuit may be dedicated hardware. Alternatively, the processing circuit may be a control circuit including a memory and a central processing unit (CPU) (also referred to as a central processor, processing unit, arithmetic unit, microprocessor, microcomputer, processor, or digital signal processor (DSP)) that executes a program stored in the memory. Here, examples of the memory include a nonvolatile or volatile semiconductor memory such as a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable programmable read only memory (EPROM), or an electrically erasable programmable read only memory (EEPROM), a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, and a digital versatile disk (DVD). 
     In the case where the processing circuit is implemented by dedicated hardware, the processing circuit is exemplified by a processing circuit  71  illustrated in  FIG. 5 .  FIG. 5  is a diagram illustrating an example of implementing the function of the wireless communication apparatus  1  illustrated in  FIG. 1  by use of a processing circuit that is dedicated hardware. Examples of the processing circuit  71  include a single circuit, a composite circuit, a programmed processor, a parallel-programmed processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), and a combination thereof. 
     In the case where the processing circuit is implemented by a control circuit including a CPU, the control circuit is exemplified by a control circuit with a configuration illustrated in  FIG. 6 .  FIG. 6  is a diagram illustrating an example of implementing the function of the wireless communication apparatus  1  illustrated in  FIG. 1  by use of a processing circuit that is a control circuit. As illustrated in  FIG. 6 , a control circuit  74  includes a memory  72  and a processor  73 . The memory  72  is a CPU. In the case where the processing circuit is implemented by the control circuit  74 , the processing circuit is implemented by the processor  73  reading and executing a program corresponding to each process of each constituent element, stored in the memory  72 . In addition, the memory  72  is also used as a temporary memory in each process performed by the processor  73 . 
     A part of each constituent element included in the wireless communication apparatus  1  may be implemented by dedicated hardware, and another part may be implemented by a control circuit including a CPU. 
     Next, a receiving operation of the wireless communication apparatus  1  will be described.  FIG. 7  is a flowchart for describing the receiving operation of the wireless communication apparatus  1  illustrated in  FIG. 1 . Here, in repetition of equalization processing and interference cancellation, each repetition unit is referred to as a stage. At the start of the receiving operation, values of indices are assumed as follows: a repetition stage index i=0, a first target level index m f =0, a second target level index m b =−1, and a detection target block index n b =0, where the first target level index is the maximum value of the number of levels to be reproduced in the front IBI, and the second target level index is the maximum value of the number of levels to be reproduced in the rear IBI. 
     The first FFT unit  18  performs FFT processing on the part of the received signal corresponding to the processing range  153  to generate a signal in the frequency domain (step S 101 ). Subsequently, the equalization weight calculation unit  21  generates equalization weights based on a transmission path estimation value estimated by use of a known signal obtained from the received signal (step S 102 ). The equalization unit  22  performs equalization processing on the received signal in the frequency domain by using the generated equalization weights (step S 103 ). Subsequently, the interference cancellation unit  23  performs a process of canceling the interference components included in the received signal by using the replicas (step S 104 ). However, because there is no generated replica at a first repetition, interference cancellation is omitted. 
     The IFFT unit  24  performs IFFT processing for converting the received signal in the frequency domain into a signal in the time domain (step S 105 ). The data extraction unit  25  extracts detection target data of a current block (step S 106 ). The LLR calculation unit  26  calculates an LLR by using the extracted detection target data (step S 107 ). 
     The first generation unit  27   a  of the replica generation unit  27  generates a replica of the front IBI for each of 0-th to m f -th levels for the next block, by using the calculated LLR (step S 108 ). In the first repetition, because m f =0, a replica of the front IBI for the 0-th level is generated. The second generation unit  27   b  of the replica generation unit  27  generates a replica of the rear IBI for each of 0-th to m b -th levels for the next block, by using the calculated LLR (step S 109 ). In the first repetition, there is no corresponding LLR obtained. Therefore, no replica of the rear IBI is generated. The second FFT unit  29  converts the obtained replicas into signals in the frequency domain, and inputs the signals to the interference cancellation unit  23  (step S 110 ). 
     The equalization unit  22  determines whether the detection target block index n b  has reached the number N b −1 of blocks in one frame (step S 111 ). If the detection target block index n b  has not reached the number N b −1 of blocks in one frame (step S 111 : No), the block index n b  is incremented and updated (step S 112 ), and the process returns to step S 101  to perform processing on the next block. If the detection target block index n b  has reached the number N b  of blocks in one frame (step S 111 : Yes), that is, if processing for one frame has been completed, the equalization unit  22  determines whether the repetition stage index i has reached the maximum number i max −1 of repetition stages (step S 113 ). If the repetition stage index i has not reached the maximum number i max −1 of repetition stages (step S 113 : No), the repetition stage index i is incremented and updated, and n b  is set such that n b =0 so as to return to the process of a 0-th detection target block. Furthermore, the second target level index m b  is made equal to the first target level index m f  so as to equally expand the target range of reproduction of the rear IBI and the target range of reproduction of the front IBI (step S 114 ). As a result, a replica of the rear IBI is generated in the subsequent process. If the repetition stage index i has reached the maximum number i max −1 of repetition stages (step S 113 : Yes), the equalization unit  22  determines whether the first target level index m f  has reached M max −1 (step S 115 ). If the first target level index m f  has not reached M max −1 (step S 115 : No), the equalization unit  22  increments and updates the first target level index m f  (step S 116 ), and returns to the process of step S 101 . If the first target level index m f  has reached M max −1 (step S 115 : Yes), the repetitive processing ends. 
       FIG. 8  is a diagram illustrating a first example of a replica to be used in an interference cancellation process to be performed by the wireless communication apparatus  1  illustrated in  FIG. 1 .  FIG. 8  illustrates an example of using 16QAM as a modulation scheme, where M max =2. When performing cancellation of interference on the n b -th detection block for the first time (in the case of m f =0 and m b =−1), the interference cancellation unit  23  performs cancellation of interference on the front IBI  104  corresponding to the first interference component by using a 0-th level replica  107 . Meanwhile, the interference cancellation unit  23  does not perform cancellation of interference on the rear IBI  105  and an ISI  106  corresponding to the second interference component because no replica has been generated for the rear IBI  105  and the ISI  106 . 
       FIG. 9  is a diagram illustrating a second example of the replica to be used in the interference cancellation process to be performed by the wireless communication apparatus  1  illustrated in  FIG. 1 . When performing cancellation of interference on the n b -th detection block for the second time (in the case of m f =0 and m b =0), the interference cancellation unit  23  performs cancellation of interference on the front IBI  104 , the rear IBI  105 , and the ISI  106  by using the 0-th level replica  107 . 
       FIG. 10  is a diagram illustrating a third example of the replica to be used in the interference cancellation process to be performed by the wireless communication apparatus  1  illustrated in  FIG. 1 . When performing cancellation of interference on the n b -th detection block for the third time (in the case of m f =1 and m b =0), the interference cancellation unit  23  performs cancellation of interference on the front IBI  104  by using the 0-th level replica  107  and a first level replica  108 , and performs cancellation of interference on the rear IBI  105  and the ISI  106  by using the 0-th level replica  107 . 
       FIG. 11  is a diagram illustrating a fourth example of the replica to be used in the interference cancellation process to be performed by the wireless communication apparatus  1  illustrated in  FIG. 1 . When performing cancellation of interference on the n b -th detection block for the fourth time (in the case of m f =1 and m b =1), the interference cancellation unit  23  performs cancellation of interference on the front IBI  104 , the rear IBI  105 , and the ISI  106  by using the 0-th level replica  107  and the first level replica  108 . 
     The first target level index m f  indicates a first range that is the range of signal components to be used by the replica generation unit  27  for reproducing the front IBI that is the first interference component. The second target level index m b  indicates a second range that is the range of signal components to be used by the replica generation unit  27  for reproducing the rear IBI that is the second interference component. The above-described operation enables the first target level index m f  to be maintained at a value equal to or larger than the second target level index m b , so that it is possible to set the first range such that the first range is equal to or larger than the second range. 
     Furthermore, accuracy in generating a replica differs between sub-symbols of a plurality of levels included in one symbol. However, in the case of generating a replica in units of one symbol, replicas for all the levels are simultaneously generated. Thus, accuracy in generating a replica decreases. Meanwhile, in the case where a replica is generated for each of the sub-symbols of the plurality of levels as signal components obtained by decomposition of an interference component as described above, generation of a replica starts from a sub-symbol of a level that is high in accuracy in replica generation, and the range of levels for which replicas are generated is expanded along with the repetitive processing. It is thus possible to increase accuracy in replica generation. As a result, the process of canceling interference is performed by use of replicas with high accuracy, so that an effect of reducing interference components is enhanced. It is thus possible to enhance reception performance. 
     Furthermore, according to the above operation, a replica of the front IBI  104  is generated by sequential use of an equalization result for an immediately preceding block at a current repetition stage. The equalization result of the immediately preceding block at the current repetition stage is higher in reliability than an equalization result for the same block in the previous repetition stage. Therefore, it is possible to generate a replica with higher accuracy. In addition, a replica of the rear IBI  105  is generated by use of an equalization result of the previous repetition stage. At this time, in the case where the range of levels to be used for generating the replicas of the rear IBI  105  is expanded simultaneously with the range of levels to be used for generating the replicas of the front IBI  104 , accuracy of signal detection in the previous repetition stage is lower than that in the current repetition stage. Thus, an effect of repetitive processing is limited. Therefore, in the first embodiment, the range of levels to be used for generating the replicas of the front IBI  104  is first expanded, and the range of levels to be used for generating the replicas of the rear IBI  105  is maintained as before. Then, the range of levels to be used for generating the replicas of the rear IBI  105  is expanded in the subsequent repetitive processing. This configuration enables an effect of repetitive processing to be enhanced, and also enables interference components included in the received signal to be further reduced. 
     It should be noted that the operation illustrated in  FIG. 7  is an example, and the order of processing can be changed within the scope of the technical idea of the present invention. For example, in the above embodiment, equalization processing is performed for each frame, and a replica is updated for each block. However, the present invention is not limited to such an example, and each loop process illustrated in  FIG. 7  can be changed also in the order of execution and conditions for performing the loop process. For example, in the above embodiment, interference cancellation is finally performed by use of all replicas of signal components of a plurality of levels obtained by decomposition of an interference component. However, the replicas of all the levels are not necessarily required to be generated. Therefore, “M max −1” in step S 115  may be a value smaller than “M max −1”. 
       FIG. 12  is a diagram illustrating a wireless communication apparatus  2  according to a variation of the first embodiment of the present invention. The wireless communication apparatus  2  illustrated in  FIG. 12  performs OFDM transmission using no CP. 
     The wireless communication apparatus  2  includes the antenna  11 , the analog front end  12 , the ADC  13 , the band limiting unit  14 , the decimation unit  15 , the timing detection unit  16 , and the frequency deviation correction unit  17 . The wireless communication apparatus  2  further includes an interference cancellation unit  32 , a first FFT unit  33 , the data separation unit  19 , a transmission path estimation unit  35 , the equalization weight calculation unit  21 , an equalization unit  36 , a first IFFT unit  37 , a data extraction unit  38 , a second FFT unit  39 , and an LLR calculation unit  40 . The wireless communication apparatus  2  further includes a replica generation unit  41 , a replica control unit  42 , a second IFFT unit  43 , a third IFFT unit  44 , a deinterleaver  45 , and an error correction decoding unit  46 . The replica generation unit  41  includes the first generation unit  27   a , the second generation unit  27   b , and the known signal addition unit  27   c . In  FIG. 12 , constituent elements similar to the constituent elements illustrated in  FIG. 1  are denoted by the same reference signs, and further description will be omitted. Hereinafter, differences from the wireless communication apparatus  1  illustrated in  FIG. 1  will be mainly described. 
     The first FFT unit  33  converts a signal in a range wider than the detection target block into a signal in the frequency domain so as to perform a process of overlap frequency domain equalization. The transmission path estimation unit  35  performs transmission path estimation, and inputs a transmission path estimation value to the equalization weight calculation unit  21  and the third IFFT unit  44 . The equalization unit  36  performs equalization processing based on calculated equalization weights. The first IFFT unit  37  performs IFFT processing on a received signal on which equalization processing has been performed, to convert the received signal into a received signal in the time domain. The data extraction unit  38  extracts detection target data from the received signal in the time domain. The second FFT unit  39  performs FFT processing with the number of points corresponding to one OFDM symbol to convert the received signal into a signal in the frequency domain again, and inputs the signal to the LLR calculation unit  40 . The LLR calculation unit  40  calculates an LLR for each subcarrier from the input signal. 
     Under the control of the replica control unit  42 , the replica generation unit  41  generates a replica of a specified level for each interference component. The replica generation unit  41  inputs the generated replicas to the equalization weight calculation unit  21  and the second IFFT unit  43 . The second IFFT unit  43  performs IFFT processing on the input replicas to convert the replicas into signals in the time domain, and inputs the signals obtained by conversion to the interference cancellation unit  32 . The third IFFT unit  44  performs IFFT processing on the transmission path estimation value input from the transmission path estimation unit  35  to convert the value into a signal in the time domain, and inputs the signal after conversion to the interference cancellation unit  32 . The interference cancellation unit  32  cancels the interference components of the received signal by using the received signals in the time domain and the replicas. At this time, the interference cancellation unit  32  convolves the replicas of the OFDM time waveform, and subtracts the convolved replicas from the received signal, by using an impulse response obtained from the transmission path estimation value obtained in the frequency domain. As a result, the interference cancellation unit  32  can cancel the interference components of the received signal. 
     As described above, according to the wireless communication apparatus  2  illustrated in  FIG. 12 , it is possible to achieve an effect similar to that of the wireless communication apparatus  1  illustrated in  FIG. 1  also in the OFDM transmission using no CP. 
       FIG. 13  is a diagram illustrating a first variation of the definitions of the interference components to be canceled from the received signal, by the wireless communication apparatus  1  illustrated in  FIG. 1 .  FIG. 14  is a diagram illustrating a second variation of the definitions of the interference components to be canceled from the received signal, by the wireless communication apparatus  1  illustrated in  FIG. 1 . The front IBI  104  and the rear IBI  105  are defined in  FIG. 4  on the assumption that a signal component of the detection target range is regarded as a signal component to be extracted. However, the present invention is not limited to such an example. Other examples are also possible as long as interference cancellation is performed in conjunction with overlap frequency domain equalization based on a predetermined criterion such as the least-squares-error criterion, on the assumption that an arbitrarily defined signal component is regarded as a signal component to be extracted. In the example of  FIG. 13 , the front IBI  104 , the rear IBI  105 , and the ISI  106  are defined on the assumption that data other than the last two symbols in a processing area are regarded as a signal component to be extracted. Furthermore, in the example of  FIG. 14 , the front IBI  104  and a front ISI  106   a  are defined as front interference components, and a center ISI  106   b , a rear ISI  106   c , and the rear IBI  105  are defined as rear interference components. 
     It should be noted that it is also possible to apply the technique of the present embodiment described above to the case where a delay amount in a delay wave exceeds a CP length in block transmission, by performing similar processing and extracting a data portion. In addition, the technique of the present embodiment can be applied not only to single carrier block transmission but also to single carrier transmission in which data are not blocked in transmission. Furthermore, the technique of the present embodiment can also be applied to a multi-antenna communication apparatus using a plurality of transmitting and receiving antennas, in consideration of inter-antenna interference. 
     Second Embodiment 
     Described below in a second embodiment is a wireless communication apparatus that performs a process of receiving a signal transmitted by use of two or more antennas, such as in the cases of multiuser transmission, spatial multiplexing transmission, and transmission diversity. Each of the cases of multiuser transmission and spatial multiplexing transmission is described below. Although description of the case of transmission diversity is omitted, a configuration for transmission diversity can be implemented by a similar function. 
       FIG. 15  is a diagram illustrating a configuration for multiple users to transmit signals in a wireless communication system according to the second embodiment of the present invention. 
       FIG. 15  illustrates wireless transmission apparatuses  300 # 0  to  300 # N t −1 of N t  users # 0  to # N t −1. When it is not necessary to distinguish between the wireless transmission apparatuses  300 # 0  to  300 # N t −1, the apparatuses are each simply referred to as the wireless transmission apparatus  300 . Each of the plurality of wireless transmission apparatuses  300  includes an error correction coding unit  201 , an interleaver  202 , a mapping unit  203 , a known signal generation unit  204 , a multiplexing unit  205 , an interpolation unit  206 , a band limiting unit  207 , a digital-to-analog converter (DAC)  208 , an analog front end  209 , and a transmission antenna  210 . In the example of  FIG. 15 , each wireless transmission apparatus  300  includes the single transmission antenna  210 . However, the number of the transmission antennas is not limited. 
     The error correction coding unit  201  performs error correction coding on a bit sequence of input transmission information, and inputs the coded bit sequence, on which error correction coding has been performed, to the interleaver  202 . The interleaver  202  performs interleaving processing on a block basis to change the order of the coded bit sequence. The interleaver  202  inputs the coded bit sequence which has been obtained to the mapping unit  203 . The mapping unit  203  performs mapping based on a multiple-value number by using the input coded bit sequence, and generates a symbol sequence by using a modulation scheme such as phase shift keying (PSK) or quadrature amplitude modulation (QAM). The known signal generation unit  204  generates a known signal sequence. The multiplexing unit  205  multiplexes the symbol sequence and the known signal sequence to generate a transmission signal, and inputs the generated transmission signal to the interpolation unit  206 . 
     The interpolation unit  206  performs an interpolation process for increasing the sampling frequency of the input transmission signal, and inputs the processed transmission signal to the band limiting unit  207 . The band limiting unit  207  performs a band limiting process on the input transmission signal, and inputs the processed transmission signal to the DAC  208 . The DAC  208  converts the input transmission signal from a digital signal to an analog signal, and inputs the converted transmission signal to the analog front end  209 . The analog front end  209  upconverts and amplifies the transmission signal, and transmits the transmission signal from the transmission antenna  210 . 
       FIG. 16  is a diagram illustrating a configuration for transmitting signals by spatial multiplexing in the wireless communication system according to the second embodiment of the present invention. Hereinafter, constituent elements similar to those of the wireless transmission apparatus  300  illustrated in  FIG. 15  are denoted by the same reference signs, and further description will be omitted. Thus, differences from the wireless transmission apparatus  300  will be mainly described. 
     A wireless transmission apparatus  310  illustrated in  FIG. 16  includes the error correction coding unit  201 , the interleaver  202 , the mapping unit  203 , a serial parallel (SP) conversion unit  211 , and a plurality of transmission processing units  301 # 0  to  301 # N t −1. The wireless transmission apparatus  310  includes N t  transmission antennas  210 # 0  to  210 # N t −1. In the wireless transmission apparatus  310 , the transmission processing units  301 # 0  to  301 # N t −1 are provided so as to correspond to the transmission antennas  210 # 0  to  210 # N t −1, respectively. Hereinafter, when it is not necessary to distinguish between the transmission processing units  301 # 0  to  301 # N t −1, the units are each simply referred to as the transmission processing unit  301 . In addition, when it is not necessary to distinguish between the transmission antennas  210 # 0  to  210 # N t −1, the antennas are each simply referred to as the transmission antenna  210 . 
     Each of the plurality of transmission processing units  301  includes the known signal generation unit  204 , the multiplexing unit  205 , the interpolation unit  206 , the band limiting unit  207 , the DAC  208 , the analog front end  209 , and the transmission antenna  210 . 
     The SP conversion unit  211  performs serial parallel conversion for converting an input serial signal into parallel signals the number of which is equal to the number N t  of the transmission antennas. The SP conversion unit  211  inputs one of the generated parallel signals to each of the transmission processing units  301 . With this configuration, a plurality of transmission signals is transmitted from the plurality of transmission antennas  210 # 0  to  210 # N t −1. The transmission signals are transmitted to receiving-side wireless communication apparatuses via a multipath propagation channel. 
       FIG. 17  is a diagram illustrating the configuration of a wireless communication apparatus  3  according to the second embodiment of the present invention. The wireless communication apparatus  3  includes N r  receiving antennas  11 # 0  to  11 # N r −1 and a plurality of reception processing units  50 # 0  to  50 # N r −1. Each reception processing unit  50  includes the analog front end  12 , the ADC  13 , the band limiting unit  14 , the decimation unit  15 , the timing detection unit  16 , and the frequency deviation correction unit  17 . The wireless communication apparatus  3  further includes a first FFT unit  51 , a data separation unit  52 , a transmission path estimation unit  53 , an equalization weight calculation unit  54 , an equalization unit  55 , an interference cancellation unit  56 , an IFFT unit  57 , a data extraction unit  58 , and an LLR calculation unit  59 . The wireless communication apparatus  3  further includes the deinterleaver  30 , the error correction decoding unit  31 , a measurement unit  60 , an ordering unit  61 , a replica control unit  62 , a replica generation unit  63 , and the second FFT unit  29 . 
     The first FFT unit  51  performs FFT processing on received signals input from the plurality of reception processing units  50  to convert the received signals into signals in the frequency domain, and inputs the received signals subjected to conversion to the data separation unit  52 . The data separation unit  52  separates the received signals into data and known signals. Then, the data separation unit  52  inputs the received signals including the data to the equalization unit  55 , and inputs the known signals to the transmission path estimation unit  53 . The transmission path estimation unit  53  calculates transmission path estimation values, and inputs the calculated transmission path estimation values to the equalization weight calculation unit  54 . The equalization weight calculation unit  54  calculates equalization weights based on the input transmission path estimation values, and inputs the calculated equalization weights to the equalization unit  55 . The equalization unit  55  performs equalization processing on the received signals by using the input equalization weights, and inputs the received signals on which equalization processing has been performed to the interference cancellation unit  56 . The interference cancellation unit  56  uses replicas input from the second FFT unit  29  to cancel interference components included in the received signals. The interference cancellation unit  56  does not perform the process of canceling the interference components until an ordering process to be described below is completed. The interference cancellation unit  56  inputs the received signals which have been processed to the IFFT unit  57 . In the case where the process has not been performed, the interference cancellation unit  56  inputs the input received signals with no change to the IFFT unit  57 . The IFFT unit  57  performs IFFT processing for converting the input received signals into signals in the time domain, and inputs the received signals which have been processed to the data extraction unit  58 . 
     The data extraction unit  58  extracts detection target data from the input received signals, and inputs the detection target data to the LLR calculation unit  59 . The data extraction unit  58  also extracts known signals from the received signals, and inputs the known signals to the measurement unit  60 . The LLR calculation unit  59  calculates LLRs by using the input received signals, and outputs the LLRs to the deinterleaver  30  and the replica generation unit  63 . The measurement unit  60  measures error powers indicating the magnitudes of interference components. The measurement unit  60  measures the magnitudes of interference components included in the received signals on which equalization processing has been performed or equalization processing and cancellation of interference components have been performed. The measurement unit  60  can calculate the error powers by using, for example, the known signals. The measurement unit  60  inputs the calculated error powers to the ordering unit  61 . The ordering unit  61  determines the order of processing the received signals based on the input error powers which are values indicating the magnitudes of the interference components. The replica control unit  62  controls the order in which the replica generation unit  63  processes the received signals, based on the order determined by the ordering unit  61 . The replica control unit  62  instructs the replica generation unit  63  to generate a replica, by using at least one of a range for reproducing the interference component, a block number of a processing target in the received signal, an identifier of the transmission antenna of a transmission source of the received signal, and an identifier of a transmission source user of the received signal. The replica control unit  62  causes the replica generation unit  63  to generate a replica by specifying the range for reproducing the interference component and a target transmission antenna number. 
     In accordance with the instruction from the replica control unit  62 , the replica generation unit  63  generates a replica by using a signal transmitted from a transmission antenna indicated by the specified transmission antenna number. The replica generation unit  63  includes N t  first generation units  65  and N t  second generation units  66 , corresponding to the N t  transmission antennas, and the known signal addition unit  27   c . Each of the first generation units  65  has a function similar to that of the first generation unit  27   a . Each of the second generation units  66  has a function similar to that of the second generation unit  27   b . The replica generation unit  63  inputs the generated replica to the second FFT unit  29  and the equalization weight calculation unit  54 . 
       FIG. 18  is a flowchart illustrating the overall operation of the wireless communication apparatus  3  illustrated in  FIG. 17 . The wireless communication apparatus  3  measures a residual error power while attempting overlap FDE (step S 201 ). Subsequently, the wireless communication apparatus  3  performs a transmission antenna ordering process (step S 202 ). For example, in the case where an index value decreases as accuracy of signal detection increases, the wireless communication apparatus  3  can determine the order of process execution such that a received signal with a smaller index value is processed earlier. After determining the processing order of the received signals, the wireless communication apparatus  3  performs repetitive equalization processing and interference cancellation processing (step S 203 ). 
       FIG. 19  is a flowchart illustrating a detailed operation of step S 201  in  FIG. 18 . The measurement unit  60  selects one of the transmission antennas  210  as a processing target (step S 204 ). Then, the first FFT unit  51  performs overlap FFT processing to obtain a signal in the frequency domain (step S 205 ). The equalization weight calculation unit  54  generates equalization weights based on a transmission path estimation value (step S 206 ). The equalization unit  55  performs equalization processing (step S 207 ). The signal on which the equalization processing has been performed is subjected to overlap IFFT processing performed by the IFFT unit  57  (step S 208 ). The data extraction unit  58  extracts a known signal, and inputs the known signal to the measurement unit  60  (step S 209 ). The measurement unit  60  calculates a difference between the input equalized known signal and a known signal known in advance, and accumulates the difference to calculate an error power (step S 210 ). The measurement unit  60  determines whether measurement for all the transmission antennas has been completed (step S 211 ). If measurement for all the transmission antennas has been completed (step S 211 : Yes), the measurement unit  60  terminates measurement of the residual error power. If measurement for all the transmission antennas has not been completed (step S 211 : No), the process returns to step S 204 . 
       FIG. 20  is a flowchart illustrating a detailed operation of step S 203  in  FIG. 18 . It should be noted that steps similar to those in the operation illustrated in  FIG. 7  are denoted by the same reference signs in  FIG. 20 , and further description will be omitted. Hereinafter, differences from the operation illustrated in  FIG. 7  will be mainly described. 
     A transmission antenna index n t  is an index of the transmission antenna  210  as a processing target, and takes a value from 0 to N t −1. When generating a replica of the front IBI  104  for the next block, the first generation unit  65  sets, as a processing target, a received signal received by a target transmission antenna in addition to a specified level (step S 250 ). When generating replicas of the ISI  106  and the rear IBI  105  for the next block, the second generation unit  66  sets, as processing targets, received signals received by target transmission antennas  0  to n t  in addition to a target level (step S 251 ). 
     If the detection target block index n b  has reached the number N b  of blocks in one frame (step S 111 : Yes), that is, if the processing for one frame has been completed, the equalization unit  55  determines whether the transmission antenna index n t  has reached the number N t −1 of antennas (step S 252 ). If the transmission antenna index n t  has reached the number N t −1 of antennas (step S 252 : Yes), the equalization unit  55  proceeds to the process of step S 113 . If the transmission antenna index n t  has not reached the number N t −1 of antennas (step S 252 : No), the equalization unit  55  increments n t  (step S 253 ). 
     In addition, if the repetition stage index i has not reached the maximum number i mak −1 of repetition stages (step S 113 : No), the value of n t  is set such that n t =0, in addition to performing the process of step S 114  (step S 254 ). 
     If the first target level index m f  has not reached M max −1 (step S 115 : No), the equalization unit  55  sets the value of n t  such that n t =0, in addition to performing the process of step S 116  (step S 255 ). 
     As described above, according to the second embodiment of the present invention, it is possible to cancel interference components not only for IBI and ISI in a signal transmitted from a target transmission antenna, but also for interference (including IBI) from a signal transmitted from another transmission antenna. In addition, the magnitude of an interference component included in a received signal is measured for each transmission antenna. Thus, the processing order of the received signals is determined based on the measurement results. As a result, a signal which is higher in detection accuracy is processed earlier, and a detection result thereof is used in performing the next detection process. Thus, it is also possible to improve accuracy of the next detection process. 
     It should be noted that the function of the measurement unit  60  described above can also be applied to the case of using one transmission antenna and one or more receiving antennas, and it is also possible to add the function of the measurement unit  60  to the configuration of the first embodiment described above. For example, after preparing two sets in which a demodulation parameter set differs between the first range and the second range and performing equalization processing based on the above-described sets, the measurement unit  60  calculates a value as an indicator of reliability, such as an error power, for each of the two sets. Then, it is also possible to select, as a final detection result, a processing result based on one of the two sets selected on the basis of the value of the indicator. 
     It should be noted that the interference cancellation process is performed in the frequency domain in the second embodiment described above. Meanwhile, the interference cancellation process may be performed in the time domain in the case where equivalent processing can be performed also in the time domain. Furthermore, the technique described in the second embodiment of the present invention can also be applied to OFDM transmission. 
     It should be noted that the hardware configuration of the wireless communication apparatus  3  according to the second embodiment described above is similar to that of the wireless communication apparatus  1  according to the first embodiment, and therefore description thereof is omitted here. The same applies to the wireless transmission apparatus  300  and the wireless transmission apparatus  310 . 
     The configuration illustrated in each of the above embodiments illustrates an example of the subject matter of the present invention, and it is possible to combine the configuration with another technique that is publicly known, and is also possible to make omissions and changes to part of the configuration without departing from the gist of the present invention. 
     For example, in the second embodiment described above, an error power calculated by use of a known signal is used as a value indicating the magnitude of an interference component remaining in a received signal. However, the present invention is not limited to such an example. For example, a value indicating the magnitude of an interference component remaining in a received signal may be calculated by use of, for example, an impulse response obtained by transmission path estimation and a norm value of equalization weights. 
     Furthermore, although the wireless communication system has been described in the above embodiment, the present invention is not limited to such an example. The technique of the present invention can be applied not only to wireless transmission but also to wired transmission. When the technique of the above embodiment is applied to wired transmission, a communication apparatus includes a transmission port instead of the above-described transmission antenna, and a reception port instead of the receiving antenna. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  2 ,  3  wireless communication apparatus;  11  antenna;  12  analog front end;  13  ADC;  14  band limiting unit;  15  decimation unit;  16  timing detection unit;  17  frequency deviation correction unit;  18 ,  33 ,  51  first FFT unit;  19 ,  52  data separation unit;  20 ,  35 ,  53  transmission path estimation unit;  21 ,  54  equalization weight calculation unit;  22 ,  36 ,  55  equalization unit;  23 ,  32 ,  56  interference cancellation unit;  24 ,  57  IFFT unit;  25 ,  38 ,  58  data extraction unit;  26 ,  40 ,  59  LLR calculation unit;  27 ,  41 ,  63  replica generation unit;  27   a ,  65  first generation unit;  27   b ,  66  second generation unit;  27   c  known signal addition unit;  28 ,  42 ,  62  replica control unit;  29 ,  39  second FFT unit;  30 ,  45  deinterleaver;  31 ,  46  error correction decoding unit;  37  first IFFT unit;  43  second IFFT unit;  44  third IFFT unit;  50  reception processing unit;  60  measurement unit;  61  ordering unit;  71  processing circuit;  72  memory;  73  processor;  101  0-th level QPSK sub-symbol;  102  first level QPSK sub-symbol;  103  16QAM symbol;  104  front IBI;  105  rear IBI;  106  ISI;  107  0-th level replica;  108  first level replica;  150  received signal;  151  first range;  152  second range;  153  processing range;  154  LLR;  155  LLR series;  201  error correction coding unit;  202  interleaver;  203  mapping unit;  204  known signal generation unit;  205  multiplexing unit;  206  interpolation unit;  207  band limiting unit;  208  DAC;  209  analog front end;  210  transmission antenna;  211  SP conversion unit;  300 ,  310  wireless transmission apparatus.