Patent Publication Number: US-2007121737-A1

Title: Transmission apparatus and peak suppression method

Description:
TECHNICAL FIELD  
      The present invention relates to a transmission apparatus and a peak suppression method, and more specifically, to a transmission apparatus and a peak suppression method where transmission signals are transmitted by an OFDM scheme.  
     BACKGROUND ART  
      Conventionally, a multicarrier communication apparatus employing an OFDM scheme is resistant to multipath and fading and realizes high quality communication, and therefore multicarrier communication apparatuses employing the OFDM scheme are attracting attention as the apparatus capable of realizing high-speed radio transmission. In OFDM scheme communication, transmission data is converted to parallel data and transmitted by being superimposed on a plurality of subcarriers, and therefore there is no correlation between subcarriers. For this reason, when the phases of subcarriers overlap one another, OFDM symbols have extremely large signal amplitudes. When the peak voltage of a signal increases in transmission due to the overlap of the phases of in this way, an amplifier with a dynamic range including peak electric power is required to amplify the transmission signal, which increases the size of the amplifier and also increases power consumption. Furthermore, an increase in peak power of a signal during transmission requires an amplifier capable of keeping linearity over an extensive area, hence an expensive amplifier.  
      Therefore, a method of suppressing peak power through processing of placing restrictions on the amplitude to reduce the amplitude of the overall transmission signal using a limiter (for example, Patent Document 1) and a method of suppressing peak voltages through processing called “clipping” to suppress only peaks are conventionally known.  
      When suppressing such peaks, a transmission apparatus that includes peak-suppressed information in data and transmits the information is known. A reception apparatus which receives data transmitted from such a transmission apparatus reconstructs the suppressed peaks using the peak-suppressed information, and can thereby decode the data without errors.  
      On the other hand, OFDM scheme communication employs a system in which a base station apparatus is reported reception quality at a communication terminal apparatus from the communication terminal apparatus on a per subcarrier basis, assigns a number of multiple subcarriers which are appropriate for each user based on the reported reception quality (frequency division user multiplexing) and selects MCS (Modulation and Coding Schemes) for each subcarrier. That is, the base station apparatus assigns each communication terminal a subcarrier having highest frequency utilization efficiency that can satisfy desired communication quality (e.g., a minimum transmission rate, error rate, etc.), selects high-speed MCS for each subcarrier, carries out data transmission and thereby realizes high-speed data communication for multi-users. 
      Patent Document 1: Unexamined Japanese Patent Publication No. HEI9-18451    

     DISCLOSURE OF INVENTION  
      Problems to be Solved by the Invention  
      However, the conventional transmission apparatus and peak suppression method include information about peak suppression in transmission data without considering the MCS of each subcarrier, and this results in a problem that, when high-MCS carrier components are suppressed, overall throughput of the system deteriorates considerably.  
      It is therefore an object of the present invention to improve overall throughput of the system through peak suppression using some frequencies in a communication band.  
      Means for Solving the Problem  
      The transmission apparatus of the present invention transmits a frequency division multiplexed transmission signal based on reception quality information indicating reception quality of a communicating party, and this transmission apparatus employs a configuration having: a determining section that determines a modulation and coding scheme parameter per frequency; a detection section that detects a peak of a transmission signal; a generation section that generates a waveform with an inverse characteristic of a waveform of the peak; a combination section that combines the waveform of the transmission signal and the waveform with the inverse characteristic at a frequency corresponding to the modulation and coding scheme parameter of a lowest transmission efficiency among the modulation and coding scheme parameters determined for respective frequencies; and a transmission section that transmits the transmission signal combined with the waveform with the inverse characteristic.  
      The peak suppression method of the present invention suppresses a peak in a frequency division multiplexed transmission signal based on reception quality information indicating reception quality of a communicating party, and this peak suppression method includes the steps of: determining a modulation and coding scheme parameter per frequency; detecting a peak of a transmission signal; generating a waveform with an inverse characteristic of a waveform of the peak; and combining the waveform of the transmission signal and the waveform with the inverse characteristic at a frequency corresponding to the modulation and coding scheme parameter of a lowest transmission efficiency among the modulation and coding scheme parameters determined for respective frequencies.  
      Advantageous Effect of the Invention  
      According to the present invention, it is possible to improve overall throughput of a system through peak suppression by suppressing peaks using some frequencies in a communication band. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a block diagram showing the configuration of a radio communication apparatus according to Embodiment 1 of the present invention;  
       FIG. 2  illustrates an MCS table according to Embodiment 1 of the present invention;  
       FIG. 3  is a flow chart showing the operation of the radio communication apparatus according to Embodiment 1 of the present invention;  
       FIG. 4  is a diagram illustrating a relationship between time and PAPR in the waveform of a transmission signal according to Embodiment 1 of the present invention;  
       FIG. 5  is a diagram illustrating a relationship between the time and amplitude in the waveform of a transmission signal according to Embodiment 1 of the present invention;  
       FIG. 6  is a diagram illustrating a relationship between the time and amplitude of a replica according to Embodiment 1 of the present invention;  
       FIG. 7  is a diagram illustrating a relationship between the time and amplitude of an inverse replica according to Embodiment 1 of the present invention;  
       FIG. 8  is a diagram illustrating subcarriers according to Embodiment 1 of the present invention;  
       FIG. 9  is a diagram illustrating a waveform after FFT of the inverse replica according to Embodiment 1 of the present invention;  
       FIG. 10  is a diagram illustrating a histogram of PAPR of a transmission signal according to Embodiment 1 of the present invention;  
       FIG. 11  is a diagram illustrating a relationship between Eb/N o  and BER of a transmission signal according to Embodiment 1 of the present invention;  
       FIG. 12  is a flow chart showing the operation of a radio communication apparatus according to Embodiment 2 of the present invention;  
       FIG. 13  is a diagram illustrating subcarriers according to Embodiment 2 of the present invention;  
       FIG. 14  is a flow chart showing the operation of a radio communication apparatus according to Embodiment 3 of the present invention;  
       FIG. 15  is a flow chart showing the operation of the radio communication apparatus according to Embodiment 3 of the present invention;  
       FIG. 16  is a block diagram showing the configuration of a radio communication apparatus according to Embodiment 4 of the present invention;  
       FIG. 17  is a block diagram showing the configuration of a radio communication apparatus according to Embodiment 5 of the present invention; and  
       FIG. 18  is a flow chart showing the operation of the radio communication apparatus according to Embodiment 5 of the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
     Embodiment 1  
       FIG. 1  is a block diagram showing the configuration of radio communication apparatus  100  according to Embodiment 1 of the present invention.  
      Coding section  101  codes transmission data at a coding rate from coding rate information input from transmission parameter determining section  123  and outputs the coded transmission data to modulation section  102 .  
      Modulation section  102  modulates transmission data input from coding section  101  according to a modulation scheme based on modulation scheme information input from transmission parameter determining section  123  and outputs the modulated transmission data to combination section  103 .  
      Based on inverse replica information, which is information about the waveform with an inverse characteristic of the waveform equal to or higher than a threshold input from FFT section  116  (hereinafter “inverse replica”), combination section  103  combines the waveform of the transmission data input from modulation section  102  and the inverse replica on the frequency axis, and outputs the combined signal to serial/parallel (hereinafter “S/P”) conversion section  104 .  
      S/P conversion section  104  converts the transmission data input from combination section  103  from a serial data format to a parallel data format and outputs the parallel data to inverse Fourier transform (hereinafter “IFFT”) section  105 .  
      IFFT section  105  is an inverse orthogonal transform section, performs IFFT on the transmission data input from S/P conversion section  104 , and outputs the transformed data to guard interval (hereinafter “GI”) insertion section  106  and peak-to-average power ratio (hereinafter “PAPR”) calculation section  109 .  
      GI insertion section  106  inserts a GI in the transmission data input from IFFT section  105  and outputs the transmission data to radio transmission processing section  107 .  
      Radio transmission processing section  107  up-converts the transmission data input from GI insertion section  106  from a baseband frequency to a radio frequency and transmits the result from antenna  108 .  
      PAPR calculation section  109  calculates PAPR from the transmission data after IFFT input from IFFT section  105  and outputs the calculation result to peak decision section  111 .  
      Cutoff instruction section  110  outputs PAPR information, which is threshold information to delete the amplitude of the transmission data, to peak decision section  111 .  
      Peak decision section  111  is a peak detection section, compares the PAPR calculation result input from PAPR calculation section  109  with threshold information input from cutoff instruction section  110  and decides whether or not there are peaks indicating PAPR equal to or higher than the threshold. Then, when there are peaks indicating PAPR equal to or higher than the threshold, peak decision section  111  outputs the waveform information of the transmission data including peaks equal to or higher than the threshold, to inverse replica generation section  112 .  
      Inverse replica generation section  112  is a waveform generation section, generates a waveform to cancel the waveform information input—that is, an inverse replica—from the waveform information input from peak decision section  111 , and outputs inverse replica information to sub-band selection section  114 .  
      Sub-band specifying section  113  instructs sub-band selection section  114  to select the frequency band of a sub-band composed of a subcarrier, to which transmission data with the lowest transmission efficiency is assigned, in a communication band from the MCS information, which is information about the MCS (MCS parameter) input from transmission parameter determining section  123 .  
      Sub-band selection section  114  selects a sub-band specified from sub-band specifying section  113 , and outputs only the inverse replica input from inverse replica generation section  112  in the selected sub-band frequency band, to bandpass filter (hereinafter “BPF”)  115 .  
      BPF  115  removes unnecessary band components outside the frequency band of the sub-band specified by sub-band specifying section  113  of the inverse replica from the inverse replica which is a canceling waveform generated by inverse replica generation section  112  from the inverse replica information input from sub-band selection section  114  and outputs the result to fast Fourier transform (hereinafter “FFT”) section  116 .  
      FFT section  116  is an orthogonal transform section, performs FFT on the inverse replica based on the inverse replica information input from sub-band selection section  114  and outputs the inverse replica to combination section  103 .  
      Radio reception processing section  118  down-converts a signal received by antenna  117  from a radio frequency to a baseband frequency and outputs the received signal to GI removing section  119 .  
      GI removing section  119  removes the GI from the received signal input from radio reception processing section  118  and outputs the signal to FFT section  120 .  
      FFT section  120  performs FFT on the received signal input from GI removing section  119  and outputs the result to demodulation section  121 .  
      Demodulation section  121  demodulates the received signal input from FFT section  120  and outputs the result to decoding section  122 .  
      Decoding section  122  decodes the received signal input from demodulation section  121  and outputs the decoded signal to transmission parameter determining section  123  and at the same time obtains received data.  
      Transmission parameter determining section  123  selects MCS which indicates a combination of a modulation scheme and coding rate, using CQI (Channel Quality Indicator) which is reception quality information showing the reception quality of the communication terminal apparatus for each subcarrier and reception power information or the like, from the received data input from decoding section  122 . That is, as shown in  FIG. 2 , transmission parameter determining section  123  includes an MCS table in which MCS, modulation schemes and coding rates are associated, and selects MCS, for each subcarrier, with reference to the MCS table with the CQI and reception power reported from the communication terminal apparatus taken into consideration. Transmission parameter determining section  123  then outputs the selected MCS of each subcarrier to sub-band specifying section  113  as MCS information. Furthermore, transmission parameter determining section  123  outputs the modulation scheme information which indicates the modulation scheme of selected MCS to modulation section  102  and outputs coding rate information which indicates the coding rate of the selected MCS to coding section  101 . In  FIG. 2 , the transmission efficiency of the MCS increases in order from 0 to 7 and MCS 7  indicates the highest transmission efficiency.  
      Next, peak suppression operation by radio communication apparatus  100  will be explained using  FIG. 3  to  FIG. 11 .  FIG. 3  is a flow chart showing peak suppression operation by radio communication apparatus  100 .  
      First, IFFT section  105  performs IFFT on transmission data (step ST 301 ).  
      Next, PAPR calculation section  109  measures PAPR (step ST 302 ).  
      Next, as shown in  FIG. 4 , peak decision section  111  decides whether or not there is a peak whose PAPR is equal to or higher than threshold (α) from the threshold information input from cutoff instruction section  110  for each symbol (step ST 303 ).  
      When there is a peak whose PAPR is equal to or higher than a threshold α, as shown in  FIG. 5 , inverse replica generation section  112  extracts waveform information  501 ,  502 ,  503 ,  504  whose amplitude is equal to or higher than threshold (β) and whose amplitude is equal to or lower than threshold (−β) in the relationship between time and amplitude of the transmission signal and generates replica  601  of waveform information  501 , replica  602  of waveform information  502 , replica  603  of waveform information  503  and replica  604  of waveform information  504  as shown in  FIG. 6  (step ST 304 ).  
      Next, inverse replica generation section  112  generates inverse replica  701  which has the inverse characteristic of replica  601 , inverse replica  702  which has the inverse characteristic of replica  602 , inverse replica  703  which has the inverse characteristic of replica  603  and inverse replica  704  which has the inverse characteristic of replica  604  as shown in FIG. 7  (step ST 305 ).  
      Next, sub-band selection section  114  selects the sub-band specified by sub-band specifying section  113  (step ST 306 ) and BPF  115  outputs only an inverse replica in the frequency band of the sub-band specified by sub-band specifying section  113 . More specifically, when MCS 6  is selected in  FIG. 2  for the transmission data assigned to subcarriers in band  1  (sub-band) and the transmission data is modulated by 16 QAM, and when MCS 3  is selected for the transmission data assigned to subcarriers in band  2  (sub-band) and the transmission data is modulated by QPSK, sub-band selection section  114  selects band  2  whose MCS is low in communication band F 3  as shown in  FIG. 8 .  
      Next, FFT section  116  performs FFT on the inverse replica of selected band  2  (step ST 307 ). By performing FFT on the inverse replica of band  2 , the waveform shown in  FIG. 9  is obtained. The inverse replica of band  1  other than band  2  is not output from sub-band selection section  114 , and therefore the waveform after FFT becomes only the solid line part in  FIG. 9 .  
      Next, combination section  103  combines the transmission signal and the inverse replica (the waveform of the solid line part in  FIG. 9 ) of band  2  subjected to FFT (step ST 308 ). In this way, by combining the inverse replica and transmission data in band  2 , the possibility of errors that may occur in the transmission data assigned to the subcarriers of band  2  increases. However, when the inverse replica and transmission data are combined in band  2 , less deterioration occurs in the error characteristic of the whole transmission data, because the inverse replica and transmission data are not combined in band  1 , as opposed to the case where the inverse replica and transmission data are combined over the whole of communication band F 3 . Furthermore, even when errors occur in the transmission data of band  2 , carrying out processing such as retransmission allows the transmission data of band  2  to be decoded without errors. On the other hand, in step ST 303 , when the PAPR is not higher than threshold (α), the inverse replica is not combined with the transmission signal.  
       FIG. 10  and  FIG. 1  show the result of a simulation.  FIG. 10  shows a histogram of PAPR when peak suppression processing (clipping) is performed all over the conventional bands and  FIG. 11  shows a relationship between the power to noise ratio (Eb/No) per bit and BER when the conventional peak suppression threshold is made variable.  
      In  FIG. 10 , P 1  shows a histogram of PAPR when peak suppression is performed with a threshold set to 4 dB; P 2  shows a histogram of PAPR when peak suppression is performed with a threshold set to 5 dB; P 3  shows a histogram of PAPR when peak suppression is performed with a threshold set to 6 dB; P 4  shows a histogram of PAPR when peak suppression is performed with a threshold set to 7 dB; P 5  shows a histogram of PAPR when peak suppression is performed with a threshold set to 8 dB; P 6  shows a histogram of PAPR when peak suppression is performed with a threshold set to 9 dB; P 7  shows a histogram of PAPR when peak suppression is performed with a threshold set to 10 dB; and P 8  shows a histogram of PAPR when no peak suppression is performed. From  FIG. 10 , it is appreciated that peak suppression eliminates PAPRs greater than thresholds. However, the elimination of peak components causes deterioration in BER, as shown in  FIG. 11 .  
      In  FIG. 11 , C 1  shows the relationship between BER and Eb/N o  where a threshold is set to 4 dB, C 2  shows the relationship between BER and Eb/No where a threshold is set to 5 dB; and C 3  shows the relationship between BER and Eb/No where a threshold is set to 8 dB. From  FIG. 11 , it is appreciated that the error rate becomes smaller when a threshold is set to 5 dB than when a threshold is set to 4 dB and the error rate becomes smaller when a threshold is set to 8 dB than when a threshold is set to 5 dB. From  FIG. 10  and  FIG. 11 , it is appreciated that PAPR can be lowered by reducing the threshold, but BER deteriorates.  
      Thus, according to this embodiment 1, it is possible to assign deterioration factors due to peak suppression to subcarriers of MCS of low transmission efficiency, so that overall throughput of the system can be improved.  
     Embodiment 2  
       FIG. 12  is a flow chart showing operation of a wireless communication apparatus when suppressing peaks. The radio communication apparatus according to this embodiment 2 has the same configuration as that in  FIG. 1 , and therefore explanations thereof will be omitted.  
      Peak suppression operation by the radio communication apparatus will be explained using  FIG. 12  and  FIG. 13 .  
      First, IFFT section  105  performs IFFT on transmission data (step ST 1201 ).  
      Next, PAPR calculation section  109  measures PAPR (step ST 1202 ).  
      Next, as shown in  FIG. 4 , peak decision section  111  decides whether or not there is a peak whose PAPR is equal to or higher than threshold (α) from threshold information input from cutoff instruction section  110  (step ST 1203 ).  
      When there is a peak whose PAPR is equal to or higher than threshold α, sub-band selection section  114  sets K=0 (step ST 1204 ).  
      Next, sub-band selection section  114  selects N (N: natural number, equal to or smaller than the total number of sub-bands in the communication band) sub-bands specified by sub-band specifying section  113  (step ST 1205 ) and outputs only inverse replicas in the frequency band of the selected N sub-bands. For example, in the case where MCS 6  is selected for transmission data assigned to subcarriers of band (sub-band)  1  and the transmission data is modulated by 16 QAM, MCS 3  is selected for transmission data assigned to subcarriers of band (sub-band)  2  and the transmission data is modulated by QPSK, and MCS 3  is selected for transmission data assigned to subcarriers of band  3  (sub-band) and the transmission data is modulated by QPSK, sub-band selection section  114  selects band  2  for which MCS with low transmission efficiency is selected.  
      Next, FFT section  116  performs FFT on an inverse replica in the frequency band of selected band  2  (step ST 1206 ). The waveform shown in  FIG. 9  can be obtained by applying FFT to the inverse replica in band  2 . Inverse replicas outside the frequency band of band  2  are not output from sub-band selection section  114 , and therefore the waveform after FFT is only the solid line part in  FIG. 9 .  
      Next, combination section  103  combines the transmission signal and the inverse replica subjected to FFT (the waveform corresponding to the solid line part in  FIG. 9 ) (step ST 1207 ).  
      Next, peak decision section  111  again decides whether or not there is a peak equal to or higher than threshold α in the transmission data subjected to IFFT after the inverse replica is combined (step ST 1208 ).  
      When there is a peak equal to or higher than threshold α in the transmission data, sub-band selection section  114  newly selects K new sub-bands (step ST 1209 ) More specifically, as shown in  FIG. 13 , sub-band selection section  114  selects band  3  for which MCS with the same transmission efficiency as the MCS of band  2  is selected as the new sub-band. When there is no band for which MCS with the same transmission efficiency as the MCS of band  2  is set, sub-band selection section  115  selects a band for which MCS with the next lowest transmission efficiency to band  2  is selected.  
      Then, the radio communication apparatus repeats the processing of steps ST 1205  to ST 1208  until there are no more peaks equal to or higher than threshold α. That is to say, until there are no peaks equal to higher than threshold α, the radio communication apparatus repeats the processing of steps ST 1205  to ST 1209  until all bands in the communication band are selected (until a maximum of N is reached).  
      In step ST 1208 , when there is no peak equal to or higher than threshold α, the radio communication apparatus concludes peak suppression processing.  
      In step ST 1203 , when there is no peak equal to or higher than threshold α, the radio communication apparatus concludes peak suppression processing.  
      In this way, in addition to the effect of above Embodiment 1, this Embodiment 2 sequentially selects new bands and expands the band in which inverse replicas are combined until there are no more peaks equal to or higher than threshold α, so that error rate characteristic of the transmission data of one band can be prevented from deteriorating.  
     Embodiment 3  
       FIG. 14  and  FIG. 15  are flow charts showing peak suppression operation by a radio communication apparatus. The radio communication apparatus according to this embodiment 3 has the configuration similar to that in  FIG. 1 , and therefore explanations thereof will be omitted.  
      Peak suppression operation by the radio communication apparatus will be explained using  FIG. 14 .  
      First, IFFT section  105  performs IFFT on transmission data (step ST 1401 ).  
      Next, PAPR calculation section  109  measures PAPR (step ST 1402 ).  
      Next, peak decision section  111  decides whether or not there is a peak whose PAPR is equal to or higher than threshold (α) from threshold information input from cutoff instruction section  110  as shown in  FIG. 4  (step ST 1403 ).  
      When PAPR is equal to or higher than threshold α, FFT section  116  performs FFT on an inverse replica (step ST 1404 ).  
      Next, combination section  103  combines the transmission signal and the inverse replica in a predetermined communication band (step ST 1405 ).  
      Next, peak decision section  111  again decides whether or not there is a peak equal to or higher than threshold α in the transmission signal after the combination of the inverse replica and transmission signal (step ST 1406 ).  
      When there is no peak equal to or higher than threshold α, sub-band selection section  114  selects K sub-bands for which the MCS having the highest transmission efficiency is selected (step ST 1407 ). More specifically, as shown in  FIG. 13 , sub-band selection section  114  selects one band  1  for which the MCS having the highest transmission efficiency is selected in the communication band.  
      Next, sub-band selection section  114  removes band  1  from all of bands  1  to  3  in the communication band and selects remaining band  2  and band  3  (step ST 1408 ).  
      Next, sub-band selection section  114  counts one every time performing the sub-band selection processing and decides whether or not the total count has reached a predetermined count (step ST 1409 ).  
      When the total count has not reached the predetermined count, sub-band selection section  114  decides whether or not a peak is detected by peak decision section  111  (step ST 1410 ).  
      When no peak is detected by peak decision section  111 , sub-band selection section  114  again selects K sub-bands for which MCS having the highest transmission efficiency is selected from the remaining sub-bands selected in the communication band (step ST 1407 ). More specifically, sub-band selection section  114  selects K sub-band of band  2  or band  3  for which MCS having the highest transmission efficiency is selected from remaining band  2  and band  3  selected in the communication band. In the case of  FIG. 13 , because MCS with the same transmission efficiency is selected for band  2  and band  3 , either one can be selected. Sub-band selection section  114  selects remaining band  3  or band  2  after removing selected band  2  or band  3  from the sub-bands to be selected (step ST 1408 ) and repeats the processing from step ST 1407  to step ST 1410  until a predetermined count is reached in step ST 1409  or a peak equal to or higher than threshold α is detected in step ST 1410 .  
      When a peak is detected by peak decision section  111  in step ST 1410 , sub-band selection section  114  returns the K sub-bands removed immediately before, as sub-bands to be selected again (step ST 1411 ). More specifically, in  FIG. 14 , when only band  3  is selected and when band  2  is excluded from the selection target immediately before, sub-band selection section  114  returns band  2  as a band to be selected and selects band  2  and band  3 .  
      Next, FFT section  116  performs FFT on the inverse replica generated by inverse replica generation section  112  (step ST 1412 ).  
      Next, combination section  103  combines the transmission signal and the inverse replica subjected to FFT (step ST 1413 ).  
      When there is a peak equal to or higher than threshold α in step ST 1406 , FFT section  116  performs FFT on the inverse replica (step ST 1412 ) and combines the inverse replica and the transmission signal (step ST 1413 ).  
      On the other hand, when the total count reaches a predetermined count in step ST 1409 , sub-band selection section  114  decides that there is no peak equal to or higher than the threshold and concludes the processing without performing peak suppression processing.  
      Furthermore, when there is no peak higher than threshold α in step ST 1403 , sub-band selection section  114  decides that there is no peak higher than the threshold and concludes processing without performing peak suppression processing.  
      In this way, in addition to the effect of above Embodiment 1, when no peak is detected after peak suppression and excessive peak suppression is performed, this embodiment 3 gradually reduces the number of sub-bands to be selected until a peak is detected and combines an inverse replica and a transmission signal when a peak is detected, so that deterioration of error rate characteristic due to excessive peak suppression can be prevented.  
     Embodiment 4  
      FIG. 16  is a block diagram showing the configuration of radio communication apparatus  1600  according to Embodiment 4 of the present invention.  
      As shown in  FIG. 16 , radio communication apparatus  1600  according to this embodiment 4 adds clipping section  1601  to radio communication apparatus  100  according to Embodiment 1 shown in  FIG. 1 . In  FIG. 16 , the same components as those in  FIG. 1  are assigned the same reference numerals and explanations thereof will be omitted.  
      Clipping section  1601  performs clipping processing on transmission data input from IFFT section  105  and outputs the result to GI insertion section  106 . Clipping section  1601  compares a preset threshold with the signal level of the transmission data, and suppresses the signal level to the threshold when the signal level is equal to or higher than the threshold, and outputs the result to GI insertion section  106 . When the signal level is lower than the threshold, transmission data is output as is to GI insertion section  106 .  
      In this way, in addition to the effect of above Embodiment 1, this embodiment 4 combines an inverse replica and transmission data and then performs clipping processing, thereby reliably suppressing peaks.  
     Embodiment 5  
       FIG. 17  is a block diagram showing the configuration of radio communication apparatus  1700  according to Embodiment 5 of the present invention.  
      As shown in  FIG. 17 , radio communication apparatus  1700  according to this embodiment 5 removes FFT section  116  from radio communication apparatus  100  according to of Embodiment 1 shown in  FIG. 1  and has S/P conversion section  1701 , IFFT section  1702  and combination section  1703  instead of combination section  103 , S/P conversion section  104  and IFFT section  105 . The same components in  FIG. 17  as those in  FIG. 1  are assigned the same reference numerals and explanations thereof will be omitted.  
      S/P conversion section  1701  converts transmission data input from modulation section  102  from a serial data format to a parallel data format and outputs the parallel data to IFFT section  1702 .  
      IFFT section  1702  performs IFFT on the transmission data input from S/P conversion section  1701  and outputs the result to combination section  1703 .  
      Combination section  1703  combines the waveform of the transmission data input from IFFT section  1702  and an inverse replica on the time axis based on inverse replica information input from BPF  115  and outputs the combined signal to GI insertion section  106 .  
      Next, peak suppression operation by radio communication apparatus  1700  will be explained using  FIG. 18 .  FIG. 18  is a flow chart showing peak suppression operation of radio communication apparatus  1700  when suppressing peaks.  
      First, IFFT section  1702  performs IFFT on transmission data (step ST 1801 ).  
      Next, PAPR calculation section  109  measures PAPR (step ST 1802 ).  
      Next, as shown in  FIG. 4 , peak decision section  111  decides whether or not there is a peak whose PAPR is equal to or higher than threshold (α) from threshold information input from cutoff instruction section  110  (step ST 1803 ).  
      When there is a peak whose PAPR is equal to or higher than threshold α, as shown in  FIG. 5 , inverse replica generation section  112  extracts waveform information whose amplitude is equal to or higher than threshold (β) and whose amplitude is equal to or lower than threshold (−β) in the relationship between time and amplitude of the transmission signal and generates a replica as shown in  FIG. 6  (step ST 1804 ).  
      Next, inverse replica generation section  112  generates an inverse replica which has the inverse characteristic of the generated replica as shown in  FIG. 7  (step ST 1805 ).  
      Next, sub-band selection section  114  selects the sub-band specified by sub-band specifying section  113  (step ST 1806 ) and BPF  115  outputs the inverse replica in which unnecessary radiation components outside the frequency band of the sub-band specified by sub-band specifying section  113  removed. More specifically, as shown in  FIG. 8 , when MCS 6  is selected for the transmission data assigned to subcarriers of band  1  and the transmission data is modulated by 16 QAM and when MCS 3  is selected for the transmission data assigned to subcarriers of band  2  and the transmission data is modulated by QPSK, sub-band selection section  114  selects band  2  for which MCS having lower transmission efficiency is selected.  
      Next, combination section  1703  combines the transmission signal and the inverse replica subjected to IFFT (step ST 1807 ).  
      In this way, according to this embodiment 5, in addition to the effect of above Embodiment 1, it is not necessary to repeat IFFT processing on the whole transmission data, thereby simplifying peak suppression processing.  
      The radio communication apparatus of above Embodiment 1 to Embodiment 5 can be applied to a base station apparatus and a communication terminal apparatus.  
      Each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip.  
      “LSI” is adopted here but this may also be referred to as “IC”, “system LSI”, “super LSI”, or “ultra LSI” depending on differing extents of integration.  
      Further, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible.  
      Further, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application in biotechnology is also possible.  
      The present application is based on Japanese Patent Application No. 2003-341654, filed on Sep. 30, 2003, the entire content of which is expressly incorporated by reference herein.  
      Industrial Applicability  
      The transmission apparatus and peak suppression method according to the present invention suppresses peaks using some frequencies in a communication band, thereby providing the effect of preventing deterioration of overall error rate characteristics of transmission data, and is useful in peak suppression.