Abstract:
An OFDM signal transmission apparatus is provided, which includes a mapping unit configured to map first signals into N subcarriers and second signals into M subcarrier(s) to form an OFDM signal, wherein N is larger than M. The first signals are each indicating a same bit of retransmission information and the second signals are each indicating a same bit of information other than retransmission information. The OFDM signal transmission apparatus further includes a transmitting unit configured to transmit the formed OFDM signal.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a transmission/reception apparatus used for digital mobile communications, and particularly relates to an OFDM (Orthogonal Frequency Division Multiplexing) transmission/reception apparatus and OFDM transmission/reception method used for OFDM-based mobile communications. 
         [0003]    2. Description of the Related Art 
         [0004]    In an OFDM-based mobile communication, transmission data is converted to a plurality of slower parallel signals, which are superimposed over their respective subcarriers and transmitted. The following is an explanation of a conventional OFDM transmission/reception apparatus with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a block diagram showing a configuration of a transmission system of a conventional OFDM transmission/reception apparatus.  FIG. 2  is a block diagram showing a configuration of a reception system of the conventional OFDM transmission/reception apparatus. For simplification, suppose a plurality of signal lines are also expressed with a single arrow. 
         [0005]    In  FIG. 1 , modulation section  11  modulates transmission data. Inverse Fast Fourier Transform (hereinafter referred to as “IFFT”) section  12  performs IFFT processing on the modulated transmission data. 
         [0006]    Transmission section  13  performs transmission processing on the IFFT-processed transmission data and transmits this data from antenna  14 . In  FIG. 2 , antenna  21  receives a radio signal. 
         [0007]    Reception section  22  performs reception processing on the signal received from antenna  21  (reception signal). Timing control section  23  controls each section according to a symbol synchronization timing acquired from reception section  22 . Fast Fourier Transform (hereinafter referred to as “FFT) section  24  performs FFT processing on the reception signal. Regarding the acquisition of symbol synchronization timing in reception section  22 , a variety of methods are proposed. However, their detailed explanations are omitted here. 
         [0008]    Synchronization detection section  25  performs synchronization detection processing on the FFT-processed reception signal and removes influences of phase rotation and amplitude variation caused by fading, etc. from the reception signal above. 
         [0009]    Thus, the conventional OFDM transmission/reception apparatus achieves large-capacity and high-quality radio communications with excellent multi-path adaptability through data transmission/reception using a plurality of subcarriers. 
         [0010]    However, in the conventional OFDM transmission/reception apparatus, important information used for communication control such as control information and retransmission information (hereinafter, simply referred to as “important information”) is multiplied by a carrier frequency signal together with user data and transmitted through one of subcarriers. 
         [0011]    Therefore, if the reception quality of subcarriers carrying the important information deteriorates in an extreme manner due to influences of fading, etc., errors may occur in bits representing control information and retransmission information, which will prevent appropriate control and retransmission requests, etc., extremely deteriorating the channel quality. 
       BRIEF SUMMARY 
       [0012]    The present invention has been implemented taking account of the points described above and it is an objective of the present invention to provide an OFDM transmission/reception apparatus capable of improving the quality of important information received by the apparatus on the receiving side. 
         [0013]    This objective is achieved by transmitting important information with two subcarriers instead of one subcarrier in the conventional manner. This objective is further achieved by using one of the two subcarriers carrying important information as the subcarrier with a carrier frequency signal (DC signal) of frequency 0 which was conventionally not used. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
           [0015]      FIG. 1  is a block diagram showing a configuration of a transmission system of a conventional OFDM transmission/reception apparatus; 
           [0016]      FIG. 2  is a block diagram showing a configuration of a reception system of the conventional OFDM transmission/reception apparatus; 
           [0017]      FIG. 3A  is a schematic diagram showing a spectrum example in the conventional OFDM system; 
           [0018]      FIG. 3B  is a schematic diagram showing a spectrum example according to Embodiment 1 of the present invention; 
           [0019]      FIG. 4  is a block diagram showing a configuration of a transmission system of an OFDM transmission/reception apparatus according to Embodiment 1 of the present invention; 
           [0020]      FIG. 5  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 1 of the present invention; 
           [0021]      FIG. 6  is a block diagram showing a configuration of a synchronization detection section in the reception system of the OFDM transmission/reception apparatus according to Embodiment 1 of the present invention; 
           [0022]      FIG. 7  is a block diagram showing a configuration of a reception system of an OFDM transmission/reception apparatus according to Embodiment 2 of the present invention; 
           [0023]      FIG. 8  is a block diagram showing a configuration of a reception system of an OFDM transmission/reception apparatus according to Embodiment 3 of the present invention; 
           [0024]      FIG. 9  is a block diagram showing a configuration of a reception system of an OFDM transmission/reception apparatus according to Embodiment 4 of the present invention; 
           [0025]      FIG. 10  is a block diagram showing a configuration of a synchronization detection section in the reception system of the OFDM transmission/reception apparatus according to Embodiment 4 of the present invention; 
           [0026]      FIG. 11  is a block diagram showing a configuration of a reception system of an OFDM transmission/reception apparatus according to Embodiment 5 of the present invention; 
           [0027]      FIG. 12  is a block diagram showing a configuration of a reception system of an OFDM transmission/reception apparatus according to Embodiment 6 of the present invention; 
           [0028]      FIG. 13  is a block diagram showing a configuration of a reception system of an OFDM transmission/reception apparatus according to Embodiment 7 of the present invention; 
           [0029]      FIG. 14  is a block diagram showing a configuration of a combination section of the reception system of the OFDM transmission/reception apparatus according to Embodiment 7 of the present invention; 
           [0030]      FIG. 15  is a schematic diagram showing a spectrum example of an OFDM transmission/reception apparatus according to Embodiment 8 of the present invention; and 
           [0031]      FIG. 16  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 8 of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    With reference now to the attached drawings, the embodiments of the present invention are explained in detail below. 
       Embodiment 1 
       [0033]    The OFDM transmission/reception apparatus according to the present embodiment transmits important information with two subcarriers and uses one of these two subcarriers carrying important information to carry a carrier frequency signal (DC signal) of angular frequency 0 which was conventionally not used. 
         [0034]    A conceivable method to prevent the reception quality of a subcarrier containing important information from extremely deteriorating due to influences of fading, etc. causing errors in the bits representing control information and retransmission information is to carry same important information through a plurality of subcarriers and for the apparatus on the receiving side to use the important information carried by the subcarrier with the best reception quality or combine all important information carried. 
         [0035]    This method may indeed improve the reception quality of important information at the apparatus on the receiving side, however causes a new problem of increasing the amount of transmission data in addition to the user data, thus reducing transmission efficiency. 
         [0036]    Therefore, the present embodiment will transmit same important information with two subcarriers and use, as one of these two subcarriers, a subcarrier with a carrier frequency signal (DC signal; hereinafter simply referred to as “DC”) of frequency 0 which was conventionally not used as a subcarrier, thus preventing deterioration of transmission efficiency. 
         [0037]    As shown in a spectrum diagram of  FIG. 3A , a normal OFDM system uses an even number of subcarriers and the subcarrier with DC shown by a dotted line in the diagram is not used as a subcarrier.  FIG. 3A  shows that important information is mapped onto the second subcarrier indicated by hatching. 
         [0038]    Thus, the present embodiment transmits important information that used to be transmitted with one subcarrier (here, the second subcarrier) with a DC subcarrier, which is conventionally not used as a subcarrier, and in this way the present embodiment can transmit same important information through two subcarriers without deteriorating the transmission efficiency. 
         [0039]      FIG. 3B  shows a spectrum diagram when same important information is carried by two subcarriers in the present embodiment. As is clear from  FIG. 3B , the same important information is multiplied not only by the second subcarrier but also by the DC subcarrier. 
         [0040]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 4  through  FIG. 6  below.  FIG. 4  is a block diagram showing a configuration of a transmission system of the OFDM transmission/reception apparatus according to Embodiment 1 of the present invention.  FIG. 5  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 1 of the present invention.  FIG. 6  is a block diagram showing a configuration of a synchronization detection section in the reception system of the OFDM transmission/reception apparatus according to Embodiment 1 of the present invention. In the present embodiment, important information is transmitted with two subcarriers; the second subcarrier and DC subcarrier, as shown in  FIG. 3B . 
         [0041]    In  FIG. 4 , modulation section  201  performs modulation processing on transmission data. Mapping control section  202  controls IFFT section  203  so that the important information is mapped onto the second subcarrier and DC subcarrier. IFFT section  203  performs IFFT processing on the modulated transmission data. Transmission section  204  performs transmission processing on the IFFT-processed transmission data and transmits the transmission-processed transmission data from antenna  205 . 
         [0042]    In  FIG. 5 , antenna  301  receives a radio signal. Reception section  302  performs reception processing on the signal received by antenna  301  (reception signal). Timing control section  303  controls each section according to a symbol synchronization timing acquired from reception section  302 . FFT section  304  performs FFT processing on the reception signal. Regarding the acquisition of symbol synchronization timing at reception section  302 , a variety of methods are proposed. The present embodiment can use any method for the acquisition of symbol synchronization timing. 
         [0043]    Synchronization detection section  305  performs synchronization detection processing (demodulation processing) on the FFT-processed reception signal and removes influences of phase rotation and amplitude variation caused by fading, etc. from the reception signal above. 
         [0044]    Important information extraction section  306  extracts important information from the reception signal subjected to synchronization detection processing important information selection section  307  detects the subcarrier with the higher reception level of the two subcarriers with important information multiplied based on reception level information detected by synchronization detection section  305 , and extracts and outputs the important information carried by the detected subcarrier. 
         [0045]    In  FIG. 6  showing the synchronization detection section, based on the symbol synchronization timing indicated by timing control section  303 , known symbol extraction section  401  extracts a signal during a known symbol interval from the FFT-processed reception signal to output the signal to multiplier  402 , and extracts a signal during a data symbol interval to output the signal to calculation section  407 . 
         [0046]    Multiplier  402  multiplies the signal of the known symbol interval of the FFT-processed reception signal by a predetermined known symbol and calculates phase rotation and amplitude variation of the reception signal influenced by fading, etc. 
         [0047]    Sum-of-squares calculation section  403  calculates a sum of squares of an I component and Q component of the output of known symbol extraction section  401 . The calculated sum of squares is output to divider  404  and root calculator  408 . 
         [0048]    Divider  404  divides the output of multiplier  402  by the output of sum-of-squares calculation section  403 . Memory  405  temporarily stores the output of divider  404 . 
         [0049]    Switch  406  outputs the output of divider  404  stored in memory  405  to calculation section  407  while the signal during the data symbol interval of the reception signal is being input to calculation section  407  based on the symbol synchronization timing indicated by timing control section  303 . 
         [0050]    Calculation section  407  generates a conjugate complex number of the output of divider  404 , multiplies the signal during the data symbol interval of the reception signal by this conjugate complex number to obtain a synchronization detection signal. 
         [0051]    Root calculator  408  carries out a root calculation on the sum of squares from sum-of-squares calculation section  403  and calculates the reception level of the reception signal. The calculated reception level is output to important information selection section  307 . 
         [0052]    Then, the operation of the OFDM transmission/reception apparatus in such a configuration is explained below. According to  FIG. 4 , the transmission data is modulated by modulation section  201 . While being controlled by mapping control section  202  so that important information is multiplied by the second subcarrier and DC subcarrier, the modulated transmission data is IFFT-processed by IFFT section  203 . The IFFT-processed transmission data is subjected to transmission processing in transmission section  204  and then transmitted from antenna  205 . 
         [0053]    The radio signal with the important information multiplied by the second subcarrier and DC subcarrier is received by antenna  301  (see  FIG. 5 ). The signal received by antenna  301  (reception signal) is subjected to reception processing by reception section  302 . The reception-processed reception signal is FFT-processed by the FFT section  304 . The FFT-processed reception signal is subjected to synchronization detection processing by synchronization detection section  305 . 
         [0054]    In important information extraction section  306 , the important information carried by the DC subcarrier and the important information carried by the second subcarrier are extracted from the reception signal subjected to synchronization detection processing. Each extracted important information is output to important information selection section  307 . In important information selection section  307 , of the important information extracted by important information extraction section  306 , the important information carried by the subcarrier with the higher reception level is output, based on the reception level information output from synchronization detection section  305 . 
         [0055]    Thus, according to the present embodiment, the apparatus on the transmitting side carries important information with two subcarriers and the apparatus on the receiving side uses, of the important information carried by the two subcarriers, the important information with the better reception level, which allows the reception quality of important information to be maintained even in a fading environment where only the reception level of a specific subcarrier deteriorates. 
         [0056]    Furthermore, when transmitting important information with two subcarriers, the present embodiment uses, as one of the subcarriers, the DC subcarrier, which was conventionally not used as a subcarrier, and in this way can transmit important information with two subcarriers without reducing the transmission efficiency. 
         [0057]    The number of subcarriers carrying same important information is not limited to 2 as described above; it goes without saying that the quality of important information will further be improved by using more subcarriers and selecting ones with better reception conditions. However, assigning many subcarriers to important information will decrease the number of subcarriers used for transmission of user data and reduce the transmission efficiency, and therefore it is most desirable to limit the number of subcarriers carrying important information to 2 and use as one of them a DC subcarrier as in the case of the present embodiment. 
       Embodiment 2 
       [0058]    The OFDM transmission/reception apparatus according to the present embodiment has the same configuration as that of the OFD transmission/reception apparatus according to Embodiment 1 except that the reception level, one of outputs of the synchronization detection section, is averaged. 
         [0059]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 7 .  FIG. 7  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 2 of the present invention. The parts in  FIG. 7  with the same configuration as that of Embodiment 1 ( FIG. 5 ) are assigned the same codes as those in  FIG. 5  and their detailed explanations are omitted. The block diagram of the transmission system is omitted. The present embodiment also uses the second subcarrier as the subcarrier other than the DC subcarrier to carry important information. 
         [0060]    In  FIG. 7 , averaging section  501  averages reception level information, one of the outputs of synchronization detection section  305  and outputs the averaged reception level information to important information selection section  307 . 
         [0061]    Thus, when comparing the reception level of subcarriers carrying important information, the present embodiment uses the averaged reception level, and in this way can improve the accuracy in comparing/determining the reception level and thus improve the reception quality of important information. In addition, the number of slots to be averaged or time interval can be determined arbitrarily. 
       Embodiment 3 
       [0062]    The OFDM transmission/reception apparatus according to the present embodiment has the same configuration as that of the OFDM transmission/reception apparatus according to Embodiment 2 except that when comparing the reception level of the DC subcarrier and second subcarrier carrying important information, if the reception level of the DC subcarrier is higher, the important information carried by the DC subcarrier is used only when the difference in the reception level between the DC subcarrier and the second subcarrier exceeds a predetermined value. 
         [0063]    Since a DC offset may be carried on a signal carried by the DC subcarrier in analog circuits of the apparatuses on the transmitting side and on the receiving side, the reception level at the apparatus on the receiving side may include an error corresponding to the DC offset component. 
         [0064]    Therefore, even if the result of a comparison of the reception level between the DC subcarrier and second subcarrier shows that the reception level of the DC subcarrier is higher, if the difference in the reception level between the DC subcarrier and second subcarrier is small, the reception level of the DC subcarrier with the DC offset component removed may fall below the reception level of the second subcarrier. 
         [0065]    Therefore, the present embodiment uses a threshold greater than a conceivable DC offset, and uses the important information carried by the DC subcarrier only when it is determined that the difference in the reception level between the DC subcarrier and the second subcarrier is sufficiently greater than the DC offset. 
         [0066]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 8 .  FIG. 8  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 3 of the present invention. The parts in  FIG. 8  with the same configuration as that of Embodiment 2 ( FIG. 7 ) are assigned the same codes as those in  FIG. 7  and their detailed explanations are omitted. The block diagram of the transmission system is omitted. The present embodiment also uses the second subcarrier as the subcarrier other than the DC subcarrier to carry important information. 
         [0067]    In  FIG. 8 , subtractor  601  performs a subtraction between the reception level of the DC subcarrier and the reception level of the second subcarrier. Subtractor  602  compares a threshold and the subtraction result, which is the output of subtractor  601 . 
         [0068]    Determination section  603  determines whether the output of subtractor  602  is positive or negative, determines whether the difference in the reception level between the DC subcarrier and the second subcarrier exceeds the threshold or not and outputs the determination result to important information selection section  307 . 
         [0069]    Based on the determination result, important information selection section  307  outputs the important information carried by the DC subcarrier when the difference in the reception level between the DC subcarrier and the second subcarrier exceeds the threshold, and outputs the important information carried by the second subcarrier when the difference in the reception level between the DC subcarrier and the second subcarrier is smaller than the threshold. 
         [0070]    In this way, the present embodiment can compare/determine the reception level taking account of influences of a DC offset by comparing the difference in the reception level between the DC subcarrier and the second subcarrier. This improves the accuracy in comparing/determining the reception level, thus improving the reception quality of important information. 
       Embodiment 4 
       [0071]    The OFDM transmission/reception apparatus according to the present embodiment has the same configuration as that of the OFDM transmission/reception apparatus according to Embodiment 2 except that a determination error, instead of the reception level, is used to select a subcarrier. 
         [0072]    As mentioned above, the signal carried by the DC subcarrier contains an error called “DC offset”, and therefore the reception quality of the DC subcarrier may be deteriorated more than other subcarriers. Thus, the present embodiment uses a determination error instead of the reception level when selecting a subcarrier. 
         [0073]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 9  and  FIG. 10 .  FIG. 9  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 4 of the present invention.  FIG. 10  is a block diagram showing a configuration of a synchronization detection section of the reception system of the OFDM transmission/reception apparatus according to Embodiment 4 of the present invention. The parts in  FIG. 9  and  FIG. 10  with the same configuration as that of Embodiment 1 or Embodiment 2 are assigned the same codes as those in Embodiment 1 or Embodiment 2 and their detailed explanations are omitted. The present embodiment also uses the second subcarrier as the subcarrier other than the DC subcarrier to carry important information. 
         [0074]    In  FIG. 9 , synchronization detection section  701  outputs determination errors of the DC subcarrier and the second subcarrier to averaging section  501 . Averaging section  501  calculates an average value of these determination errors and outputs the average value to important information selection section  307 . Important information selection section  307  selects and outputs the important information carried by the subcarrier with the smaller determination error. 
         [0075]    In  FIG. 10 , determination section  801  determines a synchronization detection signal. Subtractor  802  performs a subtraction before and after the determination of the signal, generates determination errors and outputs the generated determination errors to averaging section  501 . 
         [0076]    The calculated determination errors are averaged by averaging section  501 . This generates an average value of the determination errors. The average value of the determination errors is output to important information selection section  307 . 
         [0077]    Important information selection section  307  compares the determination error of the DC subcarrier and that of the second subcarrier and selects and outputs the important information carried by the subcarrier with the smaller determination error. 
         [0078]    Thus, the present embodiment determines which subcarrier has better reception quality based on the determination error of the DC subcarrier and that of the second subcarrier to ignore influences of a DC offset and in this way can improve the reception quality of important information. 
       Embodiment 5 
       [0079]    The OFDM transmission/reception apparatus according to the present embodiment has the same configuration as that of the OFDM transmission/reception apparatus according to Embodiment 1 except that the FFT-processed DC subcarrier signal is stripped of a DC offset and then subjected to synchronization detection processing. 
         [0080]    A DC offset has a constant value irrespective of its digital signal waveform and the probability that a digital signal will take a value of 1 or 0 is assumed to be 50%. Therefore, if FFT-processed digital signals are added up and averaged, 1 and 0 are canceled out and only the DC offset value can be detected. Thus, the present embodiment eliminates the DC offset component from the DC subcarrier signal before synchronization detection processing according to the method above. 
         [0081]    The apparatus according to the present embodiment is explained with reference to  FIG. 11 .  FIG. 11  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 5 of the present invention. The parts in  FIG. 11  with the same configuration as that of Embodiment 4 are assigned the same codes and their detailed explanations are omitted. The block diagram of the transmission system is omitted. The present embodiment also uses the second subcarrier as the subcarrier other than the DC subcarrier to carry important information. 
         [0082]    In  FIG. 11 , averaging section  901  adds up and averages the FFT-processed DC subcarrier signals. This calculated average value is a DC offset value. The number of slots added up or the time interval can be determined arbitrarily. Memory  902  stores the calculated DC offset value. 
         [0083]    Subtractor  903  reads a DC offset value in memory  902  one by one and subtracts the read DC offset value from the FFT-processed DC subcarrier signal. This makes it possible to eliminate the DC offset from the DC subcarrier signal. 
         [0084]    Moreover, when removing a DC offset from the FFT-processed signal in slot n, if the DC offset value calculated from the signal in slot n is used, the DC offset eliminating speed is slowed down. Thus, it is desirable to use a DC offset calculated from a few immediately preceding slots taking account of the fact that DC offsets are considered to be almost constant when focused on a unit slot for a long period of time. 
         [0085]    For example, averaging section  901  calculates a DC offset value for slots n- 3  to n- 1  and subtractor  903  subtracts this DC offset value from the reception signal in slot n. In this way, using a DC offset value for a few immediately preceding slots allows DC offset elimination processing without time lags. 
         [0086]    Thus, the present embodiment adds up and averages DC subcarrier signals to calculate a DC offset and eliminates this DC offset from the DC subcarrier signals, which improves the reception quality of important information. 
       Embodiment 6 
       [0087]    The OFDM transmission/reception apparatus according to the present embodiment has the same configuration as that of the OFDM transmission/reception apparatus according to Embodiment 5 except that important information carried by the DC subcarrier and important information carried by the second subcarrier are combined. 
         [0088]    Embodiments 1 through 5 describe embodiments that select the DC subcarrier or second subcarrier, whichever has a better reception condition. However, taking account of the fact that the reception quality of the DC subcarrier is also improved by eliminating DC offsets, the present embodiment combines both to obtain important information. 
         [0089]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 12 .  FIG. 12  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 6 of the present invention. The parts in  FIG. 12  with the same configuration as that of Embodiment 1 are assigned the same codes and their detailed explanations are omitted. The block diagram of the transmission system is omitted. The present embodiment also uses the second subcarrier as the subcarrier other than the DC subcarrier to carry important information. 
         [0090]    In  FIG. 12 , combination section  1001  combines the important information carried by DC subcarrier and the second subcarrier which are extracted by important information extraction section  306 , and outputs the combined important information. 
         [0091]    Thus, the present embodiment eliminates influences of DC offsets from the DC subcarrier signal and then combines the important information carried by the two subcarriers, which improves the reception quality of important information. 
       Embodiment 7 
       [0092]    The OFDM transmission/reception apparatus according to the present embodiment has the same configuration as that of the OFDM transmission/reception apparatus according to Embodiment 6 except that combination processing is carried out after weighting processing according to the reception level is carried out. 
         [0093]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 13  and  FIG. 14 .  FIG. 13  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 7 of the present invention.  FIG. 14  is a block diagram showing a configuration of a combination section of the reception system of the OFDM transmission/reception apparatus according to Embodiment 7 of the present invention. The parts in  FIG. 13  and  FIG. 14  with the same configuration as that of Embodiment 6 are assigned the same codes as those in Embodiment 6 and their detailed explanations are omitted. The present embodiment also uses the second subcarrier as the subcarrier other than the DC subcarrier to carry important information. 
         [0094]    In  FIG. 13 , reception level information, which is one of outputs of synchronization detection section  305 , is input to combination section  1101 . Combination section  1101  performs a maximum-ratio combination on important information carried by the DC subcarrier and second subcarrier. That is, combination section  1101  carries out weighting processing on each important information and then combines the weighting-processed important information. 
         [0095]    In  FIG. 14 , multiplier  1201  multiplies the important information carried by the DC subcarrier which are extracted by important information extraction section  306  by the reception level information of the DC subcarrier calculated by synchronization detection section  305 . 
         [0096]    Likewise, multiplier  1202  multiplies the important information carried by the second subcarrier which are extracted by important information extraction section  306  by the reception level information of the second subcarrier calculated by synchronization detection section  305 . Adder  1203  adds up the output of multiplier  1201  and the output of multiplier  1202  and combines weighting-processed important information. 
         [0097]    Thus, the present embodiment carries out a combination reflecting the reception level by eliminating influences of DC offsets from the DC subcarrier signal and then performing a maximum-ratio combination on the important information carried by the two subcarriers. In this way, the present embodiment can improve the reception quality of important information more than Embodiment 6. 
       Embodiment 8 
       [0098]    The OFDM transmission/reception apparatus according to the present embodiment applies a specific packet (for example, packet carrying control information, packet carrying retransmission information or packet for a user with poor channel quality, etc.) as important information carried by a plurality of subcarriers in Embodiments 1 through 7. Here, the OFDM transmission/reception apparatus according to the present embodiment is explained with reference to Embodiment 1, but the OFDM transmission/reception apparatus according to the present embodiment is also applicable to Embodiments 2 to 7. 
         [0099]    The OFDM transmission/reception apparatus according to the present embodiment is explained with reference to  FIG. 15  and  FIG. 16 .  FIG. 15  is a schematic diagram showing a spectrum example of the OFDM transmission/reception apparatus according to Embodiment 8 of the present invention.  FIG. 16  is a block diagram showing a configuration of a reception system of the OFDM transmission/reception apparatus according to Embodiment 8 of the present invention. The transmission system of the OFDM transmission/reception apparatus according to the present embodiment is mostly the same as that shown in  FIG. 4 . 
         [0100]    In  FIG. 4 , which illustrates the transmission system, a transmission signal including a specific packet is modulated by modulation section  201 . This specific packet is, for example, a signal transmitted through a control channel or a signal transmitted through multicast channels (channels received by a plurality of users). Needless to say, it is also possible to use a packet of a predetermined user, such as packet of a user having a reception system with poor reception quality, etc. 
         [0101]    Mapping control section  202  controls IFFT section  203  so that the specific packet in the modulated transmission signal is mapped onto a plurality of subcarriers. The concrete mapping method by mapping control section  202  is as follows: 
         [0102]    In mapping control section  202 , data 1 of the modulated specific packet is mapped, for example, onto two subcarriers, subcarrier # 1  and subcarrier # 1 ′ shown in  FIG. 15 . Furthermore, data 2 of the modulated specific packet is mapped, for example, onto two subcarriers, subcarrier # 2  and subcarrier # 2 ′ shown in  FIG. 15 . Likewise, each piece of data of the modulated specific packet is mapped onto either one of the two subcarriers shown in  FIG. 15 . 
         [0103]    Here, the case where there are two subcarriers onto which each piece of data of the modulated specific packet is mapped is explained, but the number of subcarriers above can further be increased. The number of subcarriers above can be determined according to various conditions such as transmission efficiency, etc. 
         [0104]    Furthermore, mapping control section  202  controls IFFT section  203  so that packets other than the specific packet above of the modulated transmission signal is mapped onto one subcarrier as in the case of the conventional method. 
         [0105]    The modulated transmission signal is controlled by mapping control section  202  as shown above and subjected to IFFT processing by IFFT section  203 . The IFFT-processed transmission signal is subjected to transmission processing by transmission section  204  and then transmitted from antenna  205 . 
         [0106]    The radio signal with each piece of data in a specific packet mapped on any two subcarriers is received by antenna  1401  in  FIG. 16 . The signal received by antenna  1401  (reception signal) is subjected to predetermined reception processing and then FFT processing by FFT section  1402 . The FFT-processed reception signal is subjected to synchronization detection processing by synchronization detection section  1403 . Here, synchronization detection processing through synchronization detection section  1403  can be replaced by delay detection processing. The reception signal subjected to synchronization detection processing (or delay detection processing) is sent to selection section  1404 . 
         [0107]    The specific packet of the reception signal above is sent from selection section  1404  to diversity section  1405 . In diversity section  1405 , diversity reception processing is performed. That is, of the data pieces mapped on two subcarriers of the specific packet, the data piece with the higher reception level is selected or both data pieces mapped on the two subcarriers of the specific packet are combined. 
         [0108]    As shown above, according to the present embodiment, the apparatus on the transmitting side carries data in a specific packet with a plurality of subcarriers and the apparatus on the receiving side selects the data piece with the better reception level of those data pieces of the specific packet above or combines the data pieces in the specific packet above, which allows the reception quality of the specific packet to be maintained even in a fading environment where only the reception level of a specific subcarrier deteriorates. This makes it possible to improve the error rate characteristic of the specific packet without reducing the transmission efficiency. 
         [0109]    As explained above, the present invention transmits important information with two subcarriers instead of one subcarrier in the conventional method and uses a DC subcarrier as one of the two subcarriers carrying important information, thus can improve the reception quality of the important information at the apparatus on the receiving side without reducing the transmission efficiency. 
         [0110]    The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
         [0111]    This application is based on the Japanese Patent Application No. HEI 11-054667 filed on Mar. 2, 1999 and the Japanese Patent Application No. HEI 11-249938 filed on Sep. 3, 1999, entire content of which is expressly incorporated by reference herein.