Abstract:
Radio-communication-system includes transmitting-station and receiving-station. The transmitting-station includes first-encoding-unit configured to generate plural parity-information by using the different-data, second-encoding-unit configured to encode each of the plural parity-information and each of the different-data to produce plural encoded-data, modulation-unit configured to modulate carriers by the plural encoded-data to generate plural modulated-signals, and multiplex-unit configured to multiplex the plural modulated-signals for outputting a multiplexed-signal. The receiving-station includes demultiplex-unit configured to demultiplex the multiplexed-signal transmitted from the transmitting-station into the plural modulated-signals, demodulation-unit configured to demodulate each of the modulated-signals demultiplexed by the demultiplex-unit to produce plural demodulated-signals, first-decoding-unit configured to decode each of the demodulated-signals according to a decoding-scheme corresponding to an encoding-scheme of the second-encoding-unit to produce plural decoded-signals, and second-decoding-unit configured to decode each of the decoded-signals according to a decoding-scheme corresponding to an encoding-scheme of the first-encoding-unit to obtain the different-data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-195193, filed Jul. 4, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to an error-correcting-coding method of a radio communication system such as a code division multiplexing (CDM) broadcast system and, more particularly, to a method of multiplexing an error-correcting-coding bit and decoding thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     If a code division multiplexing (CDM) broadcast system standardized by ITU-T (International Telecommunication Union-Radio Communication Sector) Recommendation BO. 1130-4, Digital System E. is employed in radio propagation circumstances such as urban areas where a number of multipaths exist, multiplexed signals interfere with each other due to a delay wave caused by the multipaths, and orthogonality of the multiplexed data collapse, and receiving characteristics are thereby remarkably degraded.  
         [0006]     In the CDM communication system, the signal delayed by the multipaths is effectively used by employing the RAKE receiving scheme and the diversity gain is acquired, to restrict the degradation in the receiving characteristics caused by the multipaths (for example, Jpn. Pat. Appln. KOKAI No. 2004-80360). In addition, to remove the influence of the interference caused by the multipaths, the received CDM signal is subjected to canceling which removes the interference signal from the received signal, by using a demodulation result of the multiplexed data.  
         [0007]     However, since the conventional RAKE receiving scheme and canceling need to be applied to the received signal itself, the processing needs to be executed at a high speed, with high accuracy, on the receiving side. This makes reduction of the power consumption on the receiving side difficult.  
         [0008]     Particularly, if the CDM scheme is applied to the radio communication system, spread spectrum processing to multiplex the data by code division needs to be executed at a very high speed as compared with the transmission ratio of the transmitted data. In the receiver, too, the processing needs to be executed at a high speed as compared with the transmission data ratio. This processing is therefore said to be very inefficient. In other words, since a chip which operates at a higher speed than the data rate needs to be used for the CDM, application of the CDM to the receiving on a portable terminal is not efficient in view of the processing speed.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An aspect of the present invention provides a radio communication system with a transmitting station which multiplexes different data and transmits multiplexed data to a receiving station. The transmitting station comprises a first encoding unit configured to generate plural parity information by using the different data, a second encoding unit configured to encode each of the plural parity information and each of the different data to produce plural encoded data, a modulation unit configured to modulate carriers by the plural encoded data to generate plural modulated signals, and a multiplex unit configured to multiplex the plural modulated signals for outputting a multiplexed signal. The receiving station comprises a receiving station comprising a demultiplex unit configured to demultiplex the multiplexed signal transmitted from the transmitting station into the plural modulated signals, a demodulation unit configured to demodulate each of the modulated signals demultiplexed by the demultiplex unit to produce plural demodulated signals, a first decoding unit configured to decode each of the demodulated signals according to a decoding scheme corresponding to an encoding scheme of the second encoding unit to produce plural decoded signals, and a second decoding unit configured to decode each of the decoded signals according to a decoding scheme corresponding to an encoding scheme of the first encoding unit to obtain the different data. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]      FIG. 1  is a block diagram showing a configuration of a transmitting station in a radio communication system;  
         [0011]      FIG. 2  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 1 ;  
         [0012]      FIG. 3  is a block diagram showing a configuration of a transmitting station in a radio communication system;  
         [0013]      FIG. 4  is an illustration for explanation of a signal transmitted from the transmitting station shown in  FIG. 3 ;  
         [0014]      FIG. 5  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 3 ;  
         [0015]      FIG. 6  is a block diagram showing a configuration of a transmitting station in a radio communication system;  
         [0016]      FIG. 7  is an illustration for explanation of a signal transmitted from the transmitting station shown in  FIG. 6 ;  
         [0017]      FIG. 8  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 6 ;  
         [0018]      FIG. 9  is a block diagram showing a configuration of a transmitting station in a radio communication system according to the present invention;  
         [0019]      FIG. 10  is an illustration for explanation of encoding in an encoder  90  of the transmitting station shown in  FIG. 9 ;  
         [0020]      FIG. 11  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 9 ;  
         [0021]      FIG. 12  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 9 ;  
         [0022]      FIG. 13  is an illustration for explanation of encoding in an encoder  90  of the transmitting station shown in  FIG. 9 ;  
         [0023]      FIG. 14  is an illustration for explanation of encoding in an encoder  90  of the transmitting station shown in  FIG. 9 ;  
         [0024]      FIG. 15  is an illustration for explanation of generation of parity bit sequences;  
         [0025]      FIG. 16  is a block diagram showing a configuration of a transmitting station which executes the encoding shown in  FIG. 15 ;  
         [0026]      FIG. 17  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 16 ;  
         [0027]      FIG. 18  is a block diagram showing a modified configuration of the transmitting station shown in  FIG. 9 ;  
         [0028]      FIG. 19  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 18 ;  
         [0029]      FIG. 20  is a block diagram showing a modified configuration of the transmitting station shown in  FIG. 9 ;  
         [0030]      FIG. 21  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 20 ;  
         [0031]      FIG. 22  is a block diagram showing a configuration of a transmitting station in a code division multiplexing broadcast system based on ITU-R Recommendation;  
         [0032]      FIG. 23  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 22 ;  
         [0033]      FIG. 24  is a block diagram showing a configuration of the transmitting station in the code division multiplexing broadcast system based on ITU-R Recommendation to which the transmitting station shown in  FIG. 9  is applied;  
         [0034]      FIG. 25  is a block diagram showing a configuration of a receiving station corresponding to the transmitting station shown in  FIG. 24 ;  
         [0035]      FIG. 26  is a graph showing a received bit error rate characteristic in a radio broadcast system comprising the transmitting station shown in  FIG. 24  and the receiving station shown in  FIG. 25 ; and  
         [0036]      FIG. 27  is an illustration showing a configuration of a radio communication system to which the transmitting station shown in  FIG. 9  is applied. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]     Embodiments of the present invention will be explained below with reference to the accompanying drawings.  
         [0038]      FIG. 1  shows a configuration of a transmitting station in a radio communication system. In the transmitting station, information bit sequences of different information items  11   a ,  11   b ,  11   c  are encoded by encoders  12   a,    12   b,    12   c  corresponding thereto. With the obtained encoded bit sequences, carriers are modulated by modulators  13   a,    13   b,    13   c  corresponding thereto. These modulation results are multiplexed by a multiplexer  14  and then transmitted.  
         [0039]     In the communication system employing such a transmitting station, a receiving station has a configuration shown in  FIG. 2 . In the receiving station, the receive signal multiplexed by the multiplexer  14  is separated for each of the encoded bit sequences by a demultiplexer  21 . The receive signals thus separated are detected by demodulators  22   a,    22   b,    22   c,  respectively.  
         [0040]     Metric generators  23   a,    23   b,    23   c  generate metric values on the basis of the detection results which correspond respectively to the metric generators. The metric values thus generated are decoded by decoders  24   a,    24   b,    24   c,  respectively, and desired information bit sequences  25   a,    25   b,    25   c  are thereby obtained.  
         [0041]     If the above-described encoding is executed in the transmitting station, decoding corresponding to the encoders  12   a,    12   b,    12   c  of the transmitting station can be executed independently of each other by the decoders  24   a,    24   b,    24   c,  in the receiving station. However, the receiving characteristics of the decoding depend on the only encoding of the encoded bit sequences.  
         [0042]     In general, if multiplexing is executed in the transmitting station as shown in  FIG. 1 , transmit signals corresponding to the respective encoded bit sequences are mutually orthogonalized and then multiplexed. For this reason, the multiplexed transmit signals can be separated without interference at the receiving station unless their orthogonality is broken in the communication path.  
         [0043]     If their orthogonality is broken in the communication path, the transmit signals corresponding to the multiplexed encoded bit sequences cannot be separated without interference at the receiving station. The transmit signals interfere with each other and the receiving characteristics are thereby degraded.  
         [0044]     As for the method of orthogonalizing the multiplexed transmit signals on the transmitting side, Code Division Multiple Access (CDMA) or Orthogonal Frequency Division Multiplexing (OFDM) is employed.  
         [0045]     A communication system employing the CDM scheme will be explained here.  FIG. 3  shows a configuration of the transmitting station in the communication system. In the transmitting station, information bit sequences of different information items  31   a,    31   b,    31   c  are encoded by encoders  32   a,    32   b,    32   c,  respectively.  
         [0046]     The encoded bit sequences thus obtained are multiplexed with spread code sequences  33   a,    33   b,    33   c  by multiplexers  34 ,  34   b,    34   c,  respectively, and then spread. The spread code sequences  33   a,    33   b,    33   c  orthogonalize each other.  
         [0047]     The multiplexing results thus obtained are used for modulation of carriers in modulators  35   a,    35   b,    35   c,  respectively. The modulation results are multiplexed by a multiplexer  36  and then transmitted. The transmit signal thus spread is transmitted in a state in which energies are multiplexed as shown in  FIG. 4 .  
         [0048]     In the communication system using this transmitting station, the receiving station has a configuration shown in  FIG. 5 . In this receiving station, the receive signal is multiplexed with spread code sequences  52   a,    52   b,    52   c  by despreaders  51   a,    51   b,    51   c,  respectively, and separated in accordance with the information bit sequences of information items  31   a,    31   b,    31   c.    
         [0049]     The receive signals thus separated are detected by demodulators  53   a,    53   b,    53   c,  respectively. Metric generators  54   a,    54   b,    54   c  generate metric values on the basis of the respective detection results. The metric values thus generated are decoded by decoders  55   a,    55   b,    55   c,  respectively and desired information bit sequences  56   a,    56   b,    56   c  are thereby obtained.  
         [0050]     However, if orthogonality of the multiplexed transmit signals cannot be maintained due to the multipath or for the reason that synthesis is not made between the multiplexed transmit signals, the multiplexed signals interfere with each other and cannot be separated ideally, and the receiving characteristics are therefore degraded.  
         [0051]     Next, a communication system employing the OFDM scheme will be explained here.  FIG. 6  shows a configuration of the transmitting station in the communication system. In the transmitting station, information bit sequences of different information items  61   a,    61   b,    61   c  are encoded by encoders  62   a,    62   b,    62   c,  respectively.  
         [0052]     The encoded bit sequences thus obtained are used for modulation of carriers in modulators  63   a,    63   b,    63   c,  respectively. The modulation results are subjected to inverse Fourier transform by an inverse Fourier transformer  64  such that signals in the frequency axis are transformed into signals of a time axis waveform, which are then multiplexed. Thus, orthogonality of the data multiplexed in the OFDM is maintained in the frequency axis as shown in  FIG. 7 .  
         [0053]     In the communication system employing such a transmitting station, a receiving station has a configuration shown in  FIG. 8 . In the receiving station, the receive signal is subjected to Fourier transform and separated into transmit signals in the frequency axis shown in  FIG. 7  by a Fourier transformer  81 , and desired receive signals are thereby obtained.  
         [0054]     The receive signals thus separated are detected by demodulators  82   a,    82   b,    82   c,  respectively. Metric generators  82   a,    82   b,    82   c  generate metric values on the basis of the detection results which correspond respectively to the metric generators. The metric values thus generated are decoded by decoders  84   a,    84   b,    84   c,  respectively, and desired information bit sequences  85   a,    85   b,    85   c  are thereby obtained.  
         [0055]     In this communication system, too, if the orthogonality cannot be maintained on the receiving side due to influence from the multipath in the communication path, etc., the multiplexed transmit signals interfere with each other and receiving characteristics are thereby degraded.  
         [0056]     In any one of the above-explained communication systems, multiplexing and encoding are independently executed for the multiplexed data. Since correlation information in the communication path caused by multiplexing cannot be used, optimum receiving cannot be executed.  
         [0057]     In addition, although the data to be multiplexed on the transmitting side and the receiving side need to be encoded and decoded at many costs, the correlation information of the multiplexed data, in the communication path, is not used. Therefore, only improvement effect of the receiving characteristics based on separate encoding can be obtained on the receiving side. From the viewpoint of efficiency in the transmission and receiving, in the communication system in which the data is multiplexed, the receiving characteristics of the data to be multiplexed should preferably be obtained in accordance with the data amount.  
         [0058]     The present invention provides a radio communication system capable of executing optimum receiving by effectively using the correlation information of the multiplexed data in the communication path. Improvement of the receiving characteristics can be expected without requiring a higher processing than the transmission data ratio on the receiving side, if error correction capable of reducing the influence from the interference caused by the multipath is applied to the data error which occurs due to the influence from the channel. In particular, since decoding of error correction can be executed at a processing speed proportional to the transmission data ratio, the receiving characteristics can be efficiently improved without a high-speed processing.  
         [0059]     Then, the present invention proposes a manner of optimally receiving the multiplexed data which interfere with each other by the multipath, etc., by adding the encoding based on the encoded sequences to be multiplexed to the communication system. In the proposed scheme, since the receiving characteristics of all the multiplexed data can be obtained in accordance with their processing amount, the receiving characteristics can be efficiently improved.  
         [0060]      FIG. 9  shows a configuration of a transmitting station in a radio communication system according to an embodiment of the present invention. In the transmitting station, information bit sequences of information items  91   a,    91   b,    91   c  are encoded by encoders  92   a,    92   b,    92   c,  respectively.  
         [0061]     Simultaneously with this, an encoder  90  executes encoding by using the information bit sequences of the information items  91   a,    91   b,    91   c.  In other words, the encoders  92   a,    92   b,    92   c  encode the information bit sequences of the information items  91   a,    91   b,    91   c,  respectively, while the encoder  90  executes encoding by using the information bit sequences.  
         [0062]     Encoding executed by the encoder  90  generates parity bit sequences by using the information bit sequences of the information items  91   a,    91   b,    91   c  as shown in, for example,  FIG. 10 . An encoder  92 d encodes the parity bit sequence generated by the encoder  90  in the same manner as the encoders  92   a,    92   b,    92   c  and thereby obtains an encoded bit sequence.  
         [0063]     The modulators  93   a,    93   b,    93   c,    93   d  modulate carriers by using the encoded bit sequences obtained by the encoders  92   a,    92   b,    92   c,    92   d,  respectively. The modulation results are multiplexed by a multiplexer  94  and then transmitted.  
         [0064]     In the communication system employing such a transmitting station, a receiving station has a configuration shown in  FIG. 11 . In the receiving station, the receive signal multiplexed by the multiplexer  94  is separated for each of the encoded bit sequences by a demultiplexer  111 . The receive signals thus separated are detected by demodulators  112   a,    112   b,    112   c,    112   d,  respectively.  
         [0065]     Metric generators  113   a,    113   b,    113   c,    113   d  generate metric values on the basis of the detection results which correspond respectively to the metric generators. The metric generators  113   a,    113   b,    113   c  correspond to the encoders  92   a,    92   b,    92   c,  respectively. The metric generator  113   d  corresponds to the encoder  92   d  and encoder  90 .  
         [0066]     For this reason, the metric values corresponding to the information bit sequences of the information items  91   a,    91   b,    91   c  can be obtained by the metric generators  113   a,    113   b,    113   c,  respectively. In addition, the metric value corresponding to the parity bit sequence which is output from the encoder  90  can be obtained by the metric generator  113   d.    
         [0067]     In an iterative decoder  114 , the metric values obtained by the metric generators  113   a,    113   b,    113   c,    113   d  are subjected to decoding corresponding to the encoding of the encoders  92   a,    92   b,    92   c,    92   d.  In the iterative decoder  114 , the result of decoding corresponding to the encoding of the encoders  92   a,    92   b,    92   c,    92   d  is also subjected to decoding corresponding to the encoding of the encoder  90 . Iterative decoding is executed by using the decoding results and desired information bit sequences  115   a,    115   b,    115   c  are thereby obtained.  
         [0068]      FIG. 12  shows details of the configuration of the iterative decoder  114 . In the iterative decoder  114 , the metric values obtained by the metric generators  113   a,    113   b,    113   c,    113   d  are subjected to decoding corresponding to the encoding of the encoders  92   a,    92   b,    92   c,    92   d,  by decoders  114   a,    114   b,    114   c,    114   d,  respectively.  
         [0069]     In a decoder  114   e,  the decoding results of the decoders  114   a,    114   b,    114   c,    114   d  are subjected to decoding corresponding to the encoding of the encoder  90  and bit sequences corresponding to the information items  91   a,    91   b,    91   c  are thereby obtained. On the basis of the decoding corresponding to the encoder  90 , the decoder  114   e  further decodes the decoding results of the decoders  114   a,    114   b,    114   c,    114   d  and outputs the decoding results to the decoders  114   a,    114   b,    114   c,    114   d,  respectively. The decoders  114   a,    114   b,    114   c,    114   d  executes the decoding again on the basis of the decoding results of the decoder  114   e.    
         [0070]     After that, decoding of the decoders  114   a,    114   b,    114   c,    114   d  and the decoding of the decoder  114   e  are iterated. When error is lowered below a predetermined level soon, the desired information items  115   a,    115   b,    115   c  are taken out.  
         [0071]     A method of acquiring the decoding results of the decoders  114   a,   114   b,    114   c,    114   d  and the decoding result of the decoder  114   e  will be described in detail.  
         [0072]     For example, if one bit is taken from each of different data items to be multiplexed to generate parity data satisfying even parity, the parity bit is defined in the following formula:
 
bit1⊕bit2⊕bit3⊕parity=0
 
 where bit  1 , bit  2 , bit  3  represent information bits to be multiplexed, respectively. The parity indicates the parity bit generated by the encoder  90  in  FIG. 9 . If encoding is executed in this manner, bit  1 , bit  2 , bit  3  are also encoded by the encoders  92   a,    92   b,    92   c  independently of the parity generated by the encoder  90 . Thus, they can be decoded by the decoders  114   a,    114   b,    114   c  without using the parity bit on the receiving side. 
 
         [0073]     The iterative decoding of the decoders  114   a,    114   b,    114   c,    114   d  and the decoder  114   e  will be explained here. If the parity bit sequence generated by the encoder  90  is multiplexed with the above-explained data to be multiplexed, in a receiving station shown in  FIG. 12 , the parity bit is defined on the basis of the multiplexed data.  
         [0074]     In the receiving station, first, the decoding corresponding to the encoders  92   a,    92   b,    92   c,    92   d  is executed by the decoders  114   a,    114   b,    114   c,    114   d  and a Soft-decision posteriori probability value is acquired by Maximum A-posteriori Probability (MAP) decoding, in relation to each of the multiplexed information bit sequences and the parity bit sequence generated by the encoder  90 .  
         [0075]     As algorithms for the MAP decoding, Bahl Cocke Jelinek Raviv (BCJR) algorithm, min-sum algorithm, Soft Output Viterbi Algorithm (SOVA) and the like are employed.  
         [0076]     The posteriori probability value obtained by the MAP algorithm is acquired by the following formula, in relation to each of the multiplexed information bit sequences and the parity bit sequence, in the decoders  114   a,    114   b,    114   c,    114   d. 
 
Pr[bit=a|r]=p(r|bit=a)Pr[bit=a]
 
 where r represents a metric value for each of the receive signals obtained by separating the multiplexed receive signal. p(r|bit=a) represents a probability density function of the receive signal where the information bit which is the origin of each of the encoded bits generated by the encoders  92   a,    92   b,    92   c,    92   d  is a. Pr[bit=a] represents the prior probability at which the information bit input to each of the encoders  92   a,    92   b,    92   c,    92   d  is a. Pr[bit=a|r] represents the posteriori probability at which the information bit bit=a is transmitted under the condition that r is received. 
 
         [0077]     MAP decoding is executed again by using the decoding results for the encoders  92   a,    92   b,    92   c,    92   d,  i.e. the posteriori probability values Pr[bit=a|r] as the metric values in the decoding of the decoder  114   e.  The posteriori probability values for the parity bit sequence obtained by the encoding of the encoder  90  are acquired in the following formula:
 
Pr[bit′=a|r]=Pr[bit=a|r]Pr[bit′=a]={p(r|bit=a)Pr[bit=a]}*Pr[bit′=a]
 
 Pr[bit′=a|r] represents the posteriori probability at which the information bits input to the encoder  90  are bit′=a under the condition that the receive signal r is received. Pr[bit′=a|r] represents the prior probability at which the information bits input to the encoder  90  are a. 
 
         [0078]     The data of the information bit sequences encoded by the encoders  92   a,    92   b,    92   c  are further encoded by the encoder  90  provided independently of the encoders  92   a,    92   b,    92   c.  In other words, two kinds of encoding are executed.  
         [0079]     In this case, gains Pr[bit=a] of the encoders  92   a,    92   b,    92   c  and gain Pr[bit′=a] of the encoder  90  can be obtained simultaneously, in relation to desired information bit a and the reliability can be thereby made higher.  
         [0080]     Moreover, since prior information Pr[bit=a] of the decoders  114   a,    114   b,    114   c,    114   d  and prior information Pr[bit′=a] of the decoder  114 e can be acquired independently of each other, decoding is iterated in the configuration of  FIG. 12  consisting of the decoders  114   a,    114   b,    114   c,    114   d  the decoder  114   e,  by using the gain Pr[bit′=a] acquired by the decoder  114   e  as the prior information used in the decoders  114   a,    114   b,    114   c,    114   d  and, and using the gain Pr[bit=a] acquired by the decoders  114   a,    114   b,    114   c,    114   d  as the prior information used in the decoder  114   e.  The reliability can be thereby made higher.  
         [0081]     At the first decoding, the prior probability Pr[bit=a] of the encoded bit sequences 
    Pr[bit= 0 ]= 0 . 5      Pr[bit= 1 ]= 0 . 5  
 
 the above-described decoding is iterated. 
   
 
         [0084]     In the communication system having the above-described configuration, the parity bit sequence is generated by using the information bit sequences of the respective information items  91   a,    91   b,    91   c,  the parity bit sequence and the modulation results based on the information bit sequences are multiplexed and transmitted, in the transmitting station. The parity bit sequence is decoded and the information bit sequences are decoded by using the decoded parity bit sequence, in the receiving station.  
         [0085]     Therefore, even if the orthogonality of a plurality of multiplexed information bit sequences cannot be maintained in the communication path, degradation in the receiving characteristics can be restricted. In relation to the data multiplexed in the transmitting station, the gains can be obtained by decoding the information bit sequences in the decoders  114   a,    114   b,    114   c,    114   d  and decoding the parity data in the decoder  114   e,  in the receiving station. The receiving characteristics can be thereby improved as compared with the receiving of  FIG. 2 .  
         [0086]     The processing necessary to obtain this advantage is executed in the data decoding step. A high-speed receiving which removes the influence from the interference in a state in which the data are multiplexed as seen in the prior art does not need to be executed. The processing can be implemented at the speed proportional to the transmission data ratio and the efficiency can be thereby improved.  
         [0087]     In the receiving station shown in  FIG. 12 , decoding of the decoders  114   a,    114   b,    114   c,    114   d  and decoding of the decoder  114   e  are iterated. Since the correlation of the multiplexed data in the communication path and the influence from the interference caused by the multipath or the like can be used for decoding of all of the multiplexed data items, receiving quality is improved.  
         [0088]     The signal transmitted from the transmitting station shown in  FIG. 9  is obtained by multiplexing the information bit sequences and the parity bit sequence. These bit sequences are quite independent of each other. For this reason, even the receiving station as shown in  FIG. 2  can receive the signal, similarly to the case of receiving the signal transmitted from the transmitting station shown in  FIG. 1 , without receiving the parity bit sequence alone. The receiving quality is also equal to that in the case of receiving the signal transmitted from the transmitting station shown in  FIG. 1 .  
         [0089]     For this reason, an environment in which the receiving station shown in  FIG. 2  and the transmitting station shown in  FIG. 9  exist together can be applied to the communication system. The communication system having the above-described configuration is also effective for CDM, OFDM or other multiplexing schemes.  
         [0090]     Incidentally, generation of the parity bit sequence has been explained above as an additional pattern of the parity bit as shown in  FIG. 10 . In relation to such a parity bit sequence, multiplexed data associated with a certain parity bit are also the same time as a symbol in which the data are multiplexed and transmitted. For this reason, if the reliability of receiving of the symbol is remarkably deteriorated in the communication path due to the fading or the like, the reliability of the decoding results for all of the data assigned to the symbol becomes deteriorated.  
         [0091]     Thus, to generate the parity bit sequence, bits of the used data may be made different in time, in the information bit sequences as shown in  FIG. 13 . If the parity bit sequence is generated in this manner, the bits of the data included in the symbol which are multiplexed and simultaneously transmitted, are assigned to symbols to be transmitted at different times. For this reason, even if the reliability of one symbol in a certain multiplexed transmit signal becomes deteriorated, the influence from the lower reliability can be dispersed since the signals used for the decoding of the parity bit sequence are assigned to different symbols.  
         [0092]     In addition, a generation pattern of the parity bit sequence may not be uniformly defined as shown in  FIG. 13 , but may be defined in such a manner that bits of random times are selected from the multiplexed data as shown in  FIG. 14 .  
         [0093]     If the parity bit sequence is generated, the bits used for decoding of the parity bit sequence are assigned to multiplexed symbols different in time, as shown in  FIG. 14 . For this reason, the influence from the lower reliability of the data multiplexed at the same time due to the fading or the like can be further dispersed, and the receiving characteristics can be further improved.  
         [0094]     Moreover, parity bit sequences may be generated in different encoding processings as shown in  FIG. 15 . If parity bit sequences are generated in this manner, encoding is executed in a transmitting station having a configuration shown in  FIG. 16  while decoding is executed in a receiving station having a configuration shown in  FIG. 17 .  
         [0095]     In the transmitting station shown in  FIG. 16 , information bit sequences of different information items  161   a,    161   b,    161   c  are encoded by encoders  162   a,    162   b,    162   c,  respectively.  
         [0096]     An encoder  160 A executes encoding by using the information bit sequences of the respective information items  161   a,    161   b,    161   c.  An encoder  160 B executes encoding by using the encoding result of the encoder  160 A and the information bit sequences of the respective information items  161   a,    161   b,    161   c.  As shown in  FIG. 15 , for example, a parity bit sequence is generated by using the information bit sequences of the respective information items  161   a,    161   b,    161   c,  in the encoding of the encoder  160 A while a parity bit sequence is generated by using the encoding result of the encoder  160 A and the information bit sequences of the respective information items  161   a,    161   b,    161   c,  in the encoding of the encoder  160 B.  
         [0097]     In an encoder  162 d, the parity bit sequence generated by the encoder  160 A is subjected to the same encoding as the encoders  162   a,    162   b,    162   c  and an encoded bit sequence is thereby obtained. Similarly, in an encoder  162   e,  the parity bit sequence generated by the encoder  160 B is subjected to the same encoding as the encoders  162   a,    162   b,    162   c  and an encoded bit sequence is thereby obtained.  
         [0098]     Modulators  163   a,    163   b,    163   c,    163   d,    163   e  modulate carriers by using the encoded bit sequences of the respective encoders  162   a,    162   b,    162   c,    162   d,    162   e.  These modulation results are multiplexed by a multiplexer  164  and then transmitted.  
         [0099]     In the receiving station shown in  FIG. 17 , the receive signal multiplexed by the multiplexer  164  is separated for the respective encoded bit sequences by a demultiplexer  171 . The receive signals thus separated are detected by demodulators  172   a,    172   b,    172   c,    172   d,    172   e,  respectively.  
         [0100]     On the basis of the detection results of the demodulators  172   a,    172   b,    172   c,    172   d,    172   e,  metric generators  173   a,    173   b,    173   c,    173   d,    173   e  generate metric values, respectively. The metric generators  173   a,    173   b,    173   c  correspond to the encoders  162   a,    162   b,    162   c,  respectively. The metric generator  173   d  corresponds to the encoders  162   d  and  160 A. The metric generator  173   e  corresponds to the encoders  162   e  and  160 B.  
         [0101]     For this reason, the metric values corresponding to the information bit sequences of the information bit sequences  161   a,    161   b,    161   c  are obtained by the metric generators  173   a,    173   b,    173   c.  The metric value corresponding to the parity bit sequence output from the encoder  160 A is obtained by the metric generator  173   d.  The metric value corresponding to the parity bit sequence output from the encoder  160 B is obtained by the metric generator  173   e.    
         [0102]     In an iterative decoder  174 , the metric values obtained by the metric generators  173   a,    173   b,    173   c,    173   d,    173   e  are subjected to decoding corresponding to the encoding of the encoders  162   a,    162   b,    162   c,    162   d,    162   e,  by decoders  174   a,    174   b,    174   c,    174   d,    174   e,  respectively.  
         [0103]     The decoding results of the decoders  174   a,    174   b,    174   c,    174   d  are subjected to decoding corresponding to the encoding of the encoder  160 A, by a decoder  174   f.  The decoding results of the decoders  174   a,    174   b,    174   c,    174   d,    174   e  are subjected to decoding corresponding to the encoding of the encoder  160 B, by a decoder  174   g.    
         [0104]     On the basis of the parity bit sequence corresponding to the encoder  160 A, the decoder  174   f  outputs the decoding results of the decoders  174   a,    174   b,    174   c,    174   d  to the decoders  174   a,    174   b,    174   c,    174   d,  respectively. The decoders  174   a,    174   b,    174   c,    174   d  execute decoding again, on the basis of the corrected decoding results.  
         [0105]     On the basis of the parity bit sequence corresponding to the encoder  160 B, the decoder  174   g  outputs the decoding results of the decoders  174   a,    174   b,    174   c,    174   d,    174   e  to the decoders  174   a,    174   b,    174   c,    174   d,    174   e,  respectively. The decoders  174   a,    174   b,    174   c,    174   d,    174   e  execute decoding again, on the basis of the corrected decoding results.  
         [0106]     After that, decoding of the decoders  174   a,    174   b,    174   c,    174   d,    174   e  and the error correction of the decoders  174   g  and  174   f  are iterated. When error is lowered below a predetermined level soon, the desired information items  175   a,    175   b,    175   c  are taken out.  
         [0107]     In the communication system of the above-described configuration, too, even if the orthogonality of the multiplexed information bit sequences cannot be maintained in the communication path, degradation in the receiving characteristics can be restricted, similarly to the communication system shown in  FIG. 9  or  FIG. 11 .  
         [0108]     In relation to the data multiplexed in the transmitting station, the gains can be obtained by decoding the information bit sequences in the decoders  174   a,    174   b,    174   c,    174   d,    174   e  and decoding the parity data in the decoders  174   f  and  174   g,  in the receiving station. The receiving characteristics can be thereby improved as compared with the receiving of  FIG. 2 .  
         [0109]     In addition, parity bit generation using different information bit sequences is multiplexed. For example, the reliability in the decoding result of the encoder  160 A in  FIG. 16  may be deteriorated due to influence from the fading or the like. If the decoding result of the encoder  160 B has a small influence from the fading or the like, the reliability can be improved effectively. For this reason, the influence from the lower reliability in the communication path can be further dispersed and the receiving characteristics can be improved.  
         [0110]     In the transmitting station shown in  FIG. 9 , the parity bit sequence generated by the encoder  90  is further encoded by the encoder  92   d.  However, the encoder  92   d  may be omitted in the receiving station of  FIG. 9  as seen in the transmitting station shown in  FIG. 18 . If such a transmitting station is employed, the receiving station has a configuration shown in  FIG. 19 . In other words, the decoder  114   d  corresponding to the encoder  92   d  is omitted in the receiving station shown in  FIG. 12 .  
         [0111]     According to the communication system having such a configuration, load of the encoding in the transmitting station and load of the decoding in the receiving station can be reduced. The reliability is deteriorated as compared with the communication system shown in  FIG. 9  or  FIG. 12  but the receiving characteristics can be improved since correlation of the multiplexed data in the communication path are used in the similar manner.  
         [0112]     Moreover, to disperse the deterioration in the reliability of the receiving symbol caused by successive fading in the communication path or the like, it is effective to interleave the encoded data.  FIG. 20  shows a configuration of a transmitting station which executes the interleaving processing.  
         [0113]     In the transmitting station shown in  FIG. 20 , interleavers  20   a,    20   b,    20   c,    20   d  are added to the transmitting station shown in  FIG. 9 . The interleavers  20   a,    20   b,    20   c,    20   d  interleave the encoding results of the encoders  92   a,    92   b,    92   c,    92   d,  respectively. The modulators  93   a,    93   b,    93   c,    93   d  modulate carriers by using the processing results of the interleavers  20   a,    20   b,    20   c,    20   d.    
         [0114]     On the other hand, the receiving station has a configuration shown in  FIG. 21 . In the receiving station shown in  FIG. 21 , deinterleavers  21   a,    21   b,    21   c,    21   d  are added to the receiving station shown in  FIG. 11 .  
         [0115]     The deinterleavers  21   a,    21   b,    21   c,    21   d  deinterleave the metric values acquired by the metric generators  113   a,    113   b,    113   c,    113   d,  respectively. In the iterative decoder  114 , the metric values obtained by the deinterleavers  21   a,    21   b,    21   c,    21   d  are subjected to decoding corresponding to the encoding of the encoders  92   a,    92   b,    92   c,    92   d  and the encoder  90 .  
         [0116]     According to the communication system having such a configuration, the metric values of the data assigned to the same multiplexed symbol whose reliability is successively deteriorated due to the fading or the like, can be dispersed. Therefore, influence from successive deterioration in the reliability can be dispersed and the degradation in the receiving characteristics can be restricted.  
         [0117]     Next, application of the above-described scheme to the code division multiplexing broadcast system standardized by ITU-R Recommendation BO.1130-4, Digital System E, will be explained.  FIG. 22  shows a configuration of a transmitting station and  FIG. 23  shows a configuration of a receiving station.  
         [0118]     In the configuration of  FIG. 9 , three information items  91   a,    91   b,    91   c  are input as the information from the information sources. In the code division multiplexing broadcast system, broadcast channel data  221   d ( 1 ) to  221   d (n) are multiplexed and transmitted in the CDM together with a pilot signal  221 , electronic program guide  221   a,  descramble data  221   b,  and subscriber control information  221   c,  as shown in  FIG. 22 .  
         [0119]     Information bit sequences of the electronic program guide  221   a,  descramble data  221   b,  subscriber control information  221   c,  and broadcast channel data  221   d ( 1 ) to  221   d (n) are encoded by encoders  222   a,    222   b,    222   c,    222   d ( 1 ) to  222   d (n), respectively, to obtain encoded bit sequences.  
         [0120]     Interleavers  223   a,    223   b,    223   c,    223   d ( 1 ) to  223   d (n), interleave the encoded bit sequences obtained by the encoders  222   a,    222   b,    222   c,    222   d ( 1 ) to  222   d (n), respectively. Spreaders  224 ,  224   a,    224   b,    224   c,    224   d ( 1 ) to  224   d (n), spread carriers by using the pilot signal and the processing results of the interleavers  223   a,    223   b,    223   c,    223   d ( 1 ) to  223   d (n), respectively.  
         [0121]     Modulators  225 ,  225   a,    225   b,    225   c,    225   d ( 1 ) to  225   d (n), execute modulation by using the spreading results of the spreaders  224 ,  224   a,    224   b,    224   c,    224   d ( 1 ) to  224   d (n), respectively. The modulation results of the modulators are multiplexed by a multiplexer  226  and then transmitted.  
         [0122]     All of the broadcast channel data  221   d ( 1 ) to  221   d (n) are transmitted at any time irrespective of the channel viewed by the receiving station.  
         [0123]     On the other hand, in the receiving station shown in  FIG. 23 , each of despreaders  231 ,  231   a,    231   b,    231   c,    231   d  despreads a receive signal at a timing based on the frame synchronization information, by using a despread code supplied from a receive channel controller  230 . The despreader  231  receives the signal including the pilot signal  221 . The despreader  231   a  receives the signal including the electronic program guide  221   a.  The despreader  231   b  receives the signal including the descramble data  221   b.  The despreader  231   c  receives the signal including the subscriber control information  221   c.  The despreader  231   d  receives the signal including the data of the channel selected by the user, of the broadcast channel data  221   d ( 1 ) to  221   d (n).  
         [0124]     On the basis of channel state information supplied from the receive channel controller  230 , demodulators  232 ,  232   a,    232   b,    232   c,    232   d  demodulate the despreading results of despreaders  231 ,  231   a,    231   b,    231   c,    231   d,  respectively. The pilot signal  221  is demodulated by the demodulator  232 .  
         [0125]     A frame synchronization channel estimator  233  detects the frame synchronization information and obtains channel state information from which a channel is to be estimated, on the basis of the demodulation result of the demodulator  232 , i.e., the pilot signal, and supplies the frame synchronization information and the channel state information to the despreaders  231 ,  231   a,    231   b,    231   c,    231   d  and the demodulators  232 ,  232   a,    232   b,    232   c,    232   d.    
         [0126]     On the basis of the demodulation results of the demodulators  232   a,    232   b,    232   c,    232   d,  metric generators  233   a,    233   b,    233   c,    233   d  generate metric values, respectively.  
         [0127]     Deinterleavers  234   a,    234   b,    234   c,    234   d  deinterleave the metric values obtained by the metric generators  233   a,    233   b,    233   c,    233   d,  respectively. Decoders  235   a,    235   b,    235   c,    235   d  decode the processing results of the deinterleavers  234   a,    234   b,    234   c,    234   d,  respectively.  
         [0128]     The decoders  235   a,    235   b,    235   c  execute the decoding corresponding to the encoders  222   a,    222   b,    222   c.  The decoder  235   d  selectively executes the decoding corresponding to any one of the encoders  222   d ( 1 ) to  222   d (n). As a result of the decoding, broadcast channel data item  236   d  to be viewed is obtained together with the electronic program guide  236   a,  the descramble data  236   b,  and the subscriber control information  236   c.    
         [0129]     In the receiving station having the above-described configuration, the receive channel controller  230  outputs the spread code corresponding to the broadcast channel designated by the user to the despreader  231   d,  to separate the data of the broadcast channel selected by the viewer from the transmit signal to which all of the broadcast channel data are multiplexed. The despreader  231   d  thereby executes receiving by multiplying the spread code with the receive signal.  
         [0130]     For this reason, the pilot signal  221  for estimation of the frame synchronization information and transmission path information to receive the CDM signal, the electronic program guide  221   a,  and the descramble data  221   b  and the subscriber control information  221   c  of the broadcast data are received at any time. In addition, the broadcast data of only one channel selected by the user are decoded. Therefore, five kinds of multiplexed data, of all of the data multiplexed on the transmitting side are received at any time by the receiving station.  
         [0131]     Next, application of the present invention to the above-described code division multiplexing broadcast system will be described.  FIG. 24  shows a configuration of a transmitting station and  FIG. 25  shows a configuration of a receiving station.  
         [0132]     In the transmitting station shown in  FIG. 24 , an encoder  240 , an encoder  242 , an interleaver  243 , a spreader  244  and a modulator  245  are added to the transmitting station shown in  FIG. 22 . Only differences between the transmitting station shown in  FIG. 24  and the transmitting station shown in  FIG. 22  will be explained below.  
         [0133]     In accordance with the encoding of the encoders  222   a,    222   b,    222   c,    222   d ( 1 ) to  222   d (n), the encoder  240  executes encoding by using the information bit sequences of the electronic program guide  221   a,  descramble data  221   b,  and subscriber control information  221   c,  and the broadcast channel data  221   d ( 1 ) to  221   d (n). IN the encoding executed by the encoder  240 , a parity bit sequence is generated by using the information bit sequences, as shown in, for example,  FIG. 10 ,  FIG. 13  or  FIG. 14 .  
         [0134]     In the encoder  242 , the parity bit sequence is subjected to the same encoding as the encoders  222   a,    222   b,    222   c,    222   d ( 1 ) to  222   d (n), and an encoded bit sequence is thereby obtained.  
         [0135]     The interleaver  243  interleaves the encoded bit sequence obtained by the encoder  242 . The spreader  244  spreads a carrier by using the processing result of the interleaver  243 . The modulator  245  executes modulation by using the spreading result of the spreader  244 . This modulation result is multiplexed with the modulation results of the modulators  225 ,  225   a,    225   b,    225   c,    225   d ( 1 ) to  225   d (n) by the multiplexer  226  and then transmitted.  
         [0136]     In the receiving station shown in  FIG. 25 , a despreader  251 , a demodulator  252 , a metric generator  253 , and a deinterleaver  254  are added to the receiving station shown in  FIG. 23 . In addition, an iterative decoder  255  is provided instead of the decoders  235   a,    235   b,    235   c,    235   d.    
         [0137]     Only differences between the receiving station shown in  FIG. 25  and the receiving station shown in  FIG. 23  will be explained below.  
         [0138]     The despreader  251  despreads the receive signal at a timing based on the frame synchronization information, by using the despread code supplied from the receive channel controller  230 . The despreader  251  thereby receives the signal including the parity bit sequence. The demodulator  252  demodulates the despreading result of the despreader  251 , on the basis of the channel state information supplied from the receive channel controller  230 .  
         [0139]     The metric generator  253  generates a metric value on the basis of the demodulation result of the demodulator  252 . The deinterleaver  254  deinterleaves the metric value obtained by the metric generator  253 .  
         [0140]     The iterative decoder  255  comprises decoders  255   a,    255   b,    255   c,    255   d,    255   e,    255   f.  The decoders  255   a,    255   b,    255   c,    25   d,    255   e,  decode the processing results of the deinterleavers  234   a,    234   b,    234   c,    234   d,    254 , respectively.  
         [0141]     The decoders  255   a,    255   b,    255   c  execute the decoding corresponding to the encoders  222   a,    222   b,    222   c.  The decoder  255   d  selectively executes the decoding corresponding to any one of the encoders  222   d ( 1 ) to  222   d (n). The decoder  255   e  executes the decoding corresponding to the encoder  242 .  
         [0142]     In the decoder  255   f,  the decoding results of the decoders  255   a,    255   b,    255   c,    255   d,    255   e  are subjected to the decoding corresponding to the encoding of the encoder  240  and a parity bit sequence is thereby obtained. On the basis of the parity bit sequence corresponding to the encoder  240 , the decoder  255   f  outputs the decoding results of the decoders  255   a,    255   b,    255   c,    255   d,    255   e  to the decoders  255   a,    255   b,    255   c,    255   d,    255   e,  respectively. On the basis of the corrected decoding results, the decoders  255   a,    255   b,    255   c,    255   d,    255   e  executes the decoding again.  
         [0143]     After that, the decoding of the decoders  255   a,    255   b,    255   c,    255   d,    255   e,    255   f  and the error correction of the decoder  255   f  are iterated. The error is lowered below a predetermined level soon and the electronic program guide  236   a,  the descramble data  236   b,  the subscriber control information  236   c  and the data  236   d  of the viewed broadcast channel are obtained.  
         [0144]     In the broadcast system having the above-described configuration, the parity bit sequence is generated on the basis of the information bit sequences of the electronic program guide  236   a,  the descramble data  236   b,  the subscriber control information  236   c,  and the broadcast channel data  221   d ( 1 ) to  221   d (n), and the parity bit sequence and the modulation results based on the information bit sequences are multiplexed and transmitted. In the receiving station, the parity bit sequence is decoded and the information bit sequences are decoded on the basis of the decoded parity bit sequence. Therefore, even if the orthogonality of the multiplexed information bit sequences cannot be maintained in the communication path, the degradation in the receiving characteristics can be restricted. It is assumed that the broadcast system ( 1 ) shown in  FIG. 22  and  FIG. 23  has thirty channels of the multiplexed data including the control information. It is also assumed that the broadcast system ( 2 ) shown in  FIG. 24  and  FIG. 25  has thirty channels of the multiplexed data including the control information and thirty-one channels including the parity bit sequence are multiplexed.  FIG. 26  shows receive bit error rate characteristics of both the broadcast systems.  
         [0145]     As the condition to obtain the characteristics, an urban multipath environment modeled by 3rd Generation Partnership Project (3GPP), i.e. International Mobile Telecommunication 2000 (IMT2000) is set as the communication path.  
         [0146]     Since the broadcast system ( 2 ) has more multiplexed data channels by one than the broadcast system ( 1 ), the interference amount of the data caused by the multipath is greater. In the broadcast system ( 2 ), however, the decoding of the broadcast system ( 1 ) and decoding of the new parity bit sequence are iterated in the receiving station. Therefore, the receive bit error rate characteristic is improved as shown in  FIG. 26 .  
         [0147]     Such an improvement of the characteristic indicates that since the correlation information of the multiplexed data in the communication path can be ideally used for the multiplexed data at the time of decoding in the decoder  255 f corresponding to the encoder  240 , the multiplexed receive signal can be optimally received.  
         [0148]     In the broadcast system ( 2 ), the parity bit sequence generated by the newly added encoder  240  is multiplexed similarly to the other normal data. For this reason, the receiving operation can be executed even in the receiving station shown in  FIG. 23 , similarly to the case where the transmitting station has the configuration shown in  FIG. 22 .  
         [0149]     In the broadcast system ( 2 ), the number of the receive channels in the receiving station shown in  FIG. 25  is smaller than the number of the multiplexed channels in the transmitting station shown in  FIG. 24  and all of the channels used by the encoder  240  is not used in the receiving station. In the receiving station, considering that the data of the channel which is not received by the receiving station is punctured, the decoder  255   f  may execute the decoding without using the channel data.  
         [0150]     Incidentally, the code division multiplexing broadcast system standardized by the ITU-R is composed of the system which simultaneously executes distribution of the broadcast data using a broadcast satellite and distribution of the broadcast data using a ground repeater.  
         [0151]     The transmitting station shown in  FIG. 24  can be applied to a ground broadcasting station  271  as shown in  FIG. 27 ( a ), a satellite repeater  272  as shown in  FIG. 27 ( b ), or a ground repeater  273  as shown in  FIG. 27 ( c ). In any of the configurations, the advantage of improvement of the receiving characteristic can be obtained. A receiving terminal  274  in  FIG. 27  corresponds to the receiving station in  FIG. 25 .  
         [0152]     If the transmitting station is applied to the ground broadcasting station as shown in  FIG. 27 ( a ), an existing satellite repeater and an existing ground repeater can be used without modification.  
         [0153]     The applied system of the present invention is not limited to the broadcast system. For example, it can also be applied to large-capacity relay communication using the multiplexing in repeater lines of subscriber telephones.  
         [0154]     The present invention is not limited to the embodiments described above but the constituent elements of the invention can be modified in various manners without departing from the spirit and scope of the invention. Various aspects of the invention can also be extracted from any appropriate combination of a plurality of constituent elements disclosed in the embodiments. Some constituent elements may be deleted in all of the constituent elements disclosed in the embodiments. The constituent elements described in different embodiments may be combined arbitrarily.  
         [0155]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.