Abstract:
A radio communication system includes a transmission terminal and a reception terminal. The transmission terminal divides and outputs transmission data, modulates the output data to a plurality of subcarriers, converts the modulated data to a temporal waveform of a multicarrier signal, and copies divided transmission data a predetermined number of times such that the number of divided transmission data, subsequent to being copied the predetermined number of times, is equal to the number corresponding to the plurality of subcarriers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radio communication systems, particularly a radio communication system that carries out transmission and reception of multicarrier signals, a transmission terminal, and a reception terminal. 
     2. Description of the Background Art 
     In the field of radio communication systems, various studies have conventionally been made about the reliability of transmitted and received data. 
     For example, Japanese Patent Laying-Open No. 2004-088268 discloses, in order to improve the error rate of reception data with respect to signals that are transmitted again (retransmission signal) from the sender side when transfer error is detected at the receiver side in an OFDM (Orthogonal Frequency-Division Multiplexing)-CDMA (Code Division Multiple Access) communication scheme, the technique of increasing the number of spread codes to be allocated to retransmission signals as the retransmission count increases. 
     With the widespread use of various portable communication devices including cellular phones, it is now increasingly critical to improve the reliability of data transmitted and received in a radio communication system. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the present invention is to improve the reliability of data transmitted and received in a radio communication system. 
     According to an aspect of the present invention, a radio communication system includes a transmission terminal transmitting transmission data identified as data to be transmitted, and a reception terminal receiving data transmitted by the transmission terminal. The transmission terminal includes an output unit dividing transmission data for output, a modulation unit modulating the data output from the output unit to a plurality of subcarriers, and a conversion unit converting the data modulated by the modulation unit to a temporal waveform of a multicarrier signal. The output unit divides the transmission data and copies the divided transmission data a predetermined number of times for output to the modulation unit such that the number of divided transmission data, subsequent to being copied the predetermined number of times, is equal to a number corresponding to a plurality of subcarriers. 
     According to an aspect of the present invention, a transmission terminal for transmitting transmission data identified as data to be transmitted includes an output unit dividing transmission data for output, a modulation unit modulating the data output from the output unit to a plurality of subcarriers, and a conversion unit converting the data modulated by the modulation unit to a temporal waveform of a multicarrier signal. The modulation unit adds a long training signal to each subcarrier a plurality of times. The conversion unit combines a pilot signal to the data modulated by the modulation unit to generate the multicarrier signal. The output unit divides the transmission data and copies the divided transmission data a predetermined number of times for output to the modulation unit such that the number of the divided transmission data, subsequent to being copied the predetermined number of times, is equal to a value of the number of the plurality of subcarriers minus the number of pilot signals combined to one multicarrier signal. A radio communication system including the transmission terminal also is disclosed. 
     A transmission terminal according to an aspect of the present invention transmits transmission data identified as data to be transmitted. The transmission terminal includes an output unit dividing transmission data for output, a modulation unit modulating the data output from the output unit to a plurality of subcarriers, and a conversion unit converting the data modulated by the modulation unit to a temporal waveform of a multicarrier signal. The output unit divides the transmission data and copies the divided transmission data a predetermined number of times for output to the modulation unit such that the number of divided transmission data, subsequent to being copied the predetermined number of times, is equal to the number corresponding to the plurality of subcarriers. 
     A reception terminal according to an aspect of the present invention receives data transmitted from a transmission terminal that transmits a multicarrier signal generated by modulating, by a plurality of subcarriers, divided transmission data, subsequent to being copied the predetermined number of times. The reception terminal includes a separation unit separating received data for every subcarrier, a demodulation unit demodulating data of every subcarrier separated by the separation unit, an average calculation unit calculating, for data of every subcarrier demodulated by the demodulation unit, an average of each set of data of the same copy original from the copy by the output unit, and an integration unit integrating the averages for data of the same copy original from the copy by the output unit, calculated by the average calculation unit. 
     According to another aspect of the present invention, a radio communication system includes a transmission terminal transmitting transmission data identified as data to be transmitted, and a reception terminal receiving data transmitted by the transmission terminal. The transmission terminal includes an output unit dividing transmission data for output, a modulation unit modulating the data output from the output unit to a plurality of subcarrier, and a conversion unit converting the data modulated by the modulation unit to a temporal waveform of a multicarrier signal. The modulation unit adds a long training signal to each subcarrier a plurality of times. 
     A transmission terminal according to another aspect of the present invention transmits transmission data identified as data to be transmitted. The transmission terminal includes an output unit dividing transmission data for output, a modulation unit modulating the data output from the output unit to a plurality of subcarriers, and a conversion unit converting the data modulated by the modulation unit to a temporal waveform of a multicarrier signal. The modulation unit adds a long training signal to each subcarrier a plurality of times. 
     At the transmission terminal of the present invention, transmission data is divided by a plurality of subcarriers for every plurality of times, each divided transmission data is copied a predetermined number of times, and then modulated by a plurality of subcarriers to generate a multicarrier signal, which is transmitted from the transmission terminal to the reception terminal. 
     Since the same data is transmitted by a plurality of subcarriers to the reception terminal, the reception terminal can demodulate each data in a manner of improved reliability by, for example, taking the average of the same data transmitted by a plurality of subcarriers. 
     Therefore, the reliability of transmitted and received data can be improved in a radio communication system. 
     When data is modulated by a plurality of subcarriers at the transmission terminal in the present invention, a long training signal is added a plurality of times to each subcarrier. 
     Therefore, the frequency phase error during data transmission can be reliably adjusted at the reception terminal. 
     Thus, the reliability of transmitted and received data can be improved in a radio communication system. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a hardware configuration of a terminal that carries out data transmission on a radio communication network according to an embodiment of the present invention. 
         FIG. 2A  represents a portion in a control MAC (Media Access Control) unit of  FIG. 1 , utilized when the transmission circuit transmits data. 
         FIG. 2B  represents a portion of the control MAC unit, utilized when the transmission circuit receives data. 
         FIG. 3  is a diagram to describe how a multicarrier signal is handled in a radio communication network according to an embodiment of the present invention. 
         FIG. 4  is a diagram to describe how a long training signal generated at a long training signal generation unit of  FIG. 2  is added to transmission data. 
         FIG. 5  is a diagram to describe how a guard interval is inserted to a multicarrier signal by a guard interval addition unit of  FIG. 2A . 
         FIG. 6  is a diagram to describe the manner of insertion of a guard interval to a multicarrier signal in a general radio communication network. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a radio communication network of the present invention will be described hereinafter with reference to the drawings. 
     The radio communication network of the present embodiment is directed to communication under the OFDM scheme. 
     Referring to  FIG. 1 , a terminal  1  mainly includes a host system  100  that carries out execution of an application and the like, and a communication circuit  200  that transmits and receives data and the like used in the application. 
     Host system  100  includes a CPU (Central Processing Unit) that provides overall control of the operation of host system  100 . 
     The program of each application executed by host system  100  is stored in an HD (hard disk)  102 . Host system  100  includes an RAM (Random Access Memory)  103  qualified as a work area of CPU  101 , a display  104  to provide a display of information, a speaker  105  to output sound, an input unit  106  employed for entry of information such as a key and/or button, and an interface  107  for transfer of information (data) with respect to communication circuit  200 . 
     Communication circuit  200  includes a baseband/MAC circuit  250 , an RF (Radio Frequency) circuit  205 , a balun  204 , an antenna  203 , EEPROMs (Electronically Erasable and Programmable Read Only Memory)  206  and  207 , a power supply circuit  201 , and a clock circuit  202 . 
     Clock circuit  202  supplies a clock signal to baseband/MAC circuit  250  and RF circuit  205 . Power supply circuit  201  controls the supply of power to baseband/MAC circuit  250  and RF circuit  205 . 
     RF circuit  205  transmits and receives data via antenna  203 . Balun  204  is provided between antenna  203  and RF circuit  205 . 
     Baseband/MAC circuit  250  includes a CPU  251 , an interface  252 , an external bus controller  253 , a program memory  254 , a shared memory  255 , a timer  256 , a control MAC unit  300 , an ADC (analog-digital converter)  258 , and a DAC (digital-analog converter)  259 . 
     Interface  252  is directed to host system  100 . 
     Upon receiving an instruction to transmit data onto the network from host system  100 , CPU  251  causes interface  252  to take out relevant data stored in a memory (for example, RAM  103 ) in host system  100 . Host system  100  transmits, after generating data to instruct transmission and storing the relevant data in the aforementioned memory, a transmission instruction of the relevant data to communication circuit  200 . The data output by interface  252  is temporarily stored in program memory  254  as data constituting “user data body” of the frame that is to be transmitted onto the network. 
     CPU  251  generates a frame that is to be transmitted onto the network by adding various data including a MAC header and a FCS (Frame Check Sequence) to the data stored in program memory  254 . CPU  251  stores the generated frame in program memory  254 , and sets up a flag in shared memory  255  indicating that the frame has been generated. 
     The operation in receiving data transmitted via the network at communication circuit  200  will be described hereinafter. The frame transmitted to RF circuit  205  via antenna  203  and balun  204  is converted into digital data at ADC  258 , and then delivered to control MAC unit  300 . Control MAC unit  300  carries out, on the frame converted into digital signals, detection of the frame beginning, synchronous processing of time and frequency, and then error correction decoding. Control MAC unit  300  also determines whether the transmission address (DA) of the relevant frame matches the MAC address of the relevant communication circuit  200  stored in EEPROM  206 . When determination is made of a match, control MAC unit  300  removes the MAC header and FCS from the frame, and transfers the remaining data (frame body) to program memory  254 . When determination is made of a mismatch, control MAC unit  300  discards the received frame. 
     When the received frame body is stored in program memory  254 , control MAC unit  300  sets a flag indicating such information in shared memory  255 . CPU  251  responds to the setting of this flag to transmit frame body region  320  stored in program memory  254  to host system  100  via interface  252 . 
     Referring to  FIG. 2A  corresponding to transmission, control MAC unit  300  includes, at the transmission side, a long training signal generation unit (designated as “LT generation unit” in  FIG. 2A )  301 , a data combining unit  302 , a serial-parallel conversion unit (designated as “S/P” in  FIG. 2A )  303 , a pilot signal generation unit  304 , an inverse Fourier transform unit (designated as “IFFT” in  FIG. 2A )  305 , a guard interval addition unit (designated as “GI ADDITION UNIT” in  FIG. 2A )  306 , a parallel-serial conversion unit (designated at “P/S” in  FIG. 2A )  307 , and subcarrier modulation units  3001 - 3036 . 
     At the transmission side of control MAC unit  300 , the data obtained from host system  100  and stored in program memory  254  (“TRANSMISSION DATA” in  FIG. 2A ) is combined, at data combining unit  302 , with a long training signal generated at long training signal generation unit  301 . In the radio communication system of the present embodiment, data is modulated under the multicarrier modulation scheme for transmission and reception. At control MAC unit  300  of terminal  1 , long training signals and transmission data are arranged, as shown in  FIG. 4 , in the frame of each subcarrier. 
     Referring to  FIG. 4 , the long training signal is represented as “LT” and transmission data is represented as “DATA”. A frame  400  includes a header region  401 , a frame body region  402 , and an FCS  403 . In frame body region  402 , LT  402 A and DATA  402 B are arranged alternately. LT  402 A is added for every DATA  402 B corresponding to 32 symbols. In other words, each DATA  402 B is identified as data of 32 symbols. A long training signal is added a plurality of times in frame  400 . 
     The contents of data in the signal field of frame body region  402  are shown in Table 1. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Symbol 
                 Signal 
               
               
                   
               
             
             
               
                 3:0 
                 Reserved 
               
               
                 15:4  
                 Length Field 
               
               
                 23:16 
                 CRC8 
               
               
                 29:24 
                 Tail Bits 
               
               
                   
               
             
          
         
       
     
     It is appreciated from Table 1 that data corresponding to the error correction of “CRC8” is included in the signal field. Error correction is carried out at a Viterbi decode unit  358  in the present embodiment, as will be described afterwards. 
     Referring to  FIG. 2A  again, serial-parallel conversion unit  303  divides the data combined at data combining unit  302  into 12. Serial-parallel conversion unit  303  repeats the twelve divided data three times for output to respective subcarrier modulation units  3001 - 3036 . With regards to the three sets of the 12-divided data, serial-parallel conversion unit  303  provides the data divided into 12 and corresponding to the first set to each of subcarrier modulation units  3001 - 3012 , the data divided into 12 and corresponding to another set to each of subcarrier modulation units  3013 - 3024 , and the data divided into 12 and corresponding to the last set to subcarrier modulation units  3025 - 3036 . 
     Subcarrier modulation units  3001 - 3036  carry out modulation for each subcarrier, and provide the modulated data to inverse Fourier transform unit  305 . 
     At inverse Fourier transform unit  305 , the subcarrier signal output from each of subcarrier modulation units  3001 - 3036  is subjected to inverse Fourier transform. Accordingly, the subcarrier signals output from subcarrier modulation units  3001 - 3036  are combined, and a multicarrier signal is generated. Inverse Fourier transform unit  305  combines a pilot signal generated by pilot signal generation unit  304  to the subcarrier signals output from subcarrier modulation units  3001 - 3036  to generate a multicarrier signal. The configuration of the generated multicarrier signal will be described here. 
     In  FIG. 3  provided to describe how transmission data is handled at control MAC unit  300 , the upper region corresponds to the transmission side and the lower region corresponds to the reception side. 
     At the transmission side of  FIG. 3 , 52 rectangles are indicated, including those having a numeric assigned at the top, and those in hatched representation. The rectangle with a numeric corresponds to any one of the twelve data divided at serial-parallel conversion unit  303 . The numeric at the top represents which of the twelve divided data it corresponds to. The hatched rectangle corresponds to a pilot signal generated at pilot signal generation unit  304 . The multicarrier signal generated in the present embodiment includes 16 pilot signals, among the 52 subcarriers. By the relatively large number of pilot signals included in the generated multicarrier signal of the present embodiment, phase error occurring in long-distance communication can be corrected more reliably at the terminal receiving the multicarrier signal. 
     With regards to subcarriers other than the subcarriers assigned to the 16 pilot signals among the 52 carriers indicated at the transmission side of  FIG. 3 , i.e. 36 subcarriers, the data divided into 12 at serial-parallel conversion unit  303  is respectively assigned to three subcarriers. Accordingly, data can be obtained by three subcarriers for each of the 12 divided data at the reception side. Therefore, data of higher reliability can be obtained by taking an average thereof, as will be described afterwards. 
     Although the number of repetition (the number of times of copy) of the divided data in the multicarrier signal is set to 3, the copy count is not limited to 3 in the radio communication system of the present invention. The number of times of copying should be determined in view of the trade off between the required communication efficiency of data and the required reliability in data transmission/reception, depending upon each circumstance of the radio communication system application. 
     Referring to  FIG. 2A  again, the multicarrier signal generated at inverse Fourier transform unit  305  is provided to a parallel-serial conversion unit  307 . At parallel-serial conversion unit  307 , a guard interval (a redundant signal) is inserted by guard interval addition unit  306 . The insertion of a guard interval by parallel-serial conversion unit  307  will be described hereinafter with reference to  FIG. 5 . 
     Referring to  FIG. 5 , an OFDM symbol is produced by inserting a guard interval to the multicarrier signal output from inverse Fourier transform unit  305 . In the OFDM symbol, a guard interval of 1.6 μS and multicarrier signal data of 3.2 μS are present alternately. A guard interval is a redundant signal to avoid interference between codes during data transmission. The insertion of a guard interval allows the transmitted data to be protected from multipath fading. 
     By the relatively high ratio of the guard intervals inserted in the OFDM symbol to the data of the multicarrier signals in the present embodiment, transmission data can be protected from multipath fading more reliably in the radio communication system of the present embodiment. 
       FIG. 6  schematically represents an OFDM symbol that is transmitted and received in a general data communication. Referring to  FIG. 6 , the OFDM symbol that is generally transmitted/received has a guard interval of 0.8 μS and multicarrier signal data of 3.2 μS arranged alternately. 
     In the event of transmitting transmission data in the present embodiment, transmission data is combined with a long training signal at data combining unit  302 , divided into 12 at serial-parallel conversion unit  303 , copied three times and modulated at subcarrier modulation units  3001 - 3036 , added with a pilot signal at inverse Fourier transform unit  305 , inserted with a guard interval by guard interval addition unit  306  at parallel-serial conversion unit  307 , resulting in an OFDM symbol. The generated OFDM symbol is transmitted onto the network via DAC  259 , RF circuit  205 , balun  204  and antenna  203 . 
     The configuration of control MAC  300  in association with the reception side will be described hereinafter. 
     Referring to  FIG. 2B , the data converted into digital data at ADC  258  (refer to  FIG. 1 ) is applied to synchronous processing unit  351 . At synchronous processing unit  351 , detection of the beginning of a frame as well as the well-known synchronous processing such as symbol timing synchronization and carrier frequency synchronization are carried out. The processed data is provided to a serial-parallel conversion unit  353 . At serial-parallel conversion unit  353 , the data from synchronous processing unit  351  has the guard interval removed by a GI removal unit  352 , and divided for every subcarrier to be provided to a Fourier transform unit (designated as “FFT” in  FIG. 2B )  354 . 
     At Fourier transform unit  354 , the signals of the 52 received subcarriers other than those corresponding to the 16 pilot signals described with reference to the transmission side of  FIG. 3  (i.e. 36 subcarriers) are provided to subcarrier detection units  3501 - 3536 . Pilot signal detection unit  355  detects a pilot signal among the 52 subcarrier signals output to Fourier transform unit  354 . Pilot signal detection unit  355  provides the detected pilot signal to a phase correction unit  356 . 
     Phase correction unit  356  demodulates the 12 data (divided at serial-parallel conversion unit  303 ) based on the 36 subcarrier signals output from respective subcarrier detection units  3501 - 3536 . The manner of demodulation will be described hereinafter with reference to the reception side of  FIG. 3 . 
     Referring to the reception side of  FIG. 3 , the data divided to 12 at serial-parallel conversion unit  303  is included three times each among the 36 subcarriers in the 52 subcarriers of the multicarrier signal at phase correction unit  356 . At phase correction unit  356 , an average of the data of the three subcarriers is obtained (calculated) for each of the 12 data, and demodulation is carried out based on the average. The pilot signal applied from pilot signal detection unit  355  is used for phase correction in this case. 
     Calculation of the average and data demodulation at phase correction unit  356  set forth above is under control of control unit  356 A. 
     The 12 data demodulated at phase correction unit  356  are provided to parallel-serial conversion unit  357  to be combined. 
     The data combined at parallel-serial conversion unit  357  is provided to Viterbi decode unit  358  to be subjected to error correction by Viterbi decoding, and then output to shared memory  255  via external bus controller  253 . 
     The present invention is advantageous in that the number of previous data referred to in error correction at Viterbi decode unit  358  can be set lower, for example to “40”, lower than the number of data generally referred to (for example, “70”), by the measures taken in the present embodiment such as subcarrier-modulation with the same data repeated (three times each), including relatively many pilot signals, setting the ratio of inserted guard intervals relatively larger, and setting the number of added long training signals relatively larger. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.