Abstract:
To provide high-quality communication even when the power for synchronization preamble is reduced in an OFDM communication device. To achieve symbol synchronization, an OFDM communication device obtains a zero amplitude reduced preamble signal by passing a specified synchronization preamble through an ideal low-pass filter to reduce a signal component near zero amplitude within a time domain, and time-multiplexes the obtained zero amplitude reduced preamble signal with transmit data to generate an OFDM transmit signal. A receiver section of the OFDM communication device determines the cross correlation between a receive signal and a specified synchronization preamble, which is patterned the same as the counterpart in a transmitter section of the OFDM communication device, and detects a synchronization position in accordance with the determined cross correlation.

Description:
[0001]     The present invention relates to a symbol synchronization method for use in an orthogonal frequency division multiplexing (OFDM) communication device, and more particularly to a technology that is used in an OFDM communication device, which uses a synchronization preamble to achieve symbol synchronization between a transmitter and a receiver, for the purpose of preventing the communication characteristics from deteriorating even when the synchronization preamble power is reduced.  
         [0002]      FIG. 9  schematically shows the configuration of a prior art OFDM communication device that uses a synchronization preamble to achieve symbol synchronization for OFDM communication. The transmitter section of the communication device shown in the figure comprises a preamble generator  91 , a data supplier  93 , a time multiplexer  95 , a zero insertion section  97 , an inverse fast Fourier transform (IFFT) section  99 , and a guard interval (GI) insertion section  101 .  
         [0003]     The receiver section of the communication device shown in the figure comprises a time domain preamble (tx_preamble) supply section, which includes a preamble generator  91   a , a zero insertion section  97   a , an IFFT section  99   a , and a GI insertion section  101   a , and a synchronization timing detector  107 . The synchronization timing detector  107  comprises a cross-correlation calculator  103  and a synchronization timing calculator  105 .  
         [0004]     In the configuration described above, the time multiplexer (MUX)  95  in the transmitter section time-multiplexes preamble data, which is supplied from the preamble generator  91  in a specified pattern, and transmit data, which is supplied from the data supplier  93 . The zero insertion section  97  subjects the time-multiplexed data to a zero insertion process (filling with zeros) for the purpose of avoiding interference from an external signal. The resulting data is then inverse fast Fourier transformed in the IFFT section  99 . Next, the GI insertion section  101  adds a guard interval (GI) to the transformed data in order to suppress multipath interference. A transmit OFDM signal comprising an OFDM symbol, which comprises the guard interval and information, is then generated.  
         [0005]     The receiver section generates a time domain synchronization preamble (tx_preamble) in the same pattern as the transmitter section. This synchronization preamble is generated by the preamble generator  91   a , zero insertion section  97   a , IFFT section  99   a , and GI insertion section  101   a . For the time domain synchronization preamble (tx_preamble), the cross-correlation calculator  103  calculates the cross correlation with a receive signal transmitted from the transmitter section. The synchronization timing calculator  105  determines a position that is shifted from a peak value position by a specified amount of time and generates synchronization timing data. Symbol synchronization can then be achieved between the transmitter and receiver sections of the OFDM communication device.  
         [0006]     However, the synchronization preamble (tx_preamble) used to determine the cross correlation in the receiver section requires a large number of bits because it has a Gauss distribution and wide dynamic range. Thus, the volume of calculations performed in the cross-correlation calculator  103  is huge. It is therefore proposed that the volume of calculations performed in the cross-correlation calculator  103  be reduced by quantizing the time domain synchronization preamble to one bit as a synchronization preamble (Taira, et al., “OFDM Communication System Timing Synchronization Method for Frequency-Selective Fading Environment,” Journal B of The Institute of Electronics, Information and Communication Engineers, Vol. J84-B, No. 7, pp. 1255-1264, July 2001).  
         [0007]     As described above, the calculations performed by a prior art to determine the cross correlation between a time domain synchronization preamble and receive signal are large in volume and not practical because the amplitude distributions of both signals are Gauss distributions having an average value of 0. It is therefore proposed that the synchronization preamble be used after being quantized to one bit. It is also preferred that the power for synchronization preamble transmission be minimized in the OFDM communication device for the purpose of reducing the time required for cross correlation calculations and the interference with synchronization preamble data. However, if the power for synchronization preamble transmission is reduced in a situation where the synchronization preamble is used after being quantized to one bit as described above, the bit error rate (BER) and other communication characteristics deteriorate.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made in view of the above prior art problems and provides a method for achieving symbol synchronization in an OFDM communication device while reducing the degree of characteristics deterioration even when the power for synchronization preamble transmission is reduced.  
         [0009]     One aspect of the present invention provides a transmitter for use in an OFDM communication device, which uses a synchronization preamble to achieve synchronization between the transmitter and receiver. The transmitter obtains a zero amplitude reduced preamble signal by passing a specified synchronization preamble through an ideal low-pass filter to reduce a signal component near zero amplitude within a time domain, and time-multiplexes the obtained zero amplitude reduced preamble signal with transmit data to generate an OFDM transmit signal.  
         [0010]     In the above instance, it is convenient that the ideal low-pass filter comprise an FFT section for subjecting an input signal to fast Fourier transform (FFT) and a zero substitution section for providing zero substitution for FFT section output components having a frequency higher than specified.  
         [0011]     Further, the ideal low-pass filter may comprise a table that stores values obtained when input signals pass through the ideal low-pass filter in accordance with the values of the input signals.  
         [0012]     It is also convenient that the ideal low-pass filter comprise a table that stores values obtained when input signals pass through the ideal low-pass filter in accordance with the values of the input signals.  
         [0013]     Another aspect of the present invention provides a receiver in the OFDM communication device for use with the transmitter. The receiver a synchronization timing detector for determining the cross correlation between a receive signal and a specified synchronization preamble, which is patterned the same as the counterpart in the transmitter section, and detecting a synchronization position in accordance with the determined cross correlation.  
         [0014]     In the above instance, it is convenient that the synchronization position be shifted from a peak position of the cross correlation by a specified amount of time.  
         [0015]     Still another aspect of the present invention provides an OFDM communication device that uses a synchronization preamble to achieve synchronization between a transmitter and a receiver. The OFDM communication device comprises a transmitter for obtaining a zero amplitude reduced preamble signal by passing a specified synchronization preamble through an ideal low-pass filter to reduce a signal component near zero amplitude within a time domain, and generating an OFDM transmit signal by time-multiplexing the obtained zero amplitude reduced preamble signal with transmit data and a receiver having a synchronization timing detector for determining the cross correlation between a receive signal and a specified synchronization preamble, which is patterned the same as the counterpart in the transmitter section, and detecting a synchronization position in accordance with the determined cross correlation.  
         [0016]     In the above instance, it is convenient that the ideal low-pass filter comprise an FFT section for subjecting an input signal to fast Fourier transform (FFT) and a zero substitution section for providing zero substitution for FFT section output components having a frequency higher than specified.  
         [0017]     Further, the ideal low-pass filter may comprise a table that stores values obtained when input signals pass through the ideal low-pass filter in accordance with the values of the input signals.  
         [0018]     It is also convenient that the synchronization position be shifted from a peak position of the cross correlation within the receiver by a specified amount of time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a block diagram that schematically shows the configuration of one embodiment of an OFDM communication device according to the present invention.  
         [0020]      FIG. 2  is a graph that compares the amplitude-vs.-PDF characteristic of a zero amplitude reduced preamble according to the present invention against that of a prior art.  
         [0021]      FIG. 3  is a graph that compares the power-vs.-CDF characteristic of synchronization preamble use according to the present invention against that of a prior art.  
         [0022]      FIG. 4  is a block diagram that illustrates a simulation model for performance evaluation of the present invention.  
         [0023]      FIG. 5  is a block diagram that illustrates a simulation model for performance evaluation of a prior art communication device.  
         [0024]      FIG. 6  illustrates the simulation conditions for performance evaluation.  
         [0025]      FIGS. 7A and 7B  illustrate the characteristics of a multipath fading channel that is used for performance evaluation.  
         [0026]      FIG. 8  is a graph that compares the simulation results of the present invention and prior art.  
         [0027]      FIG. 9  is a block diagram that schematically shows the configuration of a prior art OFDM communication device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  FIG. 1  schematically shows the configuration of one embodiment of an OFDM communication device according to the present invention. The transmitter section of the communication device shown in the figure comprises a synchronization signal generator  10 , a data supplier  7 , a zero insertion section  9 , a time multiplexer (MUX)  11 , an inverse fast Fourier transform (IFFT) section  13 , and a guard interval (GI) insertion section  15 . The synchronization signal generator  10  includes a preamble generator  1 , a fast Fourier transform (FFT) section  3 , and a zero substitution section  5 .  
         [0029]     The receiver section of the communication device comprises a synchronization timing detector  20 , which includes a cross-correlation calculator  19  and a synchronization timing calculator  21 , and a preamble supplier  17 , which generates a synchronization preamble (org_preamble) that is patterned the same as the counterpart generated from the preamble generator  1  in the transmitter section.  
         [0030]     In the transmitter section of the OFDM communication device shown in  FIG. 1 , the synchronization signal generator  10  generates a zero amplitude reduced preamble signal. In other words, the original preamble signal (org_preamble) for a specified pattern supplied from the preamble generator  1  is supplied to an ideal low-pass filter (ideal LPF), which comprises the FFT section  3  and zero substitution section  5 . The ideal LPF is implemented when the original preamble signal is fast Fourier transformed in the FFT section  3  and then components having higher frequencies than those in a specified pass band are subjected to zero substitution. More particularly, the ideal LPF is implemented by subjecting frequency components having frequencies higher than specified to zero substitution.  
         [0031]     Practically, the ideal LPF may comprise a table that stores values obtained when input signals pass through the ideal low-pass filter in accordance with the values of the input signals. When such a table is employed and configured to obtain an output signal in response to an input signal, an ideal LPF having a simple structure and a high response speed can be implemented.  
         [0032]     The zero amplitude reduced preamble signal X, which is obtained as described above, is supplied to the time multiplexer  11 . The transmit data fed from the data supplier  7  is subjected to zero insertion in the zero insertion section  9  as is the case with the aforementioned prior art, supplied to the time multiplexer  11 , and time-multiplexed with the above zero amplitude reduced preamble signal X. The resulting time-multiplexed signal is inverse fast Fourier transformed in the IFFT section  13 . Further, the GI insertion section  15  inserts a guard interval into the resulting signal. This produces a transmit OFDM signal.  
         [0033]     The receiver section acquires a receive signal by receiving the transmit OFDM signal, which is obtained in a manner described above, via a desired communication channel. In the cross-correlation calculator  19 , this receive signal calculates the cross correlation with a 1-bit synchronization preamble (org_preamble) that is prevalent before passage through the ideal LPF in the transmitter section. The resulting cross correlation value has a peak value for a specified timing part. The synchronization timing calculator  21  calculates a synchronization position, which is shifted from the peak value position by a specified amount of time. Symbol synchronization is then achieved between the transmitter and receiver sections.  
         [0034]      FIG. 2  compares the amplitude-vs.-PDF characteristics of the zero amplitude reduced preamble signal, which is used with the OFDM communication device according to the present invention, and the synchronization preamble signal, which is used with the prior art. The term “PDF” is an acronym for probability density function. As is obvious from  FIG. 2 , the amount of near-zero signal component of the synchronization preamble according to the present invention is smaller than that of the prior art. It can therefore be estimated that the probability of significant deterioration in the instantaneous carrier-to-noise ratio (CNR) would decrease. Thus, it is possible to reduce the power required for synchronization preamble transmission.  
         [0035]      FIG. 3  compares the power-vs.-CDF (cumulative distribution function) characteristics of the zero amplitude reduced preamble signal according to the present invention and the prior art synchronization preamble signal. It can be seen from the figure that a low-power portion of the synchronization preamble according to the present invention is reduced in terms of distribution.  
         [0036]     For comparison between the advantages provided by the present invention and the prior art, the simulation models shown in  FIGS. 4 and 5  are evaluated.  FIG. 4  shows a simulation model for a communication device that is configured in accordance with the present invention. In the configuration shown in  FIG. 4 , the configuration of a transmission section is basically the same as that of the transmission section of the communication device shown in  FIG. 1 . More specifically, the transmission section shown in  FIG. 4  comprises a preamble generator  41 , an FFT section  43 , a zero substitution section  45 , a data supplier  47 , a QPSK modulator  48 , a zero insertion section  49 , a time multiplexer  51 , an IFFT section  53 , and a GI insertion section  55 . It should be noted, however, that the QPSK modulator  48  is provided between the data supplier  47  and zero insertion section  49 .  
         [0037]     A multipath fading channel  57 , an adder  59 , a white noise (AWGN) generator  51 , a synchronization timing detector  63 , a GI eliminator  65 , an FFT section  67 , a QPSK demodulator  69 , and an uncoded BER calculator  71  are incorporated to evaluate the signal in the transmitter section shown in  FIG. 4 .  
         [0038]      FIG. 5  shows a simulation model for the prior art. As is the case with the transmitter section shown in  FIG. 9 , the transmitter section shown in  FIG. 5  comprises a preamble generator  73 , a data supplier  75 , a QPSK modulator  77 , a time multiplexer  79 , a zero insertion section  81 , an IFFT section  83 , and a GI insertion section  85 . It should be noted that the QPSK modulator  77  is provided between the data supplier  75  and time multiplexer  79 . The elements for receiving a signal from the transmitter section described above for evaluation purposes are configured the same as shown in  FIG. 4  and designated by the same reference numerals as indicated in  FIG. 4 .  
         [0039]     The synchronization timing detector  63  in  FIGS. 4 and 5  is configured the same as the synchronization timing detectors  20 ,  107 , which are shown in  FIGS. 1 and 9 , respectively. Further, the uncoded information bit error rate (uncoded BER) prevalent when the power for synchronization preamble transmission is attenuated is employed as an evaluation index.  
         [0040]      FIGS. 6 and 7  depict simulation conditions.  FIG. 6  shows various simulation conditions. In  FIG. 6 , the term “SCH” denotes a synchronization preamble transmission channel, whereas the term “DTCH” denotes a data transmission channel.  
         [0041]      FIGS. 7A and 7B  show a channel model of a multipath fading channel.  FIG. 7A  shows an impulse response waveform of a 12-path type. The “Tc” value indicates a transmit OFDM signal cycle per sample.  FIG. 7B  shows the delay time and gain of each path.  
         [0042]      FIG. 8  shows the results of the simulation described above. When the uncoded BER is 0.05, it is obvious from this figure that the power required for synchronization preamble transmission according to the present invention is approximately 1 dB smaller than in the prior art in which the employed synchronization preamble is quantized to one bit. An uncoded BER of 0.05 is equivalent to a BLER (block error rate) of 0.01 when a ½ rate Viterbi code is used. In the present invention, a 1-bit type synchronization preamble is used for cross correlation in the receiver section. Therefore, the volume of cross-correlation calculations may be the same as for the prior art 1-bit quantization type.  
         [0043]     As a synchronization preamble for symbol synchronization between a transmitter and a receiver in an OFDM communication device, the present invention uses a preamble that has passed through an ideal LPF as described above. It is therefore possible to prevent the characteristics from significantly deteriorating even when the synchronization preamble power is reduced. As a result, excellent communication quality can be maintained even when the synchronization preamble is decreased in order to reduce the calculation time and interference with data.  
       DESCRIPTION OF THE SYMBOLS  
       [0000]    
       
           1 : Preamble generator  
           3 : FFT section  
           5 : Zero substitution section  
           7 : Data supplier  
           9 : Zero insertion section  
           10 : Synchronization signal generator  
           11 : Time multiplexer  
           13 : IFFT section  
           15 : GI insertion section  
           17 : Preamble signal supplier  
           19 : Cross-correlation calculator  
           20 : Synchronization timing detector  
           21 : Synchronization timing calculator