OFDM communication device

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.

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.

FIG. 9schematically 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 generator91, a data supplier93, a time multiplexer95, a zero insertion section97, an inverse fast Fourier transform (IFFT) section99, and a guard interval (GI) insertion section101.

The receiver section of the communication device shown in the figure comprises a time domain preamble (tx_preamble) supply section, which includes a preamble generator91a, a zero insertion section97a, an IFFT section99a, and a GI insertion section101a, and a synchronization timing detector107. The synchronization timing detector107comprises a cross-correlation calculator103and a synchronization timing calculator105.

In the configuration described above, the time multiplexer (MUX)95in the transmitter section time-multiplexes preamble data, which is supplied from the preamble generator91in a specified pattern, and transmit data, which is supplied from the data supplier93. The zero insertion section97subjects 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 section99. Next, the GI insertion section101adds 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.

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 generator91a, zero insertion section97a, IFFT section99a, and GI insertion section101a. For the time domain synchronization preamble (tx_preamble), the cross-correlation calculator103calculates the cross correlation with a receive signal transmitted from the transmitter section. The synchronization timing calculator105determines 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.

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 calculator103is huge. It is therefore proposed that the volume of calculations performed in the cross-correlation calculator103be 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).

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.FIG. 1schematically 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 generator10, a data supplier7, a zero insertion section9, a time multiplexer (MUX)11, an inverse fast Fourier transform (IFFT) section13, and a guard interval (GI) insertion section15. The synchronization signal generator10includes a preamble generator1, a fast Fourier transform (FFT) section3, and a zero substitution section5.

The receiver section of the communication device comprises a synchronization timing detector20, which includes a cross-correlation calculator19and a synchronization timing calculator21, and a preamble supplier17, which generates a synchronization preamble (org_preamble) that is patterned the same as the counterpart generated from the preamble generator1in the transmitter section.

In the transmitter section of the OFDM communication device shown inFIG. 1, the synchronization signal generator10generates a zero amplitude reduced preamble signal. In other words, the original preamble signal (org_preamble) for a specified pattern supplied from the preamble generator1is supplied to an ideal low-pass filter (ideal LPF), which comprises the FFT section3and zero substitution section5. The ideal LPF is implemented when the original preamble signal is fast Fourier transformed in the FFT section3and 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.

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.

The zero amplitude reduced preamble signal X, which is obtained as described above, is supplied to the time multiplexer11. The transmit data fed from the data supplier7is subjected to zero insertion in the zero insertion section9as is the case with the aforementioned prior art, supplied to the time multiplexer11, 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 section13. Further, the GI insertion section15inserts a guard interval into the resulting signal. This produces a transmit OFDM signal.

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 calculator19, 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 calculator21calculates 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.

FIG. 2compares 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 fromFIG. 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.

FIG. 3compares 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.

For comparison between the advantages provided by the present invention and the prior art, the simulation models shown inFIGS. 4 and 5are evaluated.FIG. 4shows a simulation model for a communication device that is configured in accordance with the present invention. In the configuration shown inFIG. 4, the configuration of a transmission section is basically the same as that of the transmission section of the communication device shown inFIG. 1. More specifically, the transmission section shown inFIG. 4comprises a preamble generator41, an FFT section43, a zero substitution section45, a data supplier47, a QPSK modulator48, a zero insertion section49, a time multiplexer51, an IFFT section53, and a GI insertion section55. It should be noted, however, that the QPSK modulator48is provided between the data supplier47and zero insertion section49.

A multipath fading channel57, an adder59, a white noise (AWGN) generator51, a synchronization timing detector63, a GI eliminator65, an FFT section67, a QPSK demodulator69, and an uncoded BER calculator71are incorporated to evaluate the signal in the transmitter section shown inFIG. 4.

FIG. 5shows a simulation model for the prior art. As is the case with the transmitter section shown inFIG. 9, the transmitter section shown inFIG. 5comprises a preamble generator73, a data supplier75, a QPSK modulator77, a time multiplexer79, a zero insertion section81, an IFFT section83, and a GI insertion section85. It should be noted that the QPSK modulator77is provided between the data supplier75and time multiplexer79. The elements for receiving a signal from the transmitter section described above for evaluation purposes are configured the same as shown inFIG. 4and designated by the same reference numerals as indicated inFIG. 4.

The synchronization timing detector63inFIGS. 4 and 5is configured the same as the synchronization timing detectors20,107, which are shown inFIGS. 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.

FIGS. 6 and 7depict simulation conditions.FIG. 6shows various simulation conditions. InFIG. 6, the term “SCH” denotes a synchronization preamble transmission channel, whereas the term “DTCH” denotes a data transmission channel.

FIGS. 7A and 7Bshow a channel model of a multipath fading channel.FIG. 7Ashows an impulse response waveform of a 12-path type. The “Tc” value indicates a transmit OFDM signal cycle per sample.FIG. 7Bshows the delay time and gain of each path.

FIG. 8shows 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 1/2 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.

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