JP3533354B2 - Demodulator for OFDM packet communication - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to Orthogonal Frequency Division Multiplex (OFDM).
The present invention relates to a demodulator for OFDM packet communication used in a digital wireless communication system using an exing method, and in particular, estimates the characteristics of channels through which signal components of a plurality of subcarriers of a received OFDM signal have passed, and determines a channel for each subcarrier. The present invention relates to a demodulator for OFDM packet communication, which performs high-quality demodulation by using a characteristic estimation result.

[0002]

2. Description of the Related Art In the OFDM system, signals are multiplexed and transmitted by superimposing a large number of subcarriers having different frequencies. In the receiving device, it is general to use a fast Fourier transform (FFT) in order to separate the signal component of each subcarrier from the received OFDM signal.

In an actually transmitted OFDM signal, a large number of subcarrier signal components are arranged so as to be superimposed on each OFDM symbol. In addition, since the receiving side separates the signal components of the respective subcarriers by using the frequency difference by using the fast Fourier transform, the position in each symbol in which the signals of the respective subcarriers are arranged is not important. That is, the FF used by the receiving device for the fast Fourier transform.
If the information of all subcarriers of one symbol is included in the timing of the T window, the OFDM signal can be demodulated correctly.

Therefore, generally, in the OFDM system, a guard interval is added to the transmitted OFDM signal for each symbol. Then, during the guard interval period, a signal obtained by cyclically extending the data portion of one symbol adjacent thereto is transmitted. That is, all subcarrier components of one symbol appear cyclically in the guard interval and the one-symbol OFDM signal adjacent thereto.

Therefore, the receiving device receives the received O
Even if the timing of each symbol of the FDM signal and the timing of the FFT window used for separating each subcarrier are slightly deviated from each other, the component which is the same as the component deviating from the FFT window in one symbol is used as the signal of the guard interval in the FFT window. Since it appears inside, all subcarrier components can be detected within the FFT window.

Therefore, in the OFDM system, even if a timing deviation that falls within the guard interval occurs, demodulation can be performed without receiving interference from adjacent symbols. Therefore, in the OFDM system, even if the multipath interference occurs due to the delayed wave generated in the transmission path, it is unlikely to be affected. Conventional OFDM
The demodulator for packet communication is configured as shown in FIG. 4, for example. This demodulator for OFDM packet communication is supposed to receive an OFDM signal in a burst format as shown by (2) in FIG. 2, for example.

The operation of the demodulator for OFDM packet communication shown in FIG. 4 will be described below. The OFDM signal received by the antenna 201 is input to the reception circuit 202. The reception circuit 202 performs reception processing on the input OFDM signal. That is, the receiving circuit 202 performs processing such as frequency conversion, filtering, and quadrature detection, and outputs the received signal as a complex baseband signal. Receiver circuit 20
The complex baseband signal output from 2 is input to the synchronization processing circuit 203.

The synchronization processing circuit 203 detects the carrier frequency error and the symbol timing using the synchronization signal (S1 in FIG. 2) included in the input complex baseband signal. Then, carrier frequency error correction processing is performed on the complex baseband signal based on the detected carrier frequency error information. The synchronization processing circuit 203 outputs the complex baseband signal with the corrected carrier frequency error and the signal indicating the detected symbol timing to the guard interval removal circuit 204.

The guard interval removing circuit 204 removes the guard interval component from the complex baseband signal based on the detected symbol timing information. Actually, the input of the complex baseband signal is 10
For each FDM symbol, an FFT window having a time width obtained by subtracting the time width corresponding to the signal length of the guard interval from the length of one OFDM symbol is formed, and only the signal component passing through this FFT window is extracted. Therefore, the guard interval removal circuit 204 outputs the complex baseband signal with the guard interval removed.

The complex baseband signal output from the guard interval removing circuit 204 is a Fourier transform circuit 205.
Entered in. The Fourier transform circuit 205 performs fast Fourier transform processing on the input complex baseband signal for each OFDM symbol. By the Fourier transform, the respective signal components of a large number of subcarriers included in the input complex baseband signal are separated from each other and output from the Fourier transform circuit 205.

The complex baseband signal output from the Fourier transform circuit 205 is a synchronous detection circuit 20 for each subcarrier.
9 is input. The synchronous detection circuit 209 performs synchronous detection of the input complex baseband signal and outputs a detection signal. Further, in order to enable high-quality demodulation, the synchronous detection circuit 209 corrects amplitude fluctuation and phase rotation caused by channel characteristics such as fading for each subcarrier.
In order to realize this correction, it is necessary to estimate the actual channel characteristics.

In order to estimate the channel characteristics, a channel estimation symbol extraction circuit 206, a smoothing circuit 207, and a channel estimation circuit 208 are provided. The channel estimation symbol extraction circuit 206 inputs the complex baseband signal output from the Fourier transform circuit 205 for each subcarrier, and extracts the channel estimation signal (S2 in FIG. 2). This channel estimation signal is a known signal.

The channel estimation signal extracted by the channel estimation symbol extraction circuit 206 is input to the smoothing circuit 207. The smoothing circuit 207 smoothes the channel estimation signal in the time direction or the frequency direction in order to reduce the noise component from the input channel estimation signal. The smoothed channel estimation signal is output from the smoothing circuit 207 for each subcarrier, and the channel estimation circuit 2
08 is input.

The noise component contained in the channel estimation signal input to the smoothing circuit 207 is received by the receiving circuit 202.
Is a component such as thermal noise added to the received signal.
The channel estimation circuit 208 estimates the channel state of the transmission path through which the received OFDM signal has passed, for each subcarrier, based on the channel estimation signal for each subcarrier input from the smoothing circuit 207.

That is, since the channel estimation signal is a known signal, the channel estimation circuit 208 uses the smoothing circuit 20.
The channel estimation signal input from 7 is compared with its known signal to estimate the amplitude fluctuation and phase rotation added to the received signal on the transmission path for each channel. By referring to the channel characteristics estimated by the channel estimation circuit 208, for example, it is possible to know how the amplitude and phase of each subcarrier are affected by fading.

The synchronous detection circuit 209 inputs the information of the channel characteristic estimated for each subcarrier by the channel estimation circuit 208, and corrects the amplitude fluctuation and the phase rotation caused by the channel characteristic such as fading. Synchronous detection circuit 20
The detection signal output from 9 is input to the identification circuit 210. The identification circuit 210 determines a symbol for the data signal (S4 in FIG. 2) included in the detection signal input from the synchronous detection circuit 209, and outputs the determination result.

[0017]

As described above, the OFD
When synchronously detecting the M burst signal, the OFD is usually used.
The characteristics of each channel are estimated for each subcarrier using the known channel estimation signal (S2) provided at the beginning of the M burst signal, and the estimation result is used to obtain the signal of each subcarrier after Fourier transform. On the other hand, equalization processing and synchronous detection processing are performed.

However, in general, a noise component such as thermal noise is added to the received signal in the reception processing section for performing frequency conversion, quadrature detection, etc., so that the channel characteristics are determined using only the channel estimation signal as described above. It is impossible to accurately estimate the channel characteristics only by estimating. That is, a noise component such as thermal noise added to the received signal causes an estimation error of the channel characteristic.

This channel characteristic estimation error causes incomplete equalization processing and synchronous detection processing, resulting in deterioration of the code error rate characteristic. In the conventional device, in order to reduce the estimation error of this channel characteristic,
For example, the smoothing circuit 207 of FIG. 4 is used to smooth the received signal and suppress the noise component. By the way, when smoothing the channel estimation signal in the time direction, as shown in FIG. 2, the same channel estimation signal (S2
(1), S2 (2)) are allocated in advance, and the receiving apparatus generally smoothes the channel estimation signal in the time direction for each subcarrier over a plurality of received OFDM symbols. . That is, the channel estimation signal S2 (1) and the channel estimation signal S2 (2) are smoothed for each subcarrier.

When smoothing in the frequency direction, the channel estimation signal is smoothed among a plurality of subcarriers having different frequencies. However, in that case, since the difference between the subcarriers of the frequency characteristics of the channel is suppressed and the estimation ability of the channel characteristics deteriorates, in order to minimize the deterioration, generally smoothing is performed within a narrow frequency range as much as possible. It is performed by moving average or filtering.

However, in the case of smoothing in the time direction, in order to sufficiently suppress the noise component, a large number of channel estimation signals must be included in one packet and transmitted. The ratio of the channel estimation signal to the amount of information actually transmitted increases, and the overhead thereof causes a significant decrease in system throughput.

Further, in the case of smoothing in the frequency direction, in order to sufficiently suppress the noise component, signals of a large number of subcarriers having different frequencies are averaged with each other, so that a wide frequency range is obtained. Even if the frequency characteristics of each channel due to the difference in frequency between subcarriers are smoothed more than necessary and the noise component is sufficiently suppressed, it is inevitable that the estimation accuracy of the channel is significantly reduced.

According to the present invention, in the demodulator for OFDM packet communication as described above, even if a noise component is added to the signal by the receiving process, the channel characteristic is estimated with high accuracy without significantly lowering the throughput. OFD
The purpose is to demodulate the M signal with high quality.

[0024]

A demodulator for OFDM packet communication according to a first aspect of the present invention includes receiving means for receiving a received orthogonal frequency division multiplexed signal and generating a baseband signal,
A synchronization processing unit that performs timing and carrier frequency synchronization processing on the baseband signal, and the synchronized baseband signal are input, and each signal component of a plurality of subcarriers included in the baseband signal is separated. In a demodulator for OFDM packet communication, which comprises a Fourier transform means for outputting as a channel estimation signal for extracting a channel estimation signal which is a known signal included in the received signal separated for each subcarrier by the Fourier transform means. A first characteristic for estimating a channel characteristic for each subcarrier by using a signal extracting unit and the channel estimating signal extracted by the channel estimating signal extracting unit.
From the received signal separated for each subcarrier by the Fourier transforming means, a specific signal extracting means for extracting a specific modulated signal appearing in a specific period, and a specific modulated signal extracted by the specific signal extracting means for,
First synchronous detection means for performing equalization processing and synchronous detection processing for each subcarrier using the information on the channel characteristics estimated by the first channel estimation means, and detection output by the first synchronous detection means. Second channel estimation for estimating a channel characteristic for each subcarrier using a hard decision means for making a hard decision on the sign of a signal, and a specific modulation signal extracted by the hard decision means and the specific signal extraction means. Means, the first channel characteristic estimated by the first channel estimating means, and the second channel characteristic estimated by the second channel estimating means, and characteristic combining for generating information of the final channel characteristic. Means and the received signal separated for each subcarrier by the Fourier transform means, using the information of the final channel characteristic generated by the characteristic combining means. Characterized in that a second synchronous detection means for outputting a detection signal subjected to equalizing processing, and synchronous detection processing for each store.

When a noise component due to thermal noise or the like is added to the received signal for each subcarrier output from the Fourier transforming means, this received signal is provided for each subcarrier due to frequency selective fading. Amplitude and phase fluctuations different from each other are generated, and random amplitude and phase fluctuations due to noise components are generated. Therefore, the channel characteristic cannot be estimated with sufficient accuracy only by estimating the channel characteristic of each subcarrier from the amplitude and phase of the channel estimation signal (known signal) included in the received signal. Therefore, in the first aspect, the channel characteristics are estimated by the processing of two systems, and the estimation results are combined to obtain the final channel characteristics.

That is, the channel estimation signal extracting means of claim 1 extracts the channel estimation signal which is a known signal contained in the received signal separated for each subcarrier by the Fourier transforming means. Further, the first channel estimation means estimates the channel characteristics for each subcarrier using the channel estimation signal extracted by the channel estimation signal extraction means.

The specific signal extracting means extracts a specific modulated signal appearing in a specific period from the received signal separated for each subcarrier by the Fourier transforming means. The first synchronous detection means uses the information on the channel characteristic estimated by the first channel estimation means for the specific modulated signal extracted by the specific signal extraction means to perform equalization processing and synchronization for each subcarrier. Perform detection processing.

The hard decision means makes a hard decision on the sign of the detection signal output by the first synchronous detection means. The second channel estimation means uses the determination result of the hard determination means and the specific modulation signal extracted by the specific signal extraction means,
Channel characteristics are estimated for each subcarrier. The characteristic synthesizing means includes the first channel characteristic estimated from the known channel estimation signal by the first channel estimating means, and the second channel characteristic.
Based on the second channel characteristic estimated from the specific modulated signal by the channel estimating means of 1), the information of the final channel characteristic is generated.

Since the specific modulation signal extracted by the specific signal extracting means is not a known signal, the first channel estimating means uses the information of the first channel characteristic estimated from the known channel estimation signal to perform the specific modulation. The signal is subjected to equalization processing and synchronous detection processing by the first synchronous detection means, and the result is subjected to hard judgment by the hard judgment means. The second channel estimation means is
The second channel characteristic can be estimated for each subcarrier using the determination result of the hard decision means and the extracted specific modulation signal.

According to the first aspect, it is possible to obtain a highly accurate channel estimation result by synthesizing the channel characteristics obtained by the two processes. Since it is not necessary to increase the number of symbols of the channel estimation signal included in the packet in order to reduce the noise component of the channel estimation signal, it is possible to prevent a decrease in system throughput. According to a second aspect of the present invention, in the demodulator for OFDM packet communication according to the first aspect, the specific signal extracting means converts the low-speed modulation signal modulated by a modulation method slower than a modulation method of main data to be transmitted, into the specific modulation signal. It is characterized by extracting as.

Since the estimation accuracy of the first channel characteristic estimated from the received known channel estimation signal is not so high, a specific modulated signal which is not a known signal is input to the hard decision means via the first synchronous detection means. When making a determination, an erroneous determination may occur. However, in claim 2, since the low speed modulation signal is used as the specific modulation signal, the possibility that the hard decision means makes an erroneous decision becomes small. Therefore, the estimation accuracy of the second channel characteristic is improved.

According to a third aspect of the present invention, in the demodulator for OFDM packet communication according to the first aspect, the characteristic synthesizing unit estimates the first channel characteristic estimated by the first channel estimating unit and the second channel estimating unit. It is characterized in that the information of the final channel characteristic is generated by synthesizing the results obtained by weighting each of the estimated second channel characteristic of 1) according to the signal quality.

In the third aspect, since the first channel characteristic and the second channel characteristic are weighted according to the respective signal qualities and then combined, the estimation accuracy of the final channel characteristic is improved. For example, it is considered that noise components such as thermal noise are suppressed as the number of symbols of a signal used for channel estimation increases, and deterioration of the estimation accuracy of channel characteristics due to the noise components decreases. Therefore, the first
When the channel estimation signal of 2 symbols is used for the estimation of the channel characteristic of 1 and the specific modulation signal of 1 symbol is used for the estimation of the second channel characteristic, the number of symbols of the signal to be used is taken into consideration. Weighting may be performed so that the signal quality of the first channel characteristic is higher than the signal quality of the second channel characteristic.

According to a fourth aspect of the present invention, in the OFDM packet communication demodulating device of the first aspect, there is provided smoothing means for smoothing the channel estimation signal included in the received signal for a plurality of symbols or a plurality of subcarrier components. It is characterized by being provided. Since the smoothing means smoothes the channel estimation signal in claim 4, noise components such as thermal noise added to the reception signal by the reception processing can be reduced and the estimation accuracy of the first channel characteristic can be improved. it can.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION One embodiment of a demodulator for OFDM packet communication according to the present invention will be described with reference to FIGS. This form corresponds to all the claims.
FIG. 1 is a block diagram showing the structure of the demodulator for OFDM packet communication of this embodiment. FIG. 2 is a schematic diagram showing an example of a burst format of an OFDM signal. FIG. 3 is a phase plan view showing an arrangement example of received signals and reference signal points on the phase plane.

In this embodiment, the channel estimating signal extracting means, the first channel estimating means, the specific signal extracting means, the first synchronous detecting means, the hard decision means, the second channel estimating means, the characteristic synthesizing means of the first aspect. The means and the second synchronous detection means,
Channel estimation symbol extraction circuit 106, channel estimation circuit 109, low-speed modulation signal extraction circuit 107, synchronous detection circuit 110, hard decision circuit 111, channel estimation circuit 112, weighting synthesis circuit 113, and synchronous detection circuit 1 respectively.
Corresponding to 14. The smoothing means of claim 4 corresponds to the smoothing circuit 108.

The demodulator for OFDM packet communication shown in FIG. 1 is configured on the assumption that it receives an OFDM signal in the burst format shown as (1) in FIG. Referring to FIG. 2, this OFDM signal includes a 1-symbol synchronization signal S1, a 2-symbol channel estimation signal S2 (1), S2 (2), a 1-symbol low-speed modulation signal S3, and a data signal S4. ing.

A guard interval is added to each symbol. In the interval of the guard interval, the contents of cyclically expanding the data of the symbol adjacent thereto appear. The two-symbol channel estimation signals S2 (1) and S2 (2) are known signals that the receiving apparatus has grasped in advance, and the channel estimation signal S2 (1) and the channel estimation signal S2 (2) are the same. It is the content of.

The low speed modulation signal S3 is a signal modulated by a modulation method lower than the modulation method for the data signal S4 containing information to be transmitted, and is not a known signal. For example, for modulation of the data signal S4, QPSK (Quadrature Phase
se Shift Keying: BPSK (Binary Phase S) that is slower than se Shift Keying
hift Keying: Two-phase phase shift keying) may be used to modulate the low speed modulation signal S3.

Referring to FIG. 1, the demodulator for OFDM packet communication includes an antenna 101, a receiving circuit 102,
Synchronization processing circuit 103, guard interval removal circuit 10
4, Fourier transform circuit 105, channel estimation symbol extraction circuit 106, low-speed modulation signal extraction circuit 107, smoothing circuit 108, channel estimation circuit 109, synchronous detection circuit 1
10, a hard decision circuit 111, a channel estimation circuit 112, a weighting synthesis circuit 113, a synchronous detection circuit 114 and a discrimination circuit 115.

The OFDM signal received by the antenna 101 is input to the receiving circuit 102. The receiving circuit 102 is
Reception processing such as frequency conversion, filtering, and quadrature detection is performed on the input OFDM signal, and the reception signal is output as a complex baseband signal.

The synchronization processing circuit 103 inputs the reception signal (complex baseband signal) output from the reception circuit 102,
Synchronization signal (preamble signal) S included in the signal
1 is used to detect the carrier frequency error and the symbol timing. Then, using the detected carrier frequency error information, carrier frequency error correction processing is performed on the complex baseband signal after the reception processing. The synchronization processing circuit 103
A complex baseband signal in which the carrier frequency error is corrected and the detected symbol timing signal are output.

The guard interval removing circuit 104 removes the guard interval from the input received signal.
Specifically, the guard interval removal circuit 104 passes one OFDM symbol through the complex baseband signal input from the synchronization processing circuit 103 in the FFT window formed at the timing according to the symbol timing signal input from the synchronization processing circuit 103. The signal with the guard interval removed is extracted for each time. The time width of the FFT window is set to the length obtained by subtracting the time width of the guard interval from the length of one OFDM symbol.

Therefore, the guard interval removing circuit 10
The complex baseband signal from which the guard interval has been removed by 4 is input to the Fourier transform circuit 105. The Fourier transform circuit 105 outputs 1 to the complex baseband signal input from the guard interval removal circuit 104.
Fast Fourier transform processing is performed for each OFDM symbol. Therefore, a complex baseband signal separated for each subcarrier is obtained at the output of the Fourier transform circuit 105.

The complex baseband signal output from the Fourier transform circuit 105 and separated for each subcarrier is synchronously detected by the synchronous detection circuit 114. Further, the synchronous detection circuit 114 corrects amplitude fluctuation and phase rotation caused by channel characteristics such as fading appearing in the received signal. In order to perform this correction, it is necessary to estimate the channel characteristics for each subcarrier.

To estimate the channel characteristics, the O in FIG.
The FDM packet communication demodulator includes a channel estimation symbol extraction circuit 106, a low speed modulation signal extraction circuit 107, a smoothing circuit 108, a channel estimation circuit 109, a synchronous detection circuit 110, a hard decision circuit 111, and a channel estimation circuit 112.
And a weighting synthesis circuit 113. As shown in FIG. 1, this circuit includes two channel estimation circuits 109,
112 is provided. The channel estimation circuit 109 includes a channel estimation signal included in the received signal (S2 (1),
The channel characteristics are estimated based on S2 (2)). Also,
The channel estimation circuit 112 estimates the channel characteristics based on the low speed modulation signal (S3 in FIG. 2) included in the received signal.

Channel estimation symbol extraction circuit 106
Is a channel estimation signal included in the received signal output from the Fourier transform circuit 105 (S2 (1), S2 in FIG. 2).
2 (2)) is extracted for each subcarrier.
The smoothing circuit 108 inputs the channel estimation signal extracted by the channel estimation symbol extraction circuit 106 and performs a smoothing process on the signal. This smoothing processing is performed in order to suppress noise components such as thermal noise added to the received signal in the receiving circuit 102.

The actual smoothing process is realized by averaging a plurality of signals at different positions in the time axis direction or averaging a plurality of signals having different frequencies. For example, as shown in FIG. 2, when the same channel estimation signals S2 (1) and S2 (2) appear in one packet over a plurality of OFDM symbols, the channel estimation signal S2 (1) and the channel By averaging the estimation signal S2 (2) for each subcarrier, it is possible to average a plurality of signals at different positions in the time axis direction.

Further, by averaging a plurality of subcarrier signals, a plurality of signals having different frequencies can be averaged. However, in that case, the difference in the detected channel characteristics depending on the frequency is suppressed, and the estimation accuracy deteriorates.In order to minimize the deterioration, smoothing is performed by moving average or filtering within a narrow frequency range. It is desirable to perform processing.

Since the Fourier transform performed by the Fourier transform circuit 105 is a kind of linear transform, the channel estimation signal can be smoothed for the received signal before passing through the Fourier transform circuit 105. In that case, the smoothing circuit 108 in FIG. 1 may be omitted, and another smoothing circuit instead thereof may be provided between the receiving circuit 102 and the Fourier transform circuit 105.

The channel estimation circuit 109 is the smoothing circuit 1
The channel estimation signal for each subcarrier smoothed by 08 is input, and the channel characteristic is estimated for each subcarrier based on the input signal. That is, since the channel estimation signal included in the received signal is a known signal that the channel estimation circuit 109 grasps in advance, the channel estimation circuit 10
By comparing the known signal grasped by 9 and the channel estimation signal of the received signal, it is possible to detect a change in the amplitude or phase of the received signal due to fading on the transmission path as a channel characteristic.

On the other hand, the low speed modulation signal extraction circuit 107 receives the reception signal output from the Fourier transform circuit 105 and extracts the low speed modulation signal (S3 in FIG. 2) contained therein for each subcarrier. That is, a signal of one OFDM symbol that appears at a specific position in the burst format as shown in FIG. 2 is extracted as a low speed modulation signal. Channel estimation circuit 1
Reference numeral 12 estimates the channel characteristics based on the low-speed modulation signal extracted by the low-speed modulation signal extraction circuit 107. However, since the low-speed modulation signal is not a known signal, the channel characteristics cannot be estimated from only that signal. Therefore, the synchronous detection circuit 110 and the hard decision circuit 111 are provided.

The synchronous detection circuit 110 corrects the amplitude fluctuation and the phase rotation due to the channel characteristics such as fading for the low speed modulation signal for each subcarrier extracted by the low speed modulation signal extraction circuit 107 and performs the synchronous detection. .
At the time of this correction, the information on the channel characteristics estimated by the channel estimation circuit 109 is used. The synchronous detection circuit 110 is
The detection signal of the low speed modulation signal is output.

The hard decision circuit 111 is a synchronous detection circuit 110.
The detection signal of the low-speed modulation signal output by the above is input, the code is hard-decided, and the decision result is given to the channel estimation circuit 112. Regarding the hard decision of the hard decision circuit 111, FIG.
An example of (1) will be described. In this example, it is assumed that BPSK is used as the modulation method of the low speed modulation signal. B
In the case of a PSK-modulated low-speed modulation signal, the synchronous detection circuit 1
The detection signal output from 10 originally appears at any of the predetermined reference signal points Sa and Sb on the phase plane shown in FIG.

However, when the signal processed by the synchronous detection circuit 110 contains a noise component, the synchronous detection circuit 1
Amplitude / phase fluctuation according to the noise component occurs in the detection signal output from 10. Then, for example, R1 and R2 in FIG.
The received signal appears at the point. Therefore, the hard decision circuit 111
In the case of FIG. 3 (1), the received signal appearing at the position of the point R1 is determined as the code (for example, “0”) corresponding to the reference signal point Sa closest to that point. Further, the received signal appearing at the position of the point R2 is determined as the code (for example, "1") corresponding to the reference signal point Sb closest to the point. This hard decision removes the amplitude and phase variation caused by noise.

Note that the estimation accuracy of the channel characteristics estimated by the channel estimation circuit 109 does not become so high due to the influence of thermal noise added by the reception processing. Therefore, an error may occur in the detection signal output from the synchronous detection circuit 110 due to the estimation error generated in the channel estimation circuit 109, and the hard decision circuit 111 may make an erroneous decision. However, since the signal determined by the hard decision circuit 111 is a low-speed modulated signal that is BPSK-modulated, the number of reference signal points on the phase plane is small, and the hard decision is made unless an extremely large error occurs in the amplitude and phase of the received signal. The decision circuit 111 can make a correct hard decision without making an erroneous decision.

The channel estimation circuit 112 is the hard decision circuit 1
Hard decision result output by 11 and low speed modulation signal extraction circuit 10
Channel characteristics are estimated for each subcarrier by comparing the extracted low-speed modulation signal of No. 7 with each other. That is, how the amplitude and phase of the low-speed modulation signal are affected on the transmission path is detected for each subcarrier. In the OFDM packet communication demodulation device of FIG.
9 and the output of the channel estimation circuit 112 show the estimation results of the channel characteristics for each subcarrier.

The weighting / combining circuit 113 receives the estimation result of the channel characteristic (first channel characteristic) estimated by the channel estimation circuit 109 and the estimation result of the channel characteristic (second channel characteristic) estimated by the channel estimation circuit 112, respectively. They are weighted according to the signal quality of and then they are combined. For example, it can be considered that a noise component such as thermal noise is suppressed as the number of symbols of a signal used for channel estimation increases, and deterioration in the estimation accuracy of channel characteristics due to the noise component decreases.

Therefore, for example, when receiving the OFDM signal of the burst format shown in FIG.
Since the first channel characteristic is estimated using the channel estimation signal of the FDM symbol and the second channel characteristic is estimated using the low-speed modulation signal of 1 OFDM symbol, the signal quality of the first channel characteristic is the second. Weighting may be performed so that it is higher than the signal quality of the channel characteristics.

Specifically, the first channel characteristic is weighted with "2" and the second channel characteristic is weighted with "1" in consideration of the number of symbols of signals used for channel estimation. Just go.

Of course, such weighting is an example, and different weighting can be performed. For example, weighting proportional to the square of the number of symbols of the signal used for channel estimation or weighting proportional to the cube of the number of symbols may be performed. That is, weighting may be performed so that the channel characteristic with the highest estimation accuracy can be obtained as a result of combining the two types of channel characteristics.

In a high-speed digital wireless communication system using OFDM, the fluctuation of fading within one packet is generally small enough to be ignored. Therefore, ideally, it is considered that the estimation results of the channel characteristics estimated from a plurality of signals appearing at different time positions in one packet are the same.

Therefore, the first channel characteristic estimated from the channel estimation signal and the second channel characteristic estimated from the low speed modulation signal.
It is possible to obtain a highly accurate channel estimation result by combining with the channel characteristics of. The result of the weighting synthesis circuit 113 synthesizing the first channel characteristic and the second channel characteristic is applied to the input of the synchronous detection circuit 114.

The synchronous detection circuit 114 uses the high-precision channel estimation result input from the weighting synthesis circuit 113, and determines the channel characteristics such as fading for the received signal for each subcarrier input from the Fourier transform circuit 105. The amplitude fluctuation and the phase rotation caused by the correction are corrected and the synchronous detection is performed, and the result is output as a detection signal. The identification circuit 115 receives the detection signal output from the synchronous detection circuit 114, determines a symbol for each subcarrier signal of the detection signal, and outputs the determination result as a demodulation output.

For example, the received data signal (S in FIG. 2)
If 4) is modulated by QPSK, the discrimination circuit 1
15 is four reference signal points S on the phase plane of FIG.
a, Sb, Sc, Sd of each received signal (R1, R
The code assigned to one reference signal point closest to the point 2) is output as the determination result.

The structure of the OFDM signal received by the demodulator for OFDM packet communication of the present invention is shown in FIG.
A burst format different from (1) may be used, and the modulation method of each signal may be changed as necessary. Further, the modulation method of the low speed modulation signal S3 may be the same as the modulation method of the data signal S4 which is the main data, but it is a hard decision to modulate the low speed modulation signal S3 by a lower speed modulation method. This is desirable because the circuit 111 is less likely to make an erroneous determination.

[0067]

As described above, according to the demodulator for OFDM packet communication of the present invention, the throughput of the system is reduced even when a noise component such as thermal noise is added to the signal during reception processing. It is possible to perform channel estimation with high accuracy and demodulate an OFDM signal with high quality.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a demodulation device for OFDM packet communication according to an embodiment.

FIG. 2 is a schematic diagram showing an example of a burst format of an OFDM signal.

FIG. 3 is a phase plan view showing an arrangement example of received signals and reference signal points on the phase plane.

FIG. 4 is a block diagram showing a configuration of a conventional demodulator for OFDM packet communication.

[Explanation of symbols]

101 antenna 102 receiver circuit 103 Synchronous processing circuit 104 Guard interval removal circuit 105 Fourier transform circuit 106 channel estimation symbol extraction circuit 107 low speed modulation signal extraction circuit 108 smoothing circuit 109 channel estimation circuit 110 Synchronous detection circuit 111 Hard decision circuit 112 channel estimation circuit 113 weighting synthesis circuit 114 Synchronous detection circuit 115 identification circuit 201 antenna 202 Receiver circuit 203 Synchronous processing circuit 204 Guard interval removal circuit 205 Fourier transform circuit 206 channel estimation symbol extraction circuit 207 Smoothing circuit 208 channel estimation circuit 209 Synchronous detection circuit 210 identification circuit

Continuation of front page (56) Reference JP-A-9-98148 (JP, A) JP-A-2000-358010 (JP, A) JP-A-11-308129 (JP, A) JP-A-11-289211 (JP, A) JP-A-8-265293 (JP, A) JP-A-8-102771 (JP, A) JP-A-7-321766 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) ) H04J 11/00

Claims (4)

(57) [Claims] 1. A receiving means for inputting a received orthogonal frequency division multiplexed signal to generate a baseband signal, a synchronization processing means for synchronizing timing and carrier frequency with respect to the baseband signal, and a synchronization processing. A demodulator for OFDM packet communication, which comprises a Fourier transform unit for inputting a baseband signal and separating and outputting each signal component of a plurality of subcarriers included in the baseband signal, From the received signal separated for each, using the channel estimation signal extraction means for extracting the channel estimation signal is a known signal contained therein, and the channel estimation signal extracted by the channel estimation signal extraction means First channel estimation means for estimating channel characteristics for each subcarrier; The specific signal extracting means for extracting a specific modulated signal appearing in a specific period from the received signal separated for each subcarrier by the Fourier transforming means, and the specific modulated signal extracted by the specific signal extracting means for the first signal First synchronous detection means for performing equalization processing and synchronous detection processing for each subcarrier by using the information of the channel characteristic estimated by the channel estimation means, and the code of the detection signal output by the first synchronous detection means. A hard decision means for making a hard decision, and a second channel estimation means for estimating a channel characteristic for each subcarrier by using the decision result of the hard decision means and the specific modulation signal extracted by the specific signal extraction means, Based on the first channel characteristic estimated by the first channel estimation means and the second channel characteristic estimated by the second channel estimation means, the final channel characteristic is estimated. Characteristic combining means for generating channel characteristic information, and for the received signal separated for each subcarrier by the Fourier transform means, for each subcarrier by using the final channel characteristic information generated by the characteristic combining means, etc. A demodulator for OFDM packet communication, comprising: a second coherent detection means for performing a digitization process and a coherent detection process and outputting a detection signal. 2. The demodulator for OFDM packet communication according to claim 1, wherein the specific signal extraction means converts the low-speed modulation signal modulated by a modulation method slower than the modulation method of the main data to be transmitted, into the specific modulation signal. A demodulation device for OFDM packet communication, characterized in that: 3. The demodulator for OFDM packet communication according to claim 1, wherein said characteristic synthesizing means has the first channel characteristic estimated by said first channel estimating means, and said second channel characteristic.
Demodulator for OFDM packet communication, characterized in that the second channel characteristic estimated by the channel estimating means is combined with the result of weighting according to the signal quality to generate the information of the final channel characteristic. . 4. The demodulator for OFDM packet communication according to claim 1, further comprising smoothing means for smoothing the channel estimation signal included in the received signal for a plurality of symbols or a plurality of subcarrier components. A demodulator for OFDM packet communication, characterized by: