JP3339490B2 - OFDM demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an Orthogonal Frequency Division Multiplexin.
g: hereinafter referred to as OFDM) relates to a demodulation device of a burst communication system using a modulation method, and more particularly to an OFDM demodulation device using a simple timing recovery circuit.

[0002]

2. Description of the Related Art OFDM modulation systems are being studied as modulation systems for multimedia communications such as high-speed wireless LAN. In a high-speed wireless LAN having an information transmission rate of several tens of Mbit / s, when a conventional modulation method for performing modulation such as QPSK on a single carrier is used, a large waveform distortion over a plurality of symbols is caused by a multipath delay wave. On the contrary,
The OFDM modulation scheme is suitable for high-speed wireless transmission in order to perform multicarrier modulation in which an information signal is divided into a plurality of subcarriers, and reduce or eliminate waveform distortion due to multipath delay waves by inserting / removing a guard interval (GI). .

Generally, in a wireless LAN, signals are transmitted and received in packets. A preamble for frequency synchronization and symbol timing synchronization is provided at the head of the packet used for packet synchronization.

[0004] FIG. 6 is a diagram showing an international standard for wireless LAN.
FIG. 3 is a diagram illustrating a format of an OFDM burst signal specified in EEE802.11. In the figure, at the head of the burst data 23, a preamble 24 for timing / carrier frequency synchronization for OFDM frequency synchronization and symbol timing synchronization is arranged, and thereafter a preamble 25 for channel estimation is arranged.

Here, the frequency synchronization corrects a frequency error of the oscillator of the transceiver. OFDM signals are important because they are more likely to be degraded by frequency errors than conventional modulation signals. The symbol timing synchronization is used for demodulation processing of each OFDM burst signal.

Since it is desirable that the timing / carrier frequency synchronization preamble 24 is shorter in terms of transmission efficiency, the
A plurality of short symbols shorter than the FDM symbol length (generally, about 4 μsec) are arranged.
A propagation path estimation preamble 25 having a symbol length is arranged.

In FIG. 6, five short symbols are provided as an example, but an appropriate number is selected according to the synchronization conditions. The time of the short symbol is
Generally, the time is shorter than the time of the OFDM symbol, for example, about 1/2 or 1/4.

[0008]

When such a short symbol is used, there is an advantage that the processing delay time is short. However, in the symbol reproduction circuit, the number of averaged sambles cannot be obtained because the symbol period is short. However, there is a problem that timing detection accuracy cannot be obtained. In addition, since short symbols are used, there is a problem that a method of detecting a timing of switching from short symbols to regular OFDM symbols is complicated.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide an OFDM demodulator using a symbol reproducing circuit which can easily detect a timing / carrier frequency synchronization preamble using short symbols with high detection accuracy. is there.

Therefore, the present invention provides an OFDM demodulator for demodulating an orthogonal frequency division multiplexing (OFDM) burst signal, comprising: a quasi-synchronous detector for orthogonally detecting the OFDM modulated burst signal into two series of complex baseband signals. A carrier frequency estimating circuit for inputting the complex baseband signal and estimating a carrier frequency to compensate for a carrier frequency error, based on a correlation result between a preamble signal in the complex baseband signal and a previously stored pattern signal. A symbol timing estimation circuit that controls the width of the detection window to reproduce the timing of the burst signal, and establishes symbol synchronization for the complex baseband signal with the carrier frequency error compensated based on the output of the symbol timing estimation means. A symbol synchronization processing circuit; The force by Fourier transform, it comprises a FFT that separates the signals for each subcarrier, a subcarrier demodulator for demodulating for each subcarrier from the outputs of the FFT.

[0011]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an OFDM demodulator according to the present invention. Here, the format of the OFDM burst signal to be input is as shown in FIG.
The timing / carrier frequency synchronization preamble 24 as defined in 2.11 is a burst signal format using short symbols.

In FIG. 1, a reception OFDM is
A burst signal is input. The quadrature detection circuit 2 converts a received signal into an analog complex baseband signal using a local signal substantially close to a carrier.

A / D converters 3 and 4 sample quantize the analog complex baseband signal output from quadrature detection circuit 2. The synchronization circuit 5 receives the digitally quantized digital complex baseband signal output from the A / D converters 3 and 4 and performs a synchronization operation.

A fast Fourier transform circuit (FFT Fast Fo
The urier transform 6 performs a Fourier transform on the output signal of the synchronization circuit 5 and separates the OFDM modulated signal into a signal for each subcarrier.

When receiving the channel estimation preamble 25, the channel distortion estimating circuit 7 receives the signal separated for each subcarrier from the Fourier transform circuit 6 and estimates the channel characteristic H (ω). . Based on the estimation result, the channel distortion estimating circuit 7 outputs the channel distortion compensating coefficient 1 / H (ω) to the channel distortion compensating circuit 8.

The propagation path distortion compensating circuit 8 inputs the signal separated for each subcarrier, and outputs a coefficient 1 /
Channel distortion compensation is performed by complex multiplication of H (ω). The subcarrier demodulation circuit 9 receives the signal after distortion compensation, and performs demodulation for each subcarrier.

FIG. 2 shows the configuration of the synchronization circuit 5. In FIG. 2, the symbol timing estimating circuit 10 receives the timing / carrier frequency synchronization preamble 24 when receiving the A /
The symbol timing is reproduced based on the digital quantized digital complex baseband signal output from the D converters 3 and 4, and the timing signal is output to the symbol synchronization processing circuit 13.

The carrier frequency synchronization control circuit 11 estimates the carrier frequency using the outputs of the A / D converters 3 and 4 at the time of receiving the preamble for synchronization, and outputs a frequency error compensation signal to the complex multiplier 12.

FIG. 3 shows the configuration of the symbol timing estimation circuit 10. In FIG. 3, the complex correlation circuit 101 has A /
The digital complex baseband signal after sample quantization output from the D converters 3 and 4 is correlated with the pattern of the short symbol timing / carrier frequency synchronization preamble 24 stored in the reference signal storage circuit 104 in advance. Output the correlation result sequentially.

The timing detection circuit 103 observes a correlation result from the complex correlation circuit 101 and outputs a timing detection result.

The timing control circuit 102 receives the timing detection result from the timing detection circuit 103, outputs a detection window signal to the timing detection circuit 103, and outputs a timing signal to the symbol synchronization processing circuit 13.

The timing detection circuit 103 uses the correlation result from the complex correlation circuit 101 to calculate the timing /
The timing of the short symbol, which is the carrier frequency synchronization preamble 24, is detected.

The timing detection method will be described with reference to FIG. As shown in FIG. 4, the correlation result takes a value corresponding to the repetition of the short symbol. Here, a certain threshold is set, and when the correlation result exceeds the threshold, the peak is detected as the timing of the short symbol.

Once the timing of the short symbol is detected, the timing control circuit 17 generates a detection window signal, and gradually narrows the time width for performing peak detection (the time of the detection window). This can reduce the probability of erroneous peak detection. Further, when the correlation result does not exceed the set threshold value in a state where the width of the detection window is narrowed, this is detected as timing for switching from a short symbol to a signal in OFDM symbol units.

FIG. 5 is a block diagram of a specific propagation path distortion estimating circuit 7. In the figure, the channel estimation preamble signal 25 is input to a complex multiplier 71 of the channel estimation circuit 7. Also, the reciprocal of the pattern of the propagation path estimation preamble signal 25 is stored in the reference signal storage circuit 73 and is input to another input of the complex multiplier 71 of the propagation path estimation circuit 7. At the output of the complex multiplier 71, both input signals are multiplied to obtain an estimation result H (ω) of the propagation path characteristic. The output H (ω) of the complex multiplier 71 is
2 to calculate a coefficient 1 / H (ω) for channel distortion compensation. Then, this propagation path distortion compensation coefficient 1 / H
(Ω) and the output of the FFT 6 are multiplied by the complex multiplier 8 to compensate for the propagation path distortion and output to the subcarrier demodulator 9.

In the embodiment shown in FIG. 1, the quasi-synchronous detection circuit has a configuration in which the received signal is subjected to quadrature detection and then A / D-converted. Needless to say, a configuration in which processing is performed may be employed.

[0028]

As described above, according to the present invention, by using the preamble 24 for synchronizing the timing / carrier frequency with the short symbol shown in FIG.
As compared with the preamble in the DM symbol unit, the circuit scale of the correlation circuit and the like can be reduced and the processing delay can be reduced.

Further, in the present invention, when performing timing reproduction, the width of the timing detection operation time is limited by the detection window signal, so that the timing detection accuracy can be improved even when short symbols are used. further,
Detection of the timing of switching from a short symbol to a signal in OFDM symbol units can be easily performed without increasing the circuit scale.

Therefore, the OFDM demodulator of the present invention can provide an OFDM demodulator using a timing recovery circuit that operates stably with a small circuit scale and a small processing delay.

[Brief description of the drawings]

FIG. 1 is a block diagram of an OFDM demodulator according to the present invention.

FIG. 2 is a diagram showing an OFDM burst data format of the present invention.

FIG. 3 is a block diagram showing a configuration of a synchronization circuit 5 of FIG.

FIG. 4 is a block diagram illustrating a configuration of a timing recovery circuit 10 of FIG. 1;

FIG. 5 is a block diagram showing a configuration of a propagation path distortion estimating circuit 7 of FIG. 1;

FIG. 6 is a diagram illustrating an OFDM burst data format using a conventional short symbol.

[Description of Code] 1 Antenna 2 Quadrature Detection Circuit 3, 4 A / D Conversion 5 Synchronization Circuit 6 FFT 7 Transmission Line Distortion Estimation Circuit 8 Propagation Line Distortion Compensation Circuit 9 Subcarrier Demodulator 10 Symbol Timing Estimation Circuit 11 Carrier Frequency Estimation Circuit 12 Complex multiplier 13 Symbol synchronization processing circuit 14 Delay circuit 21 Preamble for symbol synchronization 22 Preamble for carrier frequency / propagation path estimation 23 Data

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-2001-136149 (JP, A) JP-A-2001-119366 (JP, A) JP-A-2001-53714 (JP, A) Takeshi Onizawa, Masato Mizoguchi, Kiyoji Enomoto , Tomoaki Kumagai, Satoshi Hori, Masahiro Morikura, Experimental Study of OFDM Modem for IEEE 802.11a Wireless LAN, IEICE Technical Report, Japan, The Institute of Electronics, Information and Communication Engineers, September 8, 2000 , Vol. 100 No. 278, p. 1-8 (58) Field surveyed (Int. Cl. ⁷ , DB name) H04J 11/00

Claims (2)

(57) [Claims] 1. An OFDM demodulator for demodulating an orthogonal frequency division multiplexing (OFDM) burst signal, comprising: a quasi-synchronous detector for orthogonally detecting the OFDM modulated burst signal into two series of complex baseband signals; A carrier frequency estimating circuit for inputting a signal and estimating a carrier frequency to compensate for a carrier frequency error; and based on a correlation result between a preamble signal in the complex baseband signal and a previously stored pattern signal, A symbol timing estimation circuit for controlling the width of the detection window, which is a time for enabling peak detection of the correlation result, to reproduce the timing of the burst signal; and Symbol synchronization processing for establishing symbol synchronization based on the output of the symbol timing estimating means Circuit, Fourier transform the output of the symbol synchronization processing circuit,
An OFDM demodulator, comprising: an FFT that separates a signal for each subcarrier; and a subcarrier demodulator that performs demodulation for each subcarrier from the output of the FFT. 2. The symbol timing estimating circuit detects a peak from an output in which the correlation result exceeds a predetermined threshold, detects a short symbol timing in the preamble signal, and detects the short symbol. 2. The OFDM demodulator according to claim 1, wherein when the detection is performed, a detection window is narrowed stepwise to detect a timing at which the predetermined window is not exceeded.