JPH10276165A - Ofdm signal receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simply correct a DC offset even when no null symbol is multiplexed in an OFDM signal. SOLUTION: Mixers 1, 2 generate 1st and 2nd base band signals from a received IF signal. Then the 1st and 2nd base band signals are given to HPFs 16, 17 respectively via LPFs 5, 6, and the HPFs 16, 17 eliminate a DC component being a DC offset of the mixers 1, 2 and the LPFs 5, 6. The lst and 2nd base band signals subject to DC offset correction are given to bias circuits 18, 19, from which DC biases are given to A/D converters 7, 8 so as to be a mid-point of an input range voltage to be given to the A/D converters. The A/D converters 7, 8 convert the 1st and 2nd base band signals into digital signals, and the digital signals are given to an FFT 9, where FFT arithmetic operation is conducted, an OFDM modulation wave is demodulated and outputted to a data processing circuit 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM signal receiver for receiving and demodulating a signal transmitted by OFDM modulation.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a conventional OFDM signal receiver of this kind.

In FIG. 2, a received OFDM signal is input to an RF-IF frequency converter (not shown), converted into a frequency signal in an IF band, divided into two paths, and then mixed into mixers 1 and 2. Is input to On the other hand, the local oscillator 4
Are converted by the phase converter 3 into local oscillation signals having a phase of 0 degree and a phase of 90 degrees, and are supplied to the mixers 1 and 2, respectively. The mixers 1 and 2 detect the input IF signal and the phase 0 degree and 90 degree phase signals from the phase converter 3 and convert them into a first baseband signal and a second baseband signal. As a result, in the first and second baseband signals, the DC component is the highest frequency carrier as shown in FIG.

Next, the first and second baseband signals are:
The harmonic components are removed by the LPFs 5 and 6, respectively.
The signals are input to D converters 7 and 8 and are converted into digital signals.

[0005] The digital signals from the A / D converters 7 and 8 are input to a fast Fourier transform circuit (hereinafter abbreviated as FFT) 9 to perform an FFT operation, thereby demodulating an OFDM modulated wave and a data processing circuit. It is output to 10.

On the other hand, the digital signals from the A / D converters 7 and 8 are also supplied to a first AFC circuit 11. 1st AF
The C circuit 11 detects a frequency error up to half the carrier frequency interval using the correlation of the OFDM guard interval period, and outputs the error to the adder 14.

[0007] The demodulated I and Q signals from the FFT 9 are also supplied to a second AFC circuit 12. 2nd AFC
The circuit 12 compares the reference carrier in the I and Q signals with the reference carrier stored in the carrier arrangement pattern storage unit 13, detects a carrier arrangement deviation of the reference symbol in the I and Q signals, and Adder 1 for unit error
4 is output. The adder 14 adds the error signals of the first AFC circuit 11 and the second AFC circuit 12 and generates an LPF 15
Output to The LPF 15 smoothes the error signal, and supplies the smoothed error signal to the local oscillator 4 to control the oscillation frequency to a desired frequency.

In an OFDM signal transmission / reception system, a DC offset may occur in the circuits on the transmission side and the reception side. This DC offset affects the DC component of the calculation result by the FFT and causes an error. For example, each carrier is QPSK modulated OFDM.
In the case of a signal, the demodulated DC component originally becomes positive or negative corresponding to the modulated data. However, when a DC offset occurs, the DC component becomes only plus or only minus. As a result, the modulated data corresponding to the DC component is not correctly demodulated. Also, D
The A / D converters 7 and 8 convert the OFDM signal to C / offset.
If the input voltage exceeds the input voltage range, an error may occur in demodulated data other than the DC component.

For this reason, Japanese Patent Application Laid-Open No. Hei 6-326739 describes a technique for detecting a null symbol in an OFDM signal and correcting a DC offset in accordance with the result of FFT with the null symbol.

[0010]

However, in the above-described method, when a null symbol is multiplexed in the OFDM signal, the DC offset can be corrected.
If a null symbol is not multiplexed in the FDM signal, the DC offset cannot be corrected. Also,
In order to correct the DC offset, there is a problem that an offset adjustment circuit is required separately.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and provides an OFDM signal receiver capable of easily correcting a DC offset even when a null symbol is not multiplexed in an OFDM signal. It is.

[0012]

SUMMARY OF THE INVENTION The present invention provides a conversion circuit for receiving an OFDM signal and converting the received signal into a baseband band containing no DC component, and a baseband analog signal converted by the conversion circuit. DC component removing circuit for removing the DC component, an A / D conversion circuit for converting the baseband analog signal from which the DC component has been removed by the DC component removing circuit into a digital signal, and demodulation for demodulating the converted digital signal And an OFDM signal receiver.

[0013]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
The DM signal receiver will be described.

FIG. 1 is a circuit block diagram of an OFDM signal receiver according to the present invention.
In that the frequency of the local oscillation signal from the IF is provided at the upper limit than the IF band of the OFDM signal, and the HPFs 16 and 17 and the bias circuits 18 and 19 are arranged between the LPFs 5 and 6 and the A / D converters 7 and 8. is there.

In FIG. 1, a received OFDM signal is input to an RF-IF frequency converter (not shown), converted into a frequency signal in an IF band, divided into two paths, and then mixed into mixers 1 and 2. Is input to On the other hand, the local oscillator 4
Outputs a local oscillation signal having a frequency higher than the IF band of the received OFDM signal. The generated local oscillation signal is converted into a local oscillation signal having a phase of 0 degree and a phase of 90 degree by the phase converter 3 and supplied to the mixers 1 and 2, respectively. The mixer 1 detects the input IF signal and the 0-degree phase signal from the phase converter 3 and converts the signal into a first baseband signal. Also, in the mixer 2, the input IF signal and the phase 90
The second signal is detected and converted into a second baseband signal. As a result, the first and second baseband signals are as shown in FIG.
As shown in (1), the baseband signal is a signal that does not include a DC component shifted upward from 0.

Next, the first and second baseband signals are:
After the harmonic components are removed by LPFs 5 and 6, respectively.
The signals are input to the HPFs 16 and 17. The HPFs 16 and 17 remove DC components that are DC offsets of the mixers 1 and 2 and the LPFs 5 and 6. Here, since the DC component is not included in the signal components of the first and second baseband signals, the signal is not lost.

Then, the first and second DC components are removed.
The optimum DC bias is applied to the baseband signal by the bias circuits 18 and 19 so that the baseband signal is at the midpoint of the input range voltage of the A / D converters 7 and 8 described later.

As a result, the DC offset on the receiving side can be ignored, and the input signal does not exceed the input range voltage of the A / D converter.

The first and second baseband signals whose DC offsets have been corrected are input to A / D converters 7 and 8 and are converted into digital signals.

The digital signals from the A / D converters 7 and 8 are input to a fast Fourier transform circuit (hereinafter abbreviated as FFT) 9, and the FFT 9 performs an FFT operation to demodulate an OFDM modulated wave. The data is output to the data processing circuit 10.

On the other hand, the digital signals from the A / D converters 7 and 8 are also supplied to the first AFC circuit 11. 1st AF
The C circuit 11 detects a frequency error up to half the carrier frequency interval using the correlation of the OFDM guard interval period, and outputs the error to the adder 14.

The demodulated I and Q signals from the FFT 9 are also supplied to the second AFC circuit 12. 2nd AFC
The circuit 12 compares the reference carrier in the I and Q signals with the reference carrier stored in the carrier arrangement pattern storage unit 13 to detect a carrier arrangement deviation of the reference symbol in the I and Q signals, Error of adder 14
Output to

The adder 14 adds the error signals of the first AFC circuit 11 and the second AFC circuit 12 and outputs the result to the LPF 15. The LPF 15 smoothes the error signal, and supplies the smoothed error signal to the local oscillator 4 to control the oscillation frequency to a desired frequency.

The data processing circuit 10 performs data processing in consideration of the fact that each output from the FFT 9 is converted so that the baseband signal does not include a DC component. In other words, the carrier pattern storage unit 13 stores the carrier fluctuation due to the shift of the baseband signal shown in FIG. 4, and the data processing circuit 10 uses the carrier fluctuation data from the carrier pattern storage unit 13 in To demodulate the QPSK-modulated signal from the FFT 9 and output the demodulated signal to an error correction circuit (not shown) or the like.

In this embodiment, the frequency of the local oscillation signal from the local oscillator 4 is set to the IF of the received OFDM signal.
Although the frequency is selected to be higher than the band, the OFDM signal I
A frequency lower than the F band may be selected.

Although the present embodiment has been described using a QPSK-modulated digital signal, a QAM-modulated digital signal may be used.

[0027]

According to the present invention, as described above, the DC offset can be corrected even when the null symbol is not multiplexed in the OFDM signal, and the DC offset can be corrected. Does not require a separate offset adjustment circuit, and can perform DC offset correction with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an OFDM signal receiver according to the present invention.

FIG. 2 is a block diagram showing a conventional OFDM signal receiver.

FIG. 3 is a spectrum waveform showing a relationship of frequency conversion in a conventional OFDM signal receiver.

FIG. 4 is a spectrum waveform showing a relationship of frequency conversion in the OFDM signal receiver of the present invention.

[Explanation of symbols]

 1, 2 mixer 3 phase converter 4 local oscillator 5, 6 LPF 7, 8 A / D converter 9 FFT 10 data processing unit 11 first AFC circuit 12 second AFC circuit 13 carrier arrangement pattern storage unit 14 adder 15 LPF 16, 17 HPF 18, 19 Bias circuit

Claims (3)

[Claims] An orthogonal frequency division multiplexed signal (hereinafter, OFD)
A conversion circuit that receives the M signal) and converts the received signal into a baseband that does not include a DC component, and a DC component removal circuit that removes the DC component of the baseband analog signal converted by the conversion circuit OFDM signal receiver comprising: an A / D conversion circuit that converts a baseband analog signal from which a DC component has been removed by the DC component removal circuit into a digital signal; and a demodulation circuit that demodulates the converted digital signal. . 2. An orthogonal frequency division multiplexed signal (hereinafter, OFD)
A conversion circuit that receives the M signal) and converts the received signal into a baseband that does not include a DC component, and a DC component removal circuit that removes the DC component of the baseband analog signal converted by the conversion circuit A bias circuit for applying a bias to the baseband analog signal from which the DC component has been removed by the DC component removing circuit, and an A / D converter for converting the analog signal to which the DC component has been added by the bias circuit into a digital signal An OFDM signal receiver comprising a circuit and a demodulation circuit for demodulating the converted digital signal. 3. The OFDM signal receiver according to claim 1, wherein said DC component removing circuit is constituted by an HPF.