JPH09294115A - Ofdm demodulator and its method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in a bit error due to quadrature demodulation by conducting demodulation through the use of only one channel relating to the substantial quadrature demodulation. SOLUTION: A time series signal is produced by multiplying a received OFDM wave signal with a signal ω1 outputted from a local oscillator 2. Loopback distortion is eliminated from a produced I signal by an LPF circuit 3 and the resulting signal is fed to an A/D converter circuit 4. The converted signal is given to a real part input of an FFT circuit 5, and '0' is given to an imaginary part input of the circuit 5. Thus, the circuit 5 does not calculate complex numbers but calculates only the real part. The result of calculation by the circuit 5 is outputted as a symmetrical signal for the real part and a point symmetrical for the imaginary part, with the Nyquist frequency as anaxis in an FFT window. A decoding circuit 6 applies QAM decoding to th output of the circuit 5 and provides an output of QAM decoding data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulation device for OFDM transmission, which is used for modulation / demodulation used in transmission / reception of digital signals, and particularly for transmitting a digital information signal modulated by QAM or the like using a plurality of carriers.

[0002]

2. Description of the Related Art OFDM is a frequency division multiplexing digital modulation method for transmitting digital information by using a plurality of orthogonal carrier waves, is strong against multipath, is less likely to interfere with other transmission systems, and is less susceptible to interference. Since it has characteristics such as relatively high frequency utilization efficiency, it has recently attracted attention as a modulation method suitable for mobile digital audio broadcasting and digital television broadcasting. A plurality of carrier waves can be generated using an IFFT circuit that performs inverse Fourier transform on the transmission side, and F that performs Fourier transform on reception.
The carrier wave can be separated by the FT circuit.

Due to the progress in the packaging technology of this FFT circuit, OFDM transmission is becoming a reality. OFD
In modulation of M transmission, the IFFT circuit uses oversampling because of the ease of design of an analog circuit. Further, the OFDM wave generated by the orthogonal modulation is modulated by the center frequency ω 0 as shown in FIG. 3 and transmitted.

First, FIG. 6 shows a block diagram of a conventional modulator in OFDM transmission. The transmission data is supplied to the encoding circuit 20 and encoded. The encoded signal is supplied to the real input terminal and the imaginary input terminal of the IFFT circuit 21, respectively. The output analog signals are converted into analog signals by digital / analog (D / A) conversion circuits 22 and 23, respectively.

This converted signal is supplied to low-pass filter (LPF) circuits 24 and 25, respectively, in order to remove the aliasing signal. The output of the low-pass filter (LPF) circuit 24 and the output of the local oscillator 28 are multiplied by the multiplier 26, and an I signal which is a time-series signal is output.

Similarly, a low pass filter (LPF)
The output of the circuit 25 and the output of the local oscillator 28 are rotated by 90 degrees by the phase shift circuit 29, and the output is rotated by the multiplier 2
By multiplying by 7, the Q signal, which is a time-series signal whose phase is rotated 90 degrees with respect to the I signal, is output. As shown in FIG. 3, the signal is modulated by the center frequency ω 0 and transmitted.

Next, FIG. 2 shows a block diagram of a demodulation device in conventional OFDM transmission. The OFDM wave transmitted by the OFDM transmission is input to the demodulator and then multiplied by the output of the local oscillator 9 by the multiplier 7 to output an I signal which is a time-series signal. Similarly, when the output of the local oscillator 9 is rotated by 90 degrees by the phase shift circuit 10 and the output of which the phase is rotated is multiplied by the OFDM wave by the multiplier 8, when the phase is rotated by 90 degrees with respect to the I signal. A Q signal that is a series signal is output.

At this time, the oscillation frequency of the local oscillator 9 is
The center frequency ω0 is the same as during modulation. Low pass filter (LPF) circuits 11 and 12 are provided in order to remove the aliasing signals included in both signals obtained by the quadrature demodulation, and the analog signals output from the low pass filter (LPF) circuits 11 and 12 are respectively converted into analog / digital (A / D) conversion circuits. Converted into digital signals by 13 and 14, respectively.

The respective outputs are supplied to the real number part and the imaginary number part of the FFT operation circuit 15, and the results of the real part and the imaginary part obtained by Fourier transform are decoded.
6 and outputs the decoded data after decoding PSK, QAM, etc.

[0010]

In quadrature demodulation in the OFDM demodulation device, the received OFDM wave flows through two circuits. At that time, in the two quadrature waves generated by the local oscillator and the 90-degree phase shift circuit input to the multiplier, if there is a slight phase shift, that is, there is a loss of orthogonality, the frequency generated by the local oscillator. With respect to the axis, the pair of plus and minus carriers interfere with each other in the calculation result after the Fourier transform, which causes an error in the transmission information and causes a deterioration in the bit error rate. Similarly, when a difference in gain occurs between the two waves flowing through the two systems, interference occurs between the carrier waves, leading to deterioration in the bit error rate.

The above problem is caused as the modulation method of each carrier becomes multi-valued and the frequency of the carrier becomes higher,
In the case of QAM, signal points that are more noticeable and that are closer to the outside in the constellation are more prone to error. Therefore, in the OFDM demodulator, it is an important issue to prevent bit error rate deterioration due to orthogonal demodulation.

[0012]

In order to solve the above-mentioned problems, first, a received OFDM wave and a signal generated by a local oscillator are multiplied by a multiplier to perform frequency conversion into a baseband frequency. The
Originally, only the I signal which is a time-series signal is obtained, and demodulation is performed using the two orthogonal signals, the I signal and the Q signal, as described in the prior art. In the present invention, however, the generated I signal is generated. Low pass filter only, after converting to digital,
It is assumed that the real part of the FFT circuit is input, nothing is input to the imaginary part, and the Fourier operation of only the real part is performed. In this way, demodulation is performed using only one channel in the original orthogonal demodulation.

At this time, the oscillation frequency of the signal generated by the local oscillator oscillates at a frequency lower than the lowest frequency or higher than the highest frequency in the transmission band which is a rectangular spectrum in OFDM transmission. When demodulation is performed at the extreme frequency of the transmission spectrum, the baseband band is doubled compared to the demodulation at the original center frequency. Therefore, in order to use oversampling, the low pass filter (LPF) used The size of the circuit, analog / digital (A / D) converter, and FFT window is larger than that used in conventional demodulation.
Double specifications are required.

For the I signal input to the real part of the FFT circuit, only the real part is calculated in the Fourier transform. In addition, since only the I signal is subjected to Fourier transform and oversampling is performed, a redundant signal is output in the FFT calculation result. Therefore, in the final stage of the butterfly operation of the FFT circuit, the calculation amount can be reduced by omitting the calculation for this redundant signal.

According to the above method, it is possible to take out the phase and amplitude information of the transmission signal by using only the I signal, instead of requiring the base band bandwidth twice as large as the conventional one, so that the circuit is constituted by one system. You can do it. Therefore, the received OFDM wave does not flow through a two-system circuit as in quadrature demodulation, which causes a problem of orthogonality loss due to phase shift of two orthogonal waves multiplied by a multiplier as in the conventional case. Since the problem of the gain difference between the I signal and the Q signal is solved, the interference between the frequencies can be prevented.

In quadrature demodulation, when demodulation is performed at the extreme frequency of the transmission spectrum, the number of operation points in the FFT circuit is 2 compared with the conventional case in order to perform oversampling.
However, since the demodulated signal of the OFDM wave is input only to the real part, it is not necessary to calculate the complex number composed of the real part and the imaginary part, which is originally necessary.
In the final stage of the butterfly operation of the FT circuit, it is possible to reduce the amount of operation processing, so
The calculation amount can be suppressed to be equal to or less than the calculation amount of the circuit.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the specifications of the entire transmission information device will be described. Transmission information is transmitted using 256 carriers. In order to facilitate the design of the filter in the analog circuit, double oversampling is used and 512-point IFFT operation is performed to generate an OFDM wave.

256QAM is used for modulation of each carrier, and 8-bit information is applied to one carrier, that is, 4 bits are applied to each of the real part and the imaginary part. In addition to the transmission information data, reference data for calibration and synchronization data are inserted in one symbol.

The frequency allocation of the transmission information to the IFFT calculation section is as follows when numbers are assigned in order from the lowest frequency in the IFFT window. f0 to f127 Information transmission signals to be transmitted are given. f128 to f383 Set carrier level to 0 and do not generate signal. f384 to f511 The information transmission signal to be transmitted is given.

Frequency allocation is performed as described above, and IF
An OFDM wave is generated by quadrature modulation from the I signal and the Q signal, which are time-series signals output by the FT operation. At this time, the transmission band becomes a rectangular spectrum like the transmission signal spectrum 17 in FIG. 3 centered on the frequency ω 0 of the local oscillator in quadrature modulation.

Next, an embodiment of the OFDM demodulator of the present invention will be described below with reference to the drawings. FIG. 1 is a block configuration diagram of an embodiment of an OFDM demodulator of the present invention, which is different from the orthogonal demodulator circuit of FIG. 2 in that a multiplier 1, a low-pass filter (LPF) circuit 3, and an analog / digital ( Each of the A / D) conversion circuits 4 has only one system.

A time series signal is generated by multiplying the received OFDM wave signal and the signal (oscillation frequency ω 1) output by the local oscillator 2 by the multiplier 1.

The frequency oscillated by the local oscillator 2 is lower than or equal to the lower end or higher than the upper end of the rectangular spectrum in the transmission band. ,
The oscillation frequency ω1 of the local oscillator 2 in demodulation can be in the range of the following equation 1.

[0024]

[Equation 1]

Here, when demodulation is performed at ω1 = ω0−Δf * 128, which is the lowest frequency of the transmission signal spectrum, ω1 is the lowest frequency on the transmission signal spectrum 17, as shown in FIG. Frequency.

The generated I signal has its aliasing distortion removed by a low pass filter (LPF) circuit 3.
An analog signal output from the low pass filter (LPF) circuit 3 is converted into a digital signal by an analog / digital (A / D) conversion circuit 4.

At this time, since the demodulation is performed at the lowest frequency of the spectrum, the base band band is doubled as compared with the conventional demodulation at the center frequency. Therefore, the band of the low-pass filter is doubled, the sampling clock of the analog / digital (A / D) conversion circuit 4 is also doubled to perform oversampling, and the number of operation points of the FFT circuit 5 is doubled to 1024. It becomes a point.

The converted digital signal is fed to the FFT circuit 5
Of the FFT circuit 5 is input to the imaginary part of the FFT circuit 5 as shown in FIG. As a result, in the FFT operation, the operation is not performed as a complex number, and only the real part is calculated.

As a result, in the FFT circuit 5,
In the FFT operation of 1024 points in FIG. 5, without preparing the operation table itself of the imaginary part, it is the same as when the input time series t0 to t1023 are treated as real numbers, and the operation time can be reduced. Here, FIG. 5 shows the flow of the butterfly operation which is the FFT operation, and outputs the frequency series fn to the time series signal tn. Further, at the input in the butterfly operation, bit reversal is performed as shown in FIG.

The FFT circuit 5 inputs only the real part and calculates the result, and in the FFT window,
Signals are output symmetrically about the Nyquist frequency and symmetrically about the imaginary point.

In demodulation, the frequency ω at the lowest end of the transmission band
1 is used, and because the FFT is double oversampling, the transmission signal to be demodulated is up to ¼ in the FFT window, that is, the signal from the fundamental frequency to the 255th frequency, Is not necessary.

Therefore, the necessary signal to be extracted is the 255th signal.
Since it is a signal of 256 points up to the next frequency and the rest need not be taken out, the points from f0 to f255 are calculated in the final stage 18 of the butterfly operation of 1024 points in FIG. , Skip the calculation of the remaining 768 points.

The operation result obtained by the FFT circuit 5 is supplied to the next decoding circuit 6, and after QAM decoding is performed and processed, QAM decoded data is output. At this time, F
Since only the I signal is supplied to the FT circuit 5 for calculation, the magnitude of the signal in the output result is halved.

Therefore, the transmission signal is restored by doubling this signal value. Specifically, the signal of each carrier wave, which is the result of the FFT operation, is shifted by 1 bit and input to the decoding circuit 6. IFFT on the modulation side
The correspondence between the transmission signal on the frequency sequence assigned at the time of calculation and the transmission signal on the frequency sequence restored by the decoding circuit 6 is as shown in the following Expression 2.

[0035]

[Equation 2]

As described above, the transmission signal allocated on the modulation side is demodulated in demodulation corresponding to the signals on the frequency sequence from the fundamental frequency to the 255th frequency. Regarding the oscillation frequency ω1 of the local oscillator 2, if the frequency outside the spectrum of the transmission band is set to the oscillation frequency ω1, it is not necessary to pass a DC component, and the filter design becomes easy.

If the oscillating frequency ω1 is in the range of the following equation 3 and has a spectrum out of the transmission band, the analog / digital conversion circuit 4 and the FFT circuit 5 have the same specifications as those described above, which are twice as large as the conventional specifications. , FFT circuit 5 is 1024 points F
It becomes FT.

[0038]

(Equation 3)

Regarding the transmission signal correspondence between the modulation side and the demodulation side after the FFT calculation, F is determined according to the difference between the oscillation frequency ω1 of the local oscillator 2 and the end frequency of the transmission signal spectrum.
In the window after the FT calculation, it appears by moving laterally on the frequency axis.

Further, if the oscillation frequency ω1 is a frequency outside the transmission band other than the above expression, the analog / digital (A / D) conversion circuit 4 and the FFT circuit 5
In addition, the above sampling clock and the number of calculation points are required.

[0041]

According to the present invention, since the demodulation circuit can be constructed by one system, two orthogonal waves in the conventional orthogonal demodulation,
Since the orthogonality loss due to the phase shift between the I signal and the Q signal and the error in the transmission information due to the interference between the carrier waves caused by the problem of the gain difference can be prevented, the reliability in communication can be improved.

Further, according to the present invention, since the circuit can be constructed by one system as compared with the conventional quadrature demodulation circuit, the demodulation circuit can be simplified and designed compactly, and the cost can be reduced. Can be done.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an embodiment of an OFDM demodulation device of the present invention.

FIG. 2 is a block configuration diagram of a conventional OFDM demodulation device.

FIG. 3 is a diagram showing an OFDM transmission signal spectrum.

FIG. 4 is a diagram showing a transmission signal spectrum for an example of a local oscillation frequency in OFDM demodulation according to the present invention.

FIG. 5 is a diagram showing a flow of 1024-point FFT calculation.

FIG. 6 is a block diagram of a conventional OFDM modulator.

[Explanation of symbols]

1, 7, 8, 26, 27 Multiplier 2, 9, 28 Local oscillator 3, 11, 12, 24, 25 Low-pass filter (LP
F) circuit 4,13,14 analog / digital (A / D) conversion circuit 5,15 FFT circuit 6,16 decoding circuit 10,29 90 degree phase shift circuit 17 transmission signal spectrum 18 butterfly operation final stage 20 coding circuit 21 IFFT circuit 22, 23 Digital / analog (D / A) conversion circuit

Claims (2)

[Claims] 1. A local oscillator that oscillates a frequency in a band other than a transmission band, which is a rectangular spectrum, in order to receive an OFDM wave generated and transmitted by quadrature modulation at a transmission side and demodulate transmission information in the OFDM transmission. A multiplier for multiplying the received OFDM wave by the signal from the local oscillator; a low-pass filter circuit for removing aliasing distortion of the analog signal output by the multiplier; and an output signal of the low-pass filter circuit. A demodulation circuit having one system of an analog / digital conversion circuit for converting a digital signal into a digital signal, and an I signal which is a time-series signal converted into a digital signal by the analog / digital conversion circuit and output, using only a real part An FFT circuit for performing the Fourier transform of the FFT circuit, and a real part on the frequency sequence, which is the calculation result of the FFT circuit. OFDM demodulation device characterized by comprising a decoding circuit for decoding the higher transmission signal Majinaripato. 2. In OFDM transmission, an OFDM wave generated by orthogonal modulation on the transmission side and transmitted is received, and in order to demodulate transmission information, a frequency in a band other than a transmission band, which is a rectangular spectrum, is oscillated, Of the oscillated signal and the received OFDM wave are multiplied, the aliasing distortion of the analog signal of the multiplied output is removed, the analog signal of the output from which the aliasing distortion is removed is converted into a digital signal, and I, which is a time-series signal that is demodulated by only one system and converted into the above digital signal and output
An OFDM demodulation method characterized in that a Fourier transform of only a real part is performed using a signal, and a transmission signal is decoded from a real part and an imaginary part on a frequency sequence, which is a calculation result of the Fourier transform. .