JP2781946B2 - Orthogonal frequency division multiplex signal generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an orthogonal frequency division multiplex signal generator which receives digital modulation data and generates an orthogonal frequency division multiplex signal by using a fast Fourier transform.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram of a conventional orthogonal frequency division multiplexing (hereinafter referred to as OFDM) signal generator. A conventional OFDM signal generator receives a digitally modulated data 50, performs an inverse Fourier transform, and outputs a real part 52 and an imaginary part 53 thereof as a time waveform sample sequence. , A digital / analog (hereinafter referred to as D / A) conversion circuit 5 for converting the time waveform sample sequences of the real part 52 and the imaginary part 53 into analog output signals 56 and 57, respectively.
4, 55, and low-pass filter (hereinafter referred to as LPF) circuits 58, 59 for removing aliasing signals included in the analog output signals 56, 57, and a local oscillator 60 having a phase difference of 90 °. Signal to the LPF
And a quadrature modulation circuit 61 for multiplying and synthesizing analog signals output from 58 and 59, respectively, and outputs an OFDM signal 62.

FIG. 4 shows the D / A of this OFDM signal generator.
5 shows spectra of output signals 56 and 57 of converters 54 and 55. Each narrow-band modulated wave 80 constituting the spectrum is
It is multi-level phase-modulated or multi-level amplitude-phase modulated according to digital modulation data. When Fourier transform is performed using digital data, processing is performed by discrete Fourier transform.
Performed by T.

The sampling frequency f _{s of} digital data
, The center frequency f _n (f ₀ f _{3) of} each narrow-band modulated wave
...), when the number of FFT points of the N, f _n = _n ·
f _s / N. Here, n is an integer of 0 to (N-1).

The output signals 56 and 57 of the D / A conversion circuits 54 and 55 shown in FIG. 4 are output by the LPF circuits 58 and 59 having the frequency characteristic 82 as shown by the dotted lines in FIG.
Frequency f _s / 2 or more signal is removed.

[0006]

If the number of FFT points can be made sufficiently large with respect to the number of generated narrow-band modulated waves, there is a sufficient frequency interval between the signal passing through the LPF circuit and the signal to be removed. The passing signal is hardly distorted by the LPF circuit having a characteristic of giving a sufficient amount of attenuation to the signal to be removed.

However, increasing the number of points in the FFT requires an increase in the calculation speed in order to increase the amount of calculation of the FFT circuit, which increases the cost of the FFT circuit or makes the circuit configuration difficult. There is a problem. Therefore, it is often difficult to sufficiently increase the number of points of the FFT. When an LPF circuit having a characteristic of giving a sufficient amount of attenuation to a signal to be removed is used, as shown in FIG. The narrow band modulated wave 81 near the cutoff frequency of the LPF close to f _s / 2 in the passing signal has a problem of being distorted. FIG. 2 is a graph of an example of the characteristic of the LPF circuit 58 or 59, wherein a curve 20 indicates an attenuation characteristic and a curve 21 indicates a group delay characteristic.

[0008]

In order to solve the above-mentioned problems, an OFDM signal generator according to the present invention receives digitally modulated data, performs an inverse Fourier transform, and converts a real part and an imaginary part as a time waveform sample sequence. A fast Fourier transform circuit for outputting, and a digital / digital converter for converting the time waveform sample sequences of the real part and the imaginary part into analog signals, respectively.
An analog conversion circuit, respective low-pass filter circuits for removing aliasing signals included in the analog signal, and a signal from a local oscillator having a phase difference of 90 ° multiplied by the analog signal of the LPF output to synthesize the signals. In a quadrature frequency division multiplexing signal generator for frequency-division multiplexing and transmitting a plurality of modulation signals having an orthogonal relationship with each other and comprising a quadrature modulation circuit, the attenuation and delay of the transmission characteristics of the LPF are provided before the FFT circuit. A compensation data processing circuit for processing the digital modulation data according to the amount is provided to compensate for the transmission characteristics of the LPF.

[0009]

The compensation data processing circuit cancels the transmission characteristics of the LPF circuit at the input of the FFT circuit.
Based on the attenuation and delay of the transmission characteristics, digitally modulated data is processed in advance to distort the waveform, and consequently the LP
By reducing the waveform distortion at the F output, the L
Compensate the transmission characteristics of the PF. Thus, an OFDM signal with less distortion can be generated without increasing the load on the FFT circuit.

[0010]

FIG. 1 is a block diagram of an OFDM signal generator according to an embodiment of the present invention. In this figure, the same components or corresponding parts as those in FIG. 3 are denoted by the same reference numerals.
The difference from FIG. 3 is that the compensating data processing circuit 1 is provided before the FFT circuit 51.

[0011] In the present invention, first, in the attenuation characteristic 20 and the group delay characteristic 21 of premeasured LPF circuit as shown in FIG. 2, the DC at the center frequency f _n of the narrowband modulated wave constituting the OFDM signal The ratio r _{n to} the attenuation of
And the relative delay time Δt _n with respect to the DC delay time. When each narrow-band modulated wave passes through the LPF circuit,
It is assumed that each receives approximately flat attenuation and delay in frequency according to the above values approximately. The case where each narrow-band modulated wave is QPSK-modulated will be described below.
Obviously, other cases can be easily applied. First, each narrow-band modulated wave that has passed through the LPF circuit having ideal transmission characteristics is as follows in complex number representation.

[0012]

(Equation 1)

Here, a _n and b _n are I and Q channel modulation data of the nth narrowband modulated wave, respectively. Each of the narrow-band modulated waves that have passed through the actual LPF circuit, based on the amount of attenuation and the amount of delay in the DC component, is as follows in the above approximation.

[0014]

(Equation 2)

Here, as the modulation data, a _n + j · b
Instead of _n , the compensation data processing circuit 1 of FIG.

[0016]

(Equation 3)

By calculating and using, it is possible to approximately remove the distortion of the LPF circuit. That is, the input data a _n ′ and b _n ′ of the real part and the imaginary part of the FFT circuit
As

[0018]

(Equation 4)

Is used. That is, by performing the processing of [Equation 4] in the compensation data processing circuit based on the principle of [Equation 3], the distortion of the LPF circuit is compensated in advance, so that the distortion of the LPF circuit can be reduced. it can.

[0020]

As described above, according to the present invention, the LPF
By processing the digitally modulated data according to the transmission characteristics of the circuit, distortion of the transmitted OFDM signal is reduced. This makes it possible to reduce distortion without increasing the performance load of the FFT circuit, so that design simplicity and cost reduction can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a graph showing characteristics of an LPF circuit;

FIG. 3 is a block diagram of a conventional OFDM signal generator.

FIG. 4 is a graph showing a spectrum of an output signal of a D / A conversion circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Compensation data processing circuit 50 Digital modulation data 51 FFT circuit 54, 55 D / A conversion circuit 58, 59 LPF circuit 60 Local oscillator 61 Quadrature modulation circuit 62 OFDM signal

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-307820 (JP, A) JP-A-4-2226 (JP, A) Tables JP-A-4-501348 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) H04J 11/00

Claims (1)

(57) [Claims] 1. A fast Fourier transform circuit that receives digitally modulated data, performs an inverse Fourier transform, and outputs a real part and an imaginary part as a time waveform sample sequence, and a time waveform sample sequence for each of the real part and the imaginary part. Digital-to-analog conversion circuits for converting the analog signals into analog signals, respective low-pass filter circuits for removing aliasing signals included in the respective analog signals, and signals from a local oscillator having a phase difference of 90 degrees And a quadrature modulation circuit for multiplying and synthesizing the analog signal output from the low-pass filter, and transmitting a plurality of modulated signals having an orthogonal relationship to each other by frequency division multiplexing. Before the Fourier transform circuit, the attenuation of the transmission characteristics of the low-pass filter and The compensation data processing circuit for processing compensates for the digital modulation data according to the delay amount provided,
An orthogonal frequency division multiplex signal generator, wherein output data of the compensation data processing circuit is used as input data of a real part and an imaginary part of the fast Fourier transform circuit.