JP2000252946A - Distortion compensation circuit for ofdm - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate for the nonlinear distortion generated in the post-stage of an OFDM(orthogonal frequency division multiplexing) transmission even in the case of multi-value quadrature amplitude modulation(QAM). SOLUTION: The OFDM transmission part is provided with a constant peak circuit 7 and a predistorter 9, and an OFDM reception part is provided with a peak restoration circuit 27. The constant peak circuit 7 corrects the peak value of a transmission signal converted to inverse Fourier transform so that it may be within the input range of the predistorter 9. The predistorter 9 adds the reverse characteristics of the nonlinear distortion generated in the succeeding stage (concretely, a frequency converter 13 and an amplifier 14) of the predistorter 9 to the signal corrected by the constant peak circuit 7. Meanwhile, the peak restoration circuit 27 returns the peak value of the reception signal to the value before correction in the constant peak circuit 7 before the reception signal is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to OFDM (Orthog
onal Frequency Division Multiplexing;
M), which is related to a distortion compensating circuit for OFDM that removes non-linear distortion which is a problem in the transmission unit of the device.

[0002]

2. Description of the Related Art First, an example of the configuration and operation of a transmission / reception unit of an OFDM apparatus will be described with reference to FIG. In addition,
In FIG. 5, a signal shown by a thick line represents a complex signal composed of two components of an in-phase signal I and a quadrature signal Q, and a signal shown by a thin line represents a real signal of one component.

[0003] QPSK (Quadrature Phase Shift Keyin)
g) In the case of -OFDM, a data sequence to be transmitted is encoded by an error correction encoder 1 and the sequence of transmission data is changed by an interleave circuit 2 before being input to a mapping circuit 3. The mapping circuit 3 allocates two bits transmitted on one subcarrier as I and Q data. Here, the I and Q data correspond to the real part and the imaginary part of the complex number on the frequency axis.

The I, Q output from each mapping circuit 3
The data is serial / separated every n data for the number of subcarriers.
Parallel converter 4, inverse FFT circuit 5, and parallel / serial converter 6
, The data is transformed on the time axis and inverse Fourier transformed into m complex numbers of time-series data of FFT size.

The time series data is input to a constant peak circuit 7. The constant peak circuit 7 converts the peak value of the input data so that it falls within the input range of the pre-distorter 9 at the subsequent stage and outputs the converted value. Next, the output data of the constant peak circuit 7 is input to the predistorter 9 after the symbol shaping circuit 8 adds a guard interval and performs a ramp process.

[0006] The predistorter 9 determines the inverse characteristic of the nonlinear distortion characteristic generated in the latter part of the OFDM transmission section, specifically, the frequency converter 13 and the amplifier 14.
The output is added to the input signal of.

The digital signal waveform output from the predistorter 9 is D / A converted by the D / A converter 10 to become an analog signal. After being converted into an analog signal, the analog signal is subjected to a predetermined low-pass filter 11 to remove high frequencies, converted from a baseband band to an intermediate frequency band by a quadrature modulator 12, and further converted to a radio frequency band by a frequency converter 13. And finally output at a desired level by the high power amplifier 14.

Next, the operation of the OFDM receiver will be described. The received signal is converted from a radio frequency band to an intermediate frequency band by a low noise amplifier 15 and a frequency converter 16,
Further, after being shifted to a baseband band by a quadrature detector 17 and removing a high frequency by a predetermined low-pass filter 18, the analog signal is converted into a digital signal by an A / D converter 19.

The digital signal output from the A / D converter 19 has its guard interval removed at a predetermined timing by a symbol synchronizing circuit 20, and a serial / parallel converter 21 and an FFT circuit 22 generate a m-series of time series data. N I, Q Fourier transformed from the time axis to the frequency axis
Data is obtained. This data is converted to a parallel / serial converter 23.
After that, a detection output signal is obtained by the detection circuit 24. Further, the order of the received data is returned to the original order corresponding to the transmitting side by the deinterleave circuit 25, and the error of the data generated in the transmission process is corrected by the error correction decoder 26.

OF having large peak power with respect to average power
The DM modulated signal is easily affected by nonlinear distortion in the amplifier 14. Conventional nonlinear distortion compensation is performed by a constant peak circuit 7 and a predistorter 9. The operation of the conventional distortion compensation will be described with reference to FIG. FIG. 6 shows an example of the nonlinear distortion characteristics of the amplifier 14 and the input / output characteristics of the predistorter 9. The nonlinear distortion characteristic of the amplifier 14 shows a deviation from the linear characteristic when the gain is set to 1 and the saturation point of the amplifier 14 is set to 1. The input / output characteristics of the predistorter 9 are the inverse characteristics of the nonlinear distortion characteristics of the amplifier 14, and both are line-symmetric with respect to a linear characteristic of 45 degrees. For example, when the amplitude A is input, the amplitude D is output by the predistorter 9 and the output
, And the amplitude C is output, thereby performing linearization. Here, AM-AM with respect to the input amplitude of the amplifier 14 is used.
Although the characteristics have been described, the AM-PM characteristics relating to the phase can be similarly compensated by complex multiplication performed in the predistorter 9.

Further, the constant peak circuit 7 detects the peak value of the amplitude of the input signal for each FFT size m, converts it into a constant value Acp, and outputs it. Thus, the input amplitude of the predistorter 9 can be set to a value equal to or smaller than Acp in the figure, and as a result, the predistorter 9 can operate within the linearizable region of the predistorter 9.

For details of the above-described technique, refer to, for example, a gazette of an OFDM distortion compensation circuit previously proposed by the present applicant (Japanese Patent Application No. 10-317881).

[0013]

The above-mentioned conventional OFDM
In the distortion compensating circuit for use, a constant peak circuit is arranged in the OFDM transmitting section, and the magnitude of the amplitude of the output signal of the inverse FFT circuit is limited for each FFT block. For this reason, unlike a conventional phase modulation such as QPSK, for example, a multi-level QAM (Quadrature Amplitude Mod
In the modulation, the amplitude component is also modulated.
A conventional OFDM distortion compensation circuit cannot be used.

The present invention has been made in view of such circumstances, and provides a distortion compensating circuit for OFDM that can compensate for non-linear distortion generated in a later stage of an OFDM transmitting section even in the case of multi-level QAM modulation. The purpose is to do.

[0015]

To achieve the above object, the present invention provides an OFDM distortion compensating circuit comprising a constant peak circuit, a bridistor, and a peak restoring circuit, wherein the constant peak circuit comprises an OFDM transmitting section. The signal subjected to the inverse Fourier transform is input, and the peak value of the input signal is corrected and output so as to be within the input range of the predistorter, and the predistorter is disposed downstream of the constant peak circuit. Connected to the OFDM transmitting section, and outputs the input signal with the inverse characteristic of the nonlinear distortion characteristic generated in the subsequent stage of the OFDM transmitting section. The peak restoring circuit inputs the received signal in the OFDM receiving section, and outputs the constant peak circuit. A distortion compensation circuit for OFDM, characterized in that the peak value corrected in step (1) is restored and output. Further, the present invention is characterized in that the peak restoration circuit returns the peak value to the original value based on the effective value of the received signal for each OFDM block in the OFDM receiving section. Further, the present invention, when a specific pilot signal is included in the transmission signal, the peak restoration circuit,
The OFDM receiver returns the peak value to the original value based on the received signal of the pilot signal.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An OFDM distortion compensation circuit according to one embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a transmitting / receiving section of an OFDM apparatus according to a first embodiment of the present invention. 1 that are the same as those in FIG. 5 are denoted by the same reference numerals, and description thereof will be omitted. In the present embodiment, multi-level QAM modulation, for example, 16 QAM-OFDM is assumed, and the mapping circuit 3 'of the OFDM transmission unit and the detection circuit 24' of the OFDM reception unit correspond to it. The other OFDM transmission units are the same as the conventional example in FIG.

The first embodiment of the present invention is significantly different from the conventional example in that a peak restoration circuit 27 is inserted between a symbol synchronization circuit 20 and a serial / parallel converter 21 in an OFDM receiving section. It is. This peak restoration circuit 2
Numeral 7 performs an operation of restoring the amplitude conversion by the constant peak circuit 7 of the OFDM transmission unit. If the peak restoration circuit 27 is arranged before the detection circuit 24 ', the FF
It may be placed after the T circuit 22.

Next, the configuration and operation of the peak restoration circuit 27 will be described. FIG. 2 shows a peak restoration circuit 2 of the present embodiment.
7 is a diagram illustrating a configuration example of FIG. In FIG. 2 as well, the signal shown by a thick line represents a complex signal composed of two components of an in-phase signal I and a quadrature signal Q, and the signal shown by a thin line represents a real signal of one component. As shown in FIG.
Are the amplitude calculation circuit 28, the effective value calculation circuit 29, the ROM 3
0, a delay circuit 31, and a multiplication circuit 32.

First, the peak restoration circuit 27 uses the output signal X (k) of the symbol synchronization circuit 20 as an input signal. Here, k represents a numerical value from 1 to FFT size m, and X (1)
To X (m) are sets (FFT blocks) of complex signals of the reception time-series data subjected to Fourier transform by the FFT circuit 22. The input signal X (k) is split into two in the peak restoration circuit 27, and one of the split signals is input to the amplitude calculation circuit 28. The amplitude calculation circuit 28 calculates the amplitude | X (k) | of the signal X (k) and inputs it to the effective value calculation circuit 29. The effective value calculation circuit 29 calculates the RMS value of the input | X (k) | in the FFT block, and outputs the result as Arms. This output is input to the ROM 30. The ROM 30 uses the input Arms value as an address to read and output the stored 1 ÷ Arms value. Further, the input signal X (k) of the peak restoring circuit 27 is time-adjusted by the delay circuit 31 and then multiplied by the output signal of the ROM 30 by the multiplying circuit 32.
In this way, the peak restoration circuit 27 calculates the effective value of the amplitude for the input signal for each FFT block, and X (k) ÷ A
The rms operation is performed and output.

As shown in FIG. 1, the output signal of the peak restoration circuit 27 passes through a serial / parallel converter 21, an FFT circuit 22, and a parallel / serial converter 23 and is input to a detection circuit 24 '. The signal whose effective power has become a constant value by the peak restoration circuit 27 is input to the detection circuit 24 '. Here, by approximating that the effective power of the 1FFT block of the transmission signal is approximately constant, a detection output of 16QAM is obtained from the reception signal by the detection circuit 24 '.

FIG. 3 is a block diagram of a transmission / reception unit of an OFDM apparatus according to a second embodiment of the present invention. This embodiment is different from the first embodiment in that a pilot insertion circuit 33 for inserting a pilot signal, which is known data, is arranged between a mapping circuit 3 'and a serial / parallel converter 4 in an OFDM transmission unit. That is. Also, O
In the FDM receiver, the peak restoration circuit 27 of the first embodiment is used.
Unlike this, a peak restoration circuit 34 for restoring the peak value based on the received pilot signal is placed between the parallel / serial converter 23 and the detection circuit 24 '.

In general, in multi-level QAM modulation, a pilot signal for synchronization is often inserted. Here, a case will be described where a pilot signal is inserted into a specific plurality of subcarriers out of n subcarriers.

FIG. 4 shows a peak restoration circuit 34 according to this embodiment.
FIG. 3 is a diagram showing an example of the configuration. In FIG. 4 as well, a signal shown by a thick line represents a complex signal composed of two components of an in-phase signal I and a quadrature signal Q, and a signal shown by a thin line represents a real signal of one component. As shown in FIG. 4, the peak restoration circuit 34
Pilot extraction circuit 35, division circuit 36, averaging circuit 3
7, an amplitude calculation circuit 38, a ROM 39, a delay circuit 40, and a multiplication circuit 41.

First, the peak restoring circuit 34 receives the output signal Y (k) of the parallel / serial converter 23 as an input signal, and the input signal is split into two in the peak restoring circuit 34. One of the split signals is a pilot signal. It is input to the extraction circuit 35. Pilot extraction circuit 35 extracts a received signal R (p) of a subcarrier in which a pilot signal is inserted from Y (k). Here, p is the number of the pilot signal, and represents a numerical value from 1 to the number M of subcarriers into which the bilot signal is inserted, and R (1) to R (M) represent the FFT circuit 22.
Is a received pilot signal Fourier-transformed. Next, the output of the pilot extraction circuit 35 is input to the division circuit 36. The dividing circuit 36 divides R (p) by a known pilot signal D (p) corresponding to the input signal R (p). Thereby, since the amplitude and phase values of each D (p) are determined, the modulation component in the phase direction of R (p) is removed, and R (p) and D (p) in the amplitude direction. Is obtained. The output of the dividing circuit 36 is input to the averaging circuit 37 to remove noise added on the propagation path by averaging operation. Thereafter, the amplitude calculating circuit 38 calculates the amplitude ratio | R / D | of the received pilot signal and the known pilot signal. This output is input to the ROM 39. The ROM 39 reads and outputs the stored value of 1 ÷ | R / D | using the input value of | R / D | as an address. Further, the input signal Y (k) of the peak restoring circuit 34 is time-adjusted by the delay circuit 40 and then R
The output signal of the OM 39 is multiplied. Thus, in the peak restoration circuit 34, the ratio of the amplitude of the input signal to the specified value | R
/ D | is calculated for each FFT block, and Y (k) ÷ | R
/ D | is output.

As shown in FIG. 3, the output signal of the peak restoration circuit 34 is input to the detection circuit 24 '. Detection circuit 24 '
, A signal in which the amplitude values of all the received signals are restored to the original values based on the ratio between the amplitude value of the received pilot signal and the amplitude value of the known pilot signal by the peak restoration circuit 34 is input. Then, a detection output of 16QAM is obtained from the reception signal by the detection circuit 24 '.

Although the embodiments of the present invention have been described in detail with reference to the drawings, the specific configuration is not limited to the embodiments, and changes in design and the like can be made without departing from the gist of the present invention. Even if there is, it is included in the present invention.

[0028]

According to the distortion compensating circuit for OFDM of the first aspect of the present invention, it is possible to cope with a modulation method in which the amplitude at the discrimination point such as a multi-value QAM modulation method is not constant. By using the peak restoring circuit according to claim 2, the peak value corrected by the constant peak circuit of the OFDM transmission unit is calculated as
The FDM receiver can return to the original peak value based on the effective value of the received signal for each OFDM block. By using the peak restoration circuit of claim 3, O
The peak value corrected by the constant peak circuit of the FDM transmitting unit can be returned to the original peak value based on the received pilot signal in the OFDM receiving unit. From the above, it is effective to use the peak restoration circuit according to claim 2 when the number of pilot signals is small and the number of subcarriers is large. Conversely, when the number of pilot signals is large, the peak restoration circuit according to claim 3 is effective. The use of the restoration circuit provides a highly accurate effect.

[Brief description of the drawings]

FIG. 1 is an OFD with distortion compensation according to a first embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration example of an M device.

FIG. 2 is a block diagram illustrating a configuration example of a peak restoration circuit according to the first embodiment.

FIG. 3 is an OFD with distortion compensation according to a second embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration example of an M device.

FIG. 4 is a block diagram illustrating a configuration example of a peak restoration circuit according to the second embodiment.

FIG. 5 is a block diagram illustrating a configuration example of a conventional OFDM device with distortion compensation.

FIG. 6 is a graph showing an example of input / output characteristics of a predistorter.

[Explanation of symbols]

1 ... Error correction encoder, 2 ... Interleave circuit, 3,
3 ': mapping circuit, 4, 21: serial / parallel converter, 5: inverse FFT circuit, 6, 23: parallel / serial converter, 7: constant peak circuit, 8: symbol shaping circuit, 9: predistorter, 10 ... D / A converter,
11, 18: low-pass filter, 12: quadrature modulator,
13, 16 ... frequency converter, 14 ... amplifier, 15 ...
Low noise amplifier, 17: quadrature detector, 19: A / D converter, 20: symbol synchronization circuit, 22: FFT circuit,
24, 24 ': detection circuit, 25: deinterleave circuit,
26 error correction decoder, 27, 34 peak restoration circuit, 28, 38 amplitude calculation circuit, 29 effective value calculation circuit, 30, 39 ROM, 31, 40 delay circuit,
32, 41 ... multiplication circuit, 33 ... pilot insertion circuit,
35 ... Pilot extraction circuit, 36 ... Division circuit, 37 ...
Average circuit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masato Mizoguchi 3-19-2 Nishi Shinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Masahiro Umehira 3-19 Nishi Shinjuku, Shinjuku-ku, Tokyo No. 2 Nippon Telegraph and Telephone Corporation F-term (reference) 5K022 DD01 DD13 DD18 DD19 DD23 DD24 DD33 DD34

Claims (3)

[Claims] 1. An OFDM distortion compensation circuit comprising: a constant peak circuit and a predistorter in an OFDM transmission unit; and a peak restoration circuit in an OFDM reception unit, wherein the constant peak circuit performs inverse Fourier transform. The peak value of the transmitted signal is corrected so as to be within the input range of the predistorter, and the predistorter converts the signal corrected by the constant peak circuit into a non-linear signal generated after the predistorter. Distortion compensation for OFDM, wherein the peak restoration circuit returns a peak value of the received signal to a value before correction by the constant peak circuit before detecting the received signal. circuit. 2. The OFDM distortion compensating circuit according to claim 1, wherein said peak restoring circuit restores a peak value based on an effective value of said received signal. 3. The transmission signal according to claim 1, wherein a pilot signal is included in the transmission signal, and the peak restoration circuit restores a peak value based on a reception signal of the pilot signal. OFDM distortion compensation circuit.