JP2001251274A - Nonlinear distortion compensating circuit for ofdm transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a transmission level detection circuit and to reduce both circuit scale and power consumption, when a pre-distortion compensation multiplier is decided. SOLUTION: As the nonlinear distortion has one-to-one correspondence between a transmission data series and transmission amplifier, the transmission data series is converted into the parallel data by a serial/parallel converter 22, and the multiplier of a pre-distortion circuit is directly decided from the parallel data. Thereby a look-up table 25 is prepared to store in advance the multiplier with the transmission data series defined as its address. Thus, the multiplier is readily obtained to contribute to simplification of the constitution and reduction of the power consumption of this nonlinear distortion compensation circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonlinear distortion compensating circuit for OFDM transmission, and more particularly, to a nonlinear distortion compensating circuit for OFDM transmission for effectively compensating for nonlinear distortion caused by high-output amplification of an OFDM transmission signal. It is about.

[0002]

2. Description of the Related Art OFDM is orthogonal frequency division multiplexing (Orth
Ogonal Frequency Division Multiplex), which is being used for digital broadcasting and wireless access. The features of this method are that the frequency use efficiency is high due to the multi-carrier transmission method using orthogonal frequencies, and the modulation method itself is resistant to multipath distortion.

However, since the OFDM modulated wave has a high peak factor, the back-off of the transmission amplifier (back-off:
This is one of the means for reducing the intermodulation due to the non-linear distortion of the amplifier when two or more carrier waves are simultaneously amplified, and the reduction in operating at a point lower than the saturation power is not sufficient. Then, the transmission quality deteriorates due to the nonlinear distortion of the amplifier. Further, there is a disadvantage that the side lobe level of the frequency spectrum is increased, and the adjacent channel interference amount is increased.

On the other hand, from the viewpoint of power saving, the back-off of the transmission amplifier cannot be made very large. The back-off and the non-linear distortion have a contradictory relationship, and it is necessary to set the back-off in a trade-off with the non-linear distortion.

[0005] As a conventional technique for reducing the nonlinear distortion of the transmission amplifier, there is a pre-distortion. This is a technique in which a characteristic opposite to the nonlinearity of a transmission amplifier is given to an input signal of the transmission amplifier, and a nonlinear distortion in an output of the transmission amplifier is removed as much as possible. As examples of the prior art, JP-A-11-215197 and JP-A-2000-2265.
There is a technique disclosed in Japanese Patent Application Laid-Open No. 9-209 or the like.

In these techniques, in a baseband signal before performing quadrature modulation, the power of the baseband signal is calculated from the quadrature signals I and Q, and a complex compensation coefficient corresponding to the transmission signal power is quoted from a table. This is a pre-distortion method for performing complex multiplication on the orthogonal signals I and Q for transmission. The basic operation of this predistortion will be described with reference to FIG. FIG. 3 shows the input / output characteristics of the amplifier (AM / A
M) shows an example. Since the output of the amplifier has a saturation point, it generally has a characteristic as shown in FIG. That is, linearity is exhibited in a region where the input level is low, and saturation is reached through a non-linear region. Predistortion is a method of multiplying an input level by a compensation coefficient so that the nonlinear region has as linear characteristics as possible.

For example, when the input level is x (n), if the amplifier is linear, the output level becomes y by a linear line of y = x.
(j), but due to the nonlinearity of the amplifier, y
The level will be lower than (j). To compensate for this, the input level x (n) is multiplied by the multiplier m = x (j) / x (n) (1), and the input level x () at which the output level is equal to the desired y (j) is obtained. j). That is, x (j) from which the desired output y (j) corresponding to the input level x (n) is selected is obtained from the input / output characteristics of the amplifier, and the compensation coefficient m satisfying the expression (1) is obtained in advance. The compensation coefficient for linearization can be quoted according to the input level of

By the above processing, the nonlinearity until the saturation shown in FIG. 3 is reached is improved, and the input / output characteristics can be linearized as much as possible. Similarly, regarding the phase characteristics (AM / PM) of the input / output characteristics of the amplifier, similarly, the phase characteristics of the amplifier are measured in advance, and the transmission signal is multiplied by the actually measured phase and the reverse phase according to the input level. Can be compensated.
In an actual predistortion, the amplitude and phase information is provided in the input / output characteristics of the amplifier, so that the compensation coefficient of the predistortion is a complex number.

FIG. 4 is a block diagram showing an example in which a conventional predistortion is applied to an OFDM transmission system. In FIG. 4, 1 is an OFDM modulator, 2 is a predistortion circuit, 3 is a transmission frequency conversion circuit, and 4 is a transmission amplifier. In the pre-distortion circuit 2,
The level detector 26 detects the transmission power or amplitude of the OFDM modulated wave, and the A / D converter 27 quantizes the power or amplitude information. Using this as an address, a complex compensation coefficient for predistortion is referenced from a look-up table (LUT) 28, and this is used as a complex multiplier 21
Performs complex multiplication with the OFDM transmission signal. With such a configuration, the transmission signal can be multiplied by the characteristic opposite to the non-linearity of the transmission amplifier 4, and the nonlinear distortion in the output of the transmission amplifier 4 can be reduced.

[0010] The OFDM modulator 1 is a FEC (Forwar).
d Error Correction) circuit 11, S / P (serial / parallel) conversion circuit 12, subcarrier modulation circuit 13, IFFT
(Inverse Fast Fourier Transform) circuit 14 and G /
It comprises an I (guard interval) adding circuit 15 and a waveform shaping circuit 16, which are described in JP-A-2000-2265.
Since the configuration is the same as that disclosed in Japanese Patent Application Publication No. 9 and is well known, the description thereof is omitted.

In this prior art, the level detector 2
As 6, an arithmetic circuit for calculating power or amplitude from analog diode detection or a baseband signal is required. To calculate the power, the quadrature signal I,
To calculate the sum of squares of Q and the amplitude, it is necessary to take the square root of the sum of squares of I and Q respectively.

On the other hand, the paper "Quantizastion Analysis and
Design of a Digital Predistortion Linearizer for R
F Power Amplifiers, "Lars Sundstroem, Michael Faulk
ner, Mats Johansson, IEEE Transactions on Vehicular
In Technology November 1996, it is pointed out that power information is easier to calculate than amplitude information, but that amplitude information should be used to perform highly accurate predistortion.

To calculate the amplitude of the transmission signal for level detection, it is necessary to find the square root of the sum of squares of the orthogonal signal. To perform these calculations on the baseband signal, a digital calculation circuit is required. In addition, there arises a problem that the device scale and extra power for driving the digital circuit are required.

[0014]

OFDM has become a standard for modulation schemes in fields such as digital broadcasting and wireless access, and is an important technology. The advantage of OFDM is that the frequency use efficiency is high and the modulation method itself is resistant to multipath distortion, but the modulation method is easily affected by nonlinear distortion of the transmission amplifier. It is also necessary to pay attention to the nonlinearity of the transmission amplifier from the viewpoint of adjacent channel interference. Under the circumstances described above, it is effective to perform nonlinear distortion compensation such as pre-distortion on OFDM transmission.

In particular, since the back-off of the transmission amplifier can be reduced by the pre-distortion, the power consumption of the transmission amplifier can be reduced, and the invention can be applied to a power saving system such as mobile communication. However, when a conventional pre-distortion is applied to a modulated wave having a high peak factor such as OFDM, a large dynamic range is required for a diode detector as a means for detecting the amplitude or power of a transmission signal, and an analog circuit is required. This causes a problem of characteristic variations due to the above.

Further, in the conventional pre-distortion, as described above, an arithmetic circuit for specifying the amplitude or power of a transmission signal is required, and even if digital processing is used, the circuit scale and power consumption increase.

An object of the present invention is not to directly detect the amplitude after OFDM modulation, but to refer to amplitude information calculated in advance from a transmission data sequence before performing OFDM modulation, thereby obtaining the above-described problem. It is an object of the present invention to provide a nonlinear distortion compensating circuit for OFDM transmission with free distortion that can solve the problem.

[0018]

According to the present invention, an OFD is provided.
An information quoting unit that receives a transmission data sequence before performing M-modulation as input, and cites amplitude information (or power information) of an OFDM modulated wave obtained in advance corresponding to the transmission data sequence, and a quotation by the information quoting unit. Inputting information, and a compensation coefficient quoting means for citing a compensation coefficient of the nonlinear distortion of the transmission amplifier determined in advance in accordance with the quoting information; And a nonlinear distortion compensating circuit for OFDM transmission.

Further, according to the present invention, there is provided a citing means for inputting a transmission data sequence before performing OFDM modulation, and citing a compensation coefficient for nonlinear distortion of a transmission amplifier obtained in advance corresponding to the transmission data sequence. Multiplication means for multiplying the OFDM modulated wave by the compensation coefficient cited by the citing means. A nonlinear distortion compensating circuit for OFDM transmission is obtained.

The operation of the present invention will be described. In the present invention, O determined in advance according to the transmission data of the baseband is
By citing the amplitude information of the FDM modulated wave, it is not necessary to use a conventional transmission signal level detecting means.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the first embodiment of the present invention, and the same parts as those in FIG. 4 are denoted by the same reference numerals. In FIG. 1, 1 is an OFDM modulator, 2 is a predistortion circuit, 3 is a transmission frequency conversion circuit, and 4 is a transmission amplifier. In the OFDM modulator 1, 11 is an error correction circuit (FEC), 12 is a serial / parallel conversion circuit (S / P), 13 is a subcarrier modulation circuit, and 14
Is an inverse Fourier transform circuit (IFFT), 15 is a guard interval addition circuit (G / I), and 16 is a waveform shaping circuit.

In the pre-distortion circuit 2, 2
1 is a complex multiplier, 22 is a serial / parallel conversion circuit (S / P), 2
Reference numeral 3 denotes a lookup table # 2 (LUT-2), and reference numeral 24 denotes a lookup table # 1 (LUT-1).

In the present invention, the amplitude information of the transmission signal is obtained from the transmission data sequence of the baseband without using the conventional transmission signal level detector 26 shown in FIG. 4, and predistortion is performed. . As a method for obtaining the amplitude information of the OFDM modulated wave from the transmission data sequence,
The OFDM modulation is performed using the property that one symbol is composed of the sum of N subcarrier modulation waves.

For example, taking the case where the subcarrier modulation is BPSK as an example, the OFDM modulated wave at time t is generally s (t) = ｛Σ (n = 0, N−1) dn · exp (j2πfn · t) )｝ (2) Here, Σ (n = 0, N-1) indicates that the variable n is from 0 to N
Dn indicates transmission data, and fn indicates an n-th orthogonal frequency. This equation (2) shows the case of an analog signal having a continuous time t, but the time t is sampled in order to perform digital processing.

On the other hand, assuming that the period of the parallelized data corresponding to the subcarrier is Tc, the frequency fn of the nth subcarrier is represented by fn = n / Tc (3). Further, when the time t is defined by a sampling value of t = p · Tc / N (p = 0, 1, 2,..., N) (4), the expression (21) gives s (pTc / N) = ｛ {(N = 0, N-1) dn · exp (j2πnp / N)} / N (5) This corresponds to the processing of the inverse discrete Fourier transform (IDFT).

From the above, the p-th sampling value s (pTc / N) of the transmission OFDM signal is obtained from equation (5) from the N-bit information from n = 0 to n = N-1 of the transmission data. Can be done. That is, the transmission data sequence and the OFDM modulated wave have a mapping relationship, and the amplitude information of the OFDM modulated wave also has a mapping relationship with the transmission data sequence. Using this property, the amplitude value (absolute value) | s of the transmission OFDM modulated wave corresponding to the combination of the N-bit data series
If (pTc / N) | is calculated in advance and stored in a look-up table, the information of the transmission data sequence can be obtained by inputting the information of the transmission data sequence as shown in FIG. To lookup table 23
, And the amplitude information of the OFDM modulated wave corresponding to the transmission data sequence can be referred to.

The amplitude value information is stored in a lookup table 24.
And a pre-distortion compensation coefficient corresponding to the amplitude of the OFDM modulated wave can be referred to. The predistortion compensation coefficient from the look-up table 24 is converted by the complex multiplier 21 into the ODM signal from the OFDM modulator 1.
By performing a complex multiplication on the FDM modulated wave, pre-distortion becomes possible as before.

Although the above description has been made using BPSK as an example, when the subcarrier modulation is QPSK, two bits of information data are assigned to one symbol of QPSK. Is 2N
Parallelized into branches. Similarly, when the subcarrier modulation is 16QAM, the serial-parallel conversion circuit 12 performs parallelization of 4N branches. Therefore, the serial-parallel conversion circuit 22 of the pre-distortion circuit 2 may perform the same processing.

FIG. 2 is a block diagram showing a second embodiment of the present invention. In FIG. 2, the same parts as those in FIG. 1 are denoted by the same reference numerals. In FIG. 2, the difference from the embodiment of FIG. 1 is that the figure lookup table (# 2) 23 is deleted and one lookup table 25 is provided. That is, in the example of FIG.
A look-up table for quoting the amplitude information of the FDM modulated wave and a look-up table for quoting the pre-distortion compensation coefficient are provided separately and independently. Are integrated into one look-up table in the second embodiment.

Since the look-up table can be realized by a read-only memory such as a ROM, two look-up tables can be aggregated.
As described in the description of the first embodiment, the transmission data sequence parallelized by the serial-parallel conversion circuit 22 and the amplitude information of the OFDM transmission wave have a mapping relationship. The amplitude information and the predistortion compensation coefficient also have a mapping relationship. That is, since the transmission data sequence and the pre-distortion compensation coefficient have a mapping relationship, it is possible to directly quote the pre-distortion compensation coefficient from the transmission data sequence without passing through the amplitude information. Therefore, it is possible to integrate two look-up tables as shown in FIG. 2, which significantly reduces the circuit configuration as compared with the pre-distortion of the prior art, and reduces the size and size of the device. A great effect can be obtained for power saving.

[0031]

As described above, according to the present invention,
As a means for obtaining amplitude information of a transmission signal, a serial-parallel conversion circuit and a look-up table are used to directly obtain a multiplier for predistortion from a transmission data sequence. It is smaller than a level detector, an amplitude calculation circuit, an A / D conversion circuit, and the like as means for obtaining amplitude information of a transmission signal in predistortion, and it is possible to reduce the size and power consumption of the device compared to the predistortion of the prior art. A certain effect can be obtained.

In the prior art pre-distortion, an analog detector is generally used as a means for detecting the level of a transmission baseband signal.
In the case of a modulation method having an extremely high peak factor such as a D signal, the dynamic range of the detection characteristics of the diode is insufficient, and variations in the characteristics of the diode are problematic. Since the amplitude information of the transmission signal can be obtained on a one-to-one basis from the transmission data sequence without using detection, there is an effect that the analog circuit in the related art is not affected.

[Brief description of the drawings]

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

FIG. 2 is a block diagram of a second embodiment of the present invention.

FIG. 3 is a diagram showing input / output characteristics (AM / AM) of an amplifier.

FIG. 4 is a block diagram showing an example of the related art.

[Explanation of symbols]

 Reference Signs List 1 OFDM modulator 2 Predistortion circuit 3 Transmission frequency conversion circuit 4 Transmission amplifier 11 FEC circuit 12, 22 Serial-parallel conversion circuit 13 Subcarrier modulation circuit 14 IFFT circuit 15 Guard interval addition circuit 16 Waveform shaping circuit 23, 24, 25 Lookup table

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J090 AA01 AA41 CA21 CA36 CA92 FA20 GN02 GN07 HN04 KA00 KA34 KA53 KA55 MA14 MA20 SA14 TA01 TA02 5J091 AA01 AA41 CA21 CA36 CA92 FA20 KA00 KA34 KA53 KA55 MA02 A01 TA14 CA21 CA36 CA92 FA20 KA00 KA34 KA53 KA55 MA14 MA20 SA14 TA01 TA02 5K022 DD01 DD23 DD24 5K060 BB07 HH31 HH33 KK06 LL01

Claims (6)

[Claims] 1. An information citing means for inputting a transmission data sequence before performing OFDM modulation and citing amplitude information (or power information) of an OFDM modulated wave obtained in advance corresponding to the transmission data sequence, A compensating coefficient citing means for inputting the citing information by the information citing means and citing a compensation coefficient of the nonlinear distortion of the transmission amplifier which is determined in advance in accordance with the citing information; And a multiplying means for multiplying the wave. 2. The information quoting means is a memory in which the transmission data sequence is input as an address, and the amplitude information (or power information) is stored in advance corresponding to each of the addresses. 2. The nonlinear distortion compensating circuit for OFDM transmission according to 1. 3. The compensation coefficient citing means according to claim 1, wherein the citation information is an address input, and the compensation coefficient is a memory in which the compensation coefficient is stored in advance corresponding to each of the addresses. Non-linear distortion compensation circuit for OFDM transmission. 4. A quoting means for inputting a transmission data sequence before performing OFDM modulation and citing a compensation coefficient of a nonlinear distortion of a transmission amplifier previously obtained corresponding to the transmission data sequence, and quoting by the quoting means. The calculated compensation coefficient is
A nonlinear distortion compensating circuit for OFDM transmission, comprising: multiplying means for multiplying the FDM modulated wave. 5. The OFDM transmission apparatus according to claim 4, wherein said citing means is a memory in which said transmission data sequence is input as an address, and said compensation coefficient is stored in advance corresponding to each of said addresses. Nonlinear distortion compensation circuit. 6. The OFD according to claim 2, wherein said memory is a read-only memory.
Nonlinear distortion compensation circuit for M transmission.