JPH0568894B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to multilevel modulation, especially multilevel modulation (QAM) that uses the amplitude and phase of a carrier wave as information, such as quadrature amplitude modulation. The present invention relates to a modulation device that transforms the waveform of a transmission signal in advance and sends it out in order to compensate for this.

(Prior art) As shown in Figure 7, the input/output nonlinear characteristics of a normal amplifier include a saturation characteristic of the output amplitude called AM-AM conversion, and a change in the output phase depending on the input amplitude called AM-PM conversion. There is. At a point where the input amplitude is sufficiently small from the saturation point, the amplitude characteristic is linear and there is no change in phase. However, as the input amplitude approaches the saturation point, the output amplitude saturates and the output phase begins to rotate. As a result, the transmission spectrum deteriorates and the reception characteristics deteriorate.

Figures 3a-d illustrate the effect of such a nonlinear amplifier on the signal. The modulation is assumed to be 16-level QAM. FIG. 3a shows the signal point distribution in the phase plane of the transmission signal as it should be, and FIG. 3b shows the transmission spectrum distribution at that time. FIG. 3c shows the distribution of signal points in the phase plane of the amplifier output when the operating point is near the saturation level. The signal point in FIG. 3c is distorted compared to the signal point in FIG. 3a. In the transmission spectrum at this time, as shown in FIG. 3d, odd-numbered intermodulation components such as 3rd and 5th order appear, causing interference with adjacent channels. Furthermore, since the receiver judges the signal point shown in Figure 3a as having been sent, if a signal point such as that shown in Figure 3c is sent, an error will occur due to small noise, and the reception Characteristics deteriorate.

In order to prevent deterioration of transmission spectral characteristics and reception characteristics, it is necessary to compensate for such nonlinearity of the amplifier.

Conventionally, there is a method disclosed in Japanese Patent Publication No. 58-105658 as a means for compensating for such nonlinearity and for also compensating for temporal changes in amplifier characteristics. FIG. 4 is a block diagram showing a conventional modulation device with an adaptive linearization circuit. Power transmission data series are input in parallel from the input terminal 400 . Diagonal lines on the connections in FIG. 4 indicate a plurality of connections. The transmission data series is stored in a random access memory 410 which is a first memory.
This is the address of {RAM (Random Access Memory)} and read-only memory 420 {ROM (Read Only Memory)} which is the second memory. The ROM 420 stores the original signal point arrangement as shown in FIG. ing. The output of the RAM 410 is converted into an analog signal by a digital-to-analog converter 430, and then sent to a low pass filter 43.
5, the output of the oscillator 451 is quadrature modulated by the modulator 440, and output from the terminal 401 to the nonlinear amplifier. To adaptively change the contents of RAM 410, the output of the nonlinear amplifier is connected to terminal 402.
The signal is input from the oscillator 451 and demodulated by the demodulator 460 using the output of the oscillator 451. The signal demodulated by demodulator 460 is converted into a complex digital signal by analog-to-digital converter 470. This complex digital signal is subtracted from the original signal read from the ROM 420 in a subtraction circuit 480, and the result is multiplied by a constant coefficient k in a correction amount generation circuit 490 (generally, k is a value sufficiently smaller than 1).
An adder circuit 491 adds it to the output read from 0. If the demodulated value is larger than the original value from the ROM 420, the contents of the RAM 410 are controlled to be smaller, and the demodulated value is
When the value from the ROM 420 is smaller than the value that should originally be, the content of the RAM 410 is controlled to be increased. By doing this, even if the input/output characteristics of the nonlinear amplifier change, the output of the nonlinear amplifier, that is, the input signal from the terminal 402, will always have the correct signal point arrangement as shown in Figure 3a.
The contents of RAM 410 can be controlled.

(Problems to be Solved by the Invention) However, although such conventional systems can prevent deterioration of reception characteristics, they cannot prevent deterioration of transmission spectrum.

For example, assume that the band-limited 4-level signal is as shown by the solid line in FIG. 5a. At that time, when it is subjected to distortion, it becomes like the broken line in FIG. 5a, and such changes in the locus lead to deterioration of the spectrum. RAM410
simply outputs the signal point at each symbol point, and the output of the filter 435 is as shown in FIG. 5b. If distortion is further added to this, Figure 5c
It looks like the solid line. However, since it does not match the original signal trajectory (broken line in FIG. 5c), the transmission spectrum is not sufficiently improved. In this way, the linearization circuit shown in FIG. 4 compensates only for the linearity at the symbol point, but does not compensate for the intermediate trajectory. Therefore, no consideration is given to improving the deterioration of the transmission spectrum.

SUMMARY OF THE INVENTION An object of the present invention is to provide a modulation device with a nonlinear compensation circuit that overcomes these drawbacks and improves not only reception characteristics but also transmission spectrum characteristics.

(Means for Solving the Problem) In order to solve the above-mentioned problem, a modulation device provided by the first invention of the present application includes: a shift register that stores an inputted N-value digital signal for K symbols; a counter operating at a speed faster than the symbol frequency of the digital signal; and a sample of the band-limited transmission signal that should be originally transmitted, which corresponds to the N-value digital signal, in response to the output of the shift register and the output of the counter; a first memory with fixed content that outputs a value series;
a rewritable second memory that receives the output of the shift register and the output of the counter and outputs a distorted signal that is a pre-distorted signal that is originally to be sent; a rewritable second memory that modulates a carrier wave using the distorted signal; a demodulator that demodulates the output of the amplifier to generate a demodulated signal; a subtraction circuit that subtracts the demodulated signal from the sample value series; a correction amount that determines the correction amount from the output of the subtraction circuit; It consists of a generating circuit; an adding circuit that adds the correction amount and the distortion signal; the output of this adding circuit is added to the second
The method is characterized in that the content of the second memory is written into the second memory and the content of the second memory is adaptively changed.

Further, in order to solve the above-mentioned problem, the modulation device provided by the second invention of the present application includes a shift register that stores an input N-value digital signal for K symbols; an operating counter; a counter with fixed contents that receives the signal sequence pattern in the shift register and the output of the counter and outputs a band-limited transmission signal sample value sequence that is originally to be transmitted and corresponds to the N-value digital signal; 1 memory; a rewritable second memory that receives the signal sequence pattern and the counter output and outputs a signal for compensating for nonlinearity of the amplifier;
a first addition circuit that adds the output of the first memory and the output of the second memory to output a pre-distorted signal; modulates a carrier wave using the output of the addition circuit; a demodulator that demodulates the output of the amplifier; a subtraction circuit that subtracts the output of the demodulator from the output of the first memory; and a correction amount calculated by receiving the output of the subtraction circuit. It consists of a correction amount generation circuit; a second addition circuit that adds the correction amount and the output of the second memory; the output of the second addition circuit is written to the second memory to calculate the distortion compensation amount. It is characterized by adaptively rewriting .

(Principle of the Invention) The adaptive linearization circuit of the present invention compensates for the nonlinearity of the amplifier by predistorting not only the symbol points but also the waveform of the signal between the symbol points, which was not considered in the conventional method. The aim is to do this more completely. By performing such compensation, the transmission signal waveform becomes a band-limited waveform that should originally be transmitted, and deterioration of the transmission spectrum waveform is also improved.

(Example) Next, the first and second inventions of the present application will be explained in more detail with reference to Examples.

An embodiment of the first invention of the present application is shown in FIG. 1 as a block diagram. The N-value digital signal sequence {a _o } input from the terminal 101 is accumulated in the shift register 120 for K symbols. Here, K corresponds to the length of the inverse response of the band-limiting filter used in this embodiment. The first memory, ROM 130, has a shift register 120.
1 corresponding to the symbol pattern in the shift register, addressed by signals output in parallel from
The sampled transmission signal waveform corresponding to the symbol time is received and outputted from the counter 110. The transmission signal waveform at this time is already a band-limited signal, and becomes a two-dimensional signal when N-value QAM is used as the modulation method, for example. Figure 6 shows the ROM
130 output example, 16 values, QAM real part signal 4
An example of what was searched within a point is shown as a solid line. The output of the shift register 120 and the output of the counter 110 similarly address the RAM 140, which is a second memory, and the RAM 140 outputs a complex signal waveform for one symbol corresponding to the symbol pattern in the shift register. The output of the RAM 140 is a signal waveform that has been distorted in advance to compensate for the nonlinearity of the amplifier, and returns to its original waveform by passing through the amplifier.
Figure 6 shows an example of the real part of RAM140 output.ROM1
The broken lines correspond to the 30 output examples. RAM1
The sample value sequence from 40 becomes an analog signal in a digital-to-analog converter (DA converter) 150. In the quadrature modulator 160, the output of the oscillator 161, which is a carrier wave, is transmitted to the DA converter 150.
Modulate with the output of The modulated signal is sent to terminal 102
is input to the nonlinear amplifier. A part of the output of the nonlinear amplifier is input to the terminal 103, and a complex signal is demodulated by the orthogonal demodulator 165. Demodulator 16 with analog-digital converter (AD converter) 155
5 outputs are sampled. The sampled AD converter output is compared with the ROM 130 output in a subtraction circuit 170, and the difference (ROM output - AD converter output) is output. In the correction amount generation circuit 180, the output of the subtraction circuit 170 is multiplied by p and sent to the addition circuit 19.
It is added to the RAM 140 output at 0 (p is a constant smaller than 1). The adder circuit 190 output is
By writing in the RAM 140, it also adaptively compensates for changes in amplifier characteristics over time.
When such a modulation device with a linearization circuit is used, a completely compensated transmission signal waveform is output at the amplifier output.

An embodiment of the second invention of the present application is shown in FIG. 2 as a block diagram. In the embodiment shown in FIG. 1, a pre-distorted signal is stored in the second memory RAM 140, but in the embodiment shown in FIG. 2, only the distortion component is stored in the second memory RAM 210. I'll let it happen. ROM130 which is the first memory
The output and the RAM 210 output are connected to the first adder circuit 22.
By summing at 0, a distorted signal is obtained to compensate for nonlinearities. The second addition circuit 230 adds the distortion component output from the RAM 210 and the distortion correction amount output from the correction amount generation circuit 180, and writes the addition result to the RAM 210. As a result, the amount of distortion to be compensated for is adaptively modified.

Since the distortion component is a signal whose signal level is sufficiently small compared to the signal waveform, by adopting such a configuration, the capacity of the RAM can be reduced compared to the embodiment shown in FIG.

In addition, in the subtraction circuit 170 used in the embodiment of FIG. 1 and the embodiment of FIG. Just use it.

Further, in these embodiments, the correction amount generation circuit 180
Although the explanation has been given on the assumption that the output of the subtraction circuit 170 is multiplied by a constant factor k, the same effect can be obtained by using a circuit that retains only the sign of the output of the subtraction circuit 170 and sets the magnitude to a constant small value as the correction amount generating circuit. It will be done. Although 16-level QAM modulation has been described in these embodiments, it is clear that deterioration due to nonlinear amplifiers can be prevented using a similar method for any other digital modulation method.

As described above, the modulation device of the present invention can automatically adjust the output of the nonlinear amplifier to the correct transmission signal waveform in accordance with the characteristics of the nonlinear amplifier, and adjustment is extremely easy. Further, it is also possible to follow changes in characteristics of the amplifier due to temperature.

In the embodiments shown in FIGS. 1 and 2, if there is a delay time between the digital-to-analog converter 150 and the analog-to-digital converter 155, a delay circuit is inserted between the ROM 130 and the subtraction circuit 170. Then, the ROM130 output is
It is necessary to make the delay equal to the AD converter 155 output. Furthermore, if this delay time is longer than one sample period, it is necessary to similarly delay the write address of the RAM and switch it to the read address.

(Effects of the Invention) According to the present invention, as described above, it is possible to provide a modulation device with a nonlinear compensation circuit that improves not only reception characteristics but also transmission spectrum characteristics.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the first invention of the present application, Fig. 2 is a block diagram showing an embodiment of the second invention of the present application, and Fig. 3 a, b, c, and d are 16 value
A diagram showing distortion caused by a QAM nonlinear amplifier. Figure 4 is a block diagram showing a conventional modulation device with an adaptive linearization circuit. Figures 5 a, b, and c are waveforms of various parts of a conventional modulation device with an adaptive linearization circuit. Figure 6 shows the first
A diagram showing the waveform of the memory output in the example shown in FIG.
FIG. 7 is a diagram showing the input/output characteristics of the nonlinear amplifier. 101, 103...input terminal, 102...output terminal, 110...counter, 120...shift register, 130...first memory
ROM, 140...RAM which is the second memory,
150...DA converter, 155...AD converter,
160... Modulator, 161... Transmitter, 165...
... Demodulator, 170 ... Subtraction circuit, 180 ... Correction amount generation circuit, 190 ... Adder, 210 ... Second
RAM, which is the memory of 220... first adder, 230... second adder, 400, 402...
...Input terminal, 401...Output terminal, 410...
RAM, 420... ROM, 430... DA converter, 440... Modulator, 451... Transmitter, 46
0...Demodulator, 470...AD converter, 480...
... Subtraction circuit, 490 ... Correction amount generation circuit, 435
... Bandwidth limit frilter, 491... Addition circuit.

Claims (1)

[Scope of Claims] 1. A shift register that stores an input N-value digital signal for K symbols; a counter that operates at a faster speed than the symbol frequency of the N-value digital signal; an output of the shift register and an output of the counter. a first memory with a fixed content that outputs a sample value series of a band-limited transmission signal that is originally to be transmitted and that corresponds to the N-value digital signal; a rewritable second memory that receives the signal and outputs a distorted signal that is a pre-distorted version of the signal that should originally be sent; a modulator that modulates a carrier wave using the distorted signal and outputs it to an amplifier;
a demodulator that demodulates the output of the amplifier to generate a demodulated signal; a subtraction circuit that subtracts the demodulated signal from the sample value series; a correction amount generation circuit that obtains a correction amount from the output of the subtraction circuit; and the correction amount. and an addition circuit for adding the distortion signal and the distortion signal; an output of the addition circuit is written to the second memory to adaptively change the contents of the second memory. 2. A shift register that stores the input N-value digital signal for K symbols; a counter that operates faster than the symbol frequency of the N-value digital signal; and a counter that receives the signal sequence pattern in the shift register and the counter output; a first memory with a fixed content for outputting a band-limited transmission signal sample value sequence that is originally to be transmitted corresponding to a value digital signal; a rewritable second memory that outputs a signal; a first addition circuit that adds the output of the first memory and the output of the second memory to output a predistorted signal; a modulator that modulates a carrier wave using the output of the addition circuit and outputs it to the amplifier; a demodulator that demodulates the output of the amplifier; a subtraction circuit that subtracts the output of the demodulator from the output of the first memory. a correction amount generation circuit that receives the output of the subtraction circuit and calculates a correction amount; a second addition circuit that adds the correction amount and the output of the second memory; the second addition circuit; A modulation device characterized in that the output is written into the second memory to adaptively rewrite the distortion compensation amount.