JPH0363265B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a method for modifying a transmission pattern in advance in order to compensate for nonlinearity of an amplifier in multi-level modulation, especially multi-level modulation 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 sends out signals.

Conventionally, in multilevel modulators, the output of the amplifier has been operated at a level considerably lower than the maximum output in order to avoid deterioration due to nonlinearity in the saturation region of the amplifier. This method of using an amplifier has the drawback that it consumes a large amount of power but cannot obtain a large output, resulting in poor power utilization efficiency of the amplifier.

SUMMARY OF THE INVENTION An object of the present invention is to provide an adaptive modulation device that eliminates the above-mentioned drawbacks of conventional modulators and can increase the operating level of the amplifier to obtain more output power.

According to the present invention, in multilevel digital communication, there is provided a rewritable first memory that outputs a transmission signal in response to an input signal series, and a first memory with fixed content that outputs a transmission signal in response to the input signal series. 2
a memory, and a modulation circuit that modulates a carrier wave with the output of the first memory and outputs it to a nonlinear amplifier;
a demodulation circuit that demodulates the output of the nonlinear amplifier;
a subtraction circuit that subtracts the output of the demodulation circuit from the output of the second memory; a correction amount generation circuit that generates a correction amount from the output of the subtraction circuit; To obtain an adaptive modulation device comprising an adder circuit for adding an output, and adaptively changing the contents of the first memory by writing the output of the adder circuit to the first memory. I can do it.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a diagram showing the input/output characteristics of a typical amplifier. The nonlinear characteristics of a normal amplifier are the saturation characteristic of the output amplitude called AM-AM conversion, and the AM-PM conversion.
There is a change in the output phase due to the input amplitude called conversion. At a point where the input amplitude is sufficiently small from the saturation point,
The amplitude characteristics are 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. Therefore, in multilevel transmission, the operating point has been set to a value much smaller than the saturation level to avoid this nonlinearity of the amplifier.

FIG. 2 shows the effect of a nonlinear amplifier on the signal. The modulation is assumed to be 16-level QAM. Figure 2 1 shows the distribution of signal points in the phase plane of the amplifier output when the operating point is lowered, and the second
FIG. 2 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. 2 is distorted compared to the signal point 1. Since the receiver judges that the signal point shown in FIG. 21 has been sent, if the signal point shown in FIG. 2 2 is sent, an error will occur due to small noise. The modulator of the present invention eliminates the influence of nonlinearity of the amplifier by distorting the input of the nonlinear amplifier in advance so that the correct amplitude and phase are obtained at the output as shown in Figure 2. .

FIG. 3 shows an embodiment of the present invention.

Transmission data sequences are input in parallel from the input terminal 100. In the case of 16-value QAM, it is 4 bits. Diagonal lines on the connections in FIG. 3 indicate a plurality of connections. The transmission data series is stored in the first memory, a random access memory 10 {RAM (Random
Access Memory)} and read-only memory 20 which is the second memory {ROM
(Read Only Memory)} address.
The original signal point arrangement as shown in Fig. 21 is stored in the ROM20 as complex numbers, and the RAM1
The content of 0 is a value distorted so that the nonlinear amplifier output becomes a correct signal point, and is also entered as a complex value. The output of the RAM 10 is converted into an analog signal by a digital-to-analog converter 30, and then the output of an oscillator 51 is quadrature-modulated by a modulation circuit 40 and output from a terminal 101 to a nonlinear amplifier. RAM1
In order to adaptively change the contents of
The signal is demodulated using the oscillator 51. Since the same oscillator 51 can be used for the modulation circuit 40 and the demodulation circuit 60, the demodulation circuit 60 does not require a carrier extraction circuit. Demodulation circuit 60
The demodulated signal is converted into a complex digital signal by an analog-to-digital converter 70. This demodulated complex digital signal is subtracted from the original signal read from the ROM 20 by a subtraction circuit 80.
The correction amount generation circuit 90 multiplies the result by a constant coefficient k (generally, k is a value sufficiently smaller than 1), and the addition circuit 91 adds the result to the output read from the RAM 10. If the demodulated value is ROM
When it is larger than the original value from 20
The content of RAM 10 is controlled to be small, and when the demodulated value is smaller than the value that should originally be from ROM 20, the content of RAM 10 is controlled to be enlarged. By doing this, the modulation device of the present invention ensures that 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 102, is always the correct signal as shown in FIG. The contents of the RAM 10 can be controlled so that the points are arranged. Assuming that the input/output characteristics of the nonlinear amplifier are such that a signal as shown in Fig. 2 1 is input and an output as shown in Fig. 2 2 is obtained, the contents of the RAM 10 in that case are shown in Fig. 4. When the signal points of the RAM 10, which have a signal point arrangement like this, are passed through a nonlinear amplifier, signal points as shown in FIG. 2 are obtained.

In this embodiment, the correction amount generating circuit is multiplied by a constant coefficient k, but the subtracting circuit 80
A similar effect can be obtained by using a circuit that retains only the sign of the output and sets the magnitude to a constant small value as the correction amount generating circuit. In this embodiment, 16-level QAM modulation has been described, but it is clear that deterioration due to nonlinear amplifiers can be prevented using a similar method for any other QAM modulation method.

As described above, the modulation device of the present invention can automatically adjust the output of the nonlinear amplifier to the correct signal point according to the characteristics of the nonlinear amplifier.
Adjustment is extremely easy. Further, it is also possible to follow changes in characteristics of the amplifier due to temperature.

Furthermore, in the embodiment shown in FIG. 3, if the delay time from the digital-to-analog converter 30 to the analog-to-digital converter 70 is longer than the symbol period, a delay circuit may be inserted into the output of the ROM 20 to cope with the problem. In that case, it is necessary to similarly delay the RAM write address and switch it to the read address.

[Brief explanation of drawings]

Figure 1 is a diagram showing the input/output characteristics of a nonlinear amplifier.
2. FIGS. 1 and 2 are diagrams showing distortion of a 16-value QAM signal by a nonlinear amplifier, and FIG. 3 is a diagram showing one embodiment of the present invention, with reference numerals 10, 20, 40, 50, 60,
Reference numerals 80, 90, and 91 indicate a first memory, a second memory modulation circuit, a nonlinear amplifier, a demodulation circuit, a subtraction circuit, a correction amount generation circuit, and an addition circuit, respectively. FIG. 4 is a diagram showing signal points stored in the RAM 10.

Claims (1)

[Claims] 1 In multilevel digital communication, a rewritable first device that outputs a transmission signal in response to an input signal sequence
a second memory with fixed content that outputs a transmission signal corresponding to the input signal sequence; a modulation circuit that modulates a carrier wave with the output of the first memory and outputs it to the nonlinear amplifier; a demodulation circuit that demodulates the output of the amplifier; a subtraction circuit that subtracts the output of the demodulation circuit from the output of the second memory; a correction amount generation circuit that generates a correction amount from the output of the subtraction circuit; and the correction amount. an adder circuit that adds the output of the generator circuit to the first memory output;
An adaptive modulation device characterized in that the content of the first memory is adaptively changed by writing to the memory of the first memory.