JPH0588005B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a modulation system that uses the amplitude and phase of a carrier wave as information, and in which the waveform of a communication signal is deformed in advance in order to compensate for the nonlinearity of an amplifier. Regarding the location.

(Prior Art) In recent years, as radio wave resources are running out, channels in wireless communications are becoming narrower in order to make more effective use of frequencies. If the channel band becomes narrower, a linear modulation method is preferable to a nonlinear modulation method such as FM, which has a wider band. This applies regardless of digital transmission or analog transmission. In the linear modulation method, deterioration of the transmission spectrum and reception characteristics due to nonlinearity of the amplifier become a problem.

As shown in FIG. 9, 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. 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.

FIGS. 7a to 7d show the influence of such a nonlinear amplifier on a signal using 16-value QAM as an example. FIG. 7a shows the signal point distribution in the phase plane of the transmission signal that should originally exist, and FIG. 7b shows the transmission spectrum distribution at that time. FIG. 7c 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. 7c is distorted compared to the signal point in FIG. 7a. In the transmission spectrum at this time, as shown in FIG. 7d, odd-numbered intermodulation components such as 3rd and 5th order appear, causing interference with adjacent channels. Also, since the receiver judges that the signal point shown in Figure 7a has been sent, if the signal point shown in Figure 7c is sent,
Errors occur due to small noises, and 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 for digital transmission that compensates for such nonlinearity and also compensates for temporal changes in amplifier characteristics, as disclosed in Japanese Patent Laid-Open No. 105658/1983. FIG. 6 is a block diagram of a conventional modulation device with an adaptive linearization circuit. Transmission data series are input in parallel from the input terminal 600. Diagonal lines on the connections in FIG. 6 indicate a plurality of connections. The transmission data series is stored in a first memory, a random access memory 610 {RAM (Random Access Memory)}.
and read-only memory, which is the second memory.
Memory 620 {ROM (Read Only Memory)}
address. The ROM 620 stores the original signal point arrangement as shown in FIG. ing. RAM610
The output of
35 is band-limited and the modulator 640 is the oscillator 651.
The output is orthogonally modulated and output from terminal 601 to the nonlinear amplifier. To adaptively change the contents of RAM 610, the output of the nonlinear amplifier is connected to terminal 610.
2 and is demodulated by the demodulator 660 using the output of the oscillator 651. The signal demodulated by demodulator 660 is converted into a complex digital signal by analog-to-digital converter 670. This demodulated complex digital signal is subtracted from the original signal read from the ROM 620 in a subtraction circuit 680, and the result is multiplied by a constant coefficient K in a correction amount generation circuit 690 (generally, K is a value sufficiently smaller than 1). ), is added to the output read from the RAM 610 by an adder circuit 691. If the demodulated value is
When the demodulated value is smaller than the original value from ROM 620, the content of RAM 610 is controlled to be smaller, and when the demodulated value is smaller than the original value from ROM 620, the content of RAM 610 is controlled to be enlarged. . By doing this, even if the input/output characteristics of the nonlinear amplifier change, the output of the nonlinear amplifier, that is, the terminal 60
The contents of the RAM 610 can be controlled so that the input signal from the RAM 610 has the correct signal point arrangement as shown in FIG. 7a.

(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, if a band-limited 4-level signal is shown as a solid line in FIG. 8a, when it is distorted by an amplifier, it becomes as shown in a broken line in FIG. 8a. Such trajectory changes lead to spectrum deterioration. The RAM 610 only outputs signal points at each symbol point, and the output of the filter 635 is as shown in FIG. 8b. If distortion is further added to this, the result will become as shown by the solid line in FIG. 8c. However, since it does not match the dashed line in FIG. 8c, which is the original signal trajectory, the transmission spectrum is not sufficiently improved. This is because the linear circuit shown in FIG. 6 only compensates for the linearity at the symbol point, and does not compensate for the trajectory along the way.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a modulation device that can overcome such drawbacks and compensate for the nonlinearity of the amplifier so that the transmission spectrum does not deteriorate due to the nonlinearity of the amplifier.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a modulation device: a readout address is provided by a first sampled signal sequence representing an amplitude component of a modulated signal; a first rewritable memory for outputting an amplitude distortion signal that compensates for distortion of the amplitude component of the power amplifier; a first summing circuit for summing the output of the first memory and the first sampled signal sequence; and; outputting a phase distortion signal compensating for the distortion of the phase component of the power amplifier applied to the second sampled signal sequence representing the phase component of the modulated signal, provided with a read address by the first sampled signal sequence; a second rewritable memory for summing the output of the second memory and the second sampled signal sequence; an amplitude modulator that receives the output of the first adder circuit and the output of this phase modulator and outputs a signal to the output amplifier by multiplying the two; and one of the outputs of this power amplifier. an amplitude component extraction circuit that receives a portion of the power amplifier output and outputs an amplitude component of the output; a phase change extraction circuit that receives a portion of the power amplifier output and the phase modulator output and extracts a phase change in the power amplifier; a first sampling circuit that samples the amplitude component extraction circuit output; a second sampling circuit that samples the phase change extraction circuit output; the first sampling circuit output and the first sampled signal. and the first rewritable memory output;
outputting a signal to the first rewritable memory for rewriting the contents of the first rewritable memory such that the first sample circuit output and the first sampled signal sequence are equal; a first correction signal generation circuit; receiving the second sample circuit output and the second rewritable memory output; It is characterized by comprising a second correction signal generation circuit that outputs a signal for rewriting the contents of the memory to the second rewritable memory.

(Principle of the Invention) Generally, a modulated band signal S(t) is expressed as follows, where the carrier frequency c is S(t)=Re{a(t)+jb(t))exp(j2〓fct)}=Re{R( t)・e ^j 〓(t)・exp(j2〓fct)} ……(1) can be written. Here, R(t)e ^j 〓(t) is an equivalent baseband signal expressed in polar coordinates. When S(t) passes through an amplifier with input/output nonlinear characteristics F(x), the output S′(t) is S′(t)=Re{F[R(t)e ^j 〓(t)]exp (j2〓fct)}
...(2) becomes. Here, the nonlinear distortion added to the amplitude and phase is uniquely determined once the value of the amplitude is determined. Therefore, equation (2) becomes S(t)′=Re[F ₁ (R(t)·exp{jF _2R (〓(t)} exp{j2〓fct}] …(2)′). Here, F ₁ (R(t)) and F _2R (〓(t)) are functions with input/output amplitude phase characteristics as shown in Figure 9. Therefore, Re[F ₁ {G ₁ (R(t) ))}・exp{jF _2R {G _2R (〓(t))}
}
exp {j2〓fct}]=Re[R(t)・exp{j〓(t)}・exp{j
2
〓fct}] When the output of the circuit that realizes the functions G ₁ (R(t)) and G _2R (〓(t)) is passed through an amplifier, a transmitted signal that is not distorted at the output of the amplifier is obtained.

The present invention solves (3) by accepting R(t)e ^j 〓(t) in equation (1).
Pass it through a digital circuit that realizes the function G ₁ (x)G _2R (x) in the equation, and use the nonlinear amplifier output as Re{R(t)e ^j 〓(t)
e ^j2 〓 ^fct }, and also has the function of changing the shape of the function G(x) in accordance with temporal changes in the amplifier characteristics.

(Summary of the present invention) The present invention utilizes the property of nonlinear characteristics that distortion is determined by amplitude. In response to the amplitude of the input signal, it outputs G ₁ (R(t))/R(t) and G _2R (〓(t)−〓(t)), and uses this and the input signal to distort the modulation signal. This method obtains G ₁ (R(t)) and G _2R (〓(t)), which are the amplitude and phase components of the equivalent baseband signal.Since the signal is expressed in polar coordinates, the amplitude components G ₁ (R(t)) and G _2R [〓(t)] can be obtained by multiplication and addition of the phase components.However, regarding changes in the amplitude component, the same applies to addition. Functional adaptive control is performed by rewriting a conversion table that outputs a signal correction component in accordance with the input amplitude.

(Example) An example of the present invention is shown in FIG. 1 and will be described in detail below.

Amplitude information of an equivalent baseband signal is input from an input terminal 101, and phase information is input from an input terminal 102. Both input signals are digital signals obtained by sampling and quantizing the amplitude and phase components of the equivalent baseband signal. The first rewritable memory (RAM) 111 stores a distortion component to be added to compensate for distortion to the amplitude component of the amplifier, corresponding to input amplitude information. The input sample quantized amplitude information and the first RAM output are added in the first adder 113. A first digital-to-analog converter (D/A) 114 converts the output of the adder 113 into an analog signal and smoothes it. D/A114
The output is the value of G ₁ (R(t)) in equation (2).
On the other hand, the nonlinear component with respect to the phase component in the amplifier is uniquely determined by the input amplitude. second
The RAM 121 stores distortion components to be added to compensate for distortion to the phase component of the amplifier 150 in accordance with input amplitude information. The input sample quantized phase information signal and the output of the second RAM 121 are added to the second adder 123 . A second digital-to-analog converter (D/A) 124 smoothes the output of the adder 123 into an analog signal. D/A124 output is the formula
It represents G _2R (〓(t)) in (2). D/
The A124 output goes into the phase modulator 140 and the signal
e ^jG2R (〓(t)). e ^j2 〓 ^fct is created. Phase modulator 1
For example, as shown in FIG.
A configuration with 0 and VCO220 is possible. In the amplitude modulator 130, the phase modulator output and the D/A 114
By multiplying with the output, G ₁ (R(t))・e ^jG2R( 〓(t)・^j2 〓 ^fct = G(R(t)e ^j 〓(t))e ^j2 〓 ^fct is obtained. By passing the output of amplitude modulator 130 to power amplifier 150, the distortion of amplifier 150 is canceled according to the algorithm shown in equation (3).

When the characteristics of power amplifier 150 change, the first
The contents of RAM 111 and second RAM 121 must be rewritten. Upon receiving a portion of the output of the amplifier 150, an amplitude extraction circuit 170 obtains an amplitude component of the output of the amplifier 150. For example, as shown in Figure 3, the amplitude component can be extracted by multiplying the signal that has passed through the amplitude limiting circuit and the signal that has not passed through it.
This can be done by extracting only the low frequency components. The amplitude component extraction circuit output is sample quantized in a first analog-to-digital converter (A/D) 115. A/D115 output and the first
A first correction signal generation circuit receives a signal from the RAM 111 output and an input signal from the terminal 101 and rewrites the contents of the first RAM so that the A/D 115 output and the input signal from the terminal 101 become equal. Obtained at 112. An example of the first modified signal generation circuit is shown in FIG. 4a. Subtracting the value of the first A/D converter 115 output from the value of the input amplitude signal,
The weighting circuit 420 multiplies by . (0<〓<1)
Output of weighting circuit 420 and first RAM 111
By adding the output with the adder 310 and writing the addition result to the RAM 111 as a new compensation signal, the contents of the RAM 111 can be rewritten so that the input signal from the input terminal 101 and the output from the A/D 115 are equal. can. With the above configuration, the RAM is adaptively adjusted according to changes in the characteristics of the amplifier 150.
The contents of 111 can be rewritten.

If the distortion of amplifier 150 is completely compensated,
The output of subtracter 430 becomes 0. Subtractor 43
0, when subtracting the input signal from the A/D 115 output, the adder 410 becomes a subtracter,
The weighting circuit 420 output is subtracted from the RAM 111 output.

Further, the compensation amount regarding the phase component is modified as follows. From the output of the phase modulator 140 whose phase value is 〓(t) + 〓〓(t) and the output of the amplifier 150 whose phase value is 〓(t) + 〓〓′(t), a phase change extraction circuit is extracted. At 160, the phase change in the amplifier 〓〓′(t)
- Extract 〓〓(t) (here, 〓(t) is the terminal 102
(the value of the phase signal input from ). Phase change extraction is
This can be done with a circuit as shown in FIG. The phase modulator 140 output and the amplifier 150 output are passed through amplitude limiting circuits 510 and 520, respectively, and then multiplied by a multiplier 530, thereby outputting the phase difference between the outputs of the two amplitude limiting circuits.
A second A/D 125 samples and quantizes the phase change extraction circuit output. Output of second A/D 125〓′(t)−〓〓(t) and output of second RAM 121〓〓(t)
The second modification amount generating circuit 122 compares the values with the values and modifies the contents of the RAM so that 〓〓′(t)−Δθ(t) becomes 0. A specific example of the second correction amount generation circuit 122 is shown in FIG. 4b. The output of the second RAM 121 and the output of the A/D 125 are added by an adder 460,
The value 〓〓′(t) is multiplied by the weighting circuit 450 (0<〓<1), and the second RAM 121 and the adder 44
Add at 0. By newly writing the addition result to the RAM 121, the contents of the RAM 121 can be adaptively rewritten according to changes in amplifier characteristics.

(Effects of the Invention) As explained above, the modulation device of the present invention automatically makes the output of the nonlinear amplifier have the correct transmission signal waveform in accordance with the characteristics of the nonlinear amplifier, regardless of the modulation method. be able to. Therefore, according to the present invention, it is possible to provide a modulation device that can compensate for the nonlinearity of the amplifier so that the transmission spectrum does not deteriorate due to the nonlinearity of the amplifier.
Further, the modulation device of the present invention is extremely easy to adjust, and can also be made to follow changes in amplifier characteristics due to temperature.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
2 is a diagram showing a specific example of the phase modulator in FIG. 1, FIG. 3 is a diagram showing a specific example of the amplitude component extraction circuit in FIG. FIG. 5 is a diagram showing a specific example of the first and second correction signal generation circuits, FIG. 5 is a diagram showing a specific example of the phase change extraction circuit of FIG. 1, and FIG. 6 is a diagram showing a conventional adaptive linearization circuit. Block diagram showing the modulator, Fig. 7a, b,
c and d are diagrams showing distortion caused by a 16-value QAM nonlinear amplifier, Figures 8a, b, and c are diagrams showing waveforms at various parts of a conventional modulator with an adaptive linearization circuit, and Figure 9 is a diagram showing the input of the nonlinear amplifier. FIG. 3 is a diagram showing output characteristics. 101, 102...Input terminal, 111, 121
...Rewritable memory, 112, 122...
...Correction signal generation circuit, 113, 123...Adder, 114, 124...Digital-analog converter, 115, 125...Analog-digital converter, 130...Amplitude modulator, 140...Phase modulator, 150 ... Power amplifier, 160 ... Phase change detection circuit, 170 ... Amplitude signal extraction circuit, 21
0...Differentiator, 220...Voltage controlled oscillator, 31
0,510,520...amplitude limiting circuit, 320,
530... Multiplier, 330... Low frequency unit, 4
30...Subtractor, 440...Adder, 410,
460... Adder, 420, 450... Weighting circuit, 600, 602... Input terminal, 601...
Output terminal, 610...RAM, 620...ROM,
630...Digital-to-analog converter, 635
... Bandwidth limiting filter, 640 ... Quadrature modulator, 651 ... Oscillator, 660 ... Demodulator, 67
0...Analog-digital converter, 680...
Subtractor, 690... Correction amount generation circuit, 691...
Adder.

Claims (1)

[Claims] 1 a first rewritable memory provided with a read address by a first sampled signal sequence representing an amplitude component of the modulated signal and outputting an amplitude distortion signal compensating for distortion of the amplitude component of the power amplifier; a first summing circuit for summing one memory output with said first sampled signal sequence; a second rewritable memory outputting a phase distortion signal that compensates for distortion of the phase component of the power amplifier applied to the sampled signal sequence; the second memory output and the second sampled signal sequence; a second adder circuit that adds up; a phase modulator that receives the output of the second adder circuit and outputs a phase modulation signal; inputs the output of the first adder circuit and the output of this phase modulator; an amplitude modulator that outputs a signal to the power amplifier; an amplitude component extraction circuit that receives a part of the output of the power amplifier and outputs an amplitude component of the output; a part of the output of the power amplifier; a phase change extraction circuit that receives the output of the phase modulator and extracts a phase change in the power amplifier; a first sample circuit that samples the output of the amplitude component extraction circuit; and a first sample circuit that samples the output of the phase change extraction circuit. a second sample circuit; receiving the first sample circuit output, the first sampled signal sequence and the first rewritable memory output; a first correction signal generation circuit that outputs a signal to the first rewritable memory for rewriting the contents of the first rewritable memory so that the sampled signal sequence of the first rewritable memory becomes equal to the sampled signal sequence; a signal for rewriting the contents of the second rewritable memory so that the output of the second sample circuit decreases in response to the output of the second sample circuit and the output of the second rewritable memory; A modulation device with an adaptive linearization circuit, comprising a second correction signal generation circuit that outputs to the second rewritable memory.