JPS61176221A - Compensation system for nonlinear distortion - Google Patents

Abstract

PURPOSE:To reduce the maximum amplitude of the envelope curve of a syn thetic wave by avoiding the coincidence between the clock cycles of input signals supplied to each modulator and therefore to reduce the effects of nonlinear distortions produced by a power amplifier. CONSTITUTION:To avoid the coincidence between clock cycles of input signals in a multi-carrier system or an orthogonal amplitude modulation system. Thus the increase of the peak power and the distortions are prevented for the envelop curve of a synthetic wave in comparison with the voltage sum of input signals. For instance, the fixed phases of the base band signals are varied so that a 180 deg. phase difference is secured between phase shifters 1 and 2 and multiplied by local oscillation signals 5 and 6 of different frequencies by modulators 3 and 4 for modulation. Then the image signals of the modulated base band signals are eliminated through filters 7 and 8, and these base band signals undergo the multiplication of frequency through a synthesizer 9 and then the common amplification through a power amplifier 10. Such base band signals are radiated through an antenna 11.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a digital modulation method in which nonlinear distortion is a problem.

(Prior art) Conventionally, in repeaters where nonlinear distortion is a problem, distortion is suppressed by using a high-output amplifier and providing sufficient back-off, or by using distortion compensation such as a predistorter. By adding a device, there are drawbacks such as the need to use means for suppressing distortion caused by non-linearity. For example, taking the 1.6 QAM modulation/single carrier system as an example, a FET amplifier requires a backoff amount of 8 dB, and a traveling wave tube amplifier requires a backoff amount of 11 dB or more.
In the fixed wireless system, in order to obtain an average transmission power of several watts, it is necessary to use a high-power amplifier with a saturated output of several 10 watts.

In addition, even when using a predistorter, which has a large compensation effect, as a distortion compensation device,
With the 16' QAM method, an effect of only about 2 dB could be obtained.

(Problem to be solved by the invention) In the multi-carrier system, multiple modulated waves are weighted, so their envelopes are summed by the voltage sum, for example, 6 dB for 2-wave multi-carrier, Since the peak power increases significantly by 12 dB, it was necessary to prepare a power amplifier with extremely high output. The present invention aims to improve this point.

(Means for Solving the Problems) In order to achieve the above object, the present invention makes the clock periods for each input signal in a multicarrier system or a quadrature amplitude modulation system different from each other.

(Function) Since the clock periods for the input signals do not match, the envelope of the composite wave is at a much lower level than the sum of the voltages of each input signal, thus preventing an increase in peak power and distortion.

゛ (Embodiment) Fig. 1 shows an embodiment of the present invention in a multi-carrier system, in which the fixed phase of the baseband signals is changed by phase shifters 1 and 2 so that they have a phase difference of 180°, and then
It is modulated by being multiplied by local oscillation signals 5 and 6 of different frequencies in a modulator 3.4. Filter 7
．． After removing the image signal at step 8, the signal is frequency-multiplexed at combiner 9, commonly amplified at power amplifier 1O, and then radiated from antenna 11.

FIG. 2 shows the eye aperture of a four-level baseband signal. Let the maximum level of the discrimination point be l, b (b=1
+a) Shows the maximum level of the ci envelope.

In wireless transmission systems, strict band limits are often imposed on baseband signals in order to increase frequency utilization efficiency and reduce interference between adjacent channels. becomes larger,
Since the AM component of the envelope increases, the maximum value of the envelope increases. For example, if band limitation is applied with a roll-off rate of 50%, a will be 0.5.

Figure 3 shows the envelope of the composite wave when the two modulated waves modulated by the signals shown in Figure 2 are frequency multiplexed. (a) shows when the clock phases are matched, (
b) shows the envelope when the clock phase is shifted by 180°. In (a), the peaks of the two envelopes match and the valleys of the two envelopes match, so the AM component of the envelopes increases significantly. (In bl, the peaks and valleys of the two envelopes coincide, so the AM component of the envelopes decreases. Taking the case where the envelopes have a sine wave shape as an example, Fig. 3(a) and Fig. 3( The ratio of the envelope of b) to the maximum value is (2+a)/(2+2
a)', a=0.5 and 0.9dE3, a=
The maximum amplitude in figure (b) is smaller by 1.4 dB %' at 1 and 1.8843 at a = 2. Therefore, the influence of nonlinear distortion generated in the power amplifier 10 can be reduced.

FIG. 4 shows another embodiment of the present invention, in which the peak factor of the envelope is reduced by shifting the phase of the quadrature signal and in-phase signal in the quadrature amplitude modulation method using phase shifters 1 and 2. Non-linear distortion that occurs can be reduced.

When a baseband signal as shown in FIG. 2 is orthogonally modulated, the maximum amplitude of the envelope is, if the phases of the quadrature signal and the in-phase signal are the same, then l+a=v'2U+
a). On the other hand, when the phases are shifted by 180°, the maximum amplitude is approximately v'(1+a)2+12, for example, 1.4 dB when a=0.5, 2 dB when a=l, a
='2, the envelope of the 2.5 dB modulated wave decreases.

FIG. 5 shows yet another embodiment of the present invention, in which synchronized baseband signals are transmitted to 14. By performing speed conversion and desynchronization using '15, it is possible to reduce the probability that a peak will occur in the envelope after synthesis, and as a result, it is possible to reduce nonlinear distortion occurring in the power amplifier. Since the 14° and 150 outputs are asynchronous, the probability that the peaks of the envelopes at the synthesis points of 9 coincide is greatly reduced, and the peak factor of the envelope of the synthesized wave can be reduced.

(Effects of the Invention) As explained below, by making the phases of multiple series of digital baseband signals input to the modulator different from each other or desynchronized, the envelope of the modulated wave or the frequency-multiplexed composite wave can be reduced. Since the maximum amplitude of the line can be reduced or the probability that the maximum amplitude will occur can be reduced, the influence of non-linear distortion generated in the power amplifier can be reduced.

Furthermore, if the present invention is used, it is possible to amplify a larger amount of power with the same amplifier, resulting in advantages such as the ability to greatly improve the efficiency of the power amplifier.

Normal baseband signals are often synchronized, and in that case, a very simple configuration such as adding a fixed phase shifter or delay line to the baseband signal input stage of the modulator can have a large distortion compensation effect. You can expect.

[Brief explanation of drawings]

Figure 1 shows an example of the device of the present invention in a multi-carrier system, Figure 2 is an eye aperture diagram of a 4-value roll-off waveform used in 16 QAM modulation, and Figure 3 shows modulated waves of the first and second waves. FIG. 4 shows an embodiment of the present invention in quadrature amplitude modulation, and FIG. 5 shows another embodiment in a multi-gear system. 1.2... Phase shifter or delay line, 3, 4... Modulator, 5.6... Local oscillator, 7.8... Bandpass filter, 9... Synthesizer, 10... power amplifier,
11...Antenna, 12,]3...Low pass filter,
14.15...speed conversion circuit or clock desynchronization circuit.

Claims (6)

[Claims] (1) In a multi-carrier system in which multiple input signals are modulated by a modulator, the outputs are frequency multiplexed, and then commonly amplified to obtain a transmission wave, the clock periods of the input signals entering each modulator are made to differ from each other. A nonlinear distortion compensation method characterized by: (2) The nonlinear distortion compensation method according to claim 1, wherein the clock periods are asynchronous with each other. (3) Claim 1, characterized in that the clock periods are mutually synchronized and have a predetermined phase difference.
Nonlinear distortion compensation method described in section. (4) In quadrature amplitude modulation, a nonlinear distortion compensation method is characterized in that the clock periods of the input signals of the quadrature channel and the in-phase channel are made to be different from each other. (5) The nonlinear distortion compensation method according to claim 4, wherein the clock periods are asynchronous with each other. (6) Claim 4, characterized in that the clock periods are mutually synchronized and have a predetermined phase difference.
Nonlinear distortion compensation method described in section.