JPH03261254A - Constant envelope modulator - Google Patents

Abstract

PURPOSE:To enhance the advantage of the spectrum modulation system to the utmost with simple circuit constitution by selecting the polarity of an interpolation signal depending on the result of the comparison between an output of an interpolation signal arithmetic means and a prescribed threshold level and the past output result. CONSTITUTION:A comparison means 50 compares an output of an interpolation signal arithmetic means 10 and a prescribed threshold level 90 and the polarity of an interpolation signal is selected depending on the result of comparison and the past output result. Thus, it is possible to select the polarity of the interpolation signal only when the amplitude of the interpolation signal is a prescribed value or below. When the spread spectrum system is adopted, the polarity switching signal for the interpolation signal is a signal with less correlation with respect to a spread signal. Thus, a modulation wave with a constant envelope is obtained with simple circuit constitution and a specific peak is not given in the frequency distribution, the band is not spread, and an interpolation signal capable of being spread at inverse spread is generated even in the case of adopting the spread spectrum system.

Description

[Detailed Description of the Invention] [Summary] A constant envelope modulator that modulates a baseband information signal into a constant envelope BPSK modulated wave with a constant envelope amplitude, particularly suitable for spread spectrum (SS) communication systems. Regarding constant envelope modulators, a modulated wave with a constant envelope can be obtained with a simple circuit configuration, the frequency distribution does not have a specific peak, the band does not widen, and even if a spread spectrum method is used, the The purpose of the present invention is to provide a constant envelope modulator having the function of generating a complementary signal that can be spread during spreading. a constant envelope modulator that orthogonally modulates a complementary signal Q whose amplitude is adjusted so that , a constant envelope modulator comprising the information signal generator and orthogonal modulation means for orthogonally modulating the complementary signal Q, a comparison means for comparing the output of the complementary signal calculation means with a predetermined threshold; calculation means that performs calculations based on the current output and past output of the means to obtain a polarity switching signal; and based on the polarity switching signal, the polarity of the output of the complementary signal calculation means is switched and supplied to the orthogonal modulation means. and a polarity switching means.

[Industrial application field]

The present invention provides a constant envelope modulator that modulates a baseband information signal into a constant envelope BPSK modulated wave whose envelope amplitude is constant;
In particular, the present invention relates to a constant envelope modulator suitable for spread spectrum (SS) communication systems.

In the field of satellite communications or mobile communications, there is a demand for the development of small, low power consumption transmitters.

As detailed below, a constant envelope modulator that can effectively employ the spread spectrum method and outputs a modulated wave with a constant envelope is promising to meet this demand. .

[Conventional technology]

As is well known, the spread spectrum communication method is a method for transmitting information signals by spreading the band over a much wider frequency band. One of its advantages is that a predetermined error rate can be achieved even if the S/N ratio of the transmission path is poor.

Therefore, by adopting this method, desired communication can be carried out by a relatively low-power transmitter, which greatly contributes to miniaturization and lower power consumption of radio equipment.

It is also important to reduce the power consumption of the power amplifier (HPA) itself at the final stage of the transmitter, and it is effective to use a class C amplifier for this purpose. However, since the class C amplifier performs amplification using the nonlinear region of the amplifier, fluctuations in the amplitude of the signal cause waveform distortion, which causes problems such as broadening the frequency band of the signal.

Therefore, in order to employ a class C amplifier as the power amplifier at the final stage of the transmitter, the modulator at the preceding stage must be a constant envelope modulator whose modulated output always has a constant envelope. Furthermore, when adopting a spread spectrum communication method to reduce the size and power consumption of the − layer, it is important to consider that even if a constant envelope is achieved, ■ It is necessary to avoid situations where peaks occur and the advantages of the spectrum modulation method are lost, or where a signal having a certain correlation with the spread signal is inserted and is not spread during despreading.

By the way, when the BPSK modulation method is adopted as the basic modulation method, in order to avoid widening the frequency band at the transition point of the baseband signal, band limitation is applied to -C in the baseband region or modulation signal region. , the modulated signal becomes a signal whose envelope is not constant as shown in column (C) of FIG.

(Assuming that the envelope line of the PSK modulated wave is kept constant) 1. Yazdam, et al″Con5ta
nt Envelope Band Limited BP
SK Signal”+ TEEE Trans, Co
mmun.

Vol C0M-28No, 6 PP, 889-897
(June, 1980). This can be expressed in a configuration as shown in FIG. The baseband signal I that has passed through the low-pass filter 800 for band limitation is input as one input of the quadrature modulator 120, and is also input to the arithmetic unit 100, where it is set to a predetermined amplitude value. As A, the amplitude Q of the complementary signal is calculated by A-111, and the result is input as the other input of the quadrature modulator 120. The complementary signal is shown in Figure 6 (D).
The waveform shown in I is obtained, and this and the low-pass filter 800
By performing orthogonal modulation with the signal ((B) 4M) that has passed through, a constant envelope modulated wave is obtained.

When the modulated wave outputted by the configuration of FIG. 7 is represented in a phase diagram, it is represented as a point moving on the broken line a in FIG. 8(B). As is clear from the figure, the baseband signal and the complementary signal are bottomed out with their phases shifted by 90 degrees, so the envelope of the modulated wave is not strictly constant. A to make it completely constant
This can be achieved by performing the calculation rΔ7−=111 instead of the calculation −111. The result is (C) 411i
! Shown below.

This fixed color route signal has a phase of 90° with respect to the baseband signal.
Contains different complementary signals. Therefore, normal BP
In the case of SK modulation, synchronization and carrier recovery are difficult due to the presence of a complementary signal during demodulation, but if a spread spectrum communication method is adopted in addition to this, this complementary signal has a direct current component. Since there is no correlation with the despread signal, it is spread and does not pose a problem. Therefore, this constant envelope modulator can be said to be the most suitable modulator for spread spectrum communication systems.

[Problems to be Solved by the Invention] However, if further miniaturization and lower power consumption are pursued, there is still room for improvement in the above-mentioned modulator. In the spectrum communication system, the baseband signal band shown in FIG. 9(A)N is spread as shown in column (B), so that the power of the transmitter can be reduced. However, by adding a complementary signal which is a direct current component containing only the carrier frequency as a frequency component, the frequency spectrum becomes a spectrum with a peak at the center as shown in (C) 8. This peak causes the problem of saturation of the power amplifier in the transmission line, causing distortion.

Distortion can be eliminated by reducing the overall amplitude, but it cannot be said that the performance of the amplifier is effectively utilized.

This problem can be solved by frequently switching the polarity instead of applying the complementary signal in the form of direct current, but the next problem is how to perform this switching.

First, it is conceivable to generate a random code that has no correlation with the spread signal independently of the spread signal and to switch the polarity.

In this case, since the complementary signal and the despread signal have no correlation and are spread during despreading, there is no problem of interference. However, in this case, since the switching signal is generated independently of the spread signal, there is a high probability of switching when the amplitude of the calculated complementary signal is relatively large. Therefore, a problem arises in that the band of the modulated signal is expanded.

Therefore, by switching the polarity of the complementary signal according to the state of the spread signal, only when the amplitude of the complementary signal is small,
It is conceivable to perform polarity switching. However, in this case, a certain correlation inevitably occurs between the spread signal and the switching signal, resulting in the problem that the signals are not spread during despreading.

In order to solve these problems, it is conceivable to use a polarity switching circuit that receives one period of the spread signal as an input, and performs switching at a timing that has no correlation with the spread signal and corresponds to the amplitude of the complementary signal. However, in this case, the complementary signal generation circuit would be extremely large-scale.

Therefore, it is an object of the present invention to obtain a modulated wave with a constant envelope with a simple circuit configuration, to have no specific peak in its frequency distribution, and to prevent the band from widening, and even when a spread spectrum method is adopted, it is possible to obtain a modulated wave with a constant envelope. The object of the present invention is to provide a constant envelope modulator capable of generating complementary signals that can be spread during spreading.

[Means to solve the problem]

FIG. 1 is a diagram showing the basic structure of a constant envelope modulator according to the present invention. The constant envelope modulator of the present invention quadrature modulates the baseband information signal 1 and the complementary signal Q whose amplitude is adjusted so that the envelope of the modulated signal becomes constant when quadrature modulated with the information signal 1. A conventionally known constant envelope modulator that calculates and outputs the amplitude of the complementary signal Q from the information signal I; Orthogonal modulation means 1 for orthogonal modulation
2, a comparison means 50 for comparing the output of the complementary signal calculation means 10 with a predetermined dark value 90, and a polarity switching method that performs calculations based on the current output and past output of the comparison means 50. It is characterized by comprising a calculation means 60 for converting the signal into a signal, and a polarity switching means 70 for switching the polarity of the output of the complementary signal calculation means 10 and supplying it to the orthogonal modulation means 12 based on the polarity switching signal. .

The computation in the complementary signal computation means 10 may be performed by the conventionally known computation of Q=A-III, with A as the predetermined value, but it is preferable to use the computation of Q=-17. Here, l is the magnitude of the information signal, and Q is the magnitude of the complementary signal.

[For production]

By comparing the output of the complementary signal calculating means 10 and a predetermined threshold value 90 in the comparing means 50 and switching the polarity according to the result and past output results, the complementary signal is generated only when the amplitude of the complementary signal is less than or equal to the predetermined value. It becomes possible to switch the polarity of the Further, when a spread spectrum method is adopted, the polarity switching signal for the complementary signal can be a signal with little correlation with the spread signal, as will be described in detail later.

[Embodiment] FIG. 2 is a diagram showing an embodiment of the constant envelope modulator of the present invention. A baseband signal spread by exclusive ORing with a PN code generated by a PN generator (not shown) is input to this circuit.

Low-pass filter 800 is a digital filter for band limiting. The arithmetic unit 102 receives the output I of the low-pass filter 800.
Based on this, the computation of fTr:11 is performed and the magnitude Q of the complementary signal is output. The arithmetic unit 102 uses the input I as an address, and the RO which stores the value of fl-7 as its contents.
This is realized by M. Comparator 500 compares the value input from input A and the value input from input B, and
A logic "1" is output only when the value from is large.

The output of the arithmetic unit 102 is connected to the input A of the comparator 500, and the digital value representing a predetermined threshold value is connected to the input B. The D flip-flop 600 samples the output of the comparator 500 at the rising edge of a clock input (not shown) and outputs it. The output of the D flip-flop 600 is input to the D flip-flop 602, and is sampled and output at the same timing as the D flip-flop 600.

Therefore, the output of the comparator 500 at the time of sampling is output to the D flip-flop 600, and the value one clock previous is output to the D flip-flop 602. The AND circuit 606 includes the output of the D flip-flop 600 and the output of the D flip-flop 600.
2 is logically inverted by an inverting circuit 604 and is inputted. Therefore, the output of the AND circuit 606 is in a state where the output of the arithmetic unit 102 was greater than the threshold value one clock ago and the current value is less than the threshold value, that is, the output of the arithmetic unit 102 is greater than the threshold value. The logic becomes "1" only when the state changes from to less than the threshold value.

The output of the AND circuit 606 is input to the T flip-flop 608, and the output of the AND circuit 606 is a logic "1".
”, the output is inverted. Polarity switching circuit 70
0 corresponds to the output of the T flip-flop 608, and when it is logic "1", the human power Q is output as is, and the logic "0"
When , the polarity is inverted and -Q is output. D/A converter 1
210 and 1212 convert the output (information signal) of the low-pass filter 800 and the output (complementary signal) of the polarity switching circuit 700 into analog signals, respectively, and supply the analog signals to the quadrature modulator 120. The orthogonal modulator 120 performs orthogonal modulation of the two. A multiplier 1201 multiplies the output of the oscillator 1200, which outputs a carrier frequency signal, and the information signal, and a multiplier 1203 multiplies the output of the oscillator 1200. Taking the product of the signal whose phase is delayed by 90° and the complementary signal,
The hybrid 1204 outputs both at the bottom.

FIG. 3 is a tie softening chart for explaining the operation of the circuit of FIG. 2. (A) to (F)8 are the second
This shows the state of the signals at points A-F in the figure. The dotted line represents the signal change timing of the spread baseband signal, and the flip-flops 600, 6
02 is supplied with a clock having a period of 172 of this period, for example.

The signal of (B) 4111 has the form of the signal of (A) WA with the high frequency components removed. For the signal in column (C), the amplitude of the signal in column B is I, the predetermined value is A, and Q=-1.
This is the signal Q calculated by the operation of 0, and it is maximum when the absolute value of the signal (B) @ is minimum, and minimum when the absolute value is maximum, and always takes a positive value.

(C) @ indicates the darkness value given to the comparator 500 L
is also indicated by a dashed line. D flip-flop 600,
The timing of the rising edge of the clock supplied to 602 is at the position indicated by the dotted line in the figure and at the center of the adjacent dotted line. Therefore, the output ((D)I) of the D flip-flop 600 becomes logic "1" when the level L is higher than the level of the output of the arithmetic unit 102 in this timing, and becomes logic "0'° when the level L is lower. It will be.
The output ((E)@) of the flip-flop 608 is inverted at the timing when the signal in the column (D) changes from O to 1. As a result, the output ((F)@) of the polarity switching circuit 700 becomes a signal whose polarity is inverted when the level of the signal Q ((C)a) is sufficiently low.

FIG. 4 shows the modulated wave output from the circuit of FIG. 2 on a phase diagram. Figure 3 (F) column and 4
As is clear from the figure, since the polarity of the complementary signal is appropriately switched, the peak on the frequency spectrum of the modulated wave is sufficiently attenuated, and since the timing is such that the amplitude of the complementary signal is sufficiently small, the band is broadened. is kept to a minimum.

Next, the correlation between the complementary signal generated by the circuit of FIG. 2 and the spread spectrum code will be described. FIG. 5 is a diagram showing these cross-correlation characteristics obtained by computer simulation.

As shown in the figure, since the cross-correlation is sufficiently small, if the complementary signal is despread by a spread spectrum demodulator, the complementary signal will be spread and its level will be sufficiently small, and it can be seen that no deterioration occurs in the demodulator.

〔Effect of the invention〕

As described above, according to the present invention, there is provided a constant envelope modulator that can maximize the advantages of the spectrum modulation method with a simple circuit configuration.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the principle configuration of a constant envelope modulator of the present invention, FIG. 2 is a diagram showing an embodiment of the present invention, and FIG. 3 is a timing diagram for explaining the operation of the circuit in FIG. 2. Fig. 4 is a phase diagram showing the modulated wave by the circuit of Fig. 2, Fig. 5 is a diagram showing the cross-correlation between the complementary signal and the spread spectrum code in the circuit of Fig. 2, and Fig. 6 is a phase diagram showing the modulated wave by the circuit of Fig. 2. A waveform diagram showing the waveforms of a modulated wave and a complementary signal according to the prior art. Fig. 7 is a diagram showing the III precept of a conventional constant envelope modulator. Fig. 8 is a phase diagram showing the modulated wave according to the prior art. Fig. 9 is a diagram showing a frequency spectrum of a modulated wave when a spread spectrum method is adopted in a conventional circuit. In the figure, 102... Arithmetic unit, 120... Quadrature modulator, 500... Comparator, 600, 602... D flip-flop, 608...
・T flip-flop, 700...Polarity switch, 800...Low-pass digital filter.

Claims (1)

[Claims] 1. Orthogonally modulating a baseband information signal I and a complementary signal Q whose amplitude is adjusted so that the envelope of the modulated signal becomes constant when the information signal I is orthogonally modulated together with the information signal I. Complementary signal calculation means (10) which is a constant envelope modulator and calculates and outputs the amplitude of the complementary signal Q from the information signal I;
In a constant envelope modulator comprising orthogonal modulation means (12) for orthogonally modulating the information signal I and the complementary signal Q, the output of the complementary signal calculating means (10) and a predetermined threshold (90)
a comparison means (50) for comparing the
calculation means (60) that performs calculations based on the current output and past output of the polarity switching signal to generate a polarity switching signal; and the complementary signal calculation means (10) based on the polarity switching signal.
A constant envelope modulator, comprising: polarity switching means (70) for switching the polarity of the output of the output signal and supplying the polarity to the orthogonal modulation means (12).