JPH1065577A - Synchronization circuit for spread spectrum communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain stable synchronization. SOLUTION: A correlation output outputted from an inverse spread circuit 1 is logarithmically compressed by a logarithmic compression circuit 2. A phase of a PN code generated from a PN code generator 9 based on a correlation output Sa subject to logarithmic compression is controlled to maintain the synchronization stably even when a level of an input signal is considerably fluctuated. Furthermore, the logarithmically compressed correlation output Sa is fed to a demodulator 4 so as to omit the use of an AGC amplifier in the demodulator 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronization circuit capable of obtaining stable synchronization in a spread spectrum communication system.

[0002]

2. Description of the Related Art A spread spectrum communication system is a communication system in which a spectrum limited to a certain band is spread over a wide band and used. In this communication scheme, the energy of the transmitted signal occupies a wide bandwidth far exceeding the bandwidth required for information transmission. The spread of this spectrum includes:
A code or signal unrelated to the information signal is used. On the receiving side, spectrum despreading is performed using a code or signal copy used for spectrum spreading. When performing this despreading, it is necessary to synchronize the received spectrum-spread received signal with the duplicated code or signal.

Next, FIG. 3 shows an example of a conventional synchronization / demodulation circuit in a spread spectrum communication system. This synchronization
In the demodulation circuit, synchronization can be maintained by tau dither tracking. In FIG. 3, a received input signal is multiplied by a despreading circuit 1 with a PN code generated by a PN code generator 9. This PN code is the same PN code as the PN code subjected to spread spectrum on the transmitting side. The despread input signal is used as an original modulated wave before being spread, and is input to the demodulator 4. Then, information obtained by demodulation in the demodulator 4 is output as an output signal. In this case, an AGC amplifier is provided in the demodulator 4, and the level fluctuation of the input signal received by the AGC amplifier is suppressed.

[0004] The demodulated signal output from the demodulator 4 is detected by the multiplier 101 multiplied by a low frequency pulse output from a low frequency (LF) oscillator 102 for generating tau dither. The detection output from the multiplier 101 is output to a low-pass filter (LPF) 103
Is converted into a DC control signal by a voltage controlled oscillator (VCO)
7 is controlled. This VC
The clock from O7 is supplied to a phase modulator (IPM) 104, where the phase of the clock is slightly modulated by a low-frequency pulse from the LF oscillator 102. In this case, the clock is phase-modulated, for example, about 1/10 clock cycle.

The clock output from IPM 104 is P
Supplied to the N code generator 9, the PN code generator 9 generates a PN code based on this clock. In this case, V
The CO 7 is controlled by the DC control signal output from the LPF 103 so that the phase difference between the input signal and the PN code output from the PN code generator 9 disappears. The loop operates so as to synchronize with the PN code. The loop is composed of the despreading circuit 1, the demodulator 4, the BPF 100, the multiplier 101, and the LP
F103, VCO 7 and PN code generator 9.

Incidentally, since the clock is phase-modulated by the low-frequency pulse output from the LF oscillator 102, the phase of the PN code output from the PN code generator 9 is also phase-modulated. Synchronization can be maintained by tau dither tracking that performs such phase modulation. Therefore, the operation of the tau dither tracking will be described with reference to FIG. A triangle shown by a solid line in FIG. 4 is a correlation output output from the despreading circuit 1 shown in FIG. 3 when the input signal has a normal input level. At this time, it is assumed that the PN code undergoes phase modulation by the action of the LF oscillator 102, and the phase of the PN code periodically changes between the point a1 and the point b1. Then, when the phase advances from point a1 to point b1, it is detected that the correlation output has increased.

Further, the PN code undergoes phase modulation,
Assume that the phase of the code periodically transitions between point a2 and point b2. Then, when the phase advances from point a2 to point b2, it is detected that the correlation output has decreased. Further, the PN code undergoes phase modulation, and P
It is assumed that the phase of the N code periodically changes between points a3 and b3. In this case, the phase advances from point a3 and b
When three points are reached, it is detected that the correlation outputs are at substantially the same level.

[0008] Synchronization can be maintained by performing the following control based on these facts. First, tau
At the time of dither tracking, if the correlation output increases when the phase advances, it is determined that the phase of the PN code is late,
The phase of the PN code is advanced. If the correlation output decreases when the phase advances, it is determined that the phase of the PN code is too advanced, and in this case, the phase of the PN code is delayed. Then, when tracking the tau dither, if the correlation output does not change when the phase advances, it is determined that synchronization is established.

[0009]

By the way, when the spread spectrum communication system is applied to communication with a mobile unit or the like, the power received from the transmitter always changes with the movement of the mobile unit. . This variation will be accompanied by a power level variation of more than six orders of magnitude. For example, when the received power fluctuates and increases, the correlation output output from the despreading circuit 1 shown in FIG. 3 has an increased correlation output level as shown by a broken line triangle in FIG. For example, when the level difference of the correlation output between the points a1 and b1 is A, the level difference of the correlation output between the points a1 ′ and b1 ′ increases to A ′. Since the increase in the correlation output level is supplied to the VCO 7 as it is, the increase in the correlation output level is treated as a change in the phase error signal, and the synchronization becomes unstable. There was a point.

It is an object of the present invention to provide a synchronous circuit of a spread spectrum communication system capable of maintaining stable synchronization even if the input signal level fluctuates greatly.

[0011]

In order to achieve the above object, a synchronization circuit of a spread spectrum communication system according to the present invention comprises:
Despreading means for despreading by multiplying the spread spectrum input signal by a PN code generated by a PN code generator, logarithmic compression means for logarithmically compressing the despread signal, and logarithmic compression Demodulation means for demodulating a signal output from the means, synchronization detection means for performing synchronization detection based on the signal output from the logarithmic compression means, and a voltage-controlled oscillator whose phase is controlled by the output of the synchronization detection means. And a PN code generator for generating a PN code based on a clock output from the voltage controlled oscillator.

Further, the synchronous circuit of the spread spectrum communication system is provided with a tau dither generating means for shifting the phase of the clock output from the voltage controlled oscillator only slightly periodically, and the tau dither generating means is provided. Thus, the phase of the PN code generated by the PN code generator may be shifted slightly periodically.

According to the present invention, the correlation output output from the despreading means is logarithmically compressed by the logarithmic compression means. Therefore, even if the input level of the input signal fluctuates greatly, the correlation output level is reduced. Fluctuations can be suppressed. Therefore, the synchronization can be stably maintained. The logarithmically compressed correlation output is also supplied to the demodulator, so that the normally required AGC amplifier for the demodulator can be used as the logarithmic compression means.
The GC amplifier can be omitted.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing an example of the configuration of an embodiment of a synchronization / demodulation circuit having a synchronization circuit of the spread spectrum communication system according to the present invention. In FIG. 1, a received input signal is multiplied by a despreading circuit 1 with a PN code generated by a PN code generator 9. This PN code is the same PN code as the PN code subjected to spread spectrum on the transmitting side. As a result, a correlation output is output from the despreading circuit 1. The despread correlation output is logarithmically compressed by the logarithmic compression circuit 2 and output as a correlation output Sa. The correlation output Sa is an original modulated wave before being spread-spectrum on the transmission side,
Unwanted wave components are removed by a band pass filter (BPF) 3 and input to a demodulator 4. Then, information obtained by demodulation in the demodulator 4 is output as an output signal.
In this case, the demodulator 4 has no AGC amplifier.

The correlation output S output from the BPF 3
“a” is supplied to the phase detection circuit 5 and a phase difference signal corresponding to the phase difference between the input signal and the PN code generator 9 is output.
This phase difference signal is obtained by multiply detecting the correlation output Sa by the low frequency pulse output from the tau dither generator 8. The phase difference signal output from the phase detection circuit 5 is converted into a DC control signal by a low pass filter (LPF) 6 to control the phase of a clock output from a voltage controlled oscillator (VCO) 7. The VCO 7 is further supplied with a low-frequency pulse from the tau dither generator 8 to slightly modulate the phase of the generated clock.
In this case, the clock is phase-modulated, for example, about 1/10 clock cycle.

The clock output from the VCO 7 is supplied to a PN code generator 9, and the PN code generator 9 generates a PN code based on the clock. In this case, VCO
7 is an input signal and a PN output from the PN code generator 9.
Since the phase difference from the code is controlled by the DC control signal output from the LPF 6, the input signal and P
The loop operates so that the PN code output from the N code generator 9 is synchronized. Note that the loop is a despreading circuit 1,
Logarithmic compression circuit 2, BPF3, phase detection circuit 5, LPF
6, a VCO 7, and a PN code generator 9.

Since the clock is phase-modulated by the low-frequency pulse output from the tau dither generator 8, the phase of the PN code output from the PN code generator 9 is also phase-modulated. With the tau dither tracking that performs such phase modulation, the synchronization can be maintained as described above with reference to FIG. In this case, since the correlation output output from the despreading circuit 1 is the correlation output Sa compressed by the logarithmic compression circuit 2, the synchronization can be stably maintained even if the input signal fluctuates greatly. Become.

This will be described with reference to FIGS. 2 (a) and 2 (b). FIG. 2A shows the correlation output Sa output from the logarithmic compression circuit 2 when the input signal level is normal. At this time, the correlation output Sa is converted by the operation of the tau dither generator 8 into the tau dither. The frequency of the low-frequency signal generated by the generator 8 changes finely in a sine wave form, for example, with a 1/10 clock cycle width. In this case, the amount of change in the correlation output Sa due to the tau dither modulation is the level B. FIG. 2B shows the correlation output S output from the logarithmic compression circuit 2 when the input signal level is significantly increased by 6 digits or more.
a ′, and the correlation output Sa ′ is sinusoidal at the cycle of the low-frequency signal generated by the tau dither generator 8 by the operation of the tau dither generator 8, for example, with a 1/10 clock cycle width. It changes finely. In this case, the amount of change in the correlation output Sa ′ due to the tau dither modulation is level B ′, and if the operation of the logarithmic compression circuit 2 is ideal,
As shown in FIG. 2B, the change is the same as the level B.
However, the DC component slightly increases.

Therefore, even if the input level fluctuates by six digits or more, the change is suppressed by the logarithmic compression circuit 2, so that the synchronization is stably maintained. Since the output from the despreading circuit 1 whose fluctuation is suppressed by the logarithmic compression circuit 2 is supplied to the demodulator 4, the demodulator 4 needs an AGC amplifier for preventing the level fluctuation of the input signal. do not do. Incidentally, since the correlation output Sa output from the logarithmic compression circuit 2 is logarithmically compressed, it includes even-order harmonics as shown in FIG. 2C. Although there is no problem in operation even if the even harmonics are included in this way, if necessary, the exponential expansion circuit 10 shown by a broken line is inserted in the loop to approximate a sine waveform having the fundamental period of the correlation output Sa. You may do so.

[0020]

As described above, according to the present invention, since the correlation output output from the despreading means is logarithmically compressed by the logarithmic compression means, the input level of the input signal fluctuates greatly over six digits or more. Even so, the fluctuation of the correlation output level can be suppressed. Therefore, the synchronization can be stably maintained. Further, since the logarithmically compressed correlation output is also supplied to the demodulator, the normally required AGC amplifier for the demodulator can also be used as the logarithmic compression means, and the overall circuit scale can be reduced. .

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of an embodiment of a synchronization / demodulation circuit including a synchronization circuit of a spread spectrum communication system according to the present invention.

FIG. 2 is a diagram showing a logarithmically compressed correlation output in a synchronization circuit of the spread spectrum communication system of the present invention, and a diagram showing a waveform of the correlation output.

FIG. 3 is a block diagram of a conventional synchronization / demodulation circuit of the spread spectrum communication system.

FIG. 4 is a diagram illustrating a correlation output in a synchronization / demodulation circuit of a conventional spread spectrum communication system.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 despreading circuit 2 logarithmic compression circuit 3 BPF 4 demodulator 5 phase detection circuit 6 LPF 7 VCO 8 tau dither generator 9 PN code generator 10 exponential expansion circuit

Claims (2)

[Claims] An input signal subjected to spread spectrum and P
Despreading means for despreading by multiplying by a PN code generated by an N code generator, logarithmic compression means for logarithmically compressing the despread signal, and demodulating a signal output from the logarithmic compression means Demodulation means for performing synchronization, synchronization detection means for performing synchronization detection based on a signal output from the logarithmic compression means, a voltage-controlled oscillator whose phase is controlled by the output of the synchronization detection means, and an output from the voltage-controlled oscillator. Based on the clock
A synchronization circuit for a spread spectrum communication system, comprising: the PN code generator for generating an N code. 2. Tau dither generating means for slightly shifting the phase of a clock output from the voltage controlled oscillator slightly periodically is provided.
2. The synchronous circuit according to claim 1, wherein the phase of the PN code generated by the PN code generator is shifted slightly periodically.