JPS6336584B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a demodulator in a direct sequence method among spread spectrum communication methods.

A spread spectrum demodulator is provided with a correlation detection/determination device to determine whether one signal (desired signal) necessary for the own station is being received. Since this correlation detection/determination device determines reception of a desired signal based on a predetermined threshold voltage, it is desirable that this threshold voltage be uniquely set to an optimal value.

Furthermore, since the spread spectrum communication system uses multiple access using code division, there are many signals within the same frequency band, and therefore signals other than the desired signal become noise. In the case of the direct diffusion method, this noise voltage level varies depending on the number of calls, so the input noise level to the correlation detection/judgment device also varies, and the threshold voltage must be set to the uniquely optimal value. Setting it up is not always easy.

SUMMARY OF THE INVENTION An object of the present invention is to provide a spread spectrum demodulator that can uniquely set the threshold voltage of a correlation detection/judgment device to an optimum value for a receiving input level that varies.

According to the present invention, there is provided a first means for receiving a spread spectrum signal in which transmission line noise and interference wave signals are mixed, and controlling the level of the spread spectrum signal to be constant; and an output of the first means. a second means for demodulating the signal; a third means for determining whether the output of the second means includes a desired signal based on a predetermined threshold voltage; and demodulating the second means. and fourth means for controlling the output level to be constant.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an example of a spread spectrum demodulation device using a direct sequence method according to the present invention. The input spread spectrum signal whose frequency has been converted into an intermediate frequency (IF) signal is input to the power divider 1 from the input terminal 101, where it is divided into three parts, and each divided output is supplied to the despreading demodulators 2, 3, and 4. be done. The despreading demodulators 2, 3 and 4 each include a bandpass waver with a corresponding bandwidth in order to obtain processing gain. Also,
These demodulators 2, 3 and 4 are each supplied with a PN pattern having a shift at an arbitrary bit interval from a pseudo noise (PN) generator 8 in order to despread the input signal.

The output of the despread demodulator 2 is input to a correlation detection/judgment unit 5, and the outputs of the despread demodulators 3 and 4 are input to a delay lock loop correlator 6, where the correlation between the input signal and the PN pattern is determined. The output voltage of the delay lock loop correlator 6 is supplied to a voltage controlled oscillator (VCO) 7 as its control voltage,
The oscillating output of VCO 7 is coupled to the input of PN generator 8 as a signal source. Despread demodulators 3 and 4, correlator 6, VCO 7 and PN generator 8 form a delay lock loop. The synchronization controller 9 responds to the output of the correlation detection/judgment unit 5 to generate a PN generator 8.
The output bits are swept by N (natural number) bits at a time, and when a correlation is established, the sweep is stopped. The synchronous controller 9 is composed of, for example, a majority voting circuit. Note that the output of the despreading demodulator 2 is output from the output terminal 10.
It is also guided by 2.

The correlation detection/judgment device 5 includes a square law detector 501, a low frequency filter 502, and a decision device 503 that determines the level of the input signal using a certain threshold voltage.
The relationship between the carrier-to-noise level ratio (C/N) of the input signal and the input voltage to the correlation detection/judgment device 5 is expressed as a second
As shown in the figure. The input noise voltage V _N varies as shown in the figure depending on the number of connected channels, and therefore the sum V _DN of this noise voltage and the voltage of the desired signal also varies in the same way. Now, assuming that the threshold voltage V _TH of the determiner 503 is set to be optimal at point M as shown in Fig. 2, the input C/N will be , this threshold voltage V _TH will deviate significantly from the optimum value. Normally, the noise voltage V _N of the received signal in the spread spectrum method, the sum of the desired signal voltage and the noise voltage V _DN
Since the relationship _between .

FIG. 3 is a block diagram of an embodiment of the present invention designed to overcome these difficulties. Reference numeral 12 is the spread spectrum despread demodulator shown in FIG. Before the demodulator 12, an automatic gain control (AGC) amplifier circuit 11 (AGC amplifier 11
1 and an AGC voltage controller 112). This AGC amplifier circuit 11 suppresses the output noise level of the demodulators 2, 3, and 4 (FIG. 1) to below a certain value, and conversely, the desired signal level becomes above a certain value. This situation is shown in FIG. The horizontal axis shows the input C/N, and the vertical axis shows the relative input/output level of the AGC amplifier circuit 11. In the figure, Si is the input signal level, Ni is the input noise level, So is the output signal level,
No indicates the output noise level, and So+No indicates the sum of the output signal level and the output noise level, respectively. Normally, the input C/N is
It is normal for it to be below 0dB. As mentioned above, as is clear from the figure, the sum of the signal level and the noise level, which is the output of the AGC amplifier circuit 11, is So+No.
is kept constant, and as the input noise level Ni decreases, the output signal level So increases.

FIG. 5 shows the input voltage characteristics of the determiner 503 for the signal after the processing gain has been obtained by the bandpass waveform in the despreading demodulator 2 when the AGC amplifier circuit 11 is provided as shown in FIG. 3. shows. Judgment device 503
If the threshold voltage V _TH is set to be optimal at the worst value of input C/N (point MB in Figure 5),
The error probability of the determiner for input C/Ns other than the MB point does not deteriorate compared to that for the MB point, and improves as the input C/N improves.
The desired signal of the despreading demodulator 12 has the characteristics shown in FIG.
V _DN fluctuates, so this demodulator 12
AGC amplifier circuit 13 (AGC amplifier 131
and an AGC voltage controller 132) or an amplitude control circuit is inserted, thereby keeping the output desired signal level constant.

As explained above, according to the present invention, the AGC that keeps the input level constant is installed before the despreading demodulator.
By providing the amplifier circuit, the threshold voltage of the correlation detection/judgment device can be uniquely set to an optimal value.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a spread spectrum demodulator according to the present invention, FIG. 2 is a diagram showing the relationship between the input C/N of the device in FIG. 1 and the determiner input voltage, and FIG. 3 is an implementation of the present invention. Example block diagram, Fig. 4 shows AGC for input C/N to the device shown in Fig. 3.
FIG. 5 is a diagram showing the relationship between relative input and output levels of the amplifier circuit 11, and FIG. 5 is a diagram showing the relationship of the determiner input voltage with respect to the input C/N to the device shown in FIG. In the figure, 101 and 103...input terminals, 102 and 104...output terminals, 1...power divider, 2-4...
Despreading demodulator, 5... Correlation detection determiner, 6... Delay lock loop correlator, 7... Voltage controlled oscillator, 8...
PN generator, 9...Synchronization controller, 11 and 13...
AGC amplifier circuit, 12... despreading demodulator.

Claims (1)

[Claims] 1. Receives a spread spectrum signal containing a mixture of transmission line noise and interference wave signals, and calculates this spread spectrum signal.
a first means for controlling the level of the spectrum signal to be constant; a second means for demodulating the output of the first means; and a method for determining whether or not the output of the second means includes a desired signal. A spread spectrum demodulation device comprising: third means for determining based on a predetermined threshold voltage; and fourth means for controlling the output level of the second means to be constant.