Receiver for spread spectrum communication and receiving method for the same

Claims

What is claimed is:

1. A receiver for use in an spread spectrum communication system comprising:

receiving means for receiving spread spectrum signals,

spread signal distributing means for distributing a spread spectrum signal received by said receiving means into a first spread signal, a second spread signal and a third spread signal,

pseudo noise signal generating means for generating a first pseudo noise signal having a code series substantially the same as a code series of a pseudo noise signal included in said received spread spectrum signal and having a reference phase, a second pseudo noise signal having a phase that leads the reference phase by an amount corresponding to a specified number of bits and a third pseudo noise signal having a phase that lags the reference phase by an amount corresponding to a specified number of bits,

activating means for setting a bit rate of said pseudo noise signal generating means,

first correlation detecting means for reversely spreading said first spread signal and for detecting the correlation with said first pseudo noise signal,

second correlation detecting means for reversely spreading said second spread signal and for detecting the correlation with said second pseudo noise signal,

third correlation detecting means for reversely spreading said third spread signal and for detecting the correlation with said third pseudo noise signal,

hybrid means for generating a signal depending on a difference between an output of said second correlation detecting means and an output of said third correlation detecting means,

initial seizing control means for setting a first bit rate to said activating means and synchronizing the pseudo noise signal included in said received spread spectrum signal and said first pseudo noise signal by monitoring an output of said first correlation detecting means,

phase lock sustaining control means for setting a second bit rate to said activating means and sustaining synchronization between the pseudo noise signal included in said received spread spectrum signal and said first noise signal by monitoring an output of said first correlation detecting means and an output of said hybrid means,

synchronization control means for first operating the initial seizing control means and upon completion of the operation of said initial seizing control means, starting a measurement of time by suspending execution of a synchronization sustaining control operation which is performed by a synchronization sustaining control means when an output of said first correlation detecting means becomes lower than a specified level and executing the synchronization sustaining control operation if an output of said first correlation detecting means becomes higher than said specified level within a specified time period, and

demodulating means connected to said first correlation detecting means for extracting data included in said received spread spectrum signal.

2. A receiver for use in a spread spectrum communication system according to claim 1, wherein said synchronization control means causes said initial seizing control operation to start when an output of said first correlation detecting means becomes lower than said specified level and remains lower than the specified level for a period of time exceeding a specified time period during operation of said synchronization sustaining control means.

3. A receiver for use in a spread spectrum communication system according to claim 2, wherein said synchronization control means suspends operation of said synchronization sustaining control means and starts a measurement of time when an output of said first correlation detecting means becomes lower than said specified level after said synchronization sustaining control means controls an output of said hybrid means to be within a specified range in the vicinity of the reference level and causes the synchronization sustaining control means to start operation again when a time measuring means of said first correlation detecting means becomes higher than said specified level within the specified time period.

4. A receiver for use in a spread spectrum communication system according to claim 3, wherein said synchronization control means stops operation of said synchronization sustaining control means and causes said initial seizing control means to start operating when an output of said first correlation detecting means becomes lower than said specified level before said synchronization sustaining control means controls an output of said hybrid means within said specified range in the vicinity of the reference level.

5. A receiver for use in a spread spectrum communication system according to claim 1, wherein said receiving means comprises:

oscillator means for oscillating electrical signals of a preset frequency,

frequency mixing means for mixing said received spread spectrum signal and an electrical signal oscillated by said oscillator means to produce a spread spectrum signal of low frequency, and

frequency compensation control means for controlling an output of said first correlation detecting means by adjusting the frequency of said electrical signal from time to time.

6. A receiver for use in a spread spectrum communication system according to claim 5, wherein said frequency mixing means further comprises:

level adjustment control means for controlling an output of said first correlation detecting means to a preset level,

frequency compensation control means for controlling an output of said first correlation detecting means by adjusting the frequency of said electrical signal from time to time, and

wherein said synchronization control means causes said synchronization sustaining control means, said level adjustment control means and said frequency compensation control means to start operating after said initial seizing control means has initially seized control.

7. A receiver for use in a spread spectrum communication system according to claim 6, wherein said synchronization control means adjusts a gain of said level adjustment control means to a constant value during operation of said frequency compensation control means.

8. A receiver for use in a spread spectrum communication system according to claim 1, wherein said demodulating means comprises:

Costas Loop demodulating means,

first prelock detecting means for detecting a first prelock signal when an output of said first correlation detecting means exceeds a preset level,

second prelock detecting means for detecting a second prelock signal when an amplitude of an output of a second multiplying means of said Costas Loop demodulating means is within a preset range, and

lock detecting means which detects a lock condition of said Costas Loop demodulating means when said first prelock signal and said second prelock signal are present.

9. A receiver for use in a spread spectrum communication system according to claim 8, wherein said Costas Loop demodulating means comprises,

oscillator means for oscillating a first demodulating signal,

phase shift means for generating a second demodulating signal of which the phase is shifted by 90.degree. from said first demodulating signal,

first multiplying means for multiplying a first reverse spread signal generated from a first reverse spread of said first correlation detecting means and said first demodulating signal,

second multiplying means for multiplying said first reverse spread signal and the second demodulating signal,

third multiplying means for multiplying an output of said first multiplying means and an output of said second multiplying means, and

frequency control means for controlling the oscillation frequency of said oscillator means in accordance with a mean value of an output of said third multiplying means.

10. A receiver for use in a spread spectrum communication system according to claim 9, wherein said first prelock detecting means comprises a binary encoding means, said second prelock detecting means comprises full-wave rectifying means and a binary encoding means, and said lock detecting means comprises a logic gate.

11. A receiver for use in a spread spectrum communication system comprising:

spread signal distributing means for distributing a received spread spectrum signal into at least three signals comprising a first spread signal, a second spread signal and a third spread signal,

pseudo noise signal generating means which generates pseudo noise signals having a code series which is substantially equal to a pseudo noise signal included in said received spread spectrum signal,

phase shift means for receiving a pseudo noise signal generated by said pseudo noise signal generating means for outputting at least a first pseudo noise signal having a reference phase, a second pseudo noise signal having a phase which leads the reference phase of said first pseudo noise signal by an amount corresponding to a specified number of code bits and a third pseudo noise signal having a phase which lags the reference phase of said first pseudo noise signal by an amount corresponding to a specified number of code bits,

first correlation detecting means comprising first reverse spread means for generating a first reverse spread signal by reversely spreading said first reverse spread signal with said first pseudo noise signal, first signal extracting means for extracting a signal element of said first reverse spread signal, and first detecting means for detecting by an envelope detection method an extracted signal element of said first signal extracting means,

second correlation detecting means comprising second reverse spread means for generating a second reverse spread signal by reversely spreading said second spread signal with said second pseudo noise signal, second signal extracting means for extracting a signal element of said second reverse spread signal, and second detecting means for detecting by an envelope detecting method an extracted signal element of said second signal extracting means,

third correlation detecting means comprising third reverse spread means for generating a third reverse spread signal by reversely spreading said third spread signal with said third pseudo noise signal, third signal extracting means for extracting a signal element of said third reverse spread signal and third detecting means for detecting by an envelope detection method an extracted signal element of said third signal extracting means,

hybrid means for generating a signal corresponding to a difference between an output of said second correlation detecting means and an output of said third correlation detecting means,

synchronization control means for setting a bit rate which deviates by a specified value from the bit rate of the pseudo noise signal included in said received spread spectrum signal and for executing an initial seizing control operation to adjust the bit rate to be substantially equal to the bit rate of the pseudo noise signal included in said received spread spectrum signal when an output of said first correlation detecting means becomes higher than the specified level, and for continuously executing a synchronization sustaining control operation to hold an output of said hybrid means to a reference level after the bit rate is adjusted in said initial seizing control operation by monitoring an output of said first correlation detecting means and an output of said hybrid means and adjusting the bit rates from time to time, and for suspending execution of said synchronization sustaining control operation to start a measurement of time when an output of said first correlation detecting means becomes lower than said specified level during execution of said synchronization sustaining control operation and for re-executing the synchronization sustaining control operation when an output of said first correlation detecting means becomes higher than said specified level within a specified time period, and

activating means for activating said pseudo noise generating means on the basis of the bit rate set by said synchronization control means.

12. A receiver for use in a spread spectrum communication system according to claim 11, wherein said synchronization control means executes said initial seizing control operation when an output of said first correlation detecting means becomes lower than said specified level during execution of said synchronization sustaining control operation and such condition continues for a period of time exceeding the specified time period.

13. A receiver for use in a spread spectrum communication system according to claim 12, wherein said synchronization control means suspends execution of said synchronization sustaining control operation and starts a measurement of time when an output of said first correlation detecting means becomes lower than said specified level after an output level of said hybrid means is controlled to be within a specified range in the vicinity of said reference level of said synchronization sustaining control means for executing the synchronization sustaining control operation when an output of a measuring means of the first correlation detecting means becomes higher than said specified level within the specified time period.

14. A receiver for use in spread spectrum communication system according to claim 13, wherein said synchronization control means suspends execution of the synchronization sustaining control operation and executes said initial seizing control operation when an output of first correlation detecting means becomes lower than said specified level before an output of said hybrid means is controlled to be within said specified level in the vicinity of said reference level in said synchronization sustaining control means.

15. A receiver for use in spread spectrum communication system comprising:

oscillator means for oscillating an electrical signal of a preset frequency,

frequency mixing means for mixing the received spread spectrum signal and an electrical signal oscillated by said oscillator means to generate a spread spectrum signal of low frequency,

spread signal distributing means for distributing the spread spectrum signal output from said frequency mixing means into at least a first spread signal, a second spread signal and a third spread signal,

pseudo noise signal generating means for generating a pseudo noise signal having a code train which is substantially equal to a pseudo noise signal included in said received spread spectrum signal,

phase shift means for receiving the pseudo noise signal generated from said pseudo noise signal generating means and for outputting at least a first pseudo noise signal having a reference phase, a second pseudo noise signal having a phase which leads the reference phase of said first pseudo noise signal by an amount corresponding to a specified number of code bits and a third pseudo noise signal having a phase which lags the reference phase of said first pseudo noise signal by an amount corresponding to a specified number of code bits,

first correlation detecting means comprising first reverse spread means for generating a first reverse spread signal by reversely spreading said first spread signal with said first pseudo noise signal, first signal extracting means for extracting a signal element of said first reverse spread signal, and first detecting means for detecting an extracted signal element of said first signal extracting means by an envelope detecting method,

second correlation detecting means comprising a second reverse spread means for generating a second reverse spread signal by reversely spreading said second spread signal with said second pseudo noise signal, a second signal extracting means for extracting a signal element of said second reverse spread signal and second detecting means for detecting an extracted signal element of said second signal extracting means by an envelope detecting method,

third correlation detecting means comprising a third reverse spread means for generating a third reverse spread signal by reversely spreading said third spread signal with said third pseudo noise signal, third extracting means for extracting a signal element of said third reverse spread signal and third detecting means for detecting an extracted signal element of said third extracting means by an envelope detecting method,

hybrid means for generating a signal depending on a difference between an output of said second correlation detecting means and an output of said third correlation detecting means,

synchronization control means for setting a bit rate which deviates by a specified value from the bit rate of the pseudo noise signal included in said received spread spectrum signal, and for executing an initial seizing control operation to adjust the bit rate to be substantially equal to the bit rate of the pseudo noise signal included in said received spread spectrum signal when an output of said first correlation detecting means becomes higher than a specified level, and for continuously executing, after the bit rate is adjusted in said initial seizing control operation, a synchronization sustaining control operation to hold an output of said hybrid means to a reference level and for causing a frequency compensation control means to control an output of said correlation detecting means by adjusting the frequency of said electrical signal from time to time by monitoring an output of said first correlation detecting means and an output of said hybrid means and adjusting the bit rates from time to time,

activating means for activating said pseudo noise generating means on the basis of the bit rate set by said synchronization control means, and

demodulating means connected to said synchronization control means for extracting data included in said spread spectrum signal.

16. A receiver for use in a spread spectrum communication system according to claim 15, wherein said frequency mixing means further comprises level adjusting means,

said synchronization control means further controls said level adjusting means and continuously executes, after the bit rate is adjusted in said initial seizing control operation, the synchronization sustaining control operation to control an output of said hybrid means to be substantially at the reference level by adjusting the bit rates from time to time, and further controlling the level adjusting means to control an output of said first correlation detecting means to a preset level by adjusting the gain of said level adjusting means and controlling the frequency compensation control mean to control an output of the first correlation detecting means to be a maximum by adjusting the frequency of said electrical signal from time to time, by monitoring an output of said first correlation detecting means and an output of said hybrid means.

17. A receiver for use in a spread spectrum communication system according to claim 16, wherein said synchronization control means adjusts the gain of the level adjusting means to a constant value during said frequency compensation control operation.

18. A receiver for use in a spread spectrum communication comprising:

spread signal distributing mean for distributing a received spread spectrum signal into at least a first spread signal, a second spread signal and a third spread signal,

pseudo noise signal generating means for generating a pseudo noise signal having a code series which is substantially equal to a pseudo noise signal included in said received spread spectrum signal,

phase shift means for receiving the pseudo noise signal generated from said pseudo noise signal generating means and for outputting at least a first pseudo noise signal having a reference phase, a second pseudo noise signal having a phase which leads the reference of the first pseudo noise signal by an amount corresponding to a specified number of code bits and a third pseudo noise signal having a phase which lags the reference phase of the first pseudo noise signal by an amount corresponding to a specified number of code bits,

first correlation detecting means comprising a first reverse spread means for generating a first reverse spread signal by reversely spreading said first spread signal with said first pseudo noise signal, first signal extracting means for extracting a signal element of said first reverse spread signal and first detecting means for detecting an extracted signal element of said first signal extracting means by an envelope detecting method,

second correlation detecting means comprising a second reverse spread means for generating a second reverse spread signal by reversely spreading said second spread signal with said second pseudo noise signal, second signal extracting means for extracting a signal element of said second reverse spread signal, and second detecting means for detecting an extracted signal element of said second signal extracting means by an envelope detecting method,

third correlation detecting means comprising a third reverse spread means for generating a third reverse spread signal by reversely spreading said third spread signal with said third pseudo noise signal, third signal extracting means for extracting a signal element of said third reverse spread signal and a third detecting means for detecting an extracted signal element of said third signal extracting means by an envelope detecting method,

hybrid means for generating a signal corresponding to a difference between an output of said second correlation detecting means and an output of said third correlation detecting means,

synchronization control means for setting a bit rate shifted by a specified value from the bit rate of a pseudo noise signal included in said received spread spectrum signal, and for executing an initial seizing control operation to adjust the bit rate to be substantially equal to the bit rate of the pseudo noise signal included in said received spread spectrum signal when an output of said first correlation detecting means becomes higher than a specified level and for continuously executing a synchronization sustaining control operation after the bit rate is adjusted in said initial seizing control operation to hold an output of said hybrid means to a reference level by monitoring an output of said first correlation detecting means and an output of said hybrid means and adjusting the bit rates from time to time,

activating means for activating said pseudo noise generating means based on the bit rate set by said synchronization control means,

demodulating means connected to said synchronization control means for extracting data included in said spread spectrum signal,

Costas Loop demodulating means,

first prelock detecting means for detecting a first prelock signal when an output of said first correlation detecting means exceeds the preset level,

second prelock detecting means for detecting a second prelock signal when the amplitude of an output of a multiplying means of said Costas Loop demodulating means is within the preset range, and

lock detecting means for detecting a lock condition of said Costas Loop demodulating means when said first prelock signal and second prelock signal exist.

19. A receiver for use in a spread spectrum communication system according to claim 18, wherein said Costas Loop demodulating means comprises:

oscillator means for oscillating a first demodulating signal,

second phase shift means for generating a second demodulating signal which deviates in phase by substantially 90.degree. from said demodulating signal,

first multiplying means for multiplying a first reverse spread signal generated from the first reverse spread of said first correlation detecting means and said first demodulating signal,

second multiplying means for multiplying said first reverse spread signal and said second demodulating signal,

third multiplying means for multiplying an output of said first multiplying means and an output of said second multiplying means, and

frequency control means for controlling the oscillation frequency of the oscillation means in accordance with a mean value of an output of said third multiplying means.

20. A receiver for use in a spread spectrum communication system according to claim 19, wherein said first prelock detecting means comprises binary encoding means, said second prelock detecting means comprises full-wave rectifying means and binary encoding means, and said lock detecting means comprises a logic gate.

FIELD OF THE INVENTION

The present invention relates to a receiver for use in a spread spectrum communication system.

BACKGROUND OF THE INVENTION

The spread spectrum communication (hereinafter referred to as SS communication) system is a digital transmission system and is characterized particularly in that information to be transmitted is spread into a wider frequency band than that required for transmission.

For a SS communication, data is spread in the sending side with a pseudo noise signal (hereinafter referred to as PN code) of the code series specified previously between the sending and receiving sides and such data is then reversely spread using the PN code series of the same code series in the receiving side. Therefore, if codes in the sending and receiving sides are not synchronized accurately, data cannot be transmitted and received.

Accordingly, a receiver utilizing a DLL (Delay Lock Loop) circuit has been proposed. The receiver utilizing the DLL circuit comprises a receiving circuit to receive the spread spectrum signal transmitted from a transmitter, a first correlation circuit which reversely spreads the received spread spectrum signal with the PN code train of the reference phase, a second correlation circuit which reversely spreads the received spread spectrum signal with the PN code train which is delayed by 1/2 bit from the reference phase, a third correlation circuit which reversely spreads the receiver spread spectrum signal which leads by 1/2 bit from the reference phase of the PN code train, a hybrid circuit which differentially combines outputs of the second correlation circuit and the third correlation circuit and a demodulation circuit which is connected to said correlation circuit and extracts data elements from the received spread spectrum signal. That is, the receiver first reversely spreads, in an initial seizing operation, received spread spectrum signals by generating a PN code train with a bit rate quicker than that in the sending side. When the first correlation circuit detects the peak of correlation between the spread spectrum signal which has been reversely spread and the PN code train of the reference phase, the receiver establishes a temporary synchronization by generating a PN code train with a bit rate equal to that in the sending side and finally establishes the phase lock condition by adjusting the bit rate so that an output of a hybrid circuit becomes 0 through the synchronization sustaining control.

Since a large amount of energy can be extracted from an SS communication only when correlation exists between the PN code trains of the sending and receiving sides, this communication system is widely employed, for example, for satellite communication which is resistive to noise and establishes communication, for example, using a weak signal.

For instance, when a receiver is installed into a mobile vehicle, a receiving antenna is often shielded temporarily by buildings or tunnels. Therefore, the receiver often generates asynchronization easily when it is installed in a moving object for mobile communication.

In addition, if the PN codes are asynchronized for some reason after establishing the phase lock, the conventional receiver reestablishes the phase lock again by re-executing the initial seizing control operation.

The PN code train used in SS communication is required to have sufficient resistivity to noise. Therefore, the PN code is also required to have a certain bit length which is necessary for a circulation of PN codes.

In the initial seizing control operation, correlation is detected, for example, by shifting one bit for each circulation of a PN code train. Therefore, when the bit length of a PN code is long, the receiver requires a comparatively longer period for reestablishing the phase lock. As described above, it is impossible to send or receive data for the SS communication if the codes are not accurately synchronized between the sending and receiving sides. Accordingly, the conventional receiver has a disadvantage in that communication is suspended for a considerable period if asynchronization occurs.

Moreover, when communication with a moving satellite is necessary, the frequency of received signal is deviated from the sending frequency due to a Doppler shift. The amount of such Doppler shift always changes.

As a means for tracking the frequency which always changes, an AFC (Auto Frequency Control) circuit has been proposed. However, in SS communication, it is often difficult to track the frequency by using the AFC circuit, because the carrier is not contained in the signal in many cases in SS communication. Therefore, the conventional receiver executes the frequency tracking in conjunction with the first correlation circuit and demodulation circuit. As a result, the conventional receiver also provides a disadvantage in that the receiving circuit is very complicated.

Moreover, SS communication systems often use the BPSK (Biphase Phase Shift Keying) signal. Accordingly, in many cases, the Costas Loop circuit is used as the demodulating circuit.

With reference to FIG. 2e, the conventional Costas Loop demodulating circuit is briefly described hereunder.

The Costas Loop demodulating circuit is composed of three multipliers. The first multiplier multiplies a VCO output and the reversely spread signal, while the second multiplier multiplies the signal obtained by shifting the phase of VCO output by 90.degree. and the reversely spread signal and the third multiplier multiplies outputs of the first and second multipliers. An output of the third multiplier controls VCO so that the phase difference between the VCO output and the reversely spread signal can be suppressed to zero. Namely, VCO of Costas Loop demodulating circuit tracks the virtual carrier and therefore the demodulated data can be obtained at the first multiplier output. However, since this output does not contain the amplitude information, it is impossible to judge whether VCO accurately tracks the virtual carrier (i.e. the Costas Loop demodulating circuit locks) or not.

In order to judge whether the Costas Loop demodulating circuit locks or not, the conventional receiver multiplies the signals obtained by shifting VCO output by +45.degree. and the reversely spread signal using the 4th and 5th multipliers and also multiplies the outputs of 4th and 5th multipliers using the 6th multiplier.

Accordingly, the conventional receiver further requires three multipliers and phase shifters for shifting the phases .+-.45.degree. in order to judge the lock of the Costas Loop demodulating circuit, resulting in a problem that the circuit structure is very complicated. In addition, a problem arises in that an increase of frequency used for SS communication complicates the design of the phase shifter.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to solve the problems of the prior art described previously.

It is a second object of the present invention to simplify the circuit structure of a receiver for use in a SS communication system.

It is a third object of the present invention to shorten the time until communication is recovered after the communication is suspended.

It is a fourth object of the present invention to realize tracking for the receiving frequency.

It is a fifth object of the present invention to detect a locked condition of a Costas Loop demodulating circuit with a simplified circuit.

Further objects and advantages of the present invention will be apparent from the following description with reference to the accompanying drawings to which, however, the scope of the invention is in no way limited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a detailed circuit diagram of a reverse spread circuit for use in the receiver of the present invention,

FIG. 1b is a detailed circuit diagram of a reverse spread circuit for use in the receiver of the present invention,

FIG. 1c is a detailed circuit diagram of a Costas Loop demodulating circuit for use in the receiver of the present invention,

FIG. 1d is a detailed circuit diagram of a PN code generator for use in the receiver of the present invention,

FIG. 2a is a block diagram of a receiving circuit for use in the receiver of the present invention,

FIG. 2b is a block diagram of a reverse spread circuit for use in the receiver of the present invention,

FIG. 2c is a block diagram of a Costas Loop demodulating circuit for use in the receiver of the present invention,

FIG. 2d is a block diagram of a PN code generator for use in the receiver of the present invention,

FIG. 2e is a block diagram of a conventional Costas Loop demodulating circuit,

FIG. 2f is a block diagram of a conventional PN code generator,

FIG. 2g is a block diagram of a communication system to which the receiver of the present invention may be applied,

FIG. 3a indicates waveforms of correlation conducted by a microcomputer 10 shown in FIG. 1b and FIG. 2b,

FIG. 3b is a schematic waveform relating to the synchronization sustaining processing,

FIG. 4 is a schematic waveform relating to the Doppler compensation processing,

FIG. 5a is a graph of a radiation pattern of the receiving antenna,

FIG. 5b is a schematic plan view of the antenna gain compensation data table stored in the microcomputer 10 shown in FIG. 1b and FIG. 2b,

FIG. 6a is a detailed circuit diagram of the AGC amplifier circuit shown in FIG. 1a,

FIG. 6b is a graph indicating characteristics of the AGC amplifier circuit of FIG. 6a,

FIG. 7 indicates output waveforms at respective points of the Costas Loop demodulating circuit shown in FIG. 1c, and

FIG. 8, FIG. 9, FIG. 10, FIG. 11, FIG. 12 and FIG. 13 are flowcharts which indicate some of the control executed by microcomputer 10 shown in FIG. 1b and FIG. 2b.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is outlined hereunder with reference to FIG. 2g. FIG. 2g is a block diagram illustrating the sending/receiving system in the SS (Spread Spectrum) system.

A transmitter Trn spreads the sent data with a PN (Pseudo Noise) code having a bit rate higher than that of said sent data, executed BPSK (Biphase Phase Shift Keying) to the carrier with said spread signal and then transmits the signal through a tuning circuit. The sent data used in this embodiment includes information which indicates transmission. A receiver Rec, meanwhile, mixes a radio frequency signal obtained through the tuning circuit with a local oscillation signal, then reversely spreads the signal by adding the PN code which is the same as that in the sending side, demodulates said reversely spread signal at the BPSK demodulating circuit and finally obtains the receiving data.

This embodiment relates to the receiver Rec, which receives the signal which is obtained by executing the BPSK to the carrier of 1575 MHz with the signal obtained by spreading the send data of 50 b/s (bit per second) with the PN code of 1.023 Mb/s. FIG. 1a, FIG. 1b, FIG. 1c and FIG. 1d are circuit diagrams indicating the detailed structure of a part of the device of this embodiment, while FIG. 2a, FIG. 2b, FIG. 2c and FIG. 2d are block diagrams indicating an outline of the structure of the device of this embodiment. The following description is made with reference to these drawings.

a. Generation of intermediate frequency signal

With reference to FIG. 2a, the signal received through the antenna Ant (in this embodiment, preferably circular polarized wave microstrip antenna) is mixed with a first local oscillation signal through a radio frequency amplifier 101 and a band-pass-filter 102 and is converted into a first intermediate frequency signal. TXCO 103 is a temperature-compensated crystal-controlled oscillator which oscillates the signal of 10 MHz. Namely, the first local oscillation signal 1520 MHz is obtained by multiplying 152 times such oscillated signal with a .times.152 multiplier 105 and the first intermediate frequency signal considering the Doppler shift etc. becomes the signals having the frequencies of 55.42 MHz.+-.5 kHz. The first intermediate frequency signal is filtered and amplified by the band-pass-filter 107, first intermediate frequency amplifier 108 and band-pass-filter 109, then mixed with a second local oscillation signal and converted to a second intermediate frequency signal.

SYN is a synthesizer which oscillates the second local oscillation signal and functions as the second local oscillator. Here, the output in frequency from 10 MHz of TCXO 103 is applied through buffer 104 and is multiplied by 4 times (40 MHz) with a .times.4 multiplier 113 and divided to 1/2 times (5 MHz) with a (1/2)multiplier 114. The first signal of 45 MHz can be generated by combining these signals. On the other hand, an output (5 MHz) of the (1/2)multiplier 114 is further divided to 1/50000 times by the (1/2)multiplier and (1/25000) multiplier and becomes the second signal.

Here, considered is the case that VCO 122 outputs the second local osci