Spread spectrum communication system and an apparatus for communication utilizing this system

Claims

What is claimed is:

1. In a spread spectrum communication system, a method comprising the steps of:

transmitting, in combination, a first signal modulated by a PN code and data and a second signal modulated only by said PN code and thereafter delayed with respect to said first signal by a time period corresponding to at least one chip of said PN code;

receiving and splitting into third and fourth signals said transmitted signals;

delaying by said time period one of the third and fourth signals; and

multiplying the delayed one of the third and fourth signals by the other to demodulate the data by despreading.

2. An apparatus for communication employing a spread spectrum communication system, comprising:

a transmitter for transmitting, in combination, a first signal modulated by a PN code and data and a second signal modulated only by said PN code and thereafter delayed with respect to said first signal by a time period corresponding to at least one chip of said PN code;

a receiver for splitting a received signal into third and fourth signals, delaying one of the third and fourth signals by said time period, and multiplying the delayed one of the third and fourth signals with the other to despread and demodulate the data.

3. The apparatus for communication according to claim 2, wherein

said transmitter includes:

PN code generating means for generating said PN code;

multiplier means for multiplying input data by the PN code generated by said PN code generating means;

first modulating means for modulating a carrier wave signal of a prescribed frequency by an output signal from said multiplier means;

second modulating means for modulating said carrier wave signal of the prescribed frequency by the PN code generated by said PN code generating means;

first delay means for delaying the modulated signal modulated by said second modulating means; and

combining means for combining said signal modulated by said first modulating means and said signal delayed by said first delay means; and

said receiver includes:

frequency converting means for receiving the signal transmitted from said transmitter and converting frequency of the signal to provide an intermediate frequency signal;

second delay means for delaying the intermediate frequency signal output from said frequency converting means; and

data demodulating means for multiplying the intermediate frequency signal as the output from said frequency converting means by the output signal from said delay means to output an original data component.

4. The apparatus for communication according to claim 3, wherein

said first and second modulating means includes BPSK modulating means.

5. The apparatus for communication according to claim 3, wherein

said first modulating means includes first BPSK modulating means, and

said second modulating means includes second BPSK modulating means.

6. The apparatus for communication according to claim 3, wherein said first and second modulating means include amplitude modulating means.

7. The apparatus for communication according to claim 3, wherein

said PN code generating means includes means for generating repeatedly the PN code in a period at least twice that of the band of data to be transmitted.

8. The apparatus for communication according to claim 3, wherein

said transmitter includes adding means for adding said input data to a reference signal to apply the result to said multiplier means, and

said receiver includes a bandpass filter for separating and extracting data and the PN code from an output of said data demodulating means.

9. The apparatus for communication according to claim 3, wherein

the intermediate frequency of said receiver, the delay time period of the delay means included in said transmitter and the delay time period of the delay means included in said receiver are selected to have values minimizing an unnecessary component of the output from data demodulating means represented as two variables of the intermediate frequency and the delay time period.

10. The apparatus for communication according to claim 3, wherein

said receiver includes comb filter means connected parallel to said second delay means between said frequency converting means and said data demodulating means.

11. The apparatus for communication according to claim 5, wherein

said transmitter includes carrier wave generating means for generating frequency hopped carrier wave signals and applying the same to said first and second BPSK modulating means.

12. An apparatus for communication according to claim 3, wherein

said receiver includes:

first filter means for filtering the intermediate frequency signal as the output from said frequency modulating means corresponding to a PN code generated by said PN code generating means; and

second filter means for filtering an output signal from said second delay means corresponding to said PN code.

13. The apparatus for communication according to claim 3, wherein

said data includes plural sets of mutually different data;

said multiplier means includes a plurality of multiplier means for multiplying respective sets of different data by the PN code generating means;

said second modulating means includes a plurality of second modulating means for modulating said carrier wave signal by respective ones of said plurality of multiplier means;

said first delay means includes a plurality of first delay means through which modulated signals output from said plurality of second modulating means are passed respectively; and

said combining means includes means for combining the signal modulated by said first modulating means and the signal delayed by said plurality of first delay means.

14. The apparatus for communication according to claim 3, wherein

said receiver includes:

first nearly baseband signal converting means for converting frequency of said intermediate frequency signal by a signal having the same frequency as said carrier wave signal of said transmitter to provide a nearly baseband signal;

second nearly baseband signal converting means for converting frequency of the output signal from said second delay means by a signal having the same frequency as said carrier wave signal to provide a nearly baseband signal;

first correlating means for outputting a signal correlated with said PN code to said data demodulating means based on said first nearly baseband signal; and

second correlating means for outputting a signal correlated to said PN code to said data demodulating means, based on said second nearly baseband signal.

15. The apparatus for communication according to claim 14, further comprising:

loop control means responsive to the correlated output from said first correlating means for generating a control voltage; and

local oscillation means responsive to the control voltage as the output of said loop control means for generating a carrier wave signal to be applied to said first and second nearly baseband signal converting means.

16. The apparatus for communication according to claim 15, wherein

said first nearly baseband signal converting means includes third and fourth nearly baseband signal converting means provided parallel to each other for converting said intermediate frequency signal to a nearly baseband signal by the carrier wave signal generated from said local oscillation means; and

said first correlating means includes third and fourth correlating means for applying output signals related to said PN code to said loop control means based on the outputs from said third and fourth nearly baseband signal converting means respectively, with one of the output signals being applied to said data demodulating means.

17. The apparatus for communication according to claim 3, wherein

said receiver includes:

baseband signal converting means for converting the frequency of said intermediate frequency signal by a signal having the same frequency as the carrier wave signal on said transmitter side to provide a nearly baseband signal to be applied to said delay means;

first correlating means for outputting a signal correlated to said PN code to the data demodulating means, based on said nearly baseband signal; and

second correlating means for outputting a signal correlated to said PN code to said data demodulating means, based on the output from said delay means.

18. The apparatus for communication according to claim 3, wherein

said receiver includes:

first splitting means for splitting said intermediate frequency signal into a plurality of signals;

second splitting means for splitting the output signal from said second delay means into a plurality of signals;

carrier wave signal generating means for generating a carrier wave signal including first and second components;

a plurality of first nearly baseband signal converting means for multiplying a plurality of intermediate frequency signals split by said first splitting means by the first and second components included in said carrier wave signal, and for converting the multiplied outputs to the first nearly baseband signal;

a plurality of second nearly baseband signal converting means for multiplying the plurality of delay signals split by said second splitting means by the first and second components included in said carrier wave signal, and for converting the multiplied output to the second nearly baseband signal;

correlating means for outputting a plurality of correlated signals correlated with said PN code based on said plurality of first and second nearly baseband signals;

first multiplier means for multiplying a correlated output signal which is obtained by multiplying the delayed signal and said first component by a correlated output signal which is obtained by multiplying not-delayed signal by said first component, among said output signals from said correlating means;

second multiplying means for multiplying a correlated output signal which is obtained by multiplying the delayed signal by said second component, and a correlated output signal which is obtained by multiplying the not-delayed signal and said second component, among the output signals from said correlating means; and

adding means for adding outputs from said first and second multiplier means and for applying the result to said data demodulating means.

19. The apparatus for communication according to claim 18, wherein

said data includes a plurality of serial data;

said transmitter includes S/P converting means for converting said serial data to a plurality of parallel data;

multiplier means of said transmitter includes a plurality of multiplier means for multiplying respective ones of the plurality of parallel data by said PN code;

the first modulation means of said transmitter includes a plurality of first modulating means for modulating respective ones of output signals from said plurality of multiplier means;

the first delay means of said transmitter includes a plurality of first delay means for delaying output signals from said plurality of first modulating means, respectively;

said combining means includes means for combining output signals from said plurality of first delay means with the output signals from said first modulating means;

the first splitting means of said receiver includes splitting means for splitting the intermediate frequency signal into the same number as said serial data; and

said receiver includes P/S converting means for converting parallel data output from said data demodulating means to serial data.

20. The apparatus for communication according to claim 2, wherein

said receiver includes:

first splitting means for splitting said intermediate frequency signal into a plurality of signals;

carrier wave signal generating means for generating a carrier wave signal including first and second components;

first nearly baseband signal converting means for multiplying one of the intermediate frequency signal split by said first splitting means by the first component included in said carrier wave signal and converting the multiplied output to a first nearly baseband signal;

second nearly baseband signal converting means for multiplying the other one of the intermediate frequency signals split by said first splitting means by the second component included in said carrier wave signal and converting the multiplied output to a second nearly baseband signal;

first delay means for delaying the output signal from said first nearly baseband signal converting means;

second delay means for delaying the output signal from said second nearly baseband signal converting means;

first correlating means for outputting a first correlated signal correlated with said PN code based on the output signal from said first nearly baseband signal converting means;

second correlating means for outputting a second correlated signal correlated with said PN code based on the output signal from said first delay means;

third correlating means for outputting a third correlated signal correlated with said PN code based on the output signal from said second nearly baseband signal converting means;

fourth correlating means for outputting a fourth correlated signal correlated with said PN code based on the output signal from said second delay means;

first multiplier means for multiplying output signals from said first and second correlating means;

second multiplier means for multiplying output signals from said third and fourth correlating means; and

demodulating means for adding output signals from said first and second multiplier means for demodulating the original data.

21. The apparatus for communication according to claim 20, wherein

said data includes a plurality of serial data;

said transmitter includes

S/P converting means for converting said serial data to a plurality of parallel data,

PN code generating means for generating a plurality of PN codes,

a plurality of multiplier means for multiplying the plurality of parallel data as the outputs from said S/P converting means by respective PN codes generated from said PN code generating means,

a plurality of first delay means for delaying respective output signals from said plurality of multiplier means,

combining means for combining respective output signals from said plurality of first delay means, and

transmitting means for modulating a carrier wave signal of a prescribed frequency by the output signal from said combining means; and

said receiver further includes

third splitting means for splitting said first nearly baseband signal corresponding to said plurality of serial data,

fourth splitting means for splitting said second nearly baseband signal corresponding to said plurality of serial data;

said first delay means includes a plurality of first delay means for delaying split signals except one as the output signals from said third splitting means;

said second delay means includes a plurality of second delay means for delaying split signals except one as the output signals from said fourth splitting means;

said second correlating means includes a plurality of second correlating means for outputting a plurality of said correlated signals correlated to said PN code based on the output signal from said plurality of first delay means;

said fourth correlating means includes a plurality of fourth correlating means for outputting a plurality of fourth correlated signals correlated to said PN code based on the output signal from said plurality of second delay means;

said first multiplier means includes a plurality of first multiplier means for multiplying said first correlated signal with the plurality of second correlated signals, respectively;

said second multiplier means includes a plurality of second multiplier means for multiplying said first correlated signal by said plurality of fourth correlated signals;

said demodulating means includes means for demodulating data by adding output signals respectively from said plurality of first multiplier means and from said plurality of second multiplier means; said apparatus further including

a P/S converting means for transmitting the parallel data demodulated by said data demodulating means to a serial data.

22. A spread spectrum communication system, comprising the steps of:

generating a first modulated signal modulated by binary digital data and a first PN code and a second modulated reference signal modulated by a second PN code having the same chip rate and the same code length as but different sequence from said first PN code,

transmitting said first and second signals in combination;

receiving and splitting into third and fourth signals said transmitted signals;

filtering one of the split signals corresponding to said first PN code ;

filtering the other one of said split signals corresponding to said second PN code and

multiplying respective filtered outputs to demodulate said binary digital data.

23. An apparatus for communication transmitting binary digital data employing spread spectrum communication system, comprising:

a transmitter for generating a first modulated signal modulated by said binary digital data and by a first PN code and a second modulated signal modulated by a second PN code having the same chip rate and the same code length but different sequence from said first PN code for transmitting said first and second modulated signals in combination; and

a receiver receiving and splitting into two said transmitted signal, filtering one of the split signals corresponding to said first PN code, filtering the other one of said split signals corresponding to said second PN code and for multiplying respective filtered outputs to demodulate said binary digital data.

24. The apparatus for communication according to claim 23, wherein

said transmitter includes

first PN code generating means for generating said first PN code,

second PN code generating means for generating said second PN code,

multiplier means for multiplying said binary digital data by the first PN code generated by said first PN code generating means,

first modulating means for modulating a carrier wave signal having a prescribed frequency by an output signal from said multiplier means,

second modulating means for modulating said carrier wave signal having the prescribed frequency by the second PN code generated by said second PN code generating means, and

combining means for combining output signals from said first and second modulating means; and

said receiver includes

frequency converting means receiving a signal transmitted from said transmitter for converting its frequency to provide an intermediate frequency signal,

splitting means for splitting the output signal from said frequency converting means,

first filtering means for filtering one of the split signals split by said splitting means corresponding to said first PN code,

second filter means for filtering the other one of the split signals split by said splitting means corresponding to said second PN code, and

demodulating means for multiplying output signals from said first and second filter means for demodulating the original binary digital data.

25. The apparatus for communication according to claim 24, wherein

said transmitter includes first delay means for delaying an output signal from said first modulating means to apply the same to said combining means, and

said receiver includes second delay means for delaying an output signal from said first filter means.

26. The apparatus for communication according to claim 24, wherein

said binary digital data includes a plurality of binary digital data;

said multiplier means includes a plurality of multiplier means for multiplying respective ones of said plurality of binary digital data by said first PN code;

said first modulating means includes a plurality of first modulating means for modulating said carrier wave signal having the prescribed frequency respectively by the output signals from said plurality of multiplier means;

said apparatus further comprising

a plurality of delay means for delaying modulated signals as outputs from said plurality of first modulating means, wherein

said combining means includes means for combining output signals from said plurality of delay means and a modulated signal as an output from said second modulating means.

27. Spread spectrum communication system employing frequency hopping, comprising the steps of:

(a) multiplying a PN code with a multibit data signal to provide a spread data signal;

(b) generating a frequency hopping (FH) carrier signal having a frequency that changes every preset number of data bits;

(c) modulating the FH carrier signal with the PN code and delaying the modulated signal by an arbitrary time delay longer than a time duration between carrier frequency changes;

(d) modulating the spread data signal with the FH carrier signal;

(e) combining and transmitting the modulated signals from steps (c) and (d);

(f) receiving said transmitted signal;

(g) splitting into two the received signal;

(h) delaying one of the split signals by the by the preset delay; and

(i) despreading and demodulating the delayed signal and the other one of the split signals.

28. An apparatus for transmitting data in a spread spectrum communication system employing frequency hopping, comprising:

a transmitter including:

a multiplier for multiplying a PN code with a multibit data signal to provide a spread data signal;

a carrier generator for generating a frequency hopping (FH) carrier signal having a frequency that changes every preset number of data bits;

a first modulator for modulating the FH carrier signal with the PN code and delaying the modulated signal by an arbitrary time delay longer than a time duration between carrier frequency changes;

a second modulator for modulating the spread data signal with the FH carrier signal;

a combiner for combining and transmitting the signals from the first and second modulators;

a receiver including:

means for receiving the transmitted signals and splitting into two the received signal;

a delay for delaying one of the split signals by the preset delay; and

means for despreading the delayed signal and the other one of the split signals to demodulate the multi-bit data data.

29. The apparatus for communication according to claim 28, wherein

said receiver includes

frequency converting means for receiving and converting said transmitted signal to an intermediate frequency signal;

second delay means for delaying said intermediate frequency signal by the same period of time as the delay time by said delay means on said transmitter side,

multiplier means for multiplying said intermediate frequency signal and an output signal from said second delay means, and

demodulating means for filtering an output signal from said multiplier means for despreading and demodulating the original data.

30. The apparatus for communication according to claim 29, wherein

said transmitter includes

local oscillation signal generating means for generating a local oscillation signal, and

means for converting frequency of said frequency hopping signal or of the signal obtained by modulating the data frequency hopping signal by said data by the local oscillation signal from said local oscillation signal generating means and applying the signal having its frequency converted to said combining means.

31. The method according to claim 1, wherein

a communication area is divided into a plurality of areas, a plurality of channels are discriminated from each other by varying said delay time in the same area and by varying said PN code between different areas.

32. The method according to claim 1, wherein

a communication area is divided into a plurality of areas, a plurality of channels are discriminated from each other by varying said PN code in the same area and by varying said delay time between different areas.

33. The method according to claim 1, wherein said time period may be arbitrarily set between one chip of said PN code and one bit width of said data.

34. The spread spectrum communication system according to claim 2, wherein said time period may be arbitrarily set between one chip of said PN code and one bit width of said data.

35. The method according to claim 1, wherein said data is multipled by said PN code to produce a modulating signal such that the first signal is modulated by multiplication with said modulating signal.

36. An apparatus for communication in accordance with claim 2, wherein said data is multipled by said PN code to produce a modulating signal such that the first signal is modulated by multiplication with said modulating signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to spread spectrum communication system and to an apparatus for communication utilizing this system. More specifically, the present invention relates to spread spectrum communication system using direct sequence and/or frequency hopping, and to an apparatus for communication utilizing this system.

2. Description of the Background Art

Communication using narrow band modulation system (such as AM, FM, BPSK) has been conventionally used in the field of data communication. In such a system, demodulation at the receiver can be carried out by a relatively small circuitry. However, such a system is weak against multipath fading and narrow band noise.

By contrast, in spread spectrum communication system, data spectrum is spread by a PN code at the transmitter side, while the PN code and the data are synchronized on the receiver side, so that the influence of multipath fading and narrow band noise can be reduced, which system has attracting increasing attention as a promising technique.

The method of spread spectrum communication includes direct sequence, frequency hopping, time hopping and a hybrid combining two or more of these. Direct sequence spreads the spectrum by multiplying data and the PN code having a chip rate considerably higher than data rate, of which circuitry can be implemented relatively easily as compared with those used in other methods. Use of different PN codes allows multiple access in the same band. Such multiple access is called CDMA (Code Division Multiple Access) or SSMA (Spread Spectrum Multiple Access).

FIGS. 1 and 2 are schematic block diagrams of the spread spectrum communication system using direct sequence. FIG. 1 shows the transmitter side and FIG. 2 shows the receiver side.

On the transmitter side, referring to FIG. 1, information 1 represented as a(t) is modulated at an information modulating block 2 to be turned to a signal b(t), which in turn is multiplied by a PN code represented as c(t) generated in a PN code generator 4, at a spreading block (multiplier) 3. PN code generator 4 is driven by clocks from a reference clock oscillator 5. The chip rate of the PN code c(t) is much higher than the data rate of data a(t), and therefore spectrum band of the multiplied output signal s(t) is spread as compared with b(t). The spread multiplied output signal s(t) is converted to RF by a frequency converting block 6, amplified by a power amplifier block 7 and is transmitted through an antenna 8.

On the receiver side, the signal received by an antenna 9 is amplified by an RF amplifier block 10 and is converted to an intermediate frequency at a frequency converting block 11. The signal s(t) which has been converted to the intermediate frequency is multiplied by a PN code c(t) having the same sequence as the code c(t) generated at the transmitter side, in a PN code generator 13. The PN code generated by PN code generator 13 must be synchronized in time with the PN code included in the received signal provided as an input to a despreading block (multiplier) 12. For this purpose, a time discrimination control circuit 14 having a loop structure is prepared, which constitutes, together with the PN code generator 13, a synchronizing block S. With the PN code removed at the despreading block 12, the output b(t) from the despreading block 12 is returned to the narrow band signal modulated only by the data, which signal is passed through an information demodulating block 15 to provide information 16 as represented by a(t).

Since synchronization in time is provided at despreading block 12 on the receiver side, the influence of the multipath fading which comes delayed in time can be reduced. Since the received signal is multiplied by the PN code generated by PN code generator 13, the narrow band noise input to the receiving antenna can be spread, and therefore the influence thereof can be reduced.

As described above, spectrum spreading enables communication in wider bandwidth which is strong against multipath fading and narrow band noise, and thus more effective communication can be carried out.

Details of the spread spectrum communication system shown in FIGS. 1 and 2 are described in Spread Spectrum Communication System, pp. 10-16, published by Kagaku Gijutsu Shuppansha.

The direct sequence spread spectrum communication system is stronger against multipath fading and narrow band noise as compared with the conventional narrow band communication as described above. However, it requires circuits for spreading and despreading spectrum, and since circuits in the synchronizing block employed therein generally has a loop structure, the circuits inevitably becomes large and completed as compared with the receivers for narrow band communication.

As a method of confirming synchronization of the PN code, the PN code of the received signal is multiplied by the PN code generated in the receiver, and the result is integrated. The spreading is carried out dependent on whether the result of integration is at a certain level. In other words, it takes some time to confirm synchronization, which depends on the time of integration. For example, if synchronization is to be confirmed with the chip shifted by 1/2, the maximum time necessary for confirmation is 2n.times.t(s), where t(s) represents the time (sec) of integration and n represents code length of n chips. Meanwhile, frequency hopping is considered promising as it is especially strong against multipath fading.

FIG. 3 shows a power spectrum, FIG. 3(a) shows frequency characteristic of propagation in a propagation path in a room, FIG. 3(b) shows an example of transmission of narrow band modulated wave having a band B.sub.1, and FIG. 3(c) shows spectrum obtained by frequency hopping. As shown in FIG. 3(a) in a propagation path in a room, there are frequencies of which gain is made stronger and frequencies of which gain is made weaker because of multipaths. Assume that a narrow band modulated wave having the band B.sub.1 shown in FIG. 3(b) is transmitted through the propagation path having such characteristic of propagation. This frequency is exactly in the frequency range of which gain is made weaker, and therefore C/N is degraded, causing significant degradation of bit error rate. In frequency hopping such as shown in FIG. 3(c), the frequencies f.sub.1 to f.sub.2 are divided into several slots each having at least the bandwidth of B.sub.1. Several tens to several hundreds of such slots are prepared and the frequency used is changed several bits by several bits. In this case, even when C/N of some of the slots may be degraded, remaining slots have high C/N, and therefore only a small number of bits may cause an error statistically. Errors continuous over several slots can be corrected by employing a method of error correction strong against burst error or interleave. Consequently, stable communication is ensured even in such a propagation path as shown in FIG. 3(a) which is much varied.

As described above, frequency hopping is strong in propagation paths having multipath phasing. However, the circuitry, especially the circuitry in despreading system, is much complicated and large. Therefore, frequency hopping is not popularly used except in a few special systems.

FIG. 4 is a block diagram of an acquisition circuit used in conventional frequency hopping. The acquisition circuit is described in Spread Spectrum Communication System, by Mitsuteru Yokoyama, Kagaku Gijutsu Shuppansha. This is an acquisition circuit for tracking an initial signal at the start of connection of communication. Referring to FIG. 4, a frequency hopping synthesizer 21 generates a local oscillation signal of local frequency corresponding to the hopping frequency, which local oscillation signal is applied to a power combiner 22 to be combined with a reception signal received at an antenna 23, and is converted to an intermediate frequency signal. A prescribed band component of the intermediate frequency signal is taken out by a bandpass filter (BPF) 24, which is squared by a square law 25 and integrated by an integrator 26, so that the signal energy is detected.

A search control logic 27 is provided for eliminating uncertainty of time domain and frequency domain, and it controls oscillation frequency of a VCO 29 by applying a control signal to a frequency control circuit 28 and controls clock frequency from a clock generating circuit 31 by applying a control signal to a clock control circuit 30. Oscillation output from VCO 29 is applied to frequency hopping synthesizer 21 and clock signal from clock generating circuit 31 is applied to a PN sequence generator 32. PN sequence generator 32 applies a control signal to frequency hopping synthesizer 21 based on the clock signal from clock generating circuit 31.

In the acquisition circuit shown in FIG. 4, hopping frequency is determined corresponding to the pattern of the PN sequence. For this purpose, a control signal is applied to frequency hopping synthesizer 21 from PN sequence generator 32, and when synchronization can not be established and timings do not match, phase of the PN sequence is shifted 1/2 chip by 1/2 chip, switching of the hopping frequency is made faster and time domain is searched. If the frequency is deviated, an main oscillator of frequency hopping synthesizer 21 is offset to search the frequency domain.

FIG. 5 is a block diagram of a tracking circuit. The tracking circuit is provided for keeping synchronization after the communication is initially tracked by the acquisition circuit shown in FIG. 4. The tracking circuit includes a frequency correlating network 35 in which an advanced local oscillation signal 1.sub.E (t) than the received signal r(t) and a retarded local oscillation signal 1.sub.L (t) are applied from a frequency hopping synthesizer 43 to power combiners 36 and 37, and the received signal r(t) has its frequency converted. The signal having the frequency converted in this manner have their bands restricted by bandpass filters 38 and 40, squared in square laws 39 and 41, respectively, and these signals are added by an adder 42 to be applied to a loop filter 44. Loop filter 44 removes high harmonics having the period of the hopping frequency as fundamental component from the received signal, the resulting output is applied to a VCC circuit 45, and a control signal is applied to an FH sequence generator 46. FH sequence generator 46 applies a control signal to frequency hopping synthesizer 43 based on the control signal from VCC circuit 45. The tracking circuit shown in FIG. 5 is described in the aforementioned article together with the above described acquisition circuit, and therefore detailed description is not given.

As described above, in the conventional spread spectrum communication system, the acquisition circuit shown in FIG. 4 and the tracking circuit shown in FIG. 5 are necessary, which requires large scale circuitry and makes it difficult to change the frequency at high speed. Although it is strong against multipath fading and suitable for use in a room, synchronization is lost when the communication is disconnected, and in that case, it is necessary to re-track from the start. This takes a long period of time during which the signal can not be demodulated.

Further, for despreading in the spread spectrum communication system, there are active methods of correlation such as sliding correlation and passive methods such as those using a matched filter or a correlator. In the active methods of correlation, synchronization is tracked, and thereafter synchronization is maintained by using a tracking loop such as a DLL loop.

However, in this method, the chip of the code is shifted little by little to find a timing at which codes coincide with each other for synchronization tracking, and therefore it takes long to track synchronization. Therefore, though it is suitable for communication with a fixed propagation path, it could not be applied for communication, in which propagation path changes frequently (as in residence or room), since it takes time to establish synchronization again once synchronization is lost.

FIG. 6 is a block diagram of a demodulator employing a positive despreading system used in such communication. Referring to FIG. 6, input intermediate frequency (IF) signal 1 is applied to a multiplying circuit 50 and has its frequency converted by I and Q components of a local signal generated from a local oscillator 61 and turned into baseband I component 51 and Q component 52. These two inputs are applied to an I channel correlator 53 and a Q channel correlator 54, respectively, and these are corelated by these correlators. The correlated output 55 is input to a data