Spread spectrum power line communications

Claims

I claim:

1. A spread spectrum power line carrier frequency communications method comprising the steps of:

connecting a plurality of slave units and one master unit to a common power line,

checking by a slave unit for the presence of a correlative output signal of a correlator responsive to a signal transmitted by any other slave unit on said power line when the slave unit needs to transmit a signal,

in the event that a checking slave unit does not detect a correlative output signal representing that a signal has been transmitted by another slave unit, transmitting a data signal using spread spectrum modulation with maximum length sequence, but in the event that a checking slave unit detects a correlative output signal representing that a signal has been transmitted by another slave unit, not transmitting, and

receiving via said power line by said master unit said data signal and demodulating it.

2. A spread spectrum power line carrier frequency communications method comprising the steps of:

connecting a plurality of slave units and one master unit to a common power line,

checking by each slave unit for the presence of a signal transmitted by any other slave unit and flowing through said power line when each slave unit needs to transmit a signal,

in the event that a checking slave unit detects transmission by another slave unit, not transmitting a data signal therefrom, but in the event that a checking slave unit does not detect a data signal, a transmitting slave unit, transmitting a data signal, a transmitting slave unit adding a sequential transmission code having the same code pattern as that of a first maximum length sequence transmission code of said data signal and a phase difference inherent in each slave unit to the modulated data signal which is obtained by subjecting a transmitting data signal to spread spectrum modulation with said first maximum length sequence transmission code, said transmitted data signal being transmitted to said master unit through said power line,

receiving by said master unit the spread spectrum modulated signal transmitted through said power line and demodulating it, and

said master unit discriminating each transmitting slave unit from the others using the phase difference between the maximum length sequence used when the spread spectrum modulated signal is produced and what is added to the spread spectrum modulated signal and transmitted through said power line.

3. A method according to claim 2, wherein a first slave unit detects the presence of a signal transmitted by a second slave unit and flowing through said power line by sequentially shifting the phase of said sequential transmission code and checking for the presence of a correlation therewith.

4. A system for spread spectrum power line carrier frequency communications, comprising:

a plurality of slave units and one master unit, each slave unit including:

first and second maximum length sequence generator circuits for generating maximum length sequences having phase differences inherent in said slave units and identical code patterns, respectively, with said first and second maximum length sequence generator circuits having identical clock pulses as inputs,

a modulator for spread spectrum modulating a transmitting data signal with the maximum length sequence produced by the first maximum length sequence generator circuit,

a correlator for correlating a signal supplied by a power line through a coupler with the maximum length sequence produced by said second maximum length sequence generator circuit,

a synchronizing control circuit for judging the presence of a signal transmitted by any other slave unit from the presence of a correlative output of said correlator when the phases of the maximum length sequence generated by said first maximum length sequence generator circuit and said second maximum length sequence generator circuit are varied,

a clock control circuit controlled by said synchronizing control circuit for sequentially shifting the phase of the identical clock pulses when the transmission of a data signal is needed, and

a switch circuit for sending a transmitting signal to said power line through said coupler, said transmitting signal being a combination of the spread spectrum modulated signal produced by said modulator only when said synchronizing control circuit has judged that no signal is transmitted by any other slave unit and the maximum length sequence produced by said second maximum length sequence generator circuit.

5. A system for spread spectrum power line carrier frequency communications, comprising:

a plurality of slave units and one master unit, said master unit comprising:

a first maximum length sequence generator circuit and a second maximum length sequence generator circuit responsive to respective identical clock pulses for respectively producing maximum length sequences having the same code patterns as those produced by said slave unit used for spread spectrum modulation,

a clock control circuit for outputting said respective identical clock pulses and for sequentially shifting the clock pulse phase supplied to said first and second maximum length sequence generator circuits,

a synchronizing control circuit for controlling said clock control circuit to sequentially shift the clock pulse phase,

a correlator for obtaining the correlation of a receiving signal supplied from a power line through a coupler to the maximum length sequence produced by said second maximum length sequence generator circuit and for sequentially shifting the phase of the maximum length sequence produced by said second maximum length sequence generator circuit, a correlative output being used to stop the shifting of the clock pulse phase by said synchronizing control circuit,

a phase shift control circuit for sequentially shifting the phase of the maximum length sequence produced by said first maximum length sequence generator circuit while the correlative output is being generated by said correlator, and

a demodulator for producing a receiving signal by demodulating the receiving spread spectrum modulated signal supplied by said coupler using the maximum length sequence produced by said first maximum length sequence generator circuit and stopping the operation of said phase shift control circuit according to said signal received.

6. A system according to claim 5, wherein a slave transmitting a signal is discriminated according to the output signal of said phase shift control circuit when a receiving signal is produced by said demodulator.

7. A system according to claim 5, wherein the generation of the clock pulse for use in each of said slave units and master unit is synchronous with A.C. power supplied through said power line used as a transmission line.

8. A spread spectrum power line carrier frequency line lock communications method comprising the steps of:

generating first and second clock pulses in each of a transmitter and a receiver, said first clock pulse being synchronized in phase with an A.C. supply flowing in a power line used as a transmission line and having a frequency (K.times.N) times as high as that of said A.C. supply, and said second clock pulse being synchronized in phase with said A.C. supply and having a frequency K/2 times as high as that of said A.C. supply, where N represents the maximum period length of a maximum length sequence generated in said transmitter and K represents an integer;

generating said maximum length sequence with said first clock pulse providing a basic timing thereof, said maximum length sequence having a generation period coincident with the period of "H" and "L" of said second clock pulse;

spread spectrum modulating transmission data and supplying it onto said power line; and

spread spectrum demodulating a received modulated signal from said power line by using a maximum length sequence the same as said maximum length sequence synchronized with said A.C. supply.

9. A line lock communication apparatus for a spread spectrum power line carrier frequency communication system, comprising:

a transmitter including a line lock clock generator for generating a first clock pulse which is synchronized in phase with an A.C. supply flowing in a power line utilized as a transmission line, said first clock pulse having a frequency K.times.N times as high as that of said A.C. supply, where N represents the maximum period length of a maximum length sequence used in said transmitter and K represents an integer, said line lock clock generator also generating a second clock pulse which is synchronized in phase with said A.C. supply and which represents a generation period of said maximum length sequence by changing its level between "H" and "L", a maximum length sequence generator for generating said maximum length sequence, said maximum length sequence having said first clock pulse as a basic timing thereof and having a period synchronized with a change in level between "H" and "L" of said second clock pulse, a spread spectrum modulator for product-modulating transmission data using said maximum length sequence generated by said maximum length sequence generator to thereby generate a modulated signal in which said transmission data are spread spectrum modulated over a wide band, and a coupler for supplying said spread spectrum modulated signal onto said power line; and

a receiver including a line lock clock generator and a maximum length sequence generator having the same construction as that of said transmitter, a coupler for receiving said modulated signal from said power line, and a spread spectrum demodulator for demodulating said modulated signal transferred from said coupler using said maximum length sequence generated from said maximum length sequence generator so as to provide reception data.

10. An apparatus according to claim 9, wherein said line lock clock generator in each of said transmitter and said receiver comprises:

a voltage controlled variable frequency oscillator for generating said first clock pulse,

a first frequency divider for frequency-dividing said first clock pulse to thereby generate said second clock pulse having a frequency 1/2N times as high as that of said first clock pulse,

a second frequency divider for frequency-dividing said second clock pulse into an output having a frequency 2/K times as high as that of said second clock pulse,

a phase comparator for comparing a phase of the output signal of said second frequency divider with that of said A.C. supply flowing in said power line to thereby generate an output signal corresponding to a difference in phase, and

a low-pass filter for smoothing the output signal of said phase comparator to thereby supply a control signal to said voltage controlled variable frequency oscillator.

11. An apparatus according to claim 9, wherein said maximum length sequence generator in each of said transmitter and said receiver comprises:

a shift register,

a first exclusive OR gate for exclusively ORing output signals generated from a plurality of stages of said shift register and for feeding-back a resulting output of the exclusive ORing to an input terminal of said shift register to thereby generate said maximum length sequence,

an AND gate for ANDing the output signals generated from all the stages of said shift register,

a frequency divider for frequency-dividing an output signal of said AND gate into an output signal having a frequency 1/2 times as high as that of said output signal of said AND gate,

a second exclusive OR gate for exclusively ORing said output signal of said frequency divider and said second clock pulse supplied from said line lock clock generator, and

an OR gate for ORing an output signal of said exclusive OR gate and said first clock pulse supplied from said line lock clock generator to thereby supply a resulting output signal as a basic clock to said shift register.

12. A spread spectrum power line carrier frequency communications method comprising the steps of:

connecting a plurality of slave units and one master unit to a common power line,

supplying from a slave unit to said power line, a spread spectrum modulated data signal formed by multiplicatively modulating data using an inherent gold code of said slave unit, and

monitoring by said master unit each of said slave units by successively producing the gold code inherent in each slave unit for use to multiplicatively demodulate a received spread spectrum modulated signal supplied through said power line and to discriminate a slave unit transmitting the receiving signal relative to the gold code generated.

13. A method according to claim 12, wherein said master unit locks up the gold code successively produced while the receiving signal is being generated.

14. A method according to claim 12, wherein each slave unit produces the inherent gold code by combining the phases of a first maximum length sequence and a second sequential code having patterns different from each other on a slave unit basis.

15. A system for spread spectrum power line carrier frequency communications, comprising:

a plurality of slave units, each slave unit comprising a gold code generator circuit for producing a gold code inherent in each slave unit, a modulator for spread spectrum modulating data to be transmitted using multiplicative modulation by means of the gold code, and a coupler for supplying said spread spectrum modulated signal to a power line, and

a master unit, said master unit comprising a coupler for receiving the spread spectrum modulated data from the power line, a gold code generator circuit for successively producing the gold code inherent in each slave unit, and a demodulator for demodulating the spread spectrum modulated data by multiplicative demodulation by means of the gold code produced by said gold code generator circuit of said master unit.

16. A system according to claim 15, wherein said gold code generator circuit in each slave unit comprises:

a first maximum length sequence generator circuit and a second maximum length sequence generator circuit, each formed of a shift register and a feedback circuit,

a setting circuit for setting a predetermined value inherent in each slave unit in the shift register forming said second maximum length sequence generator circuit, and

a gate circuit for combining maximum length sequences produced by said first and second maximum length sequence generator circuits and generating a gold code having a pattern inherent in each slave unit.

17. A spread spectrum power line carrier frequency communications system, comprising:

a transmitter providing a maximum length sequence and transmission data which are product-modulated so that said transmission data generate a spread spectrum modulated signal which is supplied onto a power line, and

a receiver for receiving said modulated signal from said power line and product-demodulating said modulated signal using a maximum length sequence that is the same as that used in said transmitter to thereby obtain reception data,

wherein said spread spectrum modulated signal in said transmitter is modulated again by using a clock pulse and is then transmitted to said receiver through said power line, and

wherein a frequency of a clock pulse used when said maximum length sequence is generated, a frequency of said clock pulse used when said spread spectrum modulated signal is modulated again, and a maximum code length of said maximum length sequence are of values at which a spectrum distribution of a transmitter output does not affect other equipment connected to said power line.

18. A system according to claim 17, in which the frequencies of said clock pulses used respectively for generating said maximum length sequence and for modulating said spread spectrum modulated signal again, and said maximum code length of said maximum length sequence are selected to be 280 Khz, 21 KHz, and 7 bits, respectively, to thereby set said spectrum distribution of said transmitter output so as not to affect a frequency band of an interphone system which is +15 KHz wide and has a center frequency selected to be one of 230 KHz, 270 KHz, 310 KHz, 350 KHz, 390 KHz, and 430 KHz.

19. A spread spectrum power line carrier frequency communications apparatus, comprising:

a transmitter including a clock generator for generating a first clock pulse, a maximum length sequence generator for generating a maximum length sequence in response to said first clock pulse produced by said clock generator, a spread spectrum modulator for performing spread spectrum modulation of transmission data using said maximum length sequence, a clock oscillator for generating a second clock pulse, a modulator for modulating an output of said spread spectrum modulator using said second clock pulse, and a coupler for transferring the modulated output to said power line; and

a receiver connected to said transmitter through a power line utilized as a transmission line, said receiver including a coupler for receiving the modulated output on said power line, a clock generator for generating a first clock pulse having the same frequency as said first clock pulse generated in said transmitter, a maximum length sequence generator for generating a maximum length sequence having the same code pattern as said maximum length sequence generated in said transmitter by using said first clock pulse generated in said clock generator of said receiver, a second clock oscillator for generating a second clock pulse having the same frequency as said second clock pulse generated in said clock oscillator of said transmitter, a demodulator for demodulating an output of said coupler connected to said power line by using said second clock pulse generated by said second clock oscillator to thereby isolate a spread spectrum modulated signal, and a spread spectrum demodulator for demodulating an output of said demodulator by using said maximum length sequence generated by said maximum length sequence generator to thereby isolate reception data, wherein respective frequencies of said first and second clock pulses and the maximum code length of said maximum length sequence in each of said transmitter and said receiver are of values at which a spectrum distribution of said transmission output transmitted from said transmitter has no influence on other equipment connected to said power line.

20. An apparatus according to claim 19, in which said clock generator in each of said transmitter and said receiver generates said first clock pulse in synchronism with an A.C. supply flowing in said power line.

21. A power line transmission type spread spectrum communications method in which on the side of a transmitter a maximum length sequence is produced and transmission data are subjected to multiplication modulation so as to produce a spread spectrum modulation signal which is supplied to power lines, and on the side of a receiver the same maximum length sequence as that used on the side of the transmitter and the modulation signal received through said power lines are used to subject reception data to multiplication demodulation, said method including the steps of:

at said receiver, producing a receiving signal level adjusting maximum length sequence which is synchronous with the maximum length sequence provided by said transmitter and has the same code pattern as the maximum length sequence provided by said transmitter, said receiving signal level adjusting maximum length sequence being swung in a predetermined range with the phase thereof shifted,

correlating the output between said receiving signal level adjusting maximum length sequence and said received modulation signal to obtain a signal corresponding to said received modulation signal without being affected by a noise signal, and

adjusting said received modulation signal such that the difference between said signal corresponding to said received modulation signal and a reference value is made constant.

22. A spread spectrum power line communications system, comprising:

a transmitter unit and a receiver unit which are connected through power lines utilized as a transmission path, said transmitter unit comprising:

a clock pulse generating circuit for producing a clock pulse,

a transmitting maximum length sequence generating circuit for producing a maximum length sequence with the aid of said clock pulse produced by said clock pulse generating circuit,

a modulator for spread spectrum modulating data to be transmitted using said maximum length sequence, and

a coupler for supplying the resulting spread spectrum modulated signal to said power lines; and

said receiver unit comprising:

a clock pulse generating circuit for generating a clock pulse synchronous with said clock pulse in said transmitter unit,

a receiving maximum length sequence generating circuit for producing a maximum length sequence which is the same as the maximum length sequence in said transmitter unit, with the aid of said clock pulse produced by said clock pulse generating circuit of said receiver unit,

a coupler for receiving said modulation signal supplied through said power lines,

a voltage-controlled variable gain receiving amplifier for amplifying an output of said coupler,

a clock pulse phase swinging circuit for stepping the phase of said clock pulse produced by said clock pulse generating circuit of said receiver unit in a predetermined direction to thereby swing the phase of said clock pulse,

a level controlling maximum length sequence generating circuit for producing a level controlling maximum length sequence which is the same in code pattern as said maximum length sequence generated by said receiving maximum length sequence generating circuit, with the aid of the clock pulse provided by said clock pulse phase swinging circuit,

a correlation unit for correlating said level controlling maximum length sequence with an output signal of said voltage-controlled variable gain receiving amplifier,

a detecting and smoothing circuit for detecting and smoothing an output of said correlation unit,

an error detecting circuit for applying the difference between an output signal of said detecting and smoothing circuit and a reference value as a level control signal to said voltage-controlled variable gain receiving amplifier, and

a spread spectrum demodulator for demodulating the received signal from said variable gain receiving amplifier by multiplicative demodulation using said maximum length sequence supplied by said receiving maximum length sequence generating circuit.

23. A system according to claim 22, wherein said clock pulse generating circuit in each of said transmitter unit and receiver unit is a power source synchronization clock pulse generating circuit which synchronizes the outputted clock pulse with the A.C. power which is applied to said power lines.

24. A power line transmission type spread spectrum communications method, comprising the steps of:

on a data transmitting side, subjecting a maximum length sequence and transmission data to multiplication modulation so as to produce a modulation signal in which said transmission data are spread in the form of a spectrum,

supplying said modulation signal to power lines,

on the data receiving side, subjecting a maximum length sequence which is the same as that produced at the time of data transmission and reception data of said modulation signal received through said power lines to multiplication demodulation,

correlating said modulation signal supplied to said power lines with said transmitted maximum length sequence in a correlating means,

detecting and smoothing a correlation output of said correlation means, and

when the level of a signal which is obtained by detecting and smoothing said correlation output becomes lower than a reference value, switching a frequency band used for said modulation signal.

25. A method according to claim 24, wherein one of said maximum length sequence and said modulation signal on the data transmitting side is subjected to multiplication modulation with a clock pulse, and the frequency of said clock pulse is changed to switch said frequency band used.

26. A power line transmission type spread spectrum communication system, comprising a transmitter unit and a receiver unit which are connected to each other through power lines which are utilized as a data transmission path,

said transmitting unit comprising:

a clock pulse generating circuit for generating a clock pulse;

a maximum length sequence generating circuit for generating a maximum length sequence with the aid of said clock pulse produced by said clock pulse generating circuit;

a divider for dividing said clock pulse so as to produce a frequency division output of said clock pulse;

a selector for selecting between said clock pulse produced by said clock pulse generating circuit and said frequency division output of said divider;

modulation means for subjecting an output signal of said selector, said maximum length sequence and transmission data to multiplication modulation; and

a first coupler for supplying an output of said modulator to said power lines; and

said receiver unit comprising:

a second coupler for receiving a modulation signal supplied to said power lines by said first coupler;

a correlation unit for obtaining the correlation between an output signal of said second coupler and said maximum length sequence of said transmitting unit;

a rectifying and smoothing circuit for rectifying and smoothing an output signal of said correlation unit;

a switching control circuit for switching said selector when the level of an output signal of said rectifying and smoothing circuit becomes lower than a reference value;

a synchronous maximum length sequence generating circuit for producing a demodulating maximum length sequence which is the same as the maximum length sequence used for the spread spectrum modulation of transmission data on the side of the transmitter unit; and

a demodulator in which an output signal of said coupler is multiplied by said demodulating maximum length sequence to obtain reception data.

27. A spread spectrum power line carrier frequency communications arrangement in which a transmitter multiplicatively modulates a gold code and transmitting data so as to produce a modulated signal containing spread spectrum transmitting data, the modulated signal being supplied to a power line, whereas a receiver multiplicatively demodulates receiving data from said power line using the same gold code as what is used by the transmitter to form the modulated signal transmitted through the power line and received as said receiving data, wherein the gold code transmitted by said transmitter is identical with a predetermined gold code which is determined according to the address of the transmitter, the gold code being received by the receiver corresponding to the predetermined address of said receiver so that the transmission of the address of the receiver as well as the transmitting data is made unnecessary.

28. An arrangement according to claim 27, wherein the gold codes used by the transmitter and receiver are produced by means of a clock pulse produced synchronously with A.C. power flowing through the power line utilized as a transmission line.

29. A system for spread spectrum power line carrier frequency communications, comprising:

a transmitter unit and a receiver unit connected through a power line utilized as a transmission line,

said transmitter unit comprising a clock generator circuit for producing a clock pulse, an address setting unit for setting its own address, a gold code generator circuit responsive to said clock pulse for producing a gold code according to an input from said address setting unit, a modulator circuit for spread spectrum modulating transmitting data using said gold code, and a coupler for supplying the modulated signal generated by said modulator circuit to said power line, and

said receiver unit comprising a clock generator circuit for producing a clock pulse synchronous with the clock pulse generated in said transmitter unit, an address setting unit for setting its own address, a gold code generator circuit for producing a gold code in response to the output of said address setting unit on receiving said clock pulse, a coupler for obtaining the modulated signal supplied by said transmitter unit through said power line, and a demodulator for obtaining receiving data by multiplying the output signal of said coupler by the output signal of said gold code generator circuit.

30. A system according to claim 29, wherein the clock pulse generator circuits in said transmitter and receiver units are formed with power supply synchronizing clock generator circuits which produce clock pulses synchronous with A.C. power flowing through said power line.

BACKGROUND OF THE INVENTION

This invention relates in general to power line communications. More specifically, it provides various improvements for systems wherein a central station "master" monitors and communicates with a plurality of "slave" units using a power line as a transmission line for communications.

A security system is one example of a system that requires centralized monitoring of slave units by a master. Various sensors such as infrared intrusion, window - glass damage and fire detection sensors are installed in target areas to be protected. These sensors are connected to a supervisory unit through individual transmitters and circuits respectively for centralized monitoring.

A problem of conventional "wired" master/slave systems is that, as the number of slave units and monitoring range increase, the amount of wiring required becomes excessive. A power line providing power to the various slave units can be used for communications to reduce the amount of wiring, but there are many factors which make it difficult to communicate reliably over a power line. For example, it is usually necessary to provide an arrangement for the prevention of signal line disconnections and quick detection of such disconnection faults.

Various schemes have been proposed to establish and maintain communications over a commercial power line. A transmission line generally utilizes single side band modulation for data signals, whereas a frequency or phase modulation is used for a distribution line. However, a power line is not designed for signal transmission.

It is electrically noisy, has a wide range of impedances, and its transmission characteristics fluctuate with line load. As a consequence, reliable signal transmission and particularly high speed data transmission have not been possible using conventional techniques.

There has been study undertaken in the applications of so called "spread spectrum" communications. The Journals of the Institute of Electronic and Communications Engineers of Japan, Sept/82, p 965 & Oct/82, p 1063, for example, disclose the principles of and comments on the applications of spread spectrum technology.

A spred spectrum communications system relies on so-called Pseudo-Noise (PN) diffusion or direct diffusion. Thus, a narrow-band data signal is transmitted over a wide-band transmission line by diffusing the spectrum thereof using an M sequential code as spurious noise signal, and even if the transmission characteristics of the transmission medium have a plurality of zero points resulting from the line load, a transmitted signal will not be substantially affected thereby. Moreover, even if narrow-band noise is blended with a transmitted data signal, the S/N ratio can be improved using correlation at the receiver.

However, the application of spread spectrum technology to power line communication systems permitting one master unit to simultaneously monitor a plurality of slave units still poses problems. For example, if multiple slave units simultaneously send data signals to the master unit, the data signals overlap and cannot be discriminated from one another. To prevent the slave units from sending the data signals to the master unit simultaneously, polling schemes have been used. In effect, the master takes turns looking at each slave successively to see if a given slave has a message to send to the master. Such systems require additional hardware, however, such as a CPU to control the polling, and such hardware is expensive.

SUMMARY OF THE INVENTION

The present invention provides various improvements in power line communications. Using the arrangements of the present invention, it is possible to achieve reliable and inexpensive centralized monitoring of a plurality of slave units by one master unit through a power line without the need to use a CPU (Central Processing Unit) for polling.

According to one aspect of the invention (See FIG. 4-FIG. 9), signals transmitted by a plurality of slave units to a master unit are prevented from overlapping one another. A slave unit having a message to transmit first checks for the presence of any spread spectrum modulated signal on the power line. If there is already a spread spectrum signal on the line, it does not transmit. However, if the line is judged to be free by the absence of any spread spectrum signal, it transmits its data message using spread spectrum modulation.

Each slave unit generates a first M sequential transmission code for use in spread spectrum modulating a data signal and a second M sequential transmission code having the same code pattern as that of the first M sequential transmission code. The second M sequential transmission code is added to the spread spectrum signal modulated by the first M sequential transmission code only when the data signal is produced. This "combined" signal is transmitted onto the power line. A slave unit transmitting a signal is discriminated from the others by setting the phase difference inherent in each slave unit between the M sequential transmission codes.

The phase of the second M sequential transmission code produced by each slave unit is successively shifted from one unit to the next. Whether or not any other slave unit is transmitting a signal is determined by obtaining the correlation between the signals received through the power line.

The master unit produces a first M sequential reception code for use in demodulating a received spread spectrum modulated signal, the first M sequential reception code having the same code pattern as that of the first M sequential transmission code, and a second M sequential reception code for use in correlating with the second M sequential transmission code, the second M sequential reception code having the same code pattern as that of the second M sequential transmission code. The codes are synchronized by successively varying the phase of the clock pulse providing a basis for the generation of the first and second M sequential reception codes on the basis of a period greater than the period wherein the second M sequential reception code is produced until the correlation of the second M sequential reception code to the second M sequential transmission code received from the slave unit is obtained.

The received spread spectrum modulated signal is multiplicatively demodulated while only the phase of the first M sequential reception code is shifted at least in the period wherein the code is produced when the correlation between the second M sequential transmission code and the second M sequential reception code is obtained. Phase shifting is stopped when the demodulated signal is obtained so as to extract the receiving signal, and the slave unit transmitting a signal is discriminated from the others according to the difference in phase between the first and second M sequential reception codes.

Transmission-to reception phase synchronization is secured by locking a clock pulse generator circuit installed in each of the slave units and the master unit to a power supply for supplying A.C. through the power line.

Slave units are prevented from transmitting signals simultaneously. Each slave unit is allowed to spread spectrum modulate with M sequential codes and transmit the thus modulated signal only after confirming the absence of any spread spectrum modulated signal flowing through the power line.

Each slave unit transmits data with the first and second M sequential transmission codes produced for spread spectrum modulating in such a state that each code has the phase difference inherent in each slave unit and transmits the combination of the second M sequential transmission code and the spread spectrum modulated signal added thereto so that any slave unit transmitting a signal may readily be discriminated from the others by obtaining the difference in phase between the first M sequential reception code for use in demodulating the receiving spread spectrum modulated signal on the part of the master unit and the second M sequential reception code for obtaining the correlation thereof to the second M sequential transmission code contained in the receiving signal.

Each slave unit is, if a data signal to be transmitted is produced, caused to transmit the combination of the spread spectrum modulated signal obtained by multiplicatively modulating the data signal with the first M sequential transm