 |
Claims  |
|
|
What is claimed is:
1. A system for use in conveying a tranmission signal from a transmission
station to a reception station through an electric power line for electric
power of a commercial frequency falling within a first frequency range,
said electric power line having a variable loss,
said transmission station comprising:
modulation means responsive to said transmission signal for carrying out
spread spectrum modulation of said transmission signal by the use of a
first pseudorandom code to produce a modulated signal which is subject to
said spread spectrum modulation and which is dispersed in a second
frequency range different from said first frequency range;
sychronization signal producing means for producing a sychronization
signal; and
combining means coupled to said electric power line, said modulating means,
and said synchronization signal producing means for combining said
modulated signal and said synchronization signal into a combined signal to
supply said electric power line with said combined signal;
said reception station comprising:
demodulating means coupled to said electric power line for demodulating
said combined signal into a demodulated signal by the use of a second
pseudorandom code corresponding to said first pseudorandom code, said
demodulated signal comprising a reproduction of said modulated signal and
a synchronization component representing said synchronization signal, and
having a variable level resulting from said variable loss of the electric
power line;
clock pulse generating means coupled to said demodulating means for
generating a sequence of clock pulses in response to said synchronization
component and said variable power level; and
means coupled to said clock pulse generating means and said demodulating
means for supplying said demodulating means with said second pseudorandom
code which is synchronized with said clock pulse sequence.
2. A system as claimed in claim 1, wherein said clock pulse generating
means comprises:
deriving means coupled to said demodulating means for deriving said
synchronization component from said demodulated signal;
clock control means coupled to said deriving means for producing a clock
control signal dependent on said variable level, in synchronism with said
synchronization component; and
clock signal means coupled to said clock control means for producing said
clock pulse sequence in response to said clock control signal.
3. A system as claimed in claim 2, wherein said deriving means comprises:
rectifying means for rectifying said demodulated signal into a rectified
signal; and
peak detecting means coupled to said rectifying means for successively
detecting a peak of said rectified signal to produce said synchronization
component.
4. A system as claimed in claim 3, wherein said clock control means
comprises:
average calculating means coupled to said rectifying means for calculating
an average level of said rectified signal to produce an average level
signal representative of said average voltage level, said average level
corresponding to said variable level;
comparing means coupled to said rectifying means and said average
calculating means for comparing said rectified signal with said average
level signal to produce a result signal representative of a result of said
comparison; and
means coupled to said peak detecting means and said comparing means for
producing said clock control signal in response to said result signal and
said synchronization component.
5. A system as claimed in claim 1, wherein said demodulating means
comprises:
gain controlling means coupled to said electric power line and responsive
to said combined signal for controlling the gain of said combined signal
to produce a gain controlled signal; and
means coupled to said gain controlling means and said second pseudorandom
code for extracting said demodulated signal from said combined signal.
6. A system as claimed in claim 5, wherein said gain controlling means
comprises:
variable gain means controllable by a gain control signal and responsive to
said combined signal for producing said gain controlled signal;
code generating means for generating an additional pseudorandom code which
is identical with said first pseudorandom code and which has a frequency
different from said first pseudorandom code;
multiplying means for multiplying said gain controlled signal by said
additional pseudorandom code to produce a product signal representative of
a produce of said gain controlled signal and said additional pseudorandom
code;
peak detecting means coupled to said multiplying means for detecting a peak
value of said product signal; and
means coupled to said peak detecting means for supplying said peak value to
said variable gain means as said gain control signal.
7. A system as claimed in claim 1, further comprising power control means
coupled to said combining means and said demodulating means for
controlling the gain of said combined signal in response to said
demodulated signal.
8. A system comprising an electric power line, a polling station coupled to
said electric power line for carrying out a polling operation, and a
plurality of communication stations for carrying out communication through
said electric power line under control of said polling station, said
electric power line being for transmitting electric power at a commercial
frequency falling within a first frequency range, each of said polling
station and said communication stations comprising:
modulation means responsive to said transmission signal for carrying out
spread spectrum modulation of said transmission signal by the use of a
first pseudorandom code to produce a modulated signal which is subject to
said spread spectrum modulation and which is dispersed in a second
frequency range different from said first frequency range;
synchronization signal producing means for producing a synchronization
signal; and
combining coupled to said electric power line, said modulation means, and
said synchronization signal producing means for combining said modulated
signal and said synchronization signal into a combined signal to supply
said electric power line with said combined signal;
demodulating means coupled to said electric power line for demodulating
said combined signal into a demodulated signal by the use of a second
pseudorandom code corresponding to said first pseudorandom code, said
demodulated signal comprising a reproduction of said modulated signal and
having a variable level resulting from said variable loss of the electric
power line;
clock pulse generating means coupled to said demodulation means for
producing a sequence of clock pulses with reference to a synchronization
component, conveyed by said demodulated signal, and to said variable
level; and
means coupled to said clock pulse generating means and said demodulating
means for supplying said demodulating means with said second pseudorandom
code which is synchronized with said clock pulse sequence.
9. A system as claimed in claim 8, wherein said clock pulse generating
means comprises:
deriving means coupled to said demodulating means for deriving said
synchronization component from said demodulated signal;
clock control means coupled to said deriving means for producing a clock
control signal dependent on said variable level, in synchronism with said
synchronization component; and
clock signal means coupled to said clock control means for producing said
clock pulse sequence in response to said clock control signal.
10. A system as claimed in claim 9, wherein said deriving means comprises:
rectifying means for rectifying said demodulated signal into a rectified
signal; and
peak detecting means coupled to said rectifying means for successively
detecting a peak of said rectified signal to produce said synchronization
component.
11. A system as claimed in claim 10, wherein said clock control means
comprises:
average calculating means coupled to said rectifying means for calculating
an average level of said rectified signal to produce an average level
signal representative of said average level, said average level
corresponding to said variable level;
comparing means coupled to said rectifying means and said average
calculating means for comparing said rectified signal with said average
level signal to produce a result signal representative of a result of said
comparison; and
means coupled to said peak detecting means and said comparing means for
producing said clock control signal in response to said result signal and
said synchronization component.
12. A system as claimed in claim 8, wherein said demodulating means
comprises:
gain controlling means coupled to said electric power line for controlling
a gain of said combined signal to produce a gain controlled signal; and
means coupled to said gain controlling means and said second pseudorandom
code for extracting said demodulated signal from said combined signal.
13. A system as claimed in claim 12, wherein said gain controlling means
comprises:
variable gain means controllable by a gain control signal and responsive to
said combined signal for producing said gain controlled signal;
code generating means for generating an additional pseudorandom code which
is identical with said first pseudorandom code and which has a frequency
different from said first pseudorandom code;
multiplying means for multiplying said gain controlled signal by said
additional pseudorandom code to produce a product signal representative of
a product of said gain controlled signal and said additional pseudorandom
code;
peak detecting means coupled to said multiplying means for detecting a peak
value of said product signal; and
means coupled to said peak detecting means for supplying said peak value to
said variable gain means as said gain controlled signal.
14. A system as claimed in claim 8, further comprising power control means
coupled to said combining means and said demodulating means for
controlling a gain of said combined signal in response to said demodulated
signal. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a system for use in carrying out communication
between a plurality of stations through an electric power line, such as a
power transmission line, a distribution line, or the like.
As a rule, an electric power line of the type described serves to deliver
electric power of a commercial frequency to a wide variety of loads
connected thereto. The electric power falls within a commercial frequency
band. Various attempts have been made to transmit an information signal
between stations through such an electric power line. In this case, it is
to be noted that the electric power line is put in bad circumstances for
transmission of the information signal because noises, such as a corona
noise and the like, or undesired signals, such as harmonic waves of a
commercial frequency, inevitably appear on the electric power line. In
addition, a variation of the loads gives rise to a variation of a noise
characteristic and a transmission characteristic of the electric power
line. Anyway, the characteristics of the electric power line are widely
variable with time.
Furthermore, transmission of the information signal should not adversely
affect any other systems or devices coupled to the electric power line. A
limited electric power is therefore shared with transmission of the
information signal and degrades a quality of the transmission.
In a paper contributed by Michell Lee to IEEE Transactions on Consumer
Electronics, Vol. CE-28, No. 3, August 1983, pages 409-413, and titled "A
NEW CARRIER CURRENT TRANSCEIVER I.C.," a conventional system is disclosed
which comprises a transmitter for carrying out FSK modulation to transmit
a modulated signal conveying an information signal to an A.C. line,
namely, an electric power line and a receiver for demodulating the
modulated signal by the use of a phase lock loop. The modulated signal
falls within a specific frequency band which is different from that of the
electric power.
Both of the transmitter and the receiver might be incorporated into a
station coupled to the electric power line. In this case, a plurality of
stations may be connected to the electric power line to carry out
communication therebetween.
As will later be described in conjunction with a few figures of the
accompanying drawing, the modulated signal is undesiredly deteriorated by
noises because the noises may fall within the specific frequency band. In
addition, the characteristics may vary in the specific frequency band.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a system which is capable of
carrying out communication between a transmitter and a receiver through an
electric power line at a high speed and with a high reliability.
It is another object of this invention to provide a system of the type
described, wherein communication is possible between two of stations in a
multiple access manner.
A system to which this invention is applicable is for use in conveying a
transmission signal from a transmission station to a reception station
through an electric power line for electric power of a commercial
frequency following within a first frequency region. According to this
invention, the transmission station comprises modulation means responsive
to the transmission signal for carrying out spread spectrum modulation of
the transmission signal to produce a modulated signal which is subjected
to the spread spectrum modulation and which is dispersed in a second
frequency region different from the first frequency region and sending
means coupled to the electric power line and the modulation means for
sending the modulated signal to the electric power line. The reception
station comprises extracting means coupled to the electric power line for
extracting the modulated signal from the second frequency region to
produce an extracted signal, demodulation means coupled to the extracting
means for demodulating the extracted signal into a demodulated signal
carrying the information signal, and means for deriving the transmission
signal from the demodulated signal.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 exemplifies a transmission characteristic on an electric power line;
FIG. 2 exemplifies a modulation spectrum of a modulated signal transmitted
to the electric power line;
FIG. 3 exemplifies a demodulation spectrum of a demodulated signal sent
through the electric power line having the transmission characteristic
shown in FIG. 1;
FIGS. 4(a) through 4(c) are time charts for use in describing a principle
of this invention;
FIGS. 5(a) through 5(d) exemplify power spectra corresponding to FIGS. 4(a)
through 4(c), respectively;
FIG. 6 is a block diagram of a communication system according to each of
first through third embodiments of this invention;
FIG. 7 is a block diagram of a station for use in the communication system
according to the first embodiment of this invention;
FIG. 8 is a block diagram of a station for use in the communication system
according to the second embodiment of this invention;
FIG. 9 is a block diagram of a receiver synchronization circuit for use in
the station illustrated in FIG. 8;
FIG. 10 is a block diagram of a station for use in the communication system
according to the third embodiment of this invention;
FIG. 11 is a block diagram of a communication system according to a fourth
and a fifth embodiment of this invention;
FIG. 12 is a block diagram of a polling station for use in the
communication system illustrated in FIG. 11;
FIG. 13 is a block diagram of a block station for use in combination with
the polling station illustrated in FIG. 12;
FIG. 14 is a block diagram of a polling station for use in a communication
system according to the fifth embodiment of this invention;
FIG. 15 is a block diagram of a local station for use in combination with
the polling station illustrated in FIG. 14;
FIG. 16 is a block diagram of a power control circuit which is applicable
to each of the embodiments;
FIG. 17 is a block diagram of another control circuit for carrying out
operation similar to that illustrated in FIG. 16;
FIG. 18 is a circuit diagram of a coupler for use in each embodiment;
FIG. 19 is a block diagram of an automatic gain control circuit which is
applicable to each embodiment;
FIG. 20 is a block diagram of a station for use in a communication system
according to a modification of this invention; and
FIG. 21 is a block diagram of a receiver synchronization circuit which is
applicable to each embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
PRIOR ART
Referring to FIGS. 1 through 3, description will be made as regards a
conventional method of carrying out amplitude modulation, phase
modulation, frequency modulation, or any other modulation. As shown in
FIG. 1, let an electric power line have a transmission characteristic H(f)
having a null point or zone at a high frequency f.sub.1. The high
frequency f.sub.1 is different from a commercial frequency of electric
power delivered to various loads connected to the electric power line. A
frequency band for the commercial frequency is herein called a first
frequency band.
On the other hand, let the modulation be carried out by the use of a
central frequency equal to the frequency f.sub.1 to transmit a modulated
signal to the electric power line. As shown in FIG. 2, such a modulated
signal exhibits a modulation spectrum M(f) locally laid in a frequency
band adjacent to the central frequency.
When the modulated signal is transmitted through the electric power line
having the transmission characteristic shown in FIG. 1 and is subjected to
demodulation, a demodulated signal has a demodulation spectrum S(f)
depicted in FIG. 3. The demodulation spectrum S(f) is seriously reduced or
deteriorated in comparison with the modulation spectrum M(f). Therefore,
transmission performance is degraded unfavorably.
In addition, the transmission characteristic is variable with time. As a
result, the null point irregularly moves with time. This widely varies
electric power of the demodulated signal and makes it difficult to achieve
stable communication.
From the above, it is readily understood that the conventional modulation
method is not suitable for a transmission system which carries out
transmission through an electric power line.
Principle of the Invention
Referring to FIG. 1 again and FIG. 4 afresh, a principle of this invention
is to carry out spread spectrum communication through the electric power
line. For this purpose, spread spectrum modulation and demodulation are
carried out in a transmitter and a receiver, respectively.
Such spread spectrum communication is usually used in a radio communication
system because the spread spectrum communication is strong against fading,
a local noise over a narrow band, and the like and has a high secrecy.
However, the spread spectrum communication is scarcely applied to a wire
communication system, such as an electric power line system.
The spread spectrum communication will be described hereinunder. Let the
information signal exhibit a waveform along an axis of time (t) as shown
in FIG. 4(a) and a first power spectrum P(f) along an axis of frequency
(f) as shown in FIG. 5(a). The information signal is a succession of data
pulses produced at a data rate.
As shown in FIG. 4(b), a pseudorandom code sequence is produced in
synchronism with a succession of clock pulses having a clock rate higher
than the data clock rate and is repeated at a frame period equal to a
reciprocal of the data rate. The frame period is determined by a code
length of the pseudorandom code sequence. Anyway, a single one of the data
pulses appears during each frame period.
Such a pseudorandom code may be a maximum length code known in the art and
has a second power spectrum P.sub.2 (f) along an axis of frequency, as
shown in FIG. 5(b). The second power spectrum P.sub.2 (f) has a plurality
of frequency components dispersed in a wide frequency band defined by the
code length of the pseudorandom code sequence. More specifically, a
maximum one of the frequency components is represented by f.sub.N, if the
code length is specified by N.
The pseudorandom code sequence is modulated by the information signal into
a modulated signal as shown in FIG. 4(c) according to the spread spectrum
modulation. The spread spectrum modulation is possible by a multiplier for
calculating a product between the information signal and the pseudorandom
code sequence. The modulated signal exhibits a modulation spectrum M(f)
divisible into a plurality of partial spectra depicted at M.sub.1 through
M.sub.N which are specified by main envelopes and are laid in the vicinity
of the frequency components shown in FIG. 5(b), respectively.
Each of the partial spectra M.sub.1 through M.sub.N uniformly includes a
signal component of the information signal distributed thereto in the
manner known in the art. Thus, the modulation spectrum is distributed like
a white noise to the wide frequency band proportional to the code length
of the pseudorandom code sequence.
Let the modulated signal be transmitted through the electric power line
having the transmission characteristic H(f) shown in FIG. 1. When the
modulated signal is demodulated into a demodulated signal by the receiver,
the demodulated signal has a demodulation spectrum S(f) as shown at a real
line in FIG. 5(a). The demodulation spectrum S(f) is hardly reduced
despite the fact that the transmission characteristic has the null point
at the frequency f.sub.1. This is because the modulation spectrum M(f) is
spread over the wide frequency band as exemplified in FIG. 5(c) and a
reduction of electric power of the modulated signal is very small even
when a null point or points locally appear on the electric power line.
Accordingly, it is possible with this invention to realize communication
which is strong against a selective jamming wave or noise. This means that
the communication can be carried out with a high reliability and at a high
speed.
Herein, the pseudorandom code sequence takes a plurality of phases during
the frame period and may be modified in phase into modified code sequences
which can form different pseudorandom code sequences, respectively, in the
manner which is also known in the art. Such modification is possible by
indicating initial phases of the pseudorandom code sequences.
FIRST EMBODIMENT
Referring to FIG. 6, a communication system according to a first embodiment
of this invention comprises an electric power line 30 which may be either
to power transmission line or a distribution cable. A plurality of
stations are coupled to the electric power line 30, although only two
stations are illustrated in FIG. 6 and will be referred to as first and
second stations denoted by 31 and 32, respectively. In the example being
illustrated, each of the first and second stations 31 and 32 carries out
both of transmission and reception through the electric power line 30. The
first and second stations 31 and 32 can optionally be connected to or
disconnected from the electric power line 30 by receptacles (not shown).
In other words, each station is not actively coupled to the electric power
line 30 but is passively coupled to the line 30.
It should be noted here that station addresses are preassigned to the
respective stations.
A plurality of terminal units collectively denoted by 33.sub.1 and 33.sub.2
are connected to the first and second stations 31 and 32, respectively.
From this fact, it is readily understood that communication is finally
carried out between two of the terminal units 33.sub.1 and 33.sub.2. Each
of the terminal units 33.sub.1 and 33.sub.2 comprises an input and an
output device. Despite the terminal units 33.sub.1 and 33.sub.2,
description will mainly be directed to communication between the first and
second stations 31 and 32.
Referring to FIG. 7 afresh and FIG. 6 again, each of the first and second
stations 31 and 32 comprises a transmitter 36 and a receiver 37 for
carrying out the transmission and reception, respectively, in the manner
which will later be described more in detail. A coupler 39 is common to
the transmitter 36 and the receiver 37 and operable as parts of the
transmitter 36 and the receiver 37. The coupler 39 is coupled through the
receptacle (not shown) to the electric power line 30. In addition, the
coupler 39 is operable to deliver electric power PW of the commercial
frequency to various electric devices (not shown).
The transmitter 36 comprises a multiplexer 41 supplied with input signals
from the input devices of the terminal units 33 (suffixes omitted). The
input signals are multiplexed by the multiplexer 41 into a multiplexed
signal and is sent to a transmitter multiplier 42. The multiplexed signal
may be called an information signal IS conveying information. For
simplicity of description, it will be assumed that the information signal
IS is transmitted from the first station 31 for reception by the second
station 32. In this event, the first and second stations 31 and 32 may be
referred to as an originating and a destination station, respectively.
In order to deliver the information signal IS to the destination station,
it is necessary to indicate a destination address assigned to the
destination station. To this end, the destination address is specified by
a transmission controller 43 which is operable in cooperation with the
terminal units 33. In the example being illustrated, the destination
address is sent to a first pseudorandom noise (PN) generator 46.
The first PN generator 46 may be a combination of flip flops and Exclusive
OR gates in the manner well known in the art and can generate a plurality
of pseudorandom code sequences which are equal in code length to one
another and different in phase from one another, when initial phases of
the respective pseudorandom code sequences are indicated, as suggested
before.
Taking the above into consideration, the plurality of pseudorandom code
sequences are made to correspond to the respective station addresses and
are used to specify the respective station addresses in the example being
illustrated. Each of the pseudorandom code sequences PN may be called a
pseudorandom noise sequence.
The destination address indicated by the transmission controller 43 is
given to the PN generator 46 as an initial phase signal indicative of one
of the initial phases that specifies a selected one of the pseudorandom
code sequences PN. The selected pseudorandom code sequence will be
referred to as a modulation pseudorandom code sequence and is sent to the
multiplier 42.
The transmitter multiplier 42 carries out product modulation between the
information signal IS and the selected pseudorandom code sequence PN to
produce a product signal or modulated signal MD representative of a
product therebetween. The product signal has a modulation spectrum spread
over a wide frequency band as illustrated in FIG. 5(c). Accordingly, the
product modulation may be called spread spectrum modulation. A combination
of the transmitter multiplier 42 and the first PN generator 46 may be
called a modulator 47.
An adder 48 adds the modulated signal MD to a transmitter synchronization
signal SYNC.sub.1 to supply the coupler 39 with a sum signal
representative of the sum through a transmitter amplifier 49. The
transmitter synchronization signal SYNC.sub.1 is produced in a manner to
be described later and may be formed by an additional pseudorandom code
sequence of a period which is equal to the pseudorandom code sequences
generated by the first PN generator 46 and which is different from all of
the pseudorandom code sequences. The transmitter synchronization signal
SYNC.sub.1 serves to define each frame of the selected or modulation
pseudorandom code sequence.
The coupler 39 sends the sum signal to the electric power line 30 as a
transmitter output signal after it attenuates a low frequency component of
the sum signal falling within the first frequency band for the commercial
frequency. Thus, the transmitter output signal is not superposed in
frequency on the electric power signal PW of the commercial frequency and
is widely dispersed in the second frequency band. The adder 48, the
transmitter amplifier 49, and the part of the coupler 39 may be called a
sending circuit for sending the modulated signal to the electric power
line 30.
Let the illustrated receiver 37 be used in the second or destination
station 32 (FIG. 6) rather than in the first or originating station. The
transmitter output signal arrives at the second station 32 as a receiver
input signal through the electric power line 30 and is extracted by the
coupler 39 from the electric power signal PW. The receiver input signal is
supplied through a receiver amplifier 51 to an automatic gain control
(AGC) circuit 52. The AGC circuit 52 has a dynamic range enough to
compensate for a variation of a dead loss on the electric power line 30
and produces a gain controlled signal GC having substantially constant
electric power. The AGC circuit 52 will later be described as regards its
operation and structure in detail.
The gain controlled signal GC includes the transmitter synchronization
signal SYNC.sub.1 and the selected pseudorandom code sequence PN.sub.1
modulated by the information signal IS, like the transmitter output
signal. The gain controlled signal GC is delivered from the AGC circuit 52
to a receiver synchronization circuit 54 and a receiver multiplier 56.
The receiver synchronization circuit 54 derives a clock signal and a frame
signal from the gain controlled signal GC. The frame signal reproduces the
frame specified by the transmitter synchronization signal SYNC.sub.1 while
the clock signal specifies clock components included in the gain
controlled signal GC. Operation and structure will become clear later.
The clock signal and the frame signal are supplied to a second pseudorandom
noise (PN) generator 57. The second PN generator 57 is operable in
response to an initial phase signal supplied from a receiver controller
59. The initial phase signal specifies an initial phase of a demodulation
pseudorandom code sequence PD assigned to the second or destination
station 32.
As a result, the second pseudorandom noise generator 57 supplies the
receiver multiplier 56 with the demodulation pseudorandom code sequence
PD. In the example being illustrated, the demodulation pseudorandom code
sequence PD is assumed to be coincident with the modulation pseudorandom
code sequence PN.
Under the circumstances, the receiver multiplier 56 carries out product
demodulation to demodulate the gain controlled signal GC into a
demodulated signal DM. The demodulated signal DM is filtered through a
low-pass filter 61 into a reproduction of the information signal IS. The
reproduction of the information signal IS may be called a reception signal
and is delivered through a demultiplexer 62 to a destination one of the
terminal units 33.sub.2 of the second station 32.
SECOND EMBODIMENT
Referring to FIG. 8, a station is for use as each station of a
communication system according to a second embodiment of this invention
and comprises similar parts designated by like reference numerals and
symbols. For brevity of description, the illustrated station is assumed to
be operable in cooperation with a single terminal unit 33 (FIG. 6). It is
to be noted that a preselected pseudorandom code sequence which has a
predetermined phase is used in common to the plurality of stations as
shown in FIG. 8. In other words, the preselected pseudorandom code
sequence is kept unchanged in phase. This means that a destination address
is given to each station in the form of a destination address signal AD
specifying a destination station assigned to a destination station. The
destination address signal AD is produced by the terminal unit 33 and is
followed by the information signal IS.
In addition, the transmitter 36 is operable in relation to the receiver 37
in a manner to be described later. It suffices to say that the transmitter
36 is operated only when the electric power line 30 is not used by other
stations than the illustrated station.
Let the information signal IS be transmitted from the first station 31 to
the second station 32, like in FIG. 7, and the illustrated station be at
first used as the first station 31. In this event, a transmission request
signal RQ of a logic "1" level is given prior to transmission of the
information signal IS from the terminal unit 33 of the first station 31 to
an AND gate 65 in a manner to be described later. The AND gate 65 is
supplied with the logic "1" level and the logic "0" level when the
electric power line 30 is being unused and used, respectively.
When the logic "1" level is given from the receiver 37 during presence of
the transmission request signal RQ, the AND gate 65 delivers a logic "1"
level signal to a modulator 47. The illustrated modulator 47 comprises a
timer 66 in addition to the multiplier 42 and the first PN generator 46.
The timer 66 is enabled or energized in response to the logic "1" level
signal sent from the AND gate 65 and times or measures a predetermined
duration T.sub.0. In this sense, the AND gate 65 and the logic "1" level
signal may be referred to as an energizing circuit and an energizing
signal, respectively.
After lapse of the predetermined duration T.sub.0, the timer 66 supplies
the first PN generator 46 and the terminal unit 33 with a start pulse ST
indicative of a start of operation. The first PN generator 46 delivers the
preselected pseudorandom code sequence denoted by PN' to the transmitter
multiplier 42 in synchronism with a sequence of transmitter clock pulses
CK.sub.1 which is given from the transmitter controller 43, although not
explicitly described in conjunction with FIG. 7.
Responsive to the start pulse ST, the terminal unit 33 supplies the
transmitter multiplier 42 with the destination address signal AD followed
by the information signal IS, as mentioned before. As a result, the
preselected pseudorandom code sequence PN' is modulated by the destination
address signal AD and by the information signal IS and sent as a modulated
signal MD' to the adder 48.
As illustrated in FIG. 8, a synchronization signal generator denoted by 68
is coupled to the adder 48 to generate the transmitter synchronization
signal SYNC.sub.1. The synchronization signal generator 68 is energized by
the logic "1" level signal, namely, energizing signal and begins to
produce the transmitter synchronization signal SYNC.sub.1. Consequently,
the transmitter synchronization signal SYNC.sub.1 precedes the modulated
signal MD' by the predetermined duration T.sub.0. This means that the
transmitter synchronization signal SYNC.sub.1 alone appears the
predetermined duration T.sub.0 and thereafter the modulated signal MD' is
superposed on the transmitter synchronization signal SYNC.sub.1. Anyway,
the sum signal between the transmitter synchronization signal SYNC.sub.1
and the modulated signal MD' is sent as a transmitter output signal from
the adder 48 through the transmitter amplifier 49 and the coupler 39 to
the electric power line 30.
Let the transmitter output signal mentioned above be received as a receiver
input signal by the second station 32. The illustrated receiver 37 is
assumed to be used in the second station 32 for brevity of description.
The receiver input signal is extracted from the electric power signal PW
by the coupler 39 to be sent through the receiver amplifier (not shown in
this figure) to the AGC circuit 52. The gain controlled signal GC is
delivered from the AGC circuit 52 to a receiver synchronization circuit
54'.
Temporarily referring to FIG. 9, the receiver synchronization circuit 54'
is similar to the receiver synchronization circuit 54 illustrated in FIG.
7 except that an additional multiplier 71 and an additional low-pass
filter 72 is used in the illustrated receiver synchronization circuit 54'.
The remaining part forms a delay lock loop known in the art.
More particularly, the receiver synchronization circuit 54' comprises a
local pseudorandom noise (PN) genrator 73 which is put into operation in
synchronism with a sequence of receiver clocks CK.sub.2 produced by a
voltage controlled oscillator (VCO) 75. The local PN generator 73
generates a first local PN code sequence LO.sub.1 identical with the
transmitter synchronization signal SYNC.sub.1 and a second local PN code
sequence L0.sub.2 delayed by two bits, namely, two clocks relative to the
first local PN code sequence LO.sub.1. The illustrated local PN generator
73 also generates a third local PN code sequence L0.sub.3 delayed by a
single bit relative to the first local PN code sequence LO.sub.1.
The first and the second local PN code sequences LO.sub.1 and L0.sub.2 are
delivered to first and second multipliers 76 and 77, respectively, while
the third local PN code sequence L0.sub.3 is delivered to the additional
multiplier 71. The first and the second multipliers 76 and 77 calculate
first and second products between the gain controlled signal GC and the
first local PN code sequence LO.sub.1 and between the gain controlled
signal GC and the second local PN code sequence LO.sub.2, respectively. In
other words, each of the first and the second multipliers 76 and 77
calculate correlations between the gain controlled signal GC and each of
the first and the second local PN code sequences LO.sub.1 and L0.sub.2.
A subtractor 79 subtracts the second product from the first product to
supply a loop filter 81 with a difference signal representative of a
difference between the first and the second products. The difference
signal is sent in the form of a variable voltage to the voltage controlled
oscillator 75. The voltage controlled oscillator 75 produces the receiver
clock pulses CK.sub.2 having a repetition frequency determined by the
voltage of the difference signal.
Synchronization is established when the difference between the first and
the second products becomes equal to zero, as known in the art. This
illustrated receiver synchronization circuit 54' is designed so that
synchronization is established within the predetermined duration T.sub.0.
Under the circumstances, the third local PN code sequence L0.sub.3 is phase
matched with the transmitter synchronization signal SYNC.sub.1 conveyed by
the gain controlled signal GC. In the example being illustrated, the third
local PN code sequence L0.sub.3 is delivered to a frame synchronization
circuit 83 for defining each frame of the gain controlled signal GC. For
this purpose, a sequence of frame pulses FR is produced by the frame
synchronization circuit 83 in response to the third local PN code sequence
L0.sub.3 and delivered to the second PN generator 57 (FIG. 8) together
with the receiver clock pulses CK.sub.2.
In the illustrated receiver synchronization circuit 54', the additional
multiplier 71 multiplies the third local PN code | | |
