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Claims  |
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What is claimed is:
1. A Code Shift Keying (CSK) communication system for transmitting and
receiving digital data each of which is a bit having a value of either "1"
or "0," the system comprising:
a transmitter including
means for processing each of its input bits,
means for continuously generating two M-series codes, a first M-series code
being identical to a second M-series code except in its phase,
means for selecting one of said two M-series codes depending on the value
of a particular bit being processed, and
means for outputting as a transmitted signal the selected code in a period
of time spanning the duration of the particular bit; and
a receiver including
means for recapturing the transmitted signal as a received signal,
means for obtaining two correlation signals by correlating the received
signal with each copy of the two M-series codes,
means for partitioning each correlation signal into a sequence of
consecutive periods,
means for comparing during each period the largest peak of one correlation
signal with the largest peak of the other correlation signal,
means for deciding during each period whether the received signal spanning
the period has value of "1" or "0" depending on the result of comparing
the sizes of the peaks, and
means for generating a sequence of bits each of which has value of either
"1" or "0."
2. A communication system as in claim 1, wherein said receiver comprises:
window generating means for generating periodic pulses each of whose center
is synchronized to each peak of a signal formed by performing an operation
on two correlation signals; and
peak detection means for detecting and storing location and size of said
each peak within a duration of time spanned by one of the periodic pulses
generated by said window generating means.
3. A Code Shift Keying (CSK) communication system transmitter for
processing and transmitting digital data each of which is a bit having a
value of either "1" or "0," the transmitter comprising:
M-series code generating means for continuously generating a first and a
second M-series codes in a period of time spanning the duration of a
particular bit, the first M-series code being identical to the second
M-series code in its sequence of 1's and 0's but different in its phase;
and
a switching circuit for selecting one of said two M-series codes depending
on the value of said particular bit, and for outputting, as transmitted
signal, the selected code in a period of time spanning the duration of the
particular bit.
4. A transmitter as in claim 3, wherein said M-series code generating means
comprises:
two identical M-series code generators, a first M-series code generator for
generating said first M-series code and a second M-series code generator
for generating said second M-series code; and
a phase setting circuit for causing the second M-series code generator to
generate said second M-series code with its phase different from the phase
of said first M-series code.
5. A transmitter as in claim 3, wherein said M-series code generating means
comprises:
an M-series code generator for generating said first M-series code; and
a delay element for generating said second M-series code by accepting as
its input said first M-series code, by delaying its inputted first
M-series code, and by outputting the delayed first M-series code as said
second M-series code.
6. A Code Shift Keying (CSK) communication system receiver for receiving a
signal and processing the received signal which is capable of including
series of a first and a second M-series codes which is identical to the
first code except in its phase, the receiver comprising:
a first correlator for generating a first correlation signal by correlating
a local replica of the first code and the received signal;
a second correlator for generating a second correlation signal by
correlating a local replica of the second code and the received signal;
and
a demodulator for periodically monitoring the first and the second
correlation signals, for comparing during each monitoring period the
largest peak of the first correlation signal with the largest peak of the
second correlation signal, for deciding during each monitoring period
whether the received signal spanning the monitoring period has value of
"1" or "0" depending on the result of comparing sizes of said largest
peaks, and for generating sequence of bits each of which has value of
either "1" or "0."
7. A receiver as in claim 6, wherein said demodulator comprises:
window generating means for generating periodic pulses each of whose center
is synchronized to a main peak of a signal formed by performing an
operation on two correlation signals during a period of time spanning one
of said bits; and
peak detection means for detecting said main peak within a duration of time
spanned by one of the periodic pulses.
8. A Code Shift Keying (CSK) communication system receiver which includes
two correlators for generating two correlation signals from a received
signal, a demodulator for generating sequence of data bits based on two
correlation signals and periodic timing pulses, a carrier detecting
circuit for providing signals to and accepting signals from a timing pulse
generator and accepting signals from two correlators, and the timing pulse
generator for generating the timing pulses partly based on signals within
or from the carrier detecting circuit, wherein the carrier detecting
circuit comprises:
a peak position detecting circuit including
operating means for outputting peaks of a signal formed by processing two
correlation signals,
means for detecting each position of said peaks, each peak within a single
period of time spanned by one bit, and
means for outputting a peak position detection signal;
a peak position memory buffer for storing detected peak positions over a
duration of time spanned by N data bits;
partitioning means for assigning, for each of N periods of time, M number
of subintervals, each subinterval within one period having corresponding
N-1 subintervals in other N-1 periods, all corresponding subintervals
forming a single subinterval group, and all subintervals forming M
subinterval groups;
first means for determining, for each duration of time spanning one bit of
data, which of M subintervals contain a correlation peak, whose position
is stored in said peak position memory buffer;
means for counting the total number of correlation peaks contained within
each subinterval group based on outputs from said first determining means,
and for outputting each of M counts resulting from the counting;
second means for determining if any of M counts is equal to or greater than
a prescribed number m, deciding that a carrier has been detected if one of
the counts is not less than m, and for outputting various signals
including a carrier detection signal thereafter.
9. A receiver as in claim 8, wherein said carrier detecting circuit further
comprises means for outputting non-detection signal if each of said M
counts is less than m.
10. A receiver as in claim 8, wherein said carrier detecting circuit
further comprises means for allowing those components within and dependent
on numbers m and N to switch their dependency from numbers m and N to
numbers m' and N'.
11. A receiver as in claim 8, wherein said operating means comprises:
means for adding two correlation signals;
means for evaluating and outputting the absolute value of added signals.
12. A receiver as in claim 8, wherein said operating means comprises:
means for evaluating the absolute value of each of two said correlation
signals;
means for outputting larger one of the two absolute values.
13. A receiver as in claim 8, wherein said partitioning means includes
means for preventing any one subinterval within one period from
overlapping any other subintervals within the same period.
14. A receiver as in claim 8, wherein said partitioning means includes
means for causing any one subinterval within one period to partially
overlap its adjacent subintervals within the same period.
15. A Code Shift Keying (CSK) communication system receiver which includes
two correlators for generating two correlation signals from a received
signal, a demodulator for generating a sequence of data bits based on two
correlation signals and periodic timing pulses, a carrier detecting
circuit for providing signals to and accepting signals from a timing pulse
generator and accepting signals from two correlators, and the timing pulse
generator for generating the timing pulses partly based on signals from
the carrier detecting circuit, wherein the carrier detecting circuit
comprises:
a peak position detecting circuit including
operating means for outputting peaks of a signal formed by processing two
correlation signals,
means for detecting each position of said peaks, each peak within a single
period of time spanned by one bit, and
means for outputting a peak position detection signal; a peak position
determining circuit including
partitioning means for assigning, for each of N periods of time, M number
of subintervals, each subinterval within one period having corresponding
N-1 subintervals in other N-1 periods, all corresponding subintervals
forming a single subinterval group, and all subintervals forming M
subinterval groups, and
means for determining, for each duration of time spanning one bit of data,
which of M subintervals contain a correlation peak based on said peak
position detection signal;
counting means for counting the total number of correlation peaks contained
within each subinterval group based on outputs from said peak position
determining circuit, and for outputting each of M counts resulting from
the counting;
an m/N determining circuit for determining if any of said M counts is equal
to or greater than a prescribed number m, for deciding that a carrier has
been detected if one of the counts is not less than m, and for outputting
various signals including a carrier detection signal thereafter.
16. A receiver as in claim 15, said timing pulse generator comprising:
extracting means for accepting as inputs signals from various lines
interconnecting internal components of said carrier detecting circuit and
for outputting quantities to a pulse generating means; and
pulse generating means for outputting timing pulses synchronized to said
peaks, each position of timing pulses depending on the values of outputs
from said extracting means and from carrier detecting circuit.
17. A receiver as in claim 15, said timing pulse generator comprising:
extracting means for accepting as inputs, signals from various lines
interconnecting internal components of said carrier detecting circuit and
for outputting quantities to an arithmetic operating means; and
arithmetic operating means for abstracting from its inputs and operating
on,
the number of subinterval groups each of which contains greater than or
equal to m peaks,
the number of subinterval groups,
a numeric label of each of subinterval groups,
the starting position of each subinterval group,
the end position of each subinterval group,
a numeric label of each subinterval group which does not contain m peaks,
the number of occurrences of peaks in each subinterval group,
the total number of occurrences of peaks in all subinterval groups, and
the sum of every peak amplitude in each subinterval group, so as to compute
and output a weighted mean peak position every N periods; and
means for outputting timing pulses synchronized to said peaks, each
position of timing pulses depending on the weighted mean peak position and
said outputs from carrier detecting circuit.
18. A receiver as in claim 15, wherein the arithmetic operating means
comprises means for computing the weighted mean peak position P.sub.0
according to the following equation,
##EQU5##
wherein r denotes the number of subinterval groups each of which contains
greater than or equal to m peaks,
j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u, and
v denotes a numeric label of each subinterval group which does not contain
m peaks.
19. A receiver as in claim 15, wherein the arithmetic operating means
comprises means for computing the weighted mean peak position P.sub.0
according to the following equation
##EQU6##
wherein j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u,
Vu denotes the number of occurrences of peaks in each subinterval group u,
and
V denotes the total number of occurrences of peaks in all subinterval
groups.
20. A receiver as in claim 15, wherein the arithmetic operating means
comprises means for computing the weighted mean peak position P.sub.0
according to the following equation,
##EQU7##
wherein r denotes the number of subinterval groups each of which contains
greater than or equal to m peaks,
j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u,
v denotes a numeric label of each subinterval group which does not contain
m peaks,
Vu denotes the number of occurrences of peaks in each subinterval group u,
V denotes the total number of occurrences of peaks in all subinterval
groups, and
Xu denotes the sum of every peak amplitude in each subinterval group u.
21. A receiver as in claim 15, wherein the arithmetic operating means
comprises means for computing the weighted mean peak position P.sub.0
according to the following equation,
##EQU8##
wherein r denotes the number of subinterval groups each of which contains
greater than or equal to m peaks,
j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u,
v denotse a numeric label of each subinterval group which does not contain
m peaks,
Vu denotes the number of occurrences of peaks in each subinterval group u,
V denotes the total number of occurrences of peaks in all subinterval
groups, and
Xu denotes the sum of every peak amplitude in each subinterval group u.
22. A Code Shift Keying (CSK) communication system receiver comprising:
two correlators for generating two correlation signals from a received
signal;
a carrier detecting means for receiving signals from said two correlators
and accepting signals from and providing signals to a synchronization
control circuit;
the synchronization control circuit including
first monitoring means for periodically monitoring two correlation signals,
means for generating timing pulses partly based on outputs from the first
monitoring means, and on outputs from said carrier detecting means; and a
demodulator including
first monitoring means for periodically monitoring two correlation signals,
means for generating a sequence of data bits based on outputs from the
first monitoring means and based on the periodic timing pulses.
23. A Code Shift Keying (CSK) communication method for transmitting and
receiving digital data each of which is a bit having a value of either "1"
or "0," the method comprising the steps of:
transmitting, comprising the steps of:
processing each of its input bits,
continuously generating two M-series codes, a first M-series code being
identical to a second M-series code except in its phase,
selecting one of said two M-series codes depending on the value of a
particular bit being processed, and
outputting as a transmitted signal the selected code in a period of time
spanning the duration of the particular bit; and receiving, comprising the
steps of:
recapturing the transmitted signal as a received signal,
obtaining two correlation signals by correlating the received signal with
each copy of the two M-series codes,
partitioning each correlation signal into a sequence of consecutive
periods,
comparing during each period the largest peak of one correlation signal
with the largest peak of the other correlation signal,
deciding during each period whether the received signal spanning the period
has value of "1" or "0" depending on the result of comparing the sizes of
the peaks, and
generating a sequence of bits each of which has value of either "1" or "0."
24. A communication method as in claim 23, wherein said receiving step
comprises the steps of:
generating periodic pulses each of whose center is synchronized to each
peak of a signal formed by performing an operation on two correlation
signals; and
detecting and storing the location and size of said each peak within a
duration of time spanned by one of said periodic pulses.
25. A Code Shift Keying (CSK) communication transmitting method for
processing and transmitting digital data each of which is a bit having a
value of either "1" or "0," the transmitting method comprising the steps
of:
continuously generating a first and a second M-series codes in a period of
time spanning the duration of a particular bit, the first M-series code
being identical to the second M-series code in its sequence of 1's and 0's
but different in its phase;
selecting one of said two M-series codes depending on the value of said
particular bit;
outputting, as transmitted signal, the selected code in a period of time
spanning the duration of the particular bit.
26. A transmitting method as in claim 25, wherein the step of generating
said M-series code comprises the steps of:
generating said first M-series code;
generating said second M-series code identical to the first code; and
setting the phase of the second M-series code so that the phase of the
second code is different from that of the first M-series code.
27. A transmitting method as in claim 25, wherein the step of generating
said M-series code comprises:
generating said first M-series code; and
generating said second M-series code by delaying the first M-series code;
and
producing the delayed first M-series code as said second M-series code.
28. A Code Shift Keying (CSK) communication receiving method for capturing
a signal and processing the captured signal which is capable of including
series of a first and a second M-series codes which is identical to the
first code except in its phase, the receiving method comprising the steps
of:
generating a first correlation signal by correlating a local of the first
code and the captured signal;
generating a second correlation signal by correlating a local replica of
the second code and the captured signal; and
demodulating, including the steps of:
periodically monitoring the first and the second correlation signals,
comparing during each monitoring period the largest peak of the first
correlation signal with the largest peak of the second correlation signal,
deciding during each monitoring period whether the captured signal spanning
the monitoring period has the value of "1" or "0" depending on the result
of comparing sizes of said largest peaks, and
generating sequence of bits each of which has value of either "1" or "0."
29. A receiving method as in claim 28, wherein said demodulating step
comprises the steps of:
generating periodic pulses each of whose center is synchronized to a main
peak of a signal formed by performing an operation on two correlation
signals during a period of time spanning one of said bits; and
detecting one of said largest peaks within a duration of time spanned by
one of the periodic pulses.
30. A Code Shift Keying (CSK) communication receiving method comprising
steps of generating two correlation signals from a received signal,
demodulating a sequence of data bits based on two correlation signals and
periodic timing pulses, producing synchronization signals based on timing
pulses and two correlation signals, and generating the timing pulses
partly based on synchronization signals, wherein the step of producing
synchronization signals comprises the steps of:
(1) peak position detecting, including the steps of:
processing two correlation signals,
generating peaks of a signal resulting from the processing,
detecting each position of said peaks, each peak within a single period of
time spanned by one bit, and outputting a peak position detection signal;
(2) storing detected peak positions over a duration of time spanned by N
data bits;
(3) assigning, for each of N periods of time, M number of subintervals,
each subinterval within one period having corresponding N-1 subintervals
in other N-1 periods, all corresponding subintervals forming a single
subinterval group, and all subintervals forming M subinterval groups;
(4) determining, for each duration of time spanning one period, which of M
subintervals contain one of stored detected peaks;
(5) counting, for each subinterval group, the total number of correlation
peaks contained within each subinterval group based on the intermediate
and final results of step (4);
(6) generating each of M counts resulting from the counting (5);
(7) determining if any of M counts is equal to or greater than a prescribed
number m, deciding that a carrier has been detected if one of the counts
is not less than m, and thereafter producing various synchronization
signals.
31. A receiving method as in claim 30, wherein said carrier detecting
further comprises allowing steps (2) through (7) which depends m and N to
switch their dependency from numbers m and N to numbers m' and N'.
32. A receiving method as in claim 30, wherein the processing of two
correlation signals in (1) comprises the steps of:
adding said two correlation signals;
evaluating and producing the absolute value of added signals.
33. A receiving method as in claim 30, wherein the processing of two
correlation signals in (1) comprises:
evaluating the absolute value of each of two said correlation signals;
producing larger one of the two absolute values.
34. A receiving method as in claim 30, wherein step (3) includes a step of
preventing any one subinterval within one period from overlapping any
other subintervals within the same period.
35. A receiving method as in claim 30, wherein step (3) includes a step of
causing any one subinterval within one period to partially overlap its
adjacent subintervals within the same period.
36. A Code Shift Keying (CSK) communication receiving method comprising
steps of generating two correlation signals from a received signal,
demodulating a sequence of data bits based on two correlation signals and
periodic timing pulses, producing synchronization signals based on timing
pulses and two correlation signals, and generating the timing pulses
partly based on synchronization signals, wherein the producing
synchronization signals comprises the steps of:
(1) peak position detecting including the steps of:
processing two correlation signals,
generating peaks of a signal resulting from the processing,
detecting each position of said peaks, each peak within a single period of
time spanned by one bit, and
outputting a peak position detection signal;
(2) peak position determining, comprising the steps of: assigning, for each
of N periods of time, M number of subintervals, each subinterval within
one period having corresponding N-1 subintervals in other N-1 periods, all
corresponding subintervals forming a single subinterval group, and all
subintervals forming M subinterval groups, and
determining, for each duration of time spanning one period, which of M
subintervals contain one of stored detected peaks;
(3) counting, for each subinterval group, the total number of correlation
peaks contained within each subinterval group based on the intermediate
and final results of step (2);
(4) generating each of M counts resulting from the counting in step (3);
and
(5) determining if any of M counts from step (4) is equal to or greater
than a prescribed number m, deciding that a carrier has been detected if
one of the counts is not less than m, and thereafter producing various
synchronization signals.
37. A receiving method as in claim 36, wherein the generating of timing
pulses comprises a step of synchronizing said timing pulses to said peaks
depending on a number of said synchronization signals.
38. A receiving method as in claim 36, wherein the generating of timing
pulses comprises:
(1) extracting information from said synchronization signals;
(2) abstracting from extracted signals
the number of subinterval groups each of which contains greater than or
equal to m peaks,
the number of subinterval groups,
a numeric label of each of subinterval groups,
the starting position of each subinterval group,
the end position of each subinterval group,
a numeric label of each subinterval group which does not contain m peaks,
the number of occurrences of peaks in each subinterval group,
the total number of occurrences of peaks in all subinterval groups, and
the sum of every peak amplitude in each subinterval group,
(3) computing, using the result of (2), a weighted mean peak position every
N periods; and
(4) generating timing pulses synchronized to said peaks, each position of
timing pulses depending on the weighted mean peak position and said
synchronization signals.
39. A receiving method as in claim 38, wherein computing the weighted mean
peak position P.sub.0 comprises carrying out steps of an algorithm
according to the following equation,
##EQU9##
wherein r denotes the number of subinterval groups each of which contains
greater than or equal to m peaks,
j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u, and
v denotse a numeric label of each subinterval group which does not contain
m peaks.
40. A receiving method as in claim 38, wherein computing the weighted mean
peak position P.sub.0 comprises carrying out steps of an algorithm
according to the following equation,
##EQU10##
wherein j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u,
Vu denotes the number of occurrences of peaks in each subinterval group u,
and
V denotes the total number of occurrences of peaks in all subinterval
groups.
41. A receiving method as in claim 38, wherein computing the weighted mean
peak position P.sub.0 comprises carrying out steps of an algorithm
according to the following equation,
##EQU11##
wherein r denotes the number of subinterval groups each of which contains
greater than or equal to m peaks,
j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u,
v denotse a numeric label of each subinterval group which does not contain
m peaks,
Vu denotes the number of occurrences of peaks in each subinterval group u,
V denotes the total number of occurrences of peaks in all subinterval
groups, and
Xu denotes the sum of every peak amplitude in each subinterval group u.
42. A receiving method as in claim 38, wherein computing the weighted mean
peak position P.sub.0 comprises carrying out steps of an algorithm
according to the following equation,
##EQU12##
wherein r denotes the number of subinterval groups each of which contains
greater than or equal to m peaks,
j denotes the number of subinterval groups,
u denotes a numeric label of each of subinterval groups,
LSu denotes the starting position of each subinterval group u,
LEu denotes the end position of each subinterval group u,
v denotse a numeric label of each subinterval group which does not contain
m peaks,
Vu denotes the number of occurrences of peaks in each subinterval group u,
V denotes the total number of occurrences of peaks in all subinterval
groups, and
Xu denotes the sum of every peak amplitude in each subinterval group u.
43. A Code Shift Keying (CSK) communication receiving method comprising the
steps of:
generating two correlation signals from a received signal;
processing two correlation signals and timing signals to produce
synchronization signals;
pulse generating, comprising the steps of:
(1) periodically monitoring two correlation signals,
(2) generating timing pulses partly based on the result of (1), and on
synchronization signals; and
demodulating, comprising the steps of:
(3) periodically monitoring two correlation signals independently from (1),
(4) generating a sequence of data bits based on the result of (3) and on
the periodic timing pulses. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to Code Shift Keying (CSK) Spread Spectrum (SS)
communication systems. Uses of SS communication systems include power line
communications, satellite communications, mobile communications, and
others.
2. Description of Related Art
A block diagram of one conventional SS communication system is shown in
FIG. 1(a). Timing of the signals associated with FIG. 1(a) is shown by
FIG. 1(b). The PN code T10 from a pseudo noise (PN) code generator 10 and
data stream: 20 are processed by the EX-OR gate 20. Its output signal T20
is amplified by amplifier 30, and then processed for transmission. After
the transmitted signal T30 is received, it is amplified by an amplifier
40, whose output is applied to a correlator 50. The signal T50 from the
correlator 50 is compared to a threshold value THl by a comparator 70,
which produces demodulated data T70.
In order to recover the transmitted bits, the PN code generated by
correlator 50 at the receiver must be synchronized to the PN code T10
modulated on the transmitted signal T30. However, if the transmitted
signal T30 experiences progressive deterioration, the output of the
correlator 50 will lose its synchronization (loss of lock).
In an effort to overcome deficiencies of conventional PN communication
systems, the present inventors proposed a Code Shift Keying communication
system, described as in "Highly Efficient Power Line SS Modem,"Symposium
on Spread Spectrum Technology and Its Applications, IEICE, Mar. 22, 1989.
FIG. 2 illustrates a general block diagram of such CSK spread spectrum
communication system consisting of a transmitter 200 and a receiver 270.
The transmitter includes a modulator 280, which in turn comprises the
following elements.
1) Two PN code generators 210 and 220 for producing two pseudo-noise (PN)
codes M00 and M01.
2) Selector 230 for choosing one of two codes M00 and M01, depending on its
input data i230. If the value of an incoming bit is "1," the circuit 230
will select code M00; otherwise, the circuit 230 will select M01.
The output from the modulator 280 is further processed and transmitted at
the signal transmitting interface 240. The transmitted signal T240, is
then later recaptured at a signal receiving interface 250, where PN
modulated signal T250 is recovered from the received signal T240. The
recovered signal T250 is input to the demodulator/correlator 260, where
T250 is correlated with local copies of the PN codes and demodulated to
recover the transmitted bits i230.
A conventional PN communication system is likely to lose lock in cases
where the communication "channel" (transmission path) introduces
significant amount of signal degradation. Implementations of the CSK
system as originally proposed by the inventors overcomes the difficulty
suffered by the conventional spread spectrum systems. However, the
previously proposed implementation of the CSK system is still not perfect.
The signal receiving interface 250 in the CSK system above transfers its
output to a pair of correlators (not shown) One of two correlators
multiplies the incoming signal by a local copy of M00. The other
correlator, by M01. For each received bit, one of two correlation signals
at the output of correlators will have an auto-correlation peak, and the
other will contain only cross-correlation peaks Because signal
demodulation depends on the detection of auto-correlation peaks, large
cross-correlation peaks may cause undesired errors. The system demodulator
may confuse an excessively large cross-correlation peak with an actual
auto-correlation peak. Low cross-correlation values at the outputs of
correlators can be ensured by using two PN codes M00 and M01 that have low
cross-correlation values. However, the number of existing pair of codes
which have low cross-correlation values decreases with decreasing length
of codes. For example, for codes of length 7, there exists only one
M-series code. Therefore, correlators in which short codes are used are
likely to exhibit high cross-correlation peaks.
In the system above, in order to demodulate data and to produce timing
signals (indicating the start and the end of each data bit), accurate
monitoring, or windowing, of correlation signals is desired. Because
demodulation depends on comparing relative signal peak sizes of two
correlation signals, and because a short monitoring window yields a better
contrast between two monitored correlation peaks, the length of the
monitoring window for demodulation needs to be relatively short. On the
other hand, the monitoring window for a timing signal generator, or a
synchronization control circuit, needs to be long in order to provide
stability. The provision of a long window enables the synchronization
control circuit to "average" out temporal effects of noise. Therefore, for
a CSK system with a single window monitoring scheme, optimum operation of
the demodulator will introduce instability to the synchronization control
circuit.
If a propagation path adds interferences and noise to the transmitted
signal, amplitudes of the received signal will fluctuate. Signals that are
synchronized to auto-correlation peaks are sensitive to the fluctuations.
The carrier detection circuit, which in turn depends on synchronization
condition of such signals, may generate undesirable outputs
Finally, in order to synchronize its monitoring windows to auto-correlation
peaks, a synchronization control circuit needs to: 1) center the placement
of the monitoring window about auto-correlation peaks; and 2) maintain the
current position of monitoring window about auto-correlation peaks once
the monitoring windows are centered. The former of the two operations is
related to synchronization, and the latter, to maintaining the
synchronization, otherwise called tracking.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve upon various features
of the CSK transmitter and the receiver as shown in FIG. 2.
A further object of the present invention is to allow a single code to be
used as well as two PN codes to modulate data bits. If a single code is
used, when a "0" needs to be transmitted, generated code M00 will be
modulated onto the data. However, if "1" needs to be transmitted, a
phase-shifted version of the code will be modulated.
It is another object of the present invention to provide a mechanism by
which the demodulator and the synchronization control circuit can both
monitor correlation signals independently of each other.
It is yet another object of the present invention to provide a carrier
detection circuit in which exact synchronization between the
auto-correlation peaks and the monitoring window is unnecessary. One such
carrier detection circuit according to the invention operates as follows.
1) It divides a period of time spanning one bit into several subintervals.
2) It counts the number of arrivals of auto-correlation peaks (output by a
correlator) in each subinterval.
3) It declares that a "carrier" is detected if there are at least m
arrivals in any one of the subintervals.
It is a further object of the invention to provide an improved
synchronization circuit, placed within a synchronization control circuit,
which generates timing pulses for other CSK components. Several algorithms
are provided for positioning the timing pulses relative to
auto-correlation peaks.
It is yet another object of the present invention to provide various
embodiments of tracking circuits. Once synchronization has been achieved,
the tracking circuit enables consistent production of the timing signals
which accurately reflect the start and the end of each data bit.
The present invention provides novel arrangements for various components of
CSK communication system: modulator, PN correlators, demodulator, carrier
detection circuit, and synchronization control circuit.
The modulator according to one embodiment of the present invention
generates two M-series codes, in which the second code is simply a phase
shifted version of the first code. Depending on the value ("1" or "0") of
each bit to be transmitted, the modulator selects one of the codes, and
sends out the selected M series code.
The PN correlators at the receiver outputs two correlation signals after
multiplying its input signal by local copies of the two M-series codes.
The demodulator accepts two correlation signals from the correlators. The
demodulator then detects auto-correlation peaks in the correlated signals
and uses them in determining whether the received bit has the value "1" or
"0". If one correlation signal has an auto-correlation peak at a
particular instance, the other has the cross-correlation peak as low as
1/(spread ratio). Clean detection of an actual auto-correlation peak is
possible, and the detection allows the correct demodulation of the
received signal.
When two identical phase-shifted pair of codes are used, inter-correlation
peaks may become as large as the auto-correlation at certain points within
correlation signals. However, by positioning the monitoring window (by
using synchronization signal, or timing pulses, output from
synchronization control circuit) of the demodulator such that large
inter-correlation peaks are excluded from the window, it is possible to
prevent the inter-correlation peaks from causing errors in demodulation.
The carrier detecting circuit according to one embodiment of the present
invention accomplishes the following:
It determines the position of "peaks" (to be defined later).
It divides a given data time segment into subintervals, and then determines
to which subinterval the peak belongs.
It counts the number of occurrences of peaks within each subinterval.
If an auto-correlation peak has been detected within a subinterval for more
than a predetermined number of times within a durations of N data bits, it
outputs a carrier detection signal, indicating the presence of incoming
data stream within the received signal.
One particular embodiment, used without a synchronization control circuit,
allows demodulation of data without exact synchronization of the center of
the demodulator's monitoring window to auto-correlation peaks.
The synchronization control circuit mainly comprises the circuit for
generating timing signals that mark the start and the end of each data
bit. If the output from the carrier detecting circuit indicates that a
synchronization has been established, timing pulses are produced so that
auto-correlation peaks will be located exactly half-way between two
consecutive timing pulses. Each timing pulse marks the start/end of each
data bit in the received signal.
Generation of timing pulses (otherwise called data section end signal)
depends on past locations of auto-correlation peaks. Consecutive locations
of auto-correlation peaks (during the time when data is present) may be
stored in a memory. If auto-correlation peaks consistently appear in one
of the subintervals during monitoring of N consecutive data bits, the
subsequent timing pulses are delayed in accordance with a weighted average
of the stored locations of auto-correlation peaks.
Finally, a monitoring window for demodulation and that for synchronization
tracking may be set independently from each other. This allows the
demodulator to clearly distinguish between an actual auto-correlation
peaks and an inter-correlation peak, | | |
