Note: Descriptions are shown in the official language in which they were submitted.
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SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD
BACXGROUND OF THE INVENTION
This invention relates to spread-spectrum
communications and more particularly to a system and method
for synchronously demodulating a spread-spectrum-
communications signal using a reference carrier signal
supplied on a spread-spectrum channel by the transmitter.
DESCRIPTION OF THE PRIOR ART
Referring to FIG. 1, message data, d(t), are processed
by spread-spectrum modulator 51, using a message chip code
signal, g1(t), to generate a spread-spectrum data signal.
The spread-spectrum data signal is processed by transmitter
52 using a carrier signal at a carrier frequency, fo, and
transmitted over communications channel 53.
At a receiver, a spread-spectrum demodulator 54
despreads the received spread-spectrum signal, and the
message data are recovered by synchronous data demodulator
60 as received data. The synchronous data demodulator 60
uses a reference signal for synchronously demodulating the
despread spread-spectrum signal. The square-law device 55,
bandpass filter 56 and frequency divider 57 are well known
in the art for generating a reference signal from a received
modulated data signal.
In a fading channel, such as the ionosphere or any
channel containing multipath, or more generally, any channel
in which the received signal's amplitude fluctuates with
time, synchronous demodulation is not practical since the
phase of the incoming signal typically is not the same as
the phase of the reference. In such cases differential
phase shift keying (DPSK) is employed. With DPSK the
received signal is delayed by one symbol and multiplied by
the undelayed signal. If the resulting phase is less than
90 a o-bit is declared, otherwise a 1-bit is declared.
Such a system is complex and suffer,s degradation of about
6 dB at error rates of 10-2.
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OBJECTS OF ASPECTS OF THE INVENTION
An object of an aspect of the invention is a system
and method for synchronously demodulating a modulated-
data signal embedded in a spread-spectrum-communications
signal, which performs well whether or not the signal is
fading.
Another object of an aspect of the invention is to
send a carrier signal on a separate spread-spectrum
channel for a data link for demodulating a modulated-data
signal embedded in a spread-spectrum-communications
signal for use in a fading channel.
An additional object of an aspect of the invention
is a synchronous spread-spectrum-communications system.
SUMMARY OF THE INVENTION
According to the present invention, as embodied and
broadly described herein, a spread spectrum
communications system for use over a communications
channel is provided comprising generic means, message
means, spreading means, summer means, transmitting means,
generic-spread-spectrum-processing means, message-spread-
spectrum-processing means, acquisition and tracking
means, detection means and synchronous means. The generic
means generates a generic-chip-code signal and the
message means generates a message-chip-code signal. The
spreading means generates a spread-spectrum signal by
spread-spectrum processing message data with the message-
chip-code signal. The message data and the message-chip-
code signal preferably have timing synchronized to the
generic-chip-code signal. The summer means combines the
generic-chip-code signal with the spread-spectrum-
processed signal. The transmitting means transmits the
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combined generic-chip-code signal and spread-spectrum-
processed signal, over the communications channel as a
spread-spectrum-communications signal.
At a receiver, the generic-spread-spectrum-
processing means recovers the carrier signal from the
spread-spectrum-communications signal, and the message-
spread-spectrum-processing means despreads the spread-
spectrum-
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communications signal as a modulated-data signal. The
acquisition and tracking means tracks the recovered-carrier
signal for synchronizing the generic-spread-spectrum-
processing means to the recovered-carrier signal. The
message-spread-spectrum-processing means derives
synchronization from a replica of the generic-chip-code
signal provided by the generic-spread-spectrum-processing
means. The detection means may be nonsynchronous or
synchronous, for converting the modulated-data signal to a
detected signal. The synchronous means uses the replica of
the generic-chip-code signal produced by the generic-spread-
spectrum-processing means for synchronizing the "integrating
and dumping" of the detected signal to received data.
The present invention may be extended to handle a
plurality of message data. Accordingly, the present
invention further includes a plurality of message means and
a plurality of spreading means. The plurality of message
means generates a plurality of message chip-code signals.
The plurality of message data and the plurality of message-
chip-code signals all have synchronous timing derived from
the generic-chip-code signal. The plurality of spreading
means generates a plurality of spread-spectrum signals by
spread-spectrum processing the plurality of message data
with the plurality of message-chip-code signals,
respectively.
The present invention also includes simultaneously
receiving a spread-spectrum-communications signal having a
plurality of spread-spectrum-processed signals. In this
case, the receiver further includes a plurality of message-
spread-spectrum-processing means, a plurality of detection
means and a plurality of synchronous means. The acquisition
and tracking means tracks the recovered-carrier signal for
synchronizing the generic-spread-spectrum-processing means
to the recovered-carrier signal. The plurality of inessage-
.5 spread-spectrum-processing means derive their
synchronization from the replica of the generic-chip-code
signal, from the generic-spread-spectrum-processing means.
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The plurality of message-spread-spectrum-processing means
despreads the spread-spectrum-communications signal as a
plurality of modulated-data signals, respectively. The
plurality of detection means may be synchronous or
nonsynchronous, for converting the plurality of modulated-
data signals to a plurality of detected signals. The
plurality of synchronous means uses the replica of the
generic-chip-code signal produced by the generic-spread-
spectrum-processing means for synchronizing the detection of
the plurality of detected signals to a plurality of received
data.
The present invention also includes a method for
synchronously modulating and demodulating spread spectrum
communications. The method comprises the steps of
generating a generic-chip-code signal and a message-chip-
code signal. The message data are modulo-2 added to the
message-chip-code signal to generate a spread-spectrum-
processed signal. The generic-chip-code signal and the
spread-spectrum-processed signal are combined and
transmitted on a carrier signal over the communications
channel as a spread-spectrum-communications signal.
At the receiver, the steps include recovering the
carrier signal from the spread-spectrum-communications
signal and despreading the spread-spectrum-communications
signal as a modulated-data signal. The recovered-carrier
signal is used to synchronize the step of generating a
replica of the generic-chip-code signal. More particularly,
a replica of the generic-chip-code signal is correlated with
the spread-spectrum-communications signal, which has a
generic channel def_ 3 by the generic-chip-code signal at
the transmitter. If e signal out of the generic-bandpass
filter is small, then the acquisition and tracking circuit
delays the phase of the generic-chip-code signal and the
correlation process is repeated. If the phase of the
replica of the generic-chip-code signal and the generic-
chip-code signal in the spread-spectrum-communications
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signal are the same, then the output of the generic-bandpass
filter will be at a high voltage level.
A replica of the message-chip-code signal is
synchronized to the replica of the generic-chip-code signal
for despreading the spread-spectrum-communications signal as
a modulated-data signal. The modulated-data signal is
detected as a detected signal. The recovered-carrier signal
optionally may be used to synchronously demodulate the
modulated-data signal as the detected signal. The detected
signal is synchronously converted to received data, by using
timing from the replica of the generic-chip-code signal to
control "integrating and dumping" functions of a lowpass
filter and electronic switch.
The present invention may include further the steps of
generating a plurality of message-chip-code signals. A
plurality of message data is modulo-2 added to the plurality
of message-chip-code signals, to generate a plurality of
spread-spectrum-processed signals, respectively. The
generic-chip-code signal and the plurality of spread-
spectrum-processed signals are combined and transmitted on a
carrier signal over the communications channel as the
spread-spectrum-communications signal. The plurality of
message data and the plurality of message-chip-code signals
preferably have synchronous timing to the generic-chip-code
signal.
When the spread-spectrum-communications signal includes
a plurality of spread-spectrum-processed signals, the
present invention may include further the steps of
despreading the spread-spectrum-communications signal as a
plurality of modulated-data signals. The recovered-carrier
signal is used to synchronize the step of generating a
replica of the generic-chip-code signal. A replica of the
plurality of message-chip-code signals is synchronized to
the replica of the generic-chip-code signal, for despreading
the spread-spectrum-communications signal as a plurality of
modulated-data signals. The plurality of modulated-data
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signals is detected as a plurality of received signals,
which is converted to a plurality of data.
Additional objects and advantages of aspects of the
invention are set forth in part in the description which
follows, and in part are obvious from the description, or
may be learned. by practice of the invention. The objects
and advantages of aspects of the invention also may be
realized and attained by means of the instrumentalities
and combinations particularly pointed out in the appended
claims.
In accordance with one embodiment of the present
invention, there is provided a method for recovering
message data from a spread spectrum message signal in a
spread spectrum communication system, the method
comprising:
transmitting a spread spectrum message signal and a
spread spectrum generic signal on a carrier signal, each
of the spread spectrum message and generic signals having
a chip code signal;
receiving a signal including the spread spectrum
message and generic signals;
recovering the carrier signal from the received
signal using the a replica of generic signal chip code;
and
recovering message data from the received signal
using the recovered carrier and the message signal chip
code.
In accordance with another embodiment of the present
invention, there is provided a method for recovering
message data from a spread spectrum message signal in a
spread spectrum communication system, the method
comprising:
transmitting a spread spectrum message signal and a
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spread spectrum generic signal, each having a chip code
signal;
receiving a signal including the spread spectrum
message and generic signals;
demodulating the received signal to an intermediate
frequency (IF) signal;
recovering an IF carrier from the IF signal using a
replica of the generic chip code; and
recovering message data from the received signal
using the recovered IF carrier and the message signal
chip code.
In accordance with another embodiment of the present
invention, there is provide a system comprising:
means for transmitting a spread spectrum message
signal and a spread spectrum generic signal on a carrier
signal, each of the spread spectrum message and generic
signals having a chip code signal;
means for receiving a signal including the spread
spectrum message and generic signals;
means for recovering the carrier signal from the
received signal using a replica of the generic signal
chip code; and
means for recovering message data from the received
signal using the recovered carrier and the message signal
chip code.
In accordance with another embodiment of the present
invention there is provided a system comprising:
means for transmitting a spread spectrum message
signal and a spread spectrum generic signal, each having
a chip code signal;
means for receiving a signal including the spread
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spectrum message and generic signals;
means for demodulating the received signal to an
intermediate frequency (IF) signal;
means for recovering an IF carrier from the IF
signal using a replica of the generic chip code; and
means for recovering message data from the received
signal using the recovered IF carrier and the message
signal chip code.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in.
and constitute a part of the specification, illustrate
preferred embodiments of the invention, and together with.
the description serve to explain the principles of the
invention.
FIG. 1 is a prior art scheme for synchronously
recovering message data;
FIG. 2 shows a synchronous spread-spectrum system
with a bit synchronizer, synchronized to a generic chip
code generator according to the present invention;
FIG. 3A shows a synchronous spread spectrum
transmitter system for a plurality of message data;
FIG. 3B shows a spread spectrum receiver using a
synchronous detector for receiving a plurality of spread-
spectrum processed signals;
FIG. 3C shows a spread spectrum receiver using a
nonsynchronous detector for receiving a plurality of
spread-spectrum processed signals; and
FIG. 4 shows a synchronous spread-spectrum
demodulating method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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Reference will now be made in detail to the present
preferred embodiments of the invention, examples of which
are illustrated in the accompanying drawings, wherein
like
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reference numerals indicate like elements throughout the
several views.
As illustratively shown in FIG. 2, a spread spectrum
communications system for use over a communications channel
110 is provided comprising generic means, message means,
spreading means, summer means, transmitting means, generic-
spread-spectrum-processing means, message-spread-spectrum-
processing means, acquisition and tracking means, detection
means and synchronous means. The generic means and message
means are embodied as a transmitter-generic-chip-code
generator 101 and transmitter-message-chip-code generator
102. The spreading means is shown as an EXCLUSIVE-OR device
103, which may be an EXCLUSIVE-OR gate. Summer means is a
combiner 105 and the transmitting means includes a
transmitter which is embodied as a signal source 108 coupled
to modulator 107. The transmitter-message-chip-code
generator 102 is coupled to the EXCLUSIVE-OR device 103.
The transmitter-generic-chip-code generator 101 is shown
coupled to the transmitter-message-chip-code generator 102
and the source for message data. The EXCLUSIVE-OR device
103 and the transmitter-generic-chip-code generator 101 are
coupled to the combiner 105. The modulator 107 is coupled
between the combiner 105 and the communications channel 110.
At the receiver the generic-spread-spectrum-processing
means is embodied as the receiver-generic-chip-code
generator 121, the generic mixer 123 and the generic-
bandpass filter 125. The generic mixer 123 is coupled
between the receiver-generic-chip-code generator 121 and the
generic-bandpass filter 125. The message-spread-spectrum-
processing means is embodied as a receiver-message-chip-code
generator 122, a message mixer 124 and a message-bandpass
filter 126. The message mixer 124 is coupled between the
receiver-message-chip-code generator 122 and the message-
bandpass filter 126. A power splitter 115 is coupled
between the communications channel 110, and the generic
mixer 123 and the message mixer 124.
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The acquisition and tracking means is embodied as an
acquisition and tracking circuit 131. The acquisition and
tracking circuit 131 is coupled to an output of the generic-
bandpass filter 125, and to the receiver-qeneric-chip-code
generator 121. The receiver-message=chip-code generator 122
preferably is coupled to the receiver-qeneric-chip-code
generator 121.
The detection means is embodied as a detector 139. The
detector 139 is coupled to the message-bandpass filter 126
and the generic-bandpass filter 125. The detector 139 may
be a nonsynchronous detector such as an envelope detector or
square-law detector. Alternatively, the detector 139 may be
a synchronous detector, which uses a recovered-carrier
signal from the generic-bandpass filter 125.
The synchronous means includes bit means, a lowpass
filter 128 and electronic switch 130. The bit means is
embodied as a bit synchronizer 129. The lowpass filter 128
and electronic switch 130 are coupled to the bit
synchronizer 129. The bit synchronizer 129, as shown in
FIG. 2, preferably is coupled to the receiver-generic-chip-
code generator 121. Alternatively, the bit synchronizer 129
may be coupled to an output of the detector 139.
The transmitter-generic-chip-code generator 101
generates a generic-chip-code signal, g0(t), and the
transmitter-message-chip-code generator 102 generates a
message-chip-code signal, g1(t). Synchronous timing of the
message data, d1(t), and the message-chip-code signal, in
FIG. 2, is provided by the generic-chip-code signal,
although other sources can be used such as a common clock
signal for synchronization. The EXCLUSIVE-OR device 103
generates a spread-spectrum signal by spread-spectrum
processing message data with the message-chip-code signal.
The spread-spectrum processing may be accomplished by
modulo-2 adding the message data to the message-chip-code
:5 signal. The combiner 105 combines the generic-chip-code
signal with the spread-spectrum-processed signal. The
combined generic-chip-code signal and spread-spectrum-
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processed signal may be a multilevel signal, having the
instantaneous voltage levels of the generic-chip-code signal
and the spread-spectrum-processed signal.
The modulator 107, as part of the transmitter,
modulates the combined generic-chip-code signal and spread-
spectrum-processed signal by a carrier signal, cos wot, at a
carrier frequency, fo. The modulated generic-chip-code
signal and spread-spectrum processed signal are transmitted
over the communications channel 110 as a spread-spectrum-
communications signal, xc(t). Thus, the spread-spectrum-
communications signal includes the generic-chip-code signal
and the spread-spectrum-processed signal as if they were
each modulated separately, and synchronously, on separate
carrier signals having the same carrier frequency, fo, and
transmitted over the communications channel.
At a receiver, the generic-spread-spectrum-processing
means recovers the carrier signal, cos wot, from the spread-
spectrum-communications signal, xc(t), and the message-
spread-spectrum-processing means despreads the spread-
spectrum-communications signal, xc(t), as a modulated-data
signal, d1(t). More particularly, referring to FIG. 2, the
spread-spectrum-communications signal received from the
communications channel 110, is divided by power splitter
115. The receiver-generic-chip-code generator 121 generates
a replica of the generic-chip-code signal, g0(t). The
generic mixer 123 uses the replica of the generic-chip-code
signal for despreading the spread-spectrum-communications
signal, xc(t), from the power splitter 115, as a recovered-
carrier signal. The spread-spectrum channel, of the spread-
spectrum-communications signal having the generic-chip-code
signal, g0(t) cos wot, generally does not include data so
that despreading the spread-spectrum-communications signal
produces the carrier signal, only. The generic-bandpass
filter 125 filters the recovered-carrier signal at the
carrier frequency, or equivalently, at an IF. In comparison
to the message-bandpass filter 126 which has a bandwidth
sufficiently wide for filtering a modulated-data signal, the
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generic-bandpass filter 125 can have a very narrow bandwidth
for filtering the recovered-carrier signal. The very narrow
bandwidth of the generic-bandpass filter 125 assists in
extracting the recovered-carrier signal from noise.
5, The acquisition and tracking circuit 131 acquires and
tracks the recovered-carrier signal from an output of the
generic-bandpass filter 125. The replica of the generic-
chip-code signal from the receiver-generic-chip-code
generator 121 is synchronized to the recovered-carrier
signal via acquisition and tracking circuit 131.
The receiver-message-chip-code generator 122 generates
a replica of the message-chip-code signal, gl(t). The
replica of the message-chip-code signal, gl(t), is
synchronized to the replica of the generic-chip-code signal,
g0(t), from the receiver-generic-chip-code generator 121.
Thus, the receiver-message-chip-code generator 122, via
synchronization to the receiver-generic-chip-code generator
121, has the same synchronization as the transmitter-
message-chip-code generator 102 via synchronization to the
transmitter-generic-chip-code generator 101. Accordingly,
the spread-spectrum communications channel having the
generic-chip-code signal provides coherent spread-spectrum
demodulation of the spread-spectrum channel with data.
The message mixer 124 uses the replica of the message-
chip-code signal for despreading the spread-spectrum-
communications signal from the power splitter 115, to
generate a modulated-data signal, dl(t) cos wot. The
modulated-data signal effectively is the message data
modulated by the carrier signal. The message-bandpass
filter 126 filters the modulated-data signal at the carrier
frequency, or equivalently at an intermediate frequency
(IF). Down converters, which convert the modulated-data
signal to an IF, optionally may be used without altering the
cooperative functions or teachings of the present invention.
The detector 139 demodulates the modulated-data signal
as a detected signal. The detected signal is filtered
through lowpass filter 128, sampled by electronic switch 130
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and outputted as received data, d1(t). The received data,
without errors, are identical to the message data. The
lowpass filter 128 and electronic switch 130 operate in an
"integrate and dump" function, respectively, under the
control of the bit synchronizer 129.
The bit synchronizer 129 controls the integrating and
dumping of lowpass filter 128 and electronic switch 130.
The bit synchronizer 129 preferably derives synchronization
using the replica of the generic-chip-code signal from the
receiver-generic-chip-code generator 121 as illustrated in
FIG. 2. The bit synchronizer 129 also may derive
synchronization from an output of the detector 139, as
illustrated in FIG. 1.
In a preferred embodiment, the bit synchronizer 129
receives the replica of the generic-chip-code signal, go(t),
from the receiver-generic-chip-code generator 121. The
replica of the generic-chip-code signal, by way of example,
may include a chip-code word having 8250 chips. Assuming
that there are eleven bits per chip-code word, then there
are 750 chips per bit of data. Since the replica of the
generic-chip-code signal provides information to the bit
synchronizer 129 as to where the chip-code word begins, the
bit synchronizer 129 thereby knows the timing of the
corresponding bits for synchronization.
The present invention further may include transmitting
as the spread-spectrum-communications signal, a plurality of
spread-spectrum-processed signals for handling a plurality
of message data. In this case the invention includes a
plurality of message means and a plurality of spreading
means. Referring to FIG. 3A, the plurality of message means
may be embodied as a plurality of transmitter-message-chip-
code generators and the plurality of spreading means may be
embodied as a plurality of EXCLUSIVE-OR gates. The
plurality of transmitter-message-chip-code generators
generates a plurality of message-chip-code signals. In
FIG. 3A, the plurality of transmitter-message-chip-code
generators is shown as first transmitter-message-chip-code
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generator 102 generating first message-chip-code signal,
g1(t), second transmitter-message-chip-code generator 172
generating second message-chip-code signal, q2(t), through
Nth transmitter-message-chip-code generator 182 generating
Nth message-chip-code signal, gN(t)-. The plurality of
EXCLUSIVE-OR gates is shown as first EXCLUSIVE-OR gate 103,
second EXCLUSIVE-OR gate 173, through Nth EXCLUSIVE-OR gate
183. The plurality of EXCLUSIVE-OR gates generates a
plurality of spread-spectrum-processed signals by modulo-2
adding the plurality of message data d1(t),
d2(t), ..., dN(t) with the plurality of message-chip-code
signals g1(t), g2(t), . . . , gN(t.), respectively. More
particularly, the first message data, d1(t), are modulo-2
added with the first message-chip-code signal, g1(t), the
second message data, d2(t), are modulo-2 added with the
second message-chip-code signal, q2(t), through the Nth
message data, dN(t), which are modulo-2 added with the Nth
message-chip-code signal, gN(t).
The transmitter-generic-chip-code generator 101 is
coupled to the plurality of transmitter-message-chip-code
generators and the source for the plurality of message data,
d1(t), d2(t), ..., dN(t). The generic-chip-code signal
g0(t), in a preferred embodiment, provides synchronous
timing for the plurality of message-chip-code signals g1(t),
g2(t), ..., gN(t), and the plurality of message data
d1(t), d2(t), . . . , dN(t).
The combiner 105 combines the generic-chip-code signal
and the plurality of spread-spectrum-processed signals, by
linearly adding the generic-chip-code signal with he
plurality of spread-spectrum-processed signals. The
combined signal typically is a multilevel signal, which has
the instantaneous voltage levels of the generic-chip-code
signal and the plurality of spread-spectrum-processed
signals.
The modulator 107, as part of the transmitter,
modulates the combined generic-chip-code signal and the
plurality of spread-spectrum-processed signals by a carrier
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signal, cos wot, at a carrier frequency, f 0. The modulated
generic-chip-code signal and the plurality of spread-
spectrum processed signals are transmitted over the
communications channel 110 as a spread-spectrum-
communications signal, xc(t). The spread-spectrum-
communications signal, xc(t) has the form:
N
xc(t) _{g0(t) + Fi1[gi(t) Q di(t)7} cos wot
Thus, the spread-spectrum-communications signal includes the
generic-chip-code signal and the plurality of spread-
spectrum-processed signals as if they were each modulated
separately, and synchronously, on separate carrier signals
with the same carrier frequency, fo, and transmitted over
the communications channel.
The present invention includes receiving a spread-
spectrum-communications signal which has a plurality of
spread-spectrum-processed signals. The receiver further
includes a plurality of message-spread-spectrum processing
means, a plurality of detection means and a plurality of
synchronous means. The plurality message-spread-spectrum-
processing means, as shown in FIG. 3B, may be embodied as a
plurality of message-chip-code generators, a plurality of
message mixers and a plurality of message-bandpass filters.
A mixer is connected between a respective message-chip-code
generator and message-bandpass filter. The plurality of
message mixers is coupled to the power splitter 115. More
particularly, the plurality of message-chip-code generators
is shown embodied as first message-chip-code generator 122,
second message-chip-code generator 172, through Nth message-
chip-code generator 182. The plurality of message mixers is
shown as first message mixer 124, second message mixer 174
through Nth message mixer 184. The plurality of message-
bandpass filters is shown as first message-bandpass filter
126, second message-bandpass filter 176, through Nth
message-bandpass filter 186.
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The plurality of detection means may be embodied as a
plurality of synchronous detectors which is shown as first
synchronous detector 127, second synchronous detector 177
through Nth synchronous detector 187. Each of the plurality
of synchronous detectors are coupled-to one of the plurality
message-bandpass filters.
The plurality of synchronous means may include a bit
synchronizer 129, a plurality of lowpass filters and a
plurality of electronic switches. The plurality of lowpass
filters is shown as first lowpass filter 128, second lowpass
filter 178, through Nth lowpass filter 188. The plurality
of electronic switches is shown as first electronic switch
130, second electronic switch 180 through Nth electronic
switch 190. Each of the plurality of synchronous detectors
is coupled to an output of the generic-bandpass filter 125.
The recovered-carrier signal from the generic-bandpass
filter 125 serves as the reference signal for synchronously
demodulating each of the plurality of message-data signals
by the plurality of synchronous detectors, as a plurality of
received data, dl(t), d2(t), . . . , dN(t).
The detection means alternatively may be embodied as a
plurality of nonsynchronous detectors, such as envelope
detectors 139, 189, 199, as shown in FIG. 3C. Typically,
the nonsynchronous detectors do not require the recovered-
carrier signal.
The bit synchronizer 129 derives timing from the
replica of the generic-chip-code signal, g0(t), and controls
the timing of the integrating and dumping functions of the
plurality lowpass filters and the plurality of electronic
switches.
With the use of the invention as embodied in FIG. 3B, a
generic-spread-spectrum channel, as part of the spread-
spectrum-communications signal, provides the recovered-
carrier signal, as discussed previously. The acquisition
and tracking circuit 131 acquires and tracks the recovered-
carrier signal from an output of the generic-bandpass filter
125. The replica of the generic-chip-code signal from the
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receiver-generic-chip-code generator 121 is synchronized to
the recovered-carrier signal via acquisition and tracking
circuit 131. The receiver-generic-chip-code generator 121
generates a replica of the generic-chip-code signal, g0(t),
which provides timing to bit synchronizer 129 and to the
plurality of receiver-message-chip-code generators 122, 172,
182.
The present invention also includes a method for
synchronously demodulating a spread-spectrum-communications
signal. Message data are input to the spreading means.
Referring to FIG. 4, the method comprises the steps of
generating 403 a generic-chip-code signal. The method
further includes generating 405 message data synchronized to
the generic-chip-code signal, and generating 407 a message-
chip-code signal synchronized to the generic-chip-code
signal. Message data are processed, using a spread-spectrum
modulator, with the message-chip-code signal to generate a
spread-spectrum-processed signal. The generic-chip-code
signal is combined 409 with the spread-spectrum-processed
signal. The method transmits 411 the combined generic-chip-
code signal and spread-spectrum-processed signal on a
carrier signal over the communications channel as a spread-
spectrum-communications signal.
At a receiver, the method includes recovering 413 the
carrier signal from the spread-spectrum-communications
signal and despreading 415 the spread-spectrum-
communications signal as a modulated-data signal. The
recovered-carrier signal is used to synchronize the step of
despreading the spread-spectrum-communications signal and to
optionally synchronously demodulate 417 and output 419 the
modulated-data signal as received data.
In use, the transmitter-generic-chip-code generator 101
generates the generic-chip-code signal, g0(t). Message data
are spread-spectrum processed by the EXCLUSIVE-OR device 103
with message-chip-code signal, g1(t), from transmitter-
message-chip-code generator 102. The combiner 105 combines
the generic-chip-code signal with the spread-spectrum-
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processed signal. The combined signal may be, for example,
a multilevel signal, which is generated by linearly adding
the voltage levels of the generic-chip-code signal and the
spread-spectrum-processed signal, or by adding the voltage
levels of the generic-chip-code signal with a plurality of
spread-spectrum-processed signals. The transmitter
transmits on a carrier signal having a carrier frequency,
fo, the combined generic-chip-code signal and the plurality
of spread-spectrum-processed signals. The spread-spectrum-
communications signal is transmitted through the
communications channel 110.
At the receiver, the generic-spread-spectrum-processing
means, embodied as the receiver-generic-chip-code generator
121, the generic mixer 123 and the generic-bandpass filter
125, cooperatively operate to recover the carrier signal
from the spread-spectrum-communications signal. The
message-spread-spectrum-processing means, embodied as the
receiver-message-chip-code generator 122, the message mixer
124 and the message-bandpass filter 126, cooperatively
despread the spread-spectrum-communications signal as the
modulated-data signal. The receiver-message-chip-code
generator 122 preferably is synchronized to the replica of
the generic-chip-code signal from the receiver-generic-chip-
code generator 121. A plurality of receiver-message-chip-
code generators may be employed, synchronized to the replica
of the generic-chip-code signal. The synchronous means,
embodied as the synchronous detector 127 synchronized to the
recovered-carrier signal, demodulates the modulated-data
signal as received data.
The received data are integrated and dumped by lowpass
filter 128 and electronic switch 130, under control of the
bit synchronizer 129. The bit synchronizer 129 preferably
uses the replica of the generic-chip-code signal for
synchronizing the integrate and dump functions.
It will be apparent to those skilled in the art that
various modifications can be made to the synchronous spread-
spectrum communications system and method of the instant
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invention without departing from the scope or spirit of the
invention, and it is intended that the present invention
cover modifications and variations of the synchronous
spread-spectrum communications system and method provided
they come in the scope of the appended claims and their
equivalents.
