Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SYNCHRONOUS SPREAD-SPECTRUM
COMMUNICATIONS SYSTEM AND METHOD
BACKGROUND 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, gi(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
0-bit is declared, otherwise a 1-bit is declared. Such a system is complex
anci
suffers degradation of about 6 dB at error rates of 10-2.
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-
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-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 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-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-spreadl-
spectrum-processing means. The detection means may be nonsynchronous
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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
message-spread-spectrum-processing means derive their synchronization
from the replica of the generic-chip-code signal, from the generic-spread-
spectrum-processing means. The plurality of inessage-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-
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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 defined by the generic-chip-code signal at the
transmitter. If the signal out of the generic-band pass 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 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 recieved 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.
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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
5 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 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 an aspect of the present invention, there is provided
a method for transmitting message data from a code division multiple access
transmitter, the method comprising:
providing a bit sequence;
combining each bit of the bit sequence with a plurality of chips,
producing a first signal;
providing message data;
combining each bit of the message data with a plurality of chips,
producing a second signal;
combining the first signal with the second signal to produce a spread
spectrum signal; whereby the first signal facilitates a receiver to
synchronize
to the second signal.
In accordance with another aspect of the present invention, there is
provided a transmitter for transmitting message data in a code division
multiple access format, the transmitter comprising:
a code generator that combines each bit of a bit sequence with a
plurality of chips, producing a first signal;
a mixing device that combines each bit of message data with a plurality
of chips, producing a second signal;
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a combiner that combines the first signal with the second signal to
produce a spread spectrum signal; whereby the first signal facilitates a
receiver to synchronize to the second signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated ih apAconst'stute
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. I 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;
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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
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 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-
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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.
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-generic-chip-
code generator 121. The receiver-message-chip-code generator 122
preferably is coupled to the receiver-generic-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 code signal, go(t), and the transmitter-message-chip-code generator
102 generates a message-chip-code signal, gi(t). Synchronous timing of the
message data, di(t), and the message-chip-code signal, in FIG. 2, is providecl
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-processed 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 signal. The combiner 105 combines the generic-chip-
code signal with the spread-spectrum-processed signal. The combined
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generic-chip-code signal and spread-spectrum-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, x,,(t), as a modulated-data signal,
di(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, go(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, go(t)
cos wot, generally does not include data so that despreading the spread-
spectrum-communications signal produces the carrier signal, only. The
generic-band pass 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 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.
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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
5 acquisition and tracking circuit 131.
The receiver-message-chip-code generator 122 generates a replica of
the message-chip-code signal, gi(t). The replica of the message-chip-code
signal, gi(t), is synchronized to the replica of the generic-chip-code signal,
go(t), from the receiver-generic-chip-code generator 121. Thus, the receiver-
10 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
channels 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, di(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 and outputted as received data, dl(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.
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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 generator 102 generating first message-chip-code signal, gi(t), second
transmitter-message-chip-code generator 172 generating second message-
chip-code signal, g2(t), through N th transmitter-message-chip-code generator
182 generating Nt" 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 di(t),
d2(t), ..., dN(t) with the plurality of message-chip-code signals gi(t),
92(t), ..., gN(t), respectively. More particularly, the first message data,
di(t),
are modulo-2 added with the first message-chip-code signal, gi(t), the seconci
message data, d2(t), are modulo-2 added with the second message-chip-code
signal, g2(t), through the Nt" message data, dN(t), which are modulo-2 added
with the Nth message-chip-code signal, gN(t).
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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, di(t), d2(t), ...,dN(t). The generic-chip-code
signal
go(t), in a preferred embodiment, provides synchronous timing for the
plurality
of message-chip-code signals gi(t), g2(t), ..., gN(t), and the plurality of
message data di(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 the 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 signal, cos wot, at a carrier frequency, fo. 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, x,(t) has the form:
N
xc (t) = {9p (t) + 2+ [9i (t) Q di (t) J } cos wot
i=1
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-
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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 24, second message mixer 174
through Nth message mixer 184. The plurality of inessage-bandpass filters is
shown as first message-bandpass filter 126, second message-bandpass filter
176, through Nth message-bandpass filter 186.
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 N th lowpass filter 188. The plurality of electronic
switches is
shown as first electronic switch 130, second electronic switch 180 through N
th
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, di(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, go(t), and controls the timing of the integrating and
dumping
functions of the plurality lowpass filters and the plurality of electronic
switches.
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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 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, go(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, go(t). Message data are spread-spectrum
processed by the EXCLUSIVE-OR device 103 with message-chip-code
signal, gi(t), from transmifter-message-chip-code generator 102. The
combiner 105 combines the generic-chip-code signal with the spread-
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spectrum-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
5 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.
10 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-
15 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 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.
