CA2103304A1 - Spread spectrum communication system - Google Patents
Spread spectrum communication systemInfo
- Publication number
- CA2103304A1 CA2103304A1 CA 2103304 CA2103304A CA2103304A1 CA 2103304 A1 CA2103304 A1 CA 2103304A1 CA 2103304 CA2103304 CA 2103304 CA 2103304 A CA2103304 A CA 2103304A CA 2103304 A1 CA2103304 A1 CA 2103304A1
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- Canada
- Prior art keywords
- code
- code sequence
- sequence
- carrier signal
- transmitter
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 238000004891 communication Methods 0.000 title claims abstract description 13
- 238000001228 spectrum Methods 0.000 title abstract description 25
- 239000002131 composite material Substances 0.000 claims abstract description 14
- 230000005540 biological transmission Effects 0.000 claims description 15
- 230000003595 spectral effect Effects 0.000 description 12
- 238000000034 method Methods 0.000 description 9
- 230000010363 phase shift Effects 0.000 description 5
- 230000008901 benefit Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- YVGGHNCTFXOJCH-UHFFFAOYSA-N DDT Chemical compound C1=CC(Cl)=CC=C1C(C(Cl)(Cl)Cl)C1=CC=C(Cl)C=C1 YVGGHNCTFXOJCH-UHFFFAOYSA-N 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 238000005314 correlation function Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000000737 periodic effect Effects 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 230000007480 spreading Effects 0.000 description 1
- 230000001360 synchronised effect Effects 0.000 description 1
Landscapes
- Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
Abstract
ABSTRACT
"Spread spectrum communication system"
Apparatus for generating a pseudo-noise (PN) code sequence with a relatively short repeatable code length, to facilitate signal acquisition in a receiver, but also having a spectrum that is practically indistinguishable from electromagnetic noise. A primary PN code sequence of relatively short length is combined with a secondary random or pseudo-random code sequence having a chip rate approximately equal to the code repetition rate of the primary PN code, to produce a composite code. When the composite code is used to modulate a carrier signal, the resulting frequency spectrum is similar to noise, but retains the short code property of ease of signal acquisition.
"Spread spectrum communication system"
Apparatus for generating a pseudo-noise (PN) code sequence with a relatively short repeatable code length, to facilitate signal acquisition in a receiver, but also having a spectrum that is practically indistinguishable from electromagnetic noise. A primary PN code sequence of relatively short length is combined with a secondary random or pseudo-random code sequence having a chip rate approximately equal to the code repetition rate of the primary PN code, to produce a composite code. When the composite code is used to modulate a carrier signal, the resulting frequency spectrum is similar to noise, but retains the short code property of ease of signal acquisition.
Description
t .~ : ~
PHA 40650 2 1 ~ 3 .3 ~ ~ 02.11.1993 "Spread spectrum communication system"
This invention relates to a communication system comprising a transmitter being coupled to a receiver via a transmission channel, said transmitter compAsing means for generating a PN code sequence, means for modulating a carrier signal with the PN code sequence, and means for transmitting the carrier signal via said transmission channel, the receiver comprising means for receiving the carrier signal from the transmission channel, means for demodulating said carrier signal, and means for correlating the demodulated carrier signal with a sequence being equal to the PN
code sequence.
This invention relates also to a transmitter and a coder for use in such a 10 transmission system.
A system according to the preamble is }cnown from the book "Mobile Radio Communications" by Raymond Steele, published by Pentech Press Publishers London, pp. 45-51.
Digital modulation techniques for communication are well known, and 15 include phase shift keying (PSK), where a constant amplitude carrier signal is phase shifted in accordance with a digital sequence. In binary phase shift keying (BPSK) the ~ -carrier is reversed in phase to indicate a binary change of state of a data signal. In quadriphase phase shift keying (QPSK), the modulated carrier can assume any of four phase states, as determinedi by pairs of data bits.
For security and other reasons, a modulated carrier signal may also be -~
subject to spread spectrum modulation. A spread spectrum signal is, as the name implies, spread over a wide bandwidth and is relatively immune to eavesdropping and jamming. A common technique uses a pseudo-random (PN) code sequence to obtain the desired spectral spreading. A PN sequence is a sequence of binary or quaternary digits 25 that repeats itself after a large number digits. Thus the digits in the sequence are not truly random, but if the repetition cycle of the sequence is long enough its spectrum shares many of the properties of random electromagnetic noise. In the context of a data transmitter, binary PN modulation may be effected by simply passing the data stream ~- 2 1 ~
PHA 40650 2 02.11.1993 and the bin~y PN code sequence through an exclusive OR gate, to achieve PSK
modulation of the data onto the PN code. Data bits are either inverted or not, depending ;
on the presence or absence of a logical " 1" bit in the PN code. The data symbol rate is typically many times slower than the PN code rate (referred to as the PN "chip" rate).
S The resulting digital data stream is a PN code modulated by the slower data symbol stream, and is used to modulate a carrier signal in acsordance with a digital modulation technique, such as QPSK or differential quadriphase phase shift keying (DQPSK), and the modulated ciarrier is transmitted. A variant of this method uses a quateInary PN
sequence.
The present invention is concerned with systems of this general type, and particularly with techniques for signal acquisition in such systems. Receiving and demodulating signals that have been subject to PN modulation requires that the same PN
code sequence be generated in the receiver, and correlated with received signals to extract the data modulation. One type of correlation technique employs a digital15 matched filter to compare the received digital signal with the lvcally generated version of the PN code. The digital filter proc'.uces an in-phase (I) signal and a quadrature (Q~
signal from which a digital demodulator (such as a DPSK demodulator) can derive data values. Another function of the digital matched filter is to produce correlationmeasurements from which synchronization (sync) signals can be generated and used to 20 handle multipath components in the receivec'. data signals.
Acquiring a PN mod.ulated. receivec'. signal is rendered more difficult if the code is a "long code, " that is it has a long repetition cycle. The longer the code the more it resembleg eleetromagnetic noise and, therefore, the more difficult it is to acquire in a receiver. Of course, a long code is prefeIred for the security it provides, 25 since it is virtually indistinguishable from noise by an eavesdropping receiver which has no prior h~owledge of the code. One solution to this dif~lculty is to begirl the acquisi-tion phase of operation with periodic repetitions of a short PN code, which is easier for the receiver to acquire, and then switch to the long code once the transmitter and receiver are synchronized. This minimizes the time duAng which the transmission is 30 more vulnerable to detection or interference, but facil;tates acquisition by the receiver.
The disadvantage of using the short PN code for acquisition is that the triansmission is more vulnerable to eavesdropping during this time.
The object of the present invention is to provide a transmission system ~ ;
~3~
.
PHA 40650 3 02.11.1993 according to the preamble enabling a fast acquisition by the receiver, but without being more vulnerable to eavesdropping during the time of acquisition.
Therefor the transmission system according to the invention is characterised in that the transmitter comprises means for generating a further code S sequence having a chip rate of the same order of magnitude as the code repetition rate of the PN code sequence, and means for combining the PN code sequence and the .! ' further code sequence to produce a composite code sequence, and in that the modulator is arranged for modulating the carrier signal with the composite code sequence..By appying the measures accoring to the invention, the modulated carrier 10 signal has a frequency spectrum in which the distinct spectral lines that would be produced by the PN code sequence alone are dissolv d into a larger number of closely spaced lines by the presence of the further PN code sequence. If the further code is truly random, the resulting frequency spectrum is practically continuous. In either case, the spectrum is practically indistinguishable from electromagnetic noise. A truly random 15 further sequence can be obtained by sampling a source of true noise at the repetition rate of the PN code sequence.
More specifically, the further code sequence has a chip rate approximately equal to the code repetition rate of the PN code sequence, and the combining step includes perforrning an exclusive OR operation on the PN code sequence and the further 20 code sequence.
It will be appreciated that the present invention represents a significant advance in the field of PN modulated communication. In particular, the inve~tionprovides a short code PN code sequence having the desirable secure properties of a long PN code sequence. Other aspects and advantages of the in~ention will become appiarent 25 from the following more detailed description, tia~en in conjunction with the : .: :
accompanying drawings.
Figure 1 is a graph showing the frequency spectrum of a carrier signal modulated with a pseud~noise (PN) code;
Figure 2 is a graph showing an enlarged portion of the frequenc~
30 spectrum of Figure l;
Figure 3 is a block diagram of a PN code generator in accordance with the present invention; and ~ ~
Figure 4 is a graph similar to Figure 2, but showing an enlarged portion . :~
PHA 40650 2 1 ~ 3 .3 ~ ~ 02.11.1993 "Spread spectrum communication system"
This invention relates to a communication system comprising a transmitter being coupled to a receiver via a transmission channel, said transmitter compAsing means for generating a PN code sequence, means for modulating a carrier signal with the PN code sequence, and means for transmitting the carrier signal via said transmission channel, the receiver comprising means for receiving the carrier signal from the transmission channel, means for demodulating said carrier signal, and means for correlating the demodulated carrier signal with a sequence being equal to the PN
code sequence.
This invention relates also to a transmitter and a coder for use in such a 10 transmission system.
A system according to the preamble is }cnown from the book "Mobile Radio Communications" by Raymond Steele, published by Pentech Press Publishers London, pp. 45-51.
Digital modulation techniques for communication are well known, and 15 include phase shift keying (PSK), where a constant amplitude carrier signal is phase shifted in accordance with a digital sequence. In binary phase shift keying (BPSK) the ~ -carrier is reversed in phase to indicate a binary change of state of a data signal. In quadriphase phase shift keying (QPSK), the modulated carrier can assume any of four phase states, as determinedi by pairs of data bits.
For security and other reasons, a modulated carrier signal may also be -~
subject to spread spectrum modulation. A spread spectrum signal is, as the name implies, spread over a wide bandwidth and is relatively immune to eavesdropping and jamming. A common technique uses a pseudo-random (PN) code sequence to obtain the desired spectral spreading. A PN sequence is a sequence of binary or quaternary digits 25 that repeats itself after a large number digits. Thus the digits in the sequence are not truly random, but if the repetition cycle of the sequence is long enough its spectrum shares many of the properties of random electromagnetic noise. In the context of a data transmitter, binary PN modulation may be effected by simply passing the data stream ~- 2 1 ~
PHA 40650 2 02.11.1993 and the bin~y PN code sequence through an exclusive OR gate, to achieve PSK
modulation of the data onto the PN code. Data bits are either inverted or not, depending ;
on the presence or absence of a logical " 1" bit in the PN code. The data symbol rate is typically many times slower than the PN code rate (referred to as the PN "chip" rate).
S The resulting digital data stream is a PN code modulated by the slower data symbol stream, and is used to modulate a carrier signal in acsordance with a digital modulation technique, such as QPSK or differential quadriphase phase shift keying (DQPSK), and the modulated ciarrier is transmitted. A variant of this method uses a quateInary PN
sequence.
The present invention is concerned with systems of this general type, and particularly with techniques for signal acquisition in such systems. Receiving and demodulating signals that have been subject to PN modulation requires that the same PN
code sequence be generated in the receiver, and correlated with received signals to extract the data modulation. One type of correlation technique employs a digital15 matched filter to compare the received digital signal with the lvcally generated version of the PN code. The digital filter proc'.uces an in-phase (I) signal and a quadrature (Q~
signal from which a digital demodulator (such as a DPSK demodulator) can derive data values. Another function of the digital matched filter is to produce correlationmeasurements from which synchronization (sync) signals can be generated and used to 20 handle multipath components in the receivec'. data signals.
Acquiring a PN mod.ulated. receivec'. signal is rendered more difficult if the code is a "long code, " that is it has a long repetition cycle. The longer the code the more it resembleg eleetromagnetic noise and, therefore, the more difficult it is to acquire in a receiver. Of course, a long code is prefeIred for the security it provides, 25 since it is virtually indistinguishable from noise by an eavesdropping receiver which has no prior h~owledge of the code. One solution to this dif~lculty is to begirl the acquisi-tion phase of operation with periodic repetitions of a short PN code, which is easier for the receiver to acquire, and then switch to the long code once the transmitter and receiver are synchronized. This minimizes the time duAng which the transmission is 30 more vulnerable to detection or interference, but facil;tates acquisition by the receiver.
The disadvantage of using the short PN code for acquisition is that the triansmission is more vulnerable to eavesdropping during this time.
The object of the present invention is to provide a transmission system ~ ;
~3~
.
PHA 40650 3 02.11.1993 according to the preamble enabling a fast acquisition by the receiver, but without being more vulnerable to eavesdropping during the time of acquisition.
Therefor the transmission system according to the invention is characterised in that the transmitter comprises means for generating a further code S sequence having a chip rate of the same order of magnitude as the code repetition rate of the PN code sequence, and means for combining the PN code sequence and the .! ' further code sequence to produce a composite code sequence, and in that the modulator is arranged for modulating the carrier signal with the composite code sequence..By appying the measures accoring to the invention, the modulated carrier 10 signal has a frequency spectrum in which the distinct spectral lines that would be produced by the PN code sequence alone are dissolv d into a larger number of closely spaced lines by the presence of the further PN code sequence. If the further code is truly random, the resulting frequency spectrum is practically continuous. In either case, the spectrum is practically indistinguishable from electromagnetic noise. A truly random 15 further sequence can be obtained by sampling a source of true noise at the repetition rate of the PN code sequence.
More specifically, the further code sequence has a chip rate approximately equal to the code repetition rate of the PN code sequence, and the combining step includes perforrning an exclusive OR operation on the PN code sequence and the further 20 code sequence.
It will be appreciated that the present invention represents a significant advance in the field of PN modulated communication. In particular, the inve~tionprovides a short code PN code sequence having the desirable secure properties of a long PN code sequence. Other aspects and advantages of the in~ention will become appiarent 25 from the following more detailed description, tia~en in conjunction with the : .: :
accompanying drawings.
Figure 1 is a graph showing the frequency spectrum of a carrier signal modulated with a pseud~noise (PN) code;
Figure 2 is a graph showing an enlarged portion of the frequenc~
30 spectrum of Figure l;
Figure 3 is a block diagram of a PN code generator in accordance with the present invention; and ~ ~
Figure 4 is a graph similar to Figure 2, but showing an enlarged portion . :~
2~33~
:
`` PHA 40650 4 02.11.1993 of the frequency spectrum of a carrier signal modulated with a code in accordance with the invention.
As shown in the drawings for purposes of ;llustration, the present invention is concerned with improvements in pseudo-noise (PN) communication S techniques. When a radio-frequency (rf) carrier signal is modulated with a PN code sequence, the resulting signal has a line spectrum, the amplitudes of which assume a charactenstic (sin x)/x shape. An example is shown in FIG. 1, where the curve indicated by reference numeral 10 is the frequency spectrum envelope resulting from PN modulation of a single-frequency carrier. The envelope 10 has null points 10 symmetrically spaced about the carrier frequency, and side lobes of smaller arnplitude beyond the null points.
In fact, the PN line spectrum consists of multiple spectral lines spaced by a frequency equal to the PN code repetition rate. If, for example, the PN ~ode has 1,000 chips in its repeating sequence, and if the code repeats at a rate of one megachip 15 per second, the code repetition rate will be 1 kiloHertz ~Hz), and the spectral lines in the PN frequency spectrum will be spaced apiart by 1 kHz. The width of the spectrum measured between the first nulls is twice the chip rate or, in this example, 2 M~Iz. A
fragmentary portion of the spectrum is shown in enlarged form in FIG. 2. For the slow PN code, the spectral lines, indicated at 20, are spaced sufficiently far apia~t to be 20 detectable during the short acquisition phase of operation.
In accordance with the invention, a PN code sequence generated in the transmitter is subject to a further stage of modulation by a random or pseudo-random sequence of chips at a rate approximately equal to the spectral line spacing resulting from the PN modulation process. The apparatus for performing the t~vo stages of PN
25 code generation is shown in block diagram form in Figure 3. A PN code generator 22 is driven at the slow PN chip rate, such as 1 Mchip/s. It also assumed, as in the foregoing exarnple, that the PN generator generates a PN code with a 1,000-chip repetition cycle. A further code generator 24, which may produce PN code or a truly random code, is driven at a rate equivalent to the spect~al line spacing associated with 30 the PN code generator 22. In this example, the secondary code generator would be driven at a rate of 1 kchip/s. The outputs of the PN code generator 22 and the further code generator 24 are combined in an exclusive OR gate 26, to produce a composite PN
code on output line 28. It will be understood that this composite code will then be ~ 33Q~.
PHA 40650 5 02.11.1993 modulated with digital data signals, and the resulting data modulated PN codes will be used to modulate an rf carrier signal, as indicated generally in block 30 of the drawing.
Typically, although not necessarily for purposes of this invention, there will be two separate PN codes (referred to as A and B codes), and pairs of data bits will modulate 5 the A and B codes, which will then modulate the carrier in accordance with a quadri-phase phase shift keying (QPSK) technique. However, the modulation schemes selected have no direct bearing on the present invention, which is concerned pnmarily with the generation of a short PN code having the desirable properties of a much longer code when used to modulate a carrier. `
The effect of the further code generator 24 is that each spectral line resulting from the presence of the primary PN code generator 24 will be spread across spectral band of twice the chip rate of the further PN code generator, or 2 kHz in the example. This is shown diagrammatically in Figure 4. If the further code is a PN code, the resulting secondary spectra 32 will, of course, also consist of discrete spectral lines.
15 The spacing of these lines will depend on the length of the further PN code. If this is also a 1 ,000-chip code, the spectral line spacing will be 1 Hz, which is the further PN
code repetition rate. Clearly, these overlapping spectra 26 will produce a composite spectrum much more closely resembling electromagnetic noise than the spectral lines of Figure 2. If the further code is random, the spectral lines will be replaced by a set of 20 overlapping continuous spectra, as shown in Figure 4.
At first sight, it would appear that maldng the short code more noise-like would detract from its advantage of being easier to acquire in a receiver. Fortunately this is not the case. Because the noise-like property is produced by means of a relatively slow further PN code generator, the receiver acquisition and correlation functions treat 25 the composite code no differently from a short PN code modulated with slower varying data. In the exarnple given, which is not intended to be limiting, the further code generator operates at 1 kHz. The receiver makes correlation measurements on the incoming composite code3 using only a locally generated form of the primary PN code sequence (or A and B se~quences). The further l-Khz modulation provided by the fur~er 30 code generator is treated like modulating data, which, for most of the acquisition phase, is ignored. At the end of the acquisition phase, a s~ecial data sequence is transrnitted `
and recognized by the receiver3 at which time both the transmitter and the receiver switch to the long PN code for transmission of real data.
':`~, 21~3~
PHA 40650 6 02.11.1993 It will be appreciated from the foregoing that the present invention represents a significant advance in the field of PN data transmission. In particular, applying a further, slower varying, PN code to a short PN code produces a spectrum that is practically indistinguishable from electromagnetic noise, but does not detract 5 from the receiver's ability to acquire the short code quickly. The principles of the invention may be used to advantage in a communication system in which short PN code sequences are used for an acquisition phase, or in a system in which short PN code sequences are used for the acquisition phase and for the transmission of data. It will also be appreciated that, although a specifi~ embodiment of the invention has been 10 described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.
:
`` PHA 40650 4 02.11.1993 of the frequency spectrum of a carrier signal modulated with a code in accordance with the invention.
As shown in the drawings for purposes of ;llustration, the present invention is concerned with improvements in pseudo-noise (PN) communication S techniques. When a radio-frequency (rf) carrier signal is modulated with a PN code sequence, the resulting signal has a line spectrum, the amplitudes of which assume a charactenstic (sin x)/x shape. An example is shown in FIG. 1, where the curve indicated by reference numeral 10 is the frequency spectrum envelope resulting from PN modulation of a single-frequency carrier. The envelope 10 has null points 10 symmetrically spaced about the carrier frequency, and side lobes of smaller arnplitude beyond the null points.
In fact, the PN line spectrum consists of multiple spectral lines spaced by a frequency equal to the PN code repetition rate. If, for example, the PN ~ode has 1,000 chips in its repeating sequence, and if the code repeats at a rate of one megachip 15 per second, the code repetition rate will be 1 kiloHertz ~Hz), and the spectral lines in the PN frequency spectrum will be spaced apiart by 1 kHz. The width of the spectrum measured between the first nulls is twice the chip rate or, in this example, 2 M~Iz. A
fragmentary portion of the spectrum is shown in enlarged form in FIG. 2. For the slow PN code, the spectral lines, indicated at 20, are spaced sufficiently far apia~t to be 20 detectable during the short acquisition phase of operation.
In accordance with the invention, a PN code sequence generated in the transmitter is subject to a further stage of modulation by a random or pseudo-random sequence of chips at a rate approximately equal to the spectral line spacing resulting from the PN modulation process. The apparatus for performing the t~vo stages of PN
25 code generation is shown in block diagram form in Figure 3. A PN code generator 22 is driven at the slow PN chip rate, such as 1 Mchip/s. It also assumed, as in the foregoing exarnple, that the PN generator generates a PN code with a 1,000-chip repetition cycle. A further code generator 24, which may produce PN code or a truly random code, is driven at a rate equivalent to the spect~al line spacing associated with 30 the PN code generator 22. In this example, the secondary code generator would be driven at a rate of 1 kchip/s. The outputs of the PN code generator 22 and the further code generator 24 are combined in an exclusive OR gate 26, to produce a composite PN
code on output line 28. It will be understood that this composite code will then be ~ 33Q~.
PHA 40650 5 02.11.1993 modulated with digital data signals, and the resulting data modulated PN codes will be used to modulate an rf carrier signal, as indicated generally in block 30 of the drawing.
Typically, although not necessarily for purposes of this invention, there will be two separate PN codes (referred to as A and B codes), and pairs of data bits will modulate 5 the A and B codes, which will then modulate the carrier in accordance with a quadri-phase phase shift keying (QPSK) technique. However, the modulation schemes selected have no direct bearing on the present invention, which is concerned pnmarily with the generation of a short PN code having the desirable properties of a much longer code when used to modulate a carrier. `
The effect of the further code generator 24 is that each spectral line resulting from the presence of the primary PN code generator 24 will be spread across spectral band of twice the chip rate of the further PN code generator, or 2 kHz in the example. This is shown diagrammatically in Figure 4. If the further code is a PN code, the resulting secondary spectra 32 will, of course, also consist of discrete spectral lines.
15 The spacing of these lines will depend on the length of the further PN code. If this is also a 1 ,000-chip code, the spectral line spacing will be 1 Hz, which is the further PN
code repetition rate. Clearly, these overlapping spectra 26 will produce a composite spectrum much more closely resembling electromagnetic noise than the spectral lines of Figure 2. If the further code is random, the spectral lines will be replaced by a set of 20 overlapping continuous spectra, as shown in Figure 4.
At first sight, it would appear that maldng the short code more noise-like would detract from its advantage of being easier to acquire in a receiver. Fortunately this is not the case. Because the noise-like property is produced by means of a relatively slow further PN code generator, the receiver acquisition and correlation functions treat 25 the composite code no differently from a short PN code modulated with slower varying data. In the exarnple given, which is not intended to be limiting, the further code generator operates at 1 kHz. The receiver makes correlation measurements on the incoming composite code3 using only a locally generated form of the primary PN code sequence (or A and B se~quences). The further l-Khz modulation provided by the fur~er 30 code generator is treated like modulating data, which, for most of the acquisition phase, is ignored. At the end of the acquisition phase, a s~ecial data sequence is transrnitted `
and recognized by the receiver3 at which time both the transmitter and the receiver switch to the long PN code for transmission of real data.
':`~, 21~3~
PHA 40650 6 02.11.1993 It will be appreciated from the foregoing that the present invention represents a significant advance in the field of PN data transmission. In particular, applying a further, slower varying, PN code to a short PN code produces a spectrum that is practically indistinguishable from electromagnetic noise, but does not detract 5 from the receiver's ability to acquire the short code quickly. The principles of the invention may be used to advantage in a communication system in which short PN code sequences are used for an acquisition phase, or in a system in which short PN code sequences are used for the acquisition phase and for the transmission of data. It will also be appreciated that, although a specifi~ embodiment of the invention has been 10 described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.
Claims (10)
1. Communication system comprising a transmitter being coupled to a receiver via a transmission channel, said transmitter comprising means for generating a PN code sequence, means for modulating a carrier signal with the PN code sequence, and means for transmitting the carrier signal via said transmission channel, the receiver comprising means for receiving the carrier signal from the transmission channel, means for demodulating said carrier signal, and means for correlating the demodulated carrier signal with a sequence being equal to the PN code sequence, characterised in that the transmitter comprises means for generating a further code sequence having a chip rate of the same order of magnitude as the code repetition rate of the PN code sequence, and means for combining the PN code sequence and the further code sequence to produce a composite code sequence, and in that the modulator is arranged for modulating the carrier signal with the composite code sequence.
2. Communication system according to claim 1, characterised in that the further code sequence is a further PN code sequence.
3. Communication system according to claim 1, characterised in that the secondary code sequence is truly random.
4. Communication system according to claim 1,2 or3, characterised in that the secondary code sequence has a chip rate approximately equal to the code repetition rate of the primary PN code sequence.
5. Transmission system according to one of the claims 1 to 4, characterised in that the means for combining the PN code sequence and the further code sequence comprises means for performing an exclusive OR operation on the PN code sequenceand the further code sequence.
6. Transmitter comprising means for generating a PN code sequence, means for modulating a carrier signal with the PN code sequence, and means for transmitting the carrier signal via a transmission channel, characterised in that the transmitter comprises means for generating a further code sequence having a chip rate of the same order of magnitude as the code repetition rate of the PN code sequence, and means for combining the PN code sequence and the further code sequence to produce a composite code sequence, and in that the modulator is arranged for modulating the carrier signal with the composite code sequence.
7. Transmitter according to claim 1, characterised in that the further code sequence is a further PN code sequence.
8. Transmitter according to claim 6, characterised in that the secondary code sequence is truly random.
9. Transmitter according to claim 6,7 or 8, characterised in that the secondary code sequence has a chip rate approximately equal to the code repetition rate of the primary PN code sequence.
10. Code generator comprising means for generating a PN code sequence, characterised in that the code generator further comprises means for generating a further code sequence having a chip rate of the same order of magnitude as the code repetition rate of the PN code sequence, and means for combining the PN code sequence and the further code sequence to produce a composite code sequence.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US98306892A | 1992-11-20 | 1992-11-20 | |
| US983,068 | 1992-11-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2103304A1 true CA2103304A1 (en) | 1994-05-21 |
Family
ID=25529777
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2103304 Abandoned CA2103304A1 (en) | 1992-11-20 | 1993-11-17 | Spread spectrum communication system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2103304A1 (en) |
-
1993
- 1993-11-17 CA CA 2103304 patent/CA2103304A1/en not_active Abandoned
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