Spread spectrum communications method and apparatus

• US 4,291,409 A
• Filed: 07/18/1978
• Issued: 09/22/1981
• Est. Priority Date: 06/20/1978
• Status: Expired due to Term

• Current Assignee: Mitre Corporation
• Original Assignee: Mitre Corporation
• Inventors: Weinberg, Aaron, Scales, Walter C., Keblawi, Feisal S., Ebert, Paul M., Kaminsky, Yaroslav, Fee, Joseph J.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US05/925,847
• Time in Patent Office: 1,162 Days
• Field of Search: 325/30, 325/32, 325/38, 325/42, 325/51, 325/65, 325/320, 325/321, 325/323, 325/473, 325/479, 325/13, 325/58, 179/15 BA, 179/15 AD, 179/15 AP, 364/724, 370/75, 370/97, 370/107, 343/100 CS, 343/100 SA, 343/854, 455/12, 455/13, 455/17, 455/23, 375/1, 375/2, 375/3, 375/4, 375/37, 375/38, 375/44, 375/100
• US Class Current: 375/347
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop
• CPC Class Codes: H04B 7/216 Code division or spread-spectrum multiple access [CDMA, SSMA]

Abstract

A method and apparatus employing spread spectrum techniques in a wide bandwidth communications system is disclosed. A plurality of transmitting stations are each equipped to provide a transmission signal representing a pseudo-random coded, phase modulated, message signal. The transmission signal is directed through a bandwidth which encompasses otherwise dedicated, relatively narrow bandwidth repeater channels, employed in connection with a communications satellite, to a generally fixed receiver station. At the receiving station, the incoming signal is (a) code acquired and tracked, (b) carrier acquired and tracked, (c) phase locked to the receiver local oscillator and (d) coherently demodulated to extract the desired data. The receiving station advantageously employs plural receiving elements each having a pseudo-random sequence code matched filter which significantly reduces code acquisition time by obviating the necessity of exhaustively correlating the incoming signal with a replica of the pseudo-random code word at the receiver station.

Claims

1. A spread spectrum communications system for use with a communications network having a wide bandwidth linear repeater channel comprising

a transmitting element having

means for repetitively generating a plural binary digit message signal for a sequence of binary input data, different input data resulting in different message signals and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of nonspread spectrum communications signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of nonspread spectrum communication signals unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal.

2. The spread spectrum system of claim 1 wherein said transmitting element comprises means adapted to generate a spread spectrum signal for transmission through a satellite communications system, said satellite system having said wide bandwidth repeater channel.

3. The spread spectrum system of claim 1 wherein said transmitting element generating means comprises

means for repeatedly generating at a first bit rate a selected pseudo-random code bindary signal having a selected length, said first bit rate and said selected length defining a code word cycle time,

means for generating at a second bit rate a synchronization identification word binary signal;

means for generating at a third bit rate said sequence of binary input data,

said first bit rate divided by said selected length being an integer multiple of said second bit rate and of said third bit rate, and

means for combining said code signal, said synchronization signal and said data sequence to form a repeating binary message signal, and

said modulating means comprises means for modulating a constant frequency carrier wave with said message signal to generate said spread spectrum transmission signal.

4. The spread spectrum system of claim 3 wherein

said synchronization signal generating means comprises a circulating shift register containing at least a selected synchronization word,

said data signal generating means comprises a second circulating shift register containing at least a data information sequence,

said combining means comprises a modulo two adder, and

said system further comprises means for synchronizing operation of said code signal generating means, said synchronization signal generating means, and said data signal generating means.

5. The spread spectrum system of claim 1 wherein

said message signal comprises a synchronization word and a sequence of data message bits, each combined with a repeating pseudo-random code sequence, and

said receiver station comprises

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and a second pseudo-random code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudo-random code signal, and

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudo-random code signal.

6. In a spread spectrum system having a carrier signal angle modulated by a message to be transmitted, and wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, a receiving method comprising the steps of

non-coherently acquiring and tracking said pseudorandom code sequence by employing non-coherent, recursive integration, and

thereafter acquiring and tracking the transmitted carrier frequency and phase.

7. In a spread spectrum system having a carrier signal angle modulated by a message to be transmitted, and wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, a receiving method comprising the steps of

providing a plurality of receiving elements operating in substantially non-overlapping frequency ranges,

non-coherently acquiring and tracking said pseudorandom code sequence using one of said receiving elements, and

thereafter acquiring and tracking the transmitted carrier frequency and phase using said one of said receiving elements.

8. A spread spectrum communication system for use with a communications network having a wide bandwidth repeater channel, comprising

a transmitting element having

means for repetitively generating a binary message signal from a sequence of binary input data, said message signal comprising a synchronization word and a sequence of data message bits, each combined with a repeating pseudorandom code sequence, and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of communications signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of communications signals, said communications signals being unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal,

said receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and a second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudorandom code signal, and

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudorandom code signal,

said detection means comprising

a code cycle accumulator for separately accumulating the sum of the squared values of the outputs of said code matched filters at each of a plurality of periodically occurring times, and

means for resetting the stored contents of said accumulator in response to the occurrence of the first of a selected sampling time or an accumulated stored contents greater than a first threshold value.

9. The spread spectrum system of claim 8 wherein said periodically occurring times correspond to not more than one-half the time duration between pseudo-random code bit occurrences.

10. A spread spectrum communication system for use in a communications network having a wide bandwidth repeater channel, comprising

a transmitting element having

means for repetitively generating a binary message signal from a sequence of binary input data, said message signal comprising a synchronization word and a sequence of data message bits, each combined with a repeating pseudorandom code sequence, and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of communications signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of communications signals, said communications signals being unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal,

said receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudorandom code signal, and

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudorandom code signal, and

each said code matched filter comprising

a low pass filter matched substantially to the shape of each pseudorandom code bit signal in the received signal,

a tapped analog delay line charge transfer device responsive to said low pass filter output and having a number of equally time-spaced output lines at least equal to twice the number of bits in each pseudorandom code sequence, said delay line being responsive to a clock rate corresponding to at least twice the bit rate associated with generation of said pseudorandom code, and

means for weighting said tapped outputs according to said selected pseudorandom sequence for providing output signals representing the degree of correlation of the selected pseudorandom sequence and the received signal.

11. The spread spectrum system of claim 10 wherein said detection means comprises

a code cycle accumulator for separately accumulating the sum of the squared values of the outputs of said code matched filters at each of a plurality of periodically occurring times corresponding to a code sequence,

means for resetting the stored contents of said accumulator in response to the occurrence of the first of a selected sampling time and an accumulated stored contents greater than a first threshold, and

means responsive to said accumulator output for selecting a repeating clock pulse time position corresponding to a repeating accumulator output greater than said first threshold and synchronized to a detected pseudo-random code modulated received signal,

a variable source of first clock signals,

means for generating a clock update signal, said generating means comprising

means for sampling the output of the accumulator at plural clock times centered around said repeating clock pulse for providing a difference output signal equal to the difference of each two sampled accumulator outputs, and

means for filtering said difference output,

said variable clock source being responsive to said filtered difference output for varying the frequency of said variable source, and

means for applying said clock source output for controlling timing and sampling of said tapped delay line.

12. A spread spectrum communication system for use with a communications network having a wide bandwidth repeater channel, comprising

a transmitting element having

means for repetitively generating a binary message signal from a sequence of binary input data, said message signal comprising a synchronization word and a sequence of data message bits, each combined with a repeating pseudorandom code sequence, and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of communication signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of communications signals, said communications signals being unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal,

said receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudorandom code signal,

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudorandom code signal,

a first and a second synchronization word matched filter responsive respectively to the first and second code matched filter outputs,

a first and a second data word matched filter responsive respectively to the first and second code matched filter outputs, and

means responsive to said code and data word matched filter outputs for accurately determining the frequency of said transmission signal carrier.

13. The spread spectrum system of claim 12 wherein said word matched filters comprise

means for coherently detecting each data word and plural segments of a synchronization word, and

means for incoherently combining said coherently detected words and segments for providing a signal output corresponding to the closeness of the quadrature mixing frequency to the received carrier frequency.

14. The spread spectrum system of claim 12 further comprising

a variable frequency carrier source for generating said quadrature components,

first and second message portion matched filters responsive to selected portions of the message signal,

said filter output responsive means further including means responsive to said message portion matched filters for controlling an automatic frequency control loop filter for more precisely acquiring said carrier source in frequency with said received signal, and

means responsive to said message portion matched filters for controlling a Costas loop filter for locking said carrier source in phase and frequency with said received signal.

15. A spread spectrum communication system for use with a communication network having a wide bandwidth repeater channel, comprising

a transmitting element having

means for repetitively generating a binary message signal from a sequence of binary input data, said message signal comprising a synchronization word and a sequence of data message bits, each combined with a repeating pseudorandom code sequence, and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of communication signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of communications signals, said communications signals being unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal,

said receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudorandom code signal,

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudorandom code signal,

means for coherently detecting at each of a plurality of first frequencies, a plurality of short sequences of said message signal synchronization and data words and combining and accumulating using incoherent detection the coherent outputs at each first frequency,

means for selecting from said accumulation of incoherently combined outputs a selected first frequency,

means for coherently detecting up to an entire message at each of a plurality of closely spaced second frequencies, said second frequencies being more closely spaced than said first frequencies and being centered around said selected first frequency, and for combining and accumulating the coherent outputs for each said second frequencies, and

means for selecting from said accumulation of second coherent outputs a selected second frequency,

said selected first and second selected frequencies being increasingly accurate approximations of the transmission signal carrier frequency.

16. A spread spectrum communication system for use with a communications network having a wide bandwidth repeater channel, comprising

a transmitting element having

means for repetitively generating a binary message signal from a sequence of binary input data, said message signal comprising a synchronization word and a sequence of data message bits, each combined with a repeating pseudorandom code sequence, and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of communications signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of communications signals, said communications signals being unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal,

said receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudorandom code signal,

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudorandom code signal,

a message cycle accumulation means responsive to said code, matched filter for repetitively accumulating said message data signals,

synchronization word matched filter means responsive to said accumulator for providing a synchronization word output signal,

controlled variable clock means responsive to said synchronization word output signal for providing message data clock synchronization signals,

a data word matched filter responsive to said message accumulator for providing a data output signal, and

means responsive to said message data clock synchronization signals and said data output signals for providing output message data.

17. A spread spectrum communication system for use with a communications network having a wide bandwidth repeater channel, comprising

a transmitting element having

means for repetitively generating a binary message signal from a sequence of binary input data, said message signal comprising a synchronization word and a sequence of data message bits, each combined with a repeating pseudorandom code sequence, and

modulation means for generating from said message signal a spread spectrum transmission signal, said transmission signal being adapted to be transmitted as a pseudo-noise signal simultaneously with a plurality of communications signals over said wide bandwidth repeater channel, said channel being dedicated to receiving and retransmitting said plurality of communications signals, said communications signals being unrelated to said spread spectrum signal, and

a receiver station for receiving said spread spectrum transmission signal, as retransmitted by said repeater channel, and for reconstructing said input data from said received retransmitted transmission signal,

said receiver station comprising

a plurality of substantially indentical receiver elements, each element comprising

means for forming first and second quadrature component signals of said received signal at a selected carrier frequency,

a first and a second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals and each filter being matched to the selected pseudorandom code signal,

incoherent detection means responsive to outputs of said filters for providing an output signal representing the degree of correlation of the received signal and the selected pseudorandom code signal,

a voltage controlled carrier source for forming said quadrature components, and

each source, prior to identifying a message, having a different carrier frequency output.

18. In a spread spectrum communications system wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, a receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of a received signal at a selected frequency, and

a first and a second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals, and each filter being matched to the selected pseudorandom code signal,

each said code matched filter having

a low pass filter matched substantially to the shape of each pseudorandom code bit received signal, and

a tapped analog delay line responsive to said low pass filter output and having a number of equally time-spaced output lines at least equal to twice the number of bits in each pseudorandom code sequence, and

means for weighting said tapped outputs according to said selected pseudorandom sequence for providing output signals representing the correlation of the selected pseudorandom code sequence and the received signal, and

detection means responsive to outputs of the filters for providing a data output signal representing said message.

19. In a spread spectrum communications system having a carrier signal angle modulated by a message to be transmitted, and wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, a receiver station comprising

at least one receiver element, each element comprising

means for forming first and second quadrature component signals of a received signal at a selected frequency, and

a first and a second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals, and each filter being matched to the selected pseudorandom code signal, and

detection means responsive to outputs of the filters for providing a data output signal representing said message

said detection means having

means for non-coherently acquiring and tracking said pseudorandom code sequence, and

means for thereafter acquiring and tracking said carrier frequency and phase.

20. In a spread spectrum communications system wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, a receiver station comprising

a plurality of substantially identical receiver elements, each element comprising

means for forming first and second quadrature component signals of a received signal at a selected frequency,

a first and a second pseudorandom code matched filter, each filter being responsive respectively to said first and second component signals, and each filter being matched to the selected pseudorandom code signal,

a variable frequency carrier source for forming said quadrature components, and

detection means responsive to outputs of the filters for providing a data output signal representing said message,

said sources operating in non-overlapping frequency ranges.

21. In a spread spectrum system having a carrier signal angle modulated by a message to be transmitted, and wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, the receiving method comprising the steps of

forming first and second quadrature component signals of the received signal at at least one selected carrier frequency,

for each pair of quadrature signals,

passing each quadrature component through a pseudorandom code matched filter, each filter being matched to the selected pseudorandom code signal, and

processing the filter outputs to provide data output signals representing the message,

acquiring and tracking the pseudorandom code signal,

coherently detecting, at each of a plurality of first frequencies a plurality of short sequences of said message signal and accumulating the incoherently summed outputs at each first frequency as a function of time,

selecting from said accumulation of incoherently summed outputs a selected first frequency,

coherently detecting up to an entire message at each of a plurality of closely spaced second frequencies, said second frequencies being more closely spaced than said first frequencies and being centered around said selected first frequency,

combining and accumulating the coherent outputs at said second frequencies, and

selecting from said accumulation of second coherent outputs a selected second frequency,

said selected first and second frequencies being increasingly accurate approximations of the transmission signal carrier frequency.

22. The method of claim 21 further comprising the steps of activating an automatic frequency control loop for a more precise selected carrier frequency than achieved by said first and second selected frequencies, and thereafter

activating a Costas loop filter for locking a voltage controlled oscillator in phase and frequency with said received carrier frequency.

23. In a spread spectrum system wherein the transmitted message comprises plural message bits synchronously combined with a repeating pseudorandom code sequence, the receiving method comprising the steps of

forming first and second quadrature component signals of the received signal at at least one selected carrier frequency,

for each pair of quadrature signals,

passing each quadrature component through a pseudorandom code matched filter, each filter being matched to the selected pseudorandom code signal, and

processing the filter outputs to provide data output signals representing the message,

non-coherently acquiring and tracking said pseudorandom code sequence, and

thereafter acquiring and tracking the transmitted carrier frequency and phase.