Spread spectrum correlator for recovering CCSK data from a PN spread MSK waveform

• US 4,707,839 A
• Filed: 09/26/1983
• Issued: 11/17/1987
• Est. Priority Date: 09/26/1983
• Status: Expired due to Term

• Current Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Original Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Inventors: Andren, Carl F., Olaker, David A.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/535,923
• Time in Patent Office: 1,513 Days
• Field of Search: 178/22.01, 178/22.05, 178/22.06, 178/22.13, 178/22.14, 178/22.16, 178/22.19, 364/728, 364/819, 364/824, 371/2, 371/37, 371/43, 375/1, 375/2.1, 375/90, 375/96, 375/47, 380/34, 380/42, 380/43, 380/46, 380/47
• US Class Current: 375/150
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7093 Matched filter type

Abstract

In a spread spectrum communications system employing cyclic code shift keying as its primary modulation, the transmission waveform is spread for transmission security by modulo-2 adding a pseudo-noise sequence to the CCSK data symbols prior to phase modulating onto a carrier signal for transmission. If the transmission modulation is minimum shift keying (MSK) the two components of the data stream are applied to the carrier with a differential encoding step implicit in the modulation scheme. This differential encoding characteristic makes stripping of the PN spread function prior to CCSK demodulation difficult at the receiving end. In order to demodulate this waveform in an optimum manner, an array correlator, the adjacent correlator stages of which have one chip relative time displacements of their CCSK reference waveform, is employed. In effect the array correlator becomes a parallel array of matched filters matched to each cyclic shift of the incoming waveform. By modulo-2 addition of the PN spreading waveform with the time displaced CCSK reference waveform in each stage of the correlator, the correlator can be made to match all versions of the spread symbol being received.

Claims

1. A spread spectrum demodulation apparatus for recovering information signals that have been combined with a prescribed encoding sequence and spread spectrum-modulated onto a carrier for transmission comprising:

first means for providing a replica of said prescribed encoding sequence;

second means for providing a replica of the spreading sequence through which the combined information signals have been spread spectrum-modulated onto said carrier;

third means, coupled to said first and second means, for combining said replica of said prescribed encoding sequence with said replica of said spreading sequence and producing therefrom a plurality of output signals respective ones of which correspond to respective combinations of said spreading sequence and said prescribed encoding sequence mutually shifted in time with respect to one another by successively offset time intervals; and

fourth means, coupled to said third means, for correlating the respective outputs of said third means with received unknown information signals and deriving therefrom an output representative of the original information signals.

2. An apparatus according to claim 1, wherein said third means comprises means for providing a plurality of replicas of said prescribed encoding sequence each of which is shifted in time with respect to other replicas of said plurality and combining each time shifted replica of said prescribed encoded sequence with said replica of said spreading sequence so as to produce said plurality of output signals.

3. An apparatus according to claim 2, wherein said third means comprises a recirculating shift register through respective stages of which said encoding sequence provided by said first means is successively shifted and a plurality of modulo-two adder circuits, one input of each of which is coupled to a respective stage of said shift register and another input of each of which is coupled to said second means to receive said spreading sequence, the outputs of said plurality of modulo-two adder circuits producing said plurality of output signals.

4. An apparatus according to claim 3 , wherein said fourth means comprises a plurality of respective correlator stages, each correlating a first input with a second input and generating an output representative of the correlation between said first and second inputs, said first input corresponding to said received unknown signal and said second input corresponding to the output of a respective modulo-two adder.

5. An apparatus according to claim 4, wherein said fourth means includes an output link from which said output representative of the original information signals is derived, and includes means for selectively coupling the outputs of said respective correlator stages to said output link.

6. An apparatus according to claim 5, wherein said fourth means includes means for sequentially coupling the outputs of said respective correlator stages to said output link in accordance with a prescribed timing sequence.

7. An apparatus according to claim 1, wherein said prescribed encoding sequence comprises a pseudo-noise sequence.

8. An apparatus according to claim 7, wherein said spreading sequence comprises a pseudo-noise sequence.

9. An apparatus according to claim 8, wherein said information signals are modulated as cyclic code shift keying signals.

10. An apparatus according to claim 1, wherein said information signals are modulated as pulse position modulation signals.

12. An apparatus according to claim 1, wherein said third means comprises means for providing a plurality of replicas of said spreading sequence each of which is shifted in time with respect to other replics of said plurality and combining each time-shifted replica of said spreading sequence with a respective portion of said prescribed encoding sequence so as to produce said plurality of output signals.

13. An apparatus according to claim 12, wherein said third means further comprises storage means in respective stages of which respective portions of said encoding sequence are stored, and a plurality of modulo-two adder circuits, one input of each of which is coupled to a respective stage of said storage means and another input of each of which is coupled to receive a respective time-shifted replica of said spreading sequence, the outputs of said plurality of modulo-two adder circuits, the outputs of said plurality of modulo-two adder circuits producing said plurality of output signals.

14. An apparatus according to claim 13, wherein said fourth means comprises a shift register through respective stages of which said unknown information signals are sucessively shifted, and a plurality of respective correlator stages, each correlating a first input with a second input and generating an output representative of the correlation between said first and second inputs, said first input being coupled to a respective stage of said shift register and said second input being coupled to the output of a respective one of said modulo-two adder circuits.

15. An apparatus according to claim 14, wherein said prescribed encoding sequence comprises a finite length pseudo-random code pattern and said spreading sequence comprises an effectively non-repetitive pseudo-random code sequence.

11. An apparatus according to claim I, wherein said prescribed encoding sequence comprises a finite length pseudo-random code pattern and said spreading sequence comprises an effectively non-repetitive pseudo-random code sequence.

16. A spread spectrum demodulation apparatus for recovering information signals that have been combined with a prescribed encoding sequence and spread-modulated onto a carrier for transmission comprising:

first means for providing a replica of said prescribed encoding sequence;

second means for providing a replica of the spreading sequence through which the combined information signals have been spread spectrum-modulated onto said carrier;

third means, coupled to said first and second means, for combining respective portions of said encoding sequence with said replica of said spreading sequence and producing therefrom a plurality of first output signals;

fourth means, coupled to said third means, for combining each of said plurality of first output signals with received unknown information signals and producing therefrom a plurality of second output signals; and

fifth means, coupled to said fourth means, for successively summing said second output signals to derive an output representative of the original information signals.

17. An apparatus according to claim 16, wherein said third means comprises a plurality of modulo-two adder circuits inputs of which are respectively coupled to receive respective portions of said encoding sequence and said spreading sequence, and outputs of which are coupled to supply said first output signals to said fourth means.

18. An apparatus according to claim 17, wherein said fourth means comprises a plurality of multipliers coupled to receive said first output signals and said unknown information signals and to produce said second output signals therefrom.

19. An apparatus according to claim 18, wherein said fifth means comprises a plurality of storage means successivley cascaded to receive and store the sum of the second output signals produced by said multipliers.

20. A spread spectrum signal demodulation apparatus for a recovering information signals that have been combined with a prescribed encoding sequence and spread spectrum-modulated onto a carrier for transmission comprising:

first means for storing a replica of said prescribed encoding sequence;

second means for combining said replica of said prescribed encoding sequence with signals corresponding to the spreading sequence through which the combined information signals have been modulated onto said carrier; and

third means for correlating the output of said second means with received unknown information signals to obtain an output representative of the original information signals, said third means comprising a plurality of respective correlator stages, each stage correlating a first input with a second input and generating a respective output representative of the correlation between said first and second inputs, said first input corresponding to a respective portion of the output of said second means and said second input. corresponding to said received unknown information signals, and including an output link from which said output representative of the original information signals is derived, and including means for selectively coupling the outputs of said respective correlator stages to said output link.

21. An apparatus according to cIaim 20, wherein said third means includes means for sequentially coupling the outputs of said respective correlator stages to said output link in accordance with a prescribed timing sequence.

22. An apparatus according to claim 21, wherein said prescribed encoding sequence comprises a finite length pseudo-random code pattern and said spreading sequence comprises an effectively non-repetitive pseudo-random code sequence.

23. An apparatus according to claim 22, wherein said second means comprises a plurality of modulo-two adders corresponding to the respective stages of said multiple stage storage means, one of the inputs of each of said modulo-two adders being coupled to a respective stages of said multiple stage storage means and other input of each of said modulo-two adders being coupled to receive said effectively non-repetitive pseudo-random code sequence.