Spread spectrum communication system utilizing differential code shift keying

• US 6,064,695 A
• Filed: 06/20/1997
• Issued: 05/16/2000
• Est. Priority Date: 06/20/1997
• Status: Expired due to Fees

• Current Assignee: Yitran Communications Ltd. (Allalouf Capital Ltd.)
• Original Assignee: Yitran Technologies Ltd.
• Inventors: Raphaeli, Dan
• Primary Examiner(s): Bocure, Tesfaldet
• Assistant Examiner(s): BAYARD, EMMANUEL
• Application Number:
  US08/879,522
• Time in Patent Office: 1,061 Days
• Field of Search: 375/200, 375/204, 375/205, 375/206, 375/209, 375/259, 375/295, 375/316, 375/244, 375/283, 375/330, 370/300, 370/335, 370/342, 370/441, 370/479, 370/282, 370/284
• US Class Current: 375/230
• CPC Class Codes: H04B 1/69 Spread spectrum techniques
• CPC Class Codes: H04B 1/7093 Matched filter type
• CPC Class Codes: H04B 2001/6912 using chirp
• CPC Class Codes: H04B 2201/70703 using multiple or variable rates
• CPC Class Codes: H04L 23/02 adapted for orthogonal signalling
• CPC Class Codes: H04L 27/103 Chirp modulation for spread spectrum techniques H04B1/69

Abstract

A spread spectrum data communication system utilizing a modulation technique referred to as differential code shift keying (DCSK) transmits data in the form of time shifts between consecutive circularly rotated waveforms of length T known as spreading waveforms. A plurality of bits are transmitted during each symbol period which is divided into a plurality of shift indexes with each shift index representing a particular bit pattern. The spreading waveform is rotated by an amount in accordance with the data to be transmitted or is conveyed in the difference between two consecutive rotations. A correlator employing a matched filter having a template of the chirp waveform pattern is used to detect the amount of rotation of the chirp within the received signal for each symbol. The received data is fed into a shift register and circularly rotated. For each rotation shift, the matched filter generates a correlation sum. The shift index chosen for each symbol corresponds to the shift index that yields the maximum correlation sum. Differential shift indexes are generated by subtracting the currently received shift index from the previously received shift index. The differential shift index is then decoded to yield the originally transmitted data.

Claims

1. A method of communication over a communications channel from a transmitter to a receiver both connected to the communications channel, said method comprising the steps of:

generating a plurality of symbols at the transmitter, each symbol constructed from a spreading waveform circularly shifted by an amount in accordance with the data to be conveyed by said symbol;

generating a transmission signal in accordance with said plurality of symbols;

transmitting said transmission signal onto the communications channel;

receiving said transmission signal from the communications channel at the receiver and generating a receive signal therefrom;

decoding said receive signal at the receiver by circularly shifting said received signal wherein for each circular shift, the received signal is correlated with a template corresponding to the spreading waveform so as to generate a correlation sum therefrom; and

determining receive data in accordance with the shift corresponding to the maximum correlation sum.

3. The method according to claims 1 or 2, wherein said spreading waveform comprises a chirp waveform.

4. The method according to claims 1 or 2, wherein said spreading waveform comprises a superchirp waveform constructed from a plurality of individual chirps.

2. A method of communication over a communications channel from a transmitter to a receiver both connected to the communications channel, said method comprising the steps of:

generating a plurality of symbols at the transmitter, each symbol constructed from a spreading waveform circularly shifted by an amount in accordance with the data to be conveyed by said symbol;

generating a transmission signal in accordance with said plurality of symbols for transmission over said communications channel;

receiving said transmission signal from the communications channel at the receiver and generating a receive signal therefrom; and

decoding said received signal at the receiver by correlating the received signal with a template corresponding to the spreading waveform so as to generate a differential shift index representing the time shift between consecutive circular shifts of the spreading waveform.

5. The method according to claim 2, wherein said step of decoding comprises the steps of:

circularly shifting each received symbol;

for each circular shift of the received symbol, correlating the received symbol with the template corresponding to the spreading waveform so as to generate a correlation sum;

generating a shift index corresponding to a maximum correlation sum; and

decoding the shift index to yield the original transmitted data.

6. The method according to claim 2, wherein said step of decoding comprises the steps of:

circularly shifting each received symbol;

for each circular shift of the received symbol, correlating the received symbol with the template corresponding to the spreading waveform;

generating a first shift index and a second shift index corresponding to a positive correlation sum and a negative correlation sum, respectively;

decoding said first shift index and said second shift index to yield a first data output and a second data output, receptively; and

outputting either said first shift index or said second shift index based on a maximum of said positive correlation sum and said negative correlation sum.

7. A spread spectrum communications system for communication over a communication channel, comprising:

a transmitter coupled to the communication channel, said transmitter for generating a plurality of symbols, each symbol constructed utilizing a spreading waveform circularly shifted by an amount in accordance with the data to be conveyed by said symbol; and

a receiver coupled to the communication channel, said receiver for receiving a signal from the communications channel and for decoding said plurality of symbols by circularly shifting said received signal wherein for each circular shift, the received signal is correlated with a template corresponding to the spreading waveform so as to generate correlation sum therefrom, determining receive data in accordance with the shift corresponding to the maximum correlation sum.

8. The system according to claim 7, wherein said spreading waveform comprises a chirp waveform.

9. The system according to claim 8, wherein said spreading waveform comprises a superchirp waveform constructed from a plurality of individual chirps.

10. A method of generating a signal for transmission over a communications channel from an input bit stream and utilizing a spreading waveform, said method comprising the steps of:

forming a serial stream of shift indexes from the input bit stream;

determining an initial index in accordance with each shift index within said serial stream of shift indexes;

circularly shifting the spreading waveform in accordance with the initial index; and

transmitting the circularly shifted spreading waveform onto the communication channel.

11. A method of generating a spread spectrum signal for transmission over a communications channel from an input bit stream and utilizing a spreading waveform, said method comprising the steps of:

forming a shift index from the input bit stream;

determining an initial index in accordance with the following equation;

##EQU7## circularly shifting the spreading waveform in accordance with the initial index; and

transmitting the circularly shifted spreading waveform onto the communication channel.

12. The method according to claim 11, wherein said spreading waveform comprises a chirp waveform.

13. The method according to claims 11 or 12, wherein said spreading waveform comprises a superchirp waveform constructed from a plurality of individual chirps.

14. The method according to claim 11 or 12, further comprising the step of differentiating the input bit stream to yield a differential shift index.

15. A transmitter for generating a signal for transmission over a communications channel from an input bit stream and utilizing a spreading waveform, comprising:

means for forming a shift index from each group of n bits in the input in bit stream;

means for determining an initial index in accordance with the shift index;

means for circularly shifting the spreading waveform in accordance with the initial index; and

means for transmitting the circularly shifted spreading waveform over the communication channel.

16. The transmitter according to claim 15, wherein said spreading waveform comprises a chirp waveform.

17. The transmitter according to claim 15, wherein said spreading waveform comprises a superchirp waveforn constructed from a plurality of individual chirps.

18. The transmitter according to claim 15, wherein said means for circularly shifting the spreading waveform comprises:

counting means adapted to receive the initial index; and

look up table means for outputting sample points of the spreading waveform corresponding to the output of said counting means.

19. The transmitter according to claim 15, further comprising a differentiator for differentiating the input bit stream to yield a differential shift index.

20. A receiver coupled to a communications channel for receiving a plurality of symbols, each symbol consisting of a spreading waveform circularly rotated by an amount in accordance with the data to be transmitted during a particular symbol time, said receiver comprising:

sampling means for sampling a receive input signal;

shift means for circularly rotating the output of said sampling means, said shift means having a plurality of taps;

correlation means coupled to said plurality of taps of said shift means, said correlation means generating a correlation sum for each circular shift of said shift means utilizing a template corresponding to the spreading waveform, said correlation means generating a plurality of correlation sums for each received symbol;

maximum correlation detection means for determining a maximum correlation sum from said plurality of correlation sums; and

a data decoder for decoding a shift index associated with said maximum correlation sum and generating an output therefrom.

23. The receiver according to claims 20, 21 or 22, wherein said spreading waveform comprises a chirp waveform.

24. The receiver according to claims 20, 21 or 22, wherein said spreading waveform comprises a superchirp waveform constructed from a plurality of individual chirps.

25. The receiver according to claims 20, 21 or 22, further comprising means for synchronizing said correlation means to a symbol time.

26. The receiver according to claims 20, 21 or 22, further comprising a differentiator coupled to the output of said maximum correlation detection means, said differentiator for generating a differential shift index corresponding to the time difference between two consecutive circularly rotated spreading waveforms.

27. The receiver according to claims 20, 21 or 22, further comprising an integrator coupled to the output of said data decoder, said integrator for integrating the output of said data decoder.

29. The receiver according to claims 20, 21 or 22, wherein said sampling means comprises a one bit analog to digital converter.

30. The receiver according to claims 20, 21 or 22, wherein said sampling means comprises a comparator and a sample circuit.

31. The receiver according to claims 20, 21 or 22, wherein said sampling means comprises means for generating both an I or inphase data stream and a Q or quadrature data stream, wherein said Q data stream is delayed in time with respect to said I data stream by a predetermined amount.

32. The receiver according to claims 20, 21 or 22, wherein said correlation means comprises complex correlation means.

33. The receiver according to claim 32, wherein said complex correlation means comprises means for applying a non linear function to the results of said complex correlation.

34. The receiver according to claim 33, wherein said non linear function comprises a squaring function.

35. The receiver according to claims 20 or 22, wherein said shift means comprises circular shift means.

36. The receiver according to claim 20, 21 or 22, further comprising tracking means for maintaining alignment of said correlation means to a symbol time.

21. A receiver coupled to a communications channel for receiving data encoded as a plurality of symbols, each symbol transmitted utilizing a spreading waveform, said receiver comprising:

signal splitting means for dividing a receive input signal into a plurality of frequency bands, said signal splitting means outputting a plurality of band pass signals, each band pass signal associated with a single frequency band;

sampling means for sampling said plurality of band pass signals;

correlation means for correlating the output of said sampling means for each frequency band, said correlation means generating a plurality of band correlation sums for each frequency band;

summing means for summing each plurality of band correlation sums to yield a plurality of correlation sums;

maximum correlation detection means for determining a maximum correlation sum from said plurality of correlation sums; and

data decoding means for decoding the received symbols utilizing said maximum correlation sum and generating an output therefrom.

28. The receiver according to claims 21 or 22, wherein said signal splitting means comprises a plurality of band pass filters, each band pass filter having a bandwidth and center frequency according to its frequency band.

22. A receiver coupled to a communications channel for receiving data encoded as a plurality of symbols, each symbol consisting of a spreading waveform circularly rotated by an amount in accordance with the data to be transmitted during a particular symbol time, said receiver comprising:

signal splitting means for dividing a receive input signal into a plurality of frequency bands, said signal splitting means outputting a plurality of band pass signals, each band pass signal associated with a single frequency band;

a plurality of sampling means for sampling said plurality of band pass signals;

a plurality of shift means for circularly rotating the output of each sampling means associated with each frequency band, each shift means having a plurality of taps;

a plurality of correlation means, each correlation means coupled to the output of one of said shift means, said correlation means generating a correlation sum for each circular shift of said shift means utilizing a template corresponding to the spreading waveform, each correlation means generating a plurality of band correlation sums for each received symbol;

summing means for summing each plurality of band correlation sums output by each correlation means so as to generate a plurality of correlation sums;

maximum correlation detection means for determining a maximum correlation sum from said plurality of correlation sums; and

a data decoder for decoding a shift index associated with said maximum correlation sum and generating an output therefrom.

37. A method of receiving data that is encoded as a plurality of symbols and transmitted over a communications channel, each symbol transmitted utilizing a spreading waveform, said method comprising the steps of:

dividing a receive input signal into a plurality of frequency bands, generating a plurality of band pass signals, each band pass signal associated with a single frequency band;

sampling said plurality of band pass signals to yield a sample stream;

correlating said sample stream associated with each frequency band so as to generate a plurality of band correlation sums therefrom;

summing each plurality of band correlation sums so as to generate a plurality of correlation sums;

determining a maximum correlation sum from said plurality of correlation sums; and

decoding a shift index for each received symbol utilizing said maximum correlation sum and generating an output therefrom.

39. The method according to claims 37 or 38, wherein said spreading waveform comprises a chirp waveform.

40. The method according to claims 37 or 38, wherein said spreading waveform comprises a superchirp waveform constructed from a plurality of individual chirps.

41. The method according to claims 37 or 38, further comprising the step of synchronizing said correlation to a symbol time.

42. The method according to claims 37 or 38, further comprising the step of differentiating said shift index to yield a differential shift index corresponding to the time difference between two consecutive circularly rotated spreading waveforms.

43. The method according to claims 37 or 38, further comprising the step of integrating the output of the step of decoding.

44. The method according to claims 37 or 38, wherein said step of dividing comprises the step of band pass filtering each frequency band with a bandwidth and center frequency according to each frequency band.

45. The method according to claims 37 or 38, wherein said step of sampling comprises the step of providing and applying a one bit analog to digital converter to each pass band signal.

46. The method according to claims 37 or 38, wherein said step of sampling comprises the step of providing and applying a comparator and a sample circuit to each pass band signal.

47. The method according to claims 37 or 38, wherein said step of sampling means comprises the step of generating both an I or inphase data stream and a Q or quadrature data stream, wherein said Q data stream is delayed in time with respect to said I data stream by a predetermined amount.

48. The method according to claims 37 or 38, wherein said step of correlating comprises applying a complex correlation to said sample stream.

49. The method according to claim 48, wherein said step of applying a complex correlation comprises the step of applying a non linear function to the results of said complex correlation.

50. The method according to claim 49, wherein said non linear function comprises a squaring function.

51. The receiver according to claims 37 or 38, the comprising the step of maintaining alignment of said correlation to a symbol time.

38. A method of receiving data encoded as a plurality of symbols and transmitted over a communication channel, each symbol consisting of a spreading waveform circularly rotated by an amount in accordance with the data to be transmitted during a particular symbol time, said method comprising the steps of:

dividing a receive input signal into a plurality of frequency bands, generating a plurality of band pass signals, each band pass signal associated with a single frequency band;

sampling said plurality of band pass signals to yield a sample stream;

circularly rotating said sample stream of each frequency band;

correlating the circularly rotated sample stream for each frequency band utilizing a template corresponding to the spreading waveform, generating a band correlation sum for each circular rotation to yield a plurality of band correlation sums for each symbol;

summing said plurality of band correlation sums for each frequency band so as to generate a plurality of correlation sums;

determining a maximum correlation sum from said plurality of correlation sums; and

decoding a shift index associated with said maximum correlation sum and generating an output therefrom.

52. In a spread spectrum communications system for communication over a communications channel, including a transmitter and a receiver both connected to the communications channel, a method for synchronizing the receiver, said method comprising the steps of:

transmitting a plurality of spreading waveforms onto said communications channel, said plurality of spreading waveforms having zero differential shift with respect to each other;

receiving said plurality of spreading waveforms from said communications channel so as to generate a receive signal; and

decoding said receive signal and declaring synchronization upon receiving a minimum of a predetermined number of spreading waveforms having zero differential shift between them.

53. The method according to claim 52, wherein said spreading waveform comprises a chirp waveform.

54. The method according to claim 52, wherein said spreading waveform comprises a superchirp waveform constructed from a plurality of individual chirps.

55. The method according to claim 52, wherein each spreading waveform has zero rotational shift.