Receiver for use in a code shift keying spread spectrum communications system

• US 6,671,311 B1
• Filed: 04/18/2000
• Issued: 12/30/2003
• 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, Matmor, Avner
• Primary Examiner(s): Corrielus, Jean B.
• Application Number:
  US09/551,449
• Time in Patent Office: 1,351 Days
• Field of Search: 375/130, 375/135, 375/136, 375/140, 375/142, 375/146, 375/147, 375/150, 375/219, 375/222, 375/343, 375/316, 375/295, 375/367, 375/133, 375/141, 370/203, 370/300, 370/335, 370/342, 370/441, 370/479, 370/282, 370/284
• US Class Current: 375/142
• CPC Class Codes: H04B 1/69 Spread spectrum techniques
• CPC Class Codes: H04B 1/7075 with code phase acquisition
• CPC Class Codes: H04B 1/7095 Sliding correlator 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

Abstract

A direct sequence spread spectrum receiver for use in a communication system that utilizes code shift keying (CSK) modulation. Code shift keying modulation transmits data in the form of circularly rotated spreading waveforms such as PN sequences. The data is conveyed in the amount of rotation applied to the spreading waveform before it is transmitted. While tracking, the receiver decodes the received symbols yielding the original transmitted data. The input frequency range of the receiver is divided into one or more frequency bands. For each frequency band, the received signal is filtered, digitized and split into I and Q data streams wherein the Q data stream is delayed by ¼ƒ_c. Both I and Q data streams are sampled and correlated with an adaptive template. The received data is clocked into a shift register and circularly rotated and a correlation sum is generated. The correlation output of both I and Q channels are summed over all the frequency bands and a maximum correlations is determined. A shift index is chosen corresponding to the shift index yielding the maximum correlation sum. The templated in the receiver is dynamically adapted to the varying conditions of the channel. The template used by the correlator is continuously updated at each sample time using a template adaption function.

Claims

1. A receiver for use in a code shift keying spread spectrum communication system comprising:

a front end circuit coupled to a channel and adapted to filter a receive input signal into N streams, each receive sample stream associated with one of said frequency bands;

N receiver circuits coupled to said front end circuit, each receiver circuit associated with a frequency band, each receiver circuit comprising;

a template comprising a plurality of taps;

initialization means operative to generate an initial value for said template from a plurality of training symbols stripped of rotation and phase whereby said template is initialized to said channel;

means for generating receive symbols from a corresponding one of said receive sample streams;

a correlator adapted to correlate the contents of said template with said received symbols, said correlator operative to generate a correlation sum for each relative rotation between said templated and said received symbols, for all possible transmitted rotations of a symbol;

a combiner for combining the correlation sums generated by said N receiver circuits so as to yield a total correlation value;

a maximum correlator detector operative to determine a shift index corresponding to a maximum total correlation value over all rotations of said template, said shift index subsequently decoded to yield receive data; and

wherein N is a positive integer.

2. The receiver according to claim 1, wherein said initialization means is adapted to initialize said template using an average of said plurality of training symbols stripped of rotation and phase.

3. The receiver according to claim 1, further comprising means for adapting said template subsequent to initialization by said plurality of training symbols utilizing a plurality of received symbols stripped of rotation and phase.

4. The receiver according to claim 1, wherein said front end circuit comprises N band pass filters adapted to filter said receive input signal into N frequency bands, each band pass filter having a center frequency ƒ

_c.

5. The receiver according to claim 1, wherein said plurality of training symbols used for initialization of said template is also used for acquisition of a receive sample stream within each receiver circuit.

6. The receiver according to claim 1, wherein said template is used during a process of the acquisition, the results of which are stored and used as a template subsequently during data reception.

7. The receiver according to claim 1, wherein said front end circuit comprises N analog to digital converters, each analog to digital converter adapted to generate a receive sample stream for a particular frequency band.

8. The receiver according to claim 7 wherein said analog to digital converter comprises a one bit analog to digital converter.

9. The receiver according to claim 8, wherein said one bit analog to digital converter utilizes a comparator against the average voltage of said received signal.

10. The receiver according to claim 1, wherein said means for generating received symbols comprises means for generating I and Q symbols from a corresponding one of said receive sample streams.

11. The receiver according to claim 10, wherein said means for generating I and Q symbols comprises delay means adapted to delay a corresponding one of said receive sample streams by ¼

ƒ

_c yielding said Q symbols.

12. The receiver according to claim 1, wherein said means for generating received symbols comprises means for generating real symbols from a corresponding one of said receive sample streams.

13. The receiver according to claim 1, wherein said template is adapted to be rotatable and said received symbols are adapted to be rotated.

14. The receiver according to claim 1, wherein said template is adapted to be fixed and said received symbols are adapted to be rotated.

15. The receiver according to claim 1, wherein said front end comprises analog channel coupling circuitry adapted to interface said receiver to said channel and to output said receive signal therefrom.

16. The receiver according to claim 1, wherein said means for generating received symbols comprises:

a first sampler adapted to create a binary representation of a corresponding one of said N receive sample streams so as to yield an I received symbol stream;

delay means adapted to delay a corresponding one of said N received sample streams by ¼

ƒ

_c, wherein ƒ

_c is the center frequency of the associated frequency band; and

a second sampler adapted to create a binary representation of the output of said delay means so as to yield a Q receive symbol stream.

17. The receiver according to claim 1, wherein said template comprises a plurality of taps, each tap comprising one or more bits.

18. The receiver according to claim 1, wherein said correlator is adapted to receive a number of bits R per tap from said template, wherein R is independent of the number of bits per tap stored in said template.

19. The receiver according to claim 1, further comprising a function module operative to apply a function adapted to emphasize low and high values of said correlation sums.

20. The receiver according to claim 19, wherein said function module comprises means for applying the function k(x)=sign(x)·

x², wherein x represents said correlation sums.

21. The receiver according claim 1, further comprising sign bit circuitry for detecting the phase of the received symbols and outputting a receive data bit therefrom.

22. The receiver according claim 1, further comprising means for detecting a carrier phase of the received symbols and extracting n bits therefrom.

23. The receiver according claim 1, wherein N equals three.

24. The receiver according claim 1, wherein N equals three and the corresponding three frequency bands are 4 to 9 MHz and 9 to 15 MHz and 15 to 20 MHz, with center frequencies of 8, 12 and 16 MHz, respectively.

25. The receiver according claim 1, further comprising a template adaptation module operative to modify said template utilizing the current contents of said template and said received symbols so as to dynamically adjust said template to said channel.

26. The receiver according to claim 25, wherein said template adaptation module comprises means for applying an averaging function to said template and said received symbols.

27. The receiver according to claim 26, wherein said averaging function comprises

28. The receiver according claim 1, wherein said initialization means comprises means for applying and averaging function to said template and said received symbols.

29. The receiver according to claim 28, wherein said averaging function comprises

30. The receiver according to claim 1, wherein said front end circuit comprises a bi-directional filter adapted to filter said receive signal.

31. The receiver according to claim 1, wherein said front end circuit comprises:

a bi-directional filter adapted to filter said receive signal; and

a preamplifier adapted to amplify the signal output of said bi-directional filter before said signal is divided into multiple frequency bands.

32. The receiver according to claim 1, further comprising means for providing the correlation results of each relative rotation of said template and said received symbols.

33. In a code shift keying spread spectrum communications system, a method of receiving, said method comprising the steps of:

filtering an input signal received from a channel into N frequency bands and generating N receive sample streams therefrom, each receive sample stream associated with one of said frequency bands;

for each frequency band;

providing a template comprising a plurality of taps;

generating an initial value for said template from a first plurality of de-rotate and phase stripped training symbols thereby adapting said template to said channel;

synchronizing a symbol clock in response to a second plurality of training symbols;

generating receive symbols from a corresponding one of said receive sample streams in accordance with said symbol clock;

correlating the contents of said template with said received symbols so as to generate a correlation sum for each relative rotation between said template and said received symbols, for all possible transmitted rotations of a symbol;

combining the N correlation sums so as to yield a total correlation sum;

determining a shift index corresponding to a maximum total correlation sum over all rotations of said template;

decoding said shift index to yield receive data; and

wherein N is a positive integer.

34. The method according to claim 33, further comprising modifying said template utilizing the current contents of said template and said received symbol so as to enable said template to dynamically adjust said template to changes in said channel.

35. The method according to claim 33, wherein said step of generating receive symbols comprises the steps of:

sampling a corresponding one of said N receive sample streams so as to yield an I receive symbol stream;

delaying a corresponding one of said N receive sample streams by ¼

ƒ

_c to generate a delayed signal, wherein ƒ

_c is the center frequency of the associated frequency band; and

sampling said delayed signal so as to yield a Q receive symbol stream.

36. The method according to claim 33, wherein said template comprises a plurality of taps, each tap comprising one or more bits.

37. The method according to claim 33, further comprising the step of modifying said template utilizing the current contents of said template and said received symbols so as to dynamically adjust said template to said channel.

38. The method according to claim 37, wherein said step of modifying said template comprises applying an averaging function to said template and said received symbols.

39. The method according to claim 38, wherein said averaging functions comprises

40. The method according to claim 33, wherein said step of correlating comprises receiving a number of bits R per tap from said template, wherein R is independent of the number of bits per tap stored in said template.

41. The method according to claim 33, further comprising the step of applying a function adapted to emphasize low and high values of said correlation sum.

42. The method according to claim 41, wherein said function comprises k(x)=sign(x)·

x², wherein x represents said correlation sums.

43. The method according to claim 33, further comprising the step of detecting the phase of the received symbols and outputting a receive data bit therefrom.

44. The method according to claim 33, wherein N equals three.

45. The method according to claim 33, wherein N equals three and the corresponding three frequency bands are 4 to 9 MHz, 9 to 15 MHz and 15 to 20 MHz, with center frequency of 8, 12 and 16 MHz, respectively.

46. The method according to claim 33, wherein said template is adapted to be rotatable and said received symbols are adapted to be fixed.

47. The method according to claim 33, wherein said template is adapted to be fixed and said received symbols are adapted to be rotated.

48. A code shift keying spread spectrum communication transceiver, comprising:

a transmitter adapted to generate symbols for transmission over a channel, said symbols comprising a spreading waveform circularly shifted in accordance with the data to be conveyed by said symbol; and

a receiver, comprising;

a front end circuit coupled to said channel and adapted to filter a receive input signal into N frequency bands, said front end circuit operative to generate N receive sample streams, each receive sample stream associated with one of said frequency bands;

N receiver circuits coupled to said front end circuit, each receiver circuit associated with a frequency band, each receiver circuit comprising;

a template comprising a plurality of taps;

acquisition means operative to generate an initial value for said template from a plurality of training symbols that have been stripped of rotation and phase whereby said template is adapted to said channel;

means for generating receive symbols from a corresponding one of said receive sample streams;

a correlator adapted to correlate the contents of said template with said receive symbols, said correlator operative to generate a correlation sum for each relative rotation between said template and said received symbols, for all possible transmitted rotations of a symbol;

a combiner for combining the correlation sums generated by said N receiver circuits so as to yield a total correlation sum;

a maximum correlator detector operative to determine a shift index corresponding to a maximum total correlation sum over all rotations of said template, said shift index subsequently decoded to yield receive data; and

wherein N is a positive integer.

49. The transceiver according to claim 48, wherein said spreading waveform comprises a chirp waveform.

50. The transceiver according to claim 48, wherein said transmitter is operative to use linear shifts.

51. The transceiver according to claim 48, wherein said transmitter is operative to use circular shifts.

52. The transceiver according to claim 48, wherein said receiver is operative to use linear shifts.

53. The transceiver according to claim 48, wherein said receiver is operative to use circular shifts.

54. The transceiver according to claim 48, wherein said spreading waveform comprises a PN sequence.

55. The transceiver according to claim 48, further comprising a serial peripheral interface for communicating data and control information between a host and said transceiver.

56. The transceiver according to claim 48, further comprising a bi-directional filter adapted to band pass filter a receive signal during reception and to filter a transmit signal during transmission.

57. The transceiver according to claim 48, further comprising means for providing the correlation results of each relative rotation of said template and said received symbols.