Method and apparatus for efficient synchronization in spread spectrum communications

• US 6,567,482 B1
• Filed: 03/05/1999
• Issued: 05/20/2003
• Est. Priority Date: 03/05/1999
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Popovic', Branislav M.
• Primary Examiner(s): PHU, PHUONG M
• Application Number:
  US09/263,106
• Time in Patent Office: 1,537 Days
• Field of Search: 375/343, 375/367, 375/134, 375/137, 375/140, 375/142, 375/143, 375/145, 375/148, 375/149, 375/150, 375/152, 370/515, 370/509, 370/514, 370/350
• US Class Current: 375/343
• CPC Class Codes: H04B 1/708 Parallel implementation
• CPC Class Codes: H04B 1/7093 Matched filter type
• CPC Class Codes: H04B 7/2668 Arrangements for Wireless Code-Division Multiple Access [CDMA] System Synchronisation, for code acquisition H04B1/7075, for code tracking H04B1/7085
• CPC Class Codes: H04J 13/0014 Golay
• CPC Class Codes: H04L 7/041 using special codes as synchronising signal
• CPC Class Codes: H04W 56/00 Synchronisation arrangements
• CPC Class Codes: H04W 74/08 Non-scheduled access, e.g. ALOHA hybrid access H04W74/02

Abstract

The present invention employs one or more complementary sequences, e.g., Golay pairs of sequences, to provide accurate and efficient synchronization between radio transceivers. A matched filter corresponding to a complementary pair of sequences is used to correlate the received signal with one of the complementary pair of sequences. A peak output is detected from the matched filter, and the detected peak is used to generate a timing estimate for synchronizing transmissions between transceivers. Each of the complementary sequences has minimal aperiodic autocorrelation sidelobe values for all non-zero delays of that complementary sequence and a maximal autocorrelation main lobe value for a zero delay of that complementary sequence. The one sequence may be used as a preamble portion of a random access message transmitted by a mobile station to a base station over a random access channel. The sequence may also be used as a synchronization code transmitted by the base over a synchronization channel and used by mobile stations to obtain synchronization. An efficient Golay correlator and a bank of efficient Golay correlators for correlating respectively with a single sequence and with a set of orthogonal sequences are disclosed.

Claims

1. A method for correlating at a first transceiver a received signal transmitted by a second transceiver, comprising:

using a matched filter corresponding to a complementary pair of sequences to correlate the received signal with one of the complementary pair of sequences, where the complementary pair of sequences has a property that the sum of their aperiodic autocorrelations equals zero for all non-zero shifts;

detecting a peak output from the matched filter,

using the detected peak output to generate a timing estimate for synchronizing transmissions between the first and second transceivers; and

using complex correlation values corresponding to detected peak outputs as fading channel coefficients in demodulating a message included in the received signal.

2. The method in claim 1, wherein the matched filter correlates the received signal with both complementary sequences at the same time.

4. The method in claim 1, wherein the first transceiver is a base station and the second transceiver is a mobile station, and wherein the one sequence is used as a preamble portion of a random access message transmitted by the mobile station to the base station over a random access channel.

5. The method in claim 4, wherein the complementary sequences are Golay sequences of a length 2 or longer.

6. The method in claim 5, wherein each of the Golay sequences is generated by concatenating a pair of shorter Golay sequences.

7. The method in claim 1, wherein the first transceiver is a mobile station and the second transceiver is a base station, and wherein the one sequence is used as a synchronization sequence transmitted by the base station over a broadcast channel.

8. The method in claim 1, further comprising:

employing a plurality of the matched filters, each corresponding to an orthogonal, complementary pair of sequences, wherein the peak output is detected from one of the matched filters.

3. The method in clam 1, wherein the matched filter correlates the received signal with the complementary sequences recursively.

9. A method for correlating at a first transceiver a received signal transmitted by a second transceiver, comprising:

using a matched filter corresponding to a complementary pair of sequences to correlate the received signal with one of the complementary pair of sequences;

detecting a peak output from the matched filter, and using the detected peak output to generate a timing estimate for synchronizing transmissions between the first and second transceivers,

wherein the complementary pair of sequences is a complementary pair of sequences determined in accordance with the following;

10. A method for generating a synchronization sequence at a first transceiver for transmission to a second transceiver, comprising:

generating a sequence from a pair of sequences having minimal aperiodic autocorrelation values for all non-zero delays of that sequence and a maximal autocorrelation value for a zero delay of that binary sequence; and

transmitting a signal from the first transceiver using the sequence to permit a second transceiver to synchronize with the first transceiver,

wherein the sequence is obtained from a pair of sequences of length L=2^N, where N is a positive integer, and

wherein the pair of sequences is determined in accordance with the following;

11. The method in claim 10, wherein the first transceiver is contained in a mobile radio and the second transceiver is contained in a base station, and the mobile station transmits the sequence over a random access channel associated with the base station.

12. The method in claim 11, wherein the sequence is known by both the base station and the mobile station.

13. The method in claim 12, wherein the base station performs a correlation process upon signals received from the mobile station using the sequence to detect a time in the correlation process when the a maximal autocorrelation value is obtained.

14. The method in claim 11, wherein the sequence is used in a preamble of random access burst transmitted by the mobile station over the random access channel.

15. The method in claim 10, further comprising:

generating a set of sequences of length L, each sequence having a minimal aperiodic autocorrelation value for all non-zero delays of that sequence and a maximal autocorrelation value for a zero delay of that sequence,

wherein the first transceiver transmits one of the sequences in the set to permit the second transceiver to synchronize with the first transceiver.

16. The method in claim 15, wherein the sequences in the set are orthogonal.

17. The method in claim 10, wherein the first transceiver is contained in a base station and the second transceiver is contained in a mobile radio, and wherein the base station broadcasts the sequence on a broadcast channel to permit the mobile transceiver to synchronize at least roughly with the base station.

18. The method in claim 10, wherein the length is 4096.

19. The method in claim 10, where a₀(k)=A(k) and b₀(k)=B(k) form a pair of binary sequences of length T and the delay D_n=T*2^P_n.

20. The method in claim 10, where a set of 2^N orthogonal sequences is obtained from a common permutation vector P_n and a set of 2^N−

1 weighting vectors W_n, wherein the set of weighting vectors is an N-bit long binary representation of different, even positive integers or different, odd, positive integers taken from a set of integers {0,1, . . . , 2<sup>N−

1</sup>}, and wherein a zero in the binary representation is mapped to one of {−

1, +1} and a one in the binary representation is mapped to the other of {−

1,+1}.

21. The method in claim 20, where a₀(k)=A(k) and b₀(k)=B(k) form a pair of binary sequences of length T and the delay D_n=T*2^P_n.

22. The method in claim 19, further comprising:

directly generating the pair of sequences using a sequence generator that requires only N memory elements.

23. The method in claim 19, further comprising:

directly generating the pair of sequences using a sequence generator that includes a modulo 2^N counter.

24. The method in claim 19, wherein A(k)=B(k).

25. A method for synchronizing a first transceiver to a second transceiver, comprising:

correlating a received signal with one of a pair of sequences using an efficient correlation procedure providing a minimal number of mathematical operations, where the pair of sequences has a property that the sum of their aperiodic autocorrelations equals zero for all non-zero time shifts;

detecting at what point in time during the correlating a maximal autocorrelation value is obtained; and

synchronizing a next transmission to the second transceiver based on the detected time,

wherein the pair of sequences are determined in accordance with the following;

26. The method in claim 25, wherein the correlating step is performed efficiently to reduce a number of data processing steps required to perform the correlating.

27. The method in claim 25, wherein the correlating step is performed efficiently to reduce an amount of memory required to perform the correlating.

28. The method in claim 25, wherein the correlating step includes correlating the pair of sequences simultaneously with the input signal.

29. The method in claim 25, wherein the pair of sequences are Golay sequences, and wherein the one of the Golay sequences is used as a primary synchronization sequence.

30. In a direct sequence, spread spectrum radio transceiver, a sequence correlator efficiently correlating a received spread spectrum signal with a pair of complementary sequences of length L=2^N with N being any positive integer, comprising:

N serially concatenated processing stages, each stage having first and second parallel processing branches;

each stage of the first processing branch including a delay line having plural memory elements coupled to a corresponding adder;

wherein in each processing stage, samples of an input signal are successively stored in memory elements of the corresponding delay line, the content of a last memory element being input to the adder and to the subtractor of the same processing stage; and

wherein the input signal is multiplied in the multiplier in the same stage by a corresponding weighting coefficient, and the output of the multiplier is fed as an input to the subtractor and to the adder in the same stage.

31. The correlator in claim 30, wherein the output of the first processing branch at the N-th processing stage produces a cross-correlation of the received spread spectrum signal with a first sequence from a pair of sequences, and the output of the second processing branch at the N-th processing stage produces a cross-correlation of the received spread spectrum signal with a second sequence from a pair of sequences.

32. The correlator in claim 31, wherein the sequences are determined in accordance with the following:

33. The correlator in claim 31, wherein the weighting coefficient is a complex conjugate of W_n.

36. The correlator in claim 34, wherein the first correlator is a correlator as defined in claim 30.

43. The method in claim 42, wherein the Golay correlator corresponds to the correlator defined in claim 30.

55. The bank of correlators in claim 53, wherein each correlator corresponds to the correlator in claim 30.

34. A correlator correlating at a first transceiver a received spread spectrum signal transmitted by at a second transceiver using one of the sequences from a pair of sequences of length L=2^N, where N is any positive integer, and where the one sequence is represented as a concatenation of two shorter, constituent sequences of length X, each constituent sequence having been repeated L/X times and modulated by +1 or −

1 according to a signature sequence of length Y=L/X, comprising;

a first correlator of a length X receiving the spread spectrum signal and generating an intermediate pair of correlation values corresponding to the constituent sequences;

a first selector alternately supplying one of the intermediate pair of correlation values during each of successive time windows based on a current value of an interleaving sequence of length Y=L/X whose elements belong to the set {0,1}, where an order of concatenation of the constituent sequences is determined by the interleaving sequence;

a multiplier multiplying the selected intermediate correlation values with an element of the signature sequence to generate a multiplied correlation output; and

a summer summing the multiplied correlation output with a feedback output of a delay line having X memory elements and providing the resulting sum as an input to the delay line,

wherein a final correlation value corresponds to the resulting sum after Y successive time windows.

35. The correlator in claim 34, wherein n=12, L=4096, X=256, and Y=16.

37. The correlator in claim 34, wherein the correlator correlates the received spread spectrum signal with plural orthogonal sequences generated using plural signatures, further comprising:

a plurality of multipliers each multiplying the selected intermediate correlation values with an element of a different signature sequence to generate a corresponding multiplied correlation output;

a plurality of summers, each summer corresponding to one of the multipliers and summing the multiplied correlation output with a feedback output of a corresponding delay line having X memory elements and providing the resulting sum as an input to the delay line,

wherein a final correlation value for each of the orthogonal sequences from each of the summers corresponds to each summer'"'"'s resulting sum after Y successive time windows.

38. The correlator in claim 34, wherein there are 16 signatures, 16 multipliers, 16 summers, and 16 delay lines producing 16 correlation values corresponding to 16 orthogonal sequences.

39. The correlator in claim 38, wherein there are 32 signatures, 32 multipliers, 32 summers, and 32 delay lines producing 32 correlation values corresponding to 32 orthogonal sequences.

40. The correlator in claim 39, wherein the first selector output is coupled to 16 of the 32 multipliers along with 16 of the signatures, further comprising:

a second selector alternately supplying the other of the intermediate pair of correlation values during each of the successive time windows based on a current value of a second interleaving sequence of length Y=L/X whose elements belong to the set {0,1}, where an order of concatenation of the constituent sequences is determined by the interleaving sequence,

wherein the second selector output is coupled to the remaining 16 of the 32 multipliers along with the remaining 16 signatures.

41. The correlator in claim 34, wherein the pair of sequences is a Golay pair and the first correlator is an efficient Golay correlator using a reduced amount of required memory or data processing operations.

42. A method for correlating at a first transceiver a received spread spectrum signal transmitted by at a second transceiver using one of the sequences from a pair of Golay complementary sequences of length L=2^N, where N is any positive integer, comprising:

representing the one Golay sequence as a concatenation of two shorter, constituent complementary sequences A(k) and B(k) of length X, each constituent sequence having been repeated L/X times and modulated by +1 or −

1 according to a signature sequence of length Y=L/X, where an order of concatenation is determined by an interleaving sequence of length Y=L/X, whose elements are 0 or 1, comprising;

using a Golay correlator of a length X receiving the spread spectrum signal to generate an intermediate pair of correlation values corresponding to the constituent Golay complementary sequences;

storing the intermediate correlation values at a first correlation output of the Golay correlator in a first group of Y=(L/X)−

1 or less serially concatenated delay lines, where each delay line has X memory elements;

storing the intermediate correlation values at a second correlation output of the Golay correlator in a second group of Y=(L/X) −

1 or less serially concatenated delay lines, where each delay line has X memory elements;

multiplying the first correlator output and the outputs of the first group of delay lines by respective elements of a reverted version of the signature sequence;

summing only those multiplied intermediate correlation values from the first correlation output and attached delay lines which correspond to non-zero elements of a reverted version of the interleaving sequence, and only those multiplied intermediate correlation values from the second correlation output and attached delay lines which correspond to non-zero elements of the inverted and reverted version of the interleaving sequence, where the first element of the reverted version of the interleaving sequence corresponds to the correlator output and the last element of such sequence corresponds to the output of the last delay line in the cascade; and

outputting a result of the summing step as a final correlation value.

44. The method in claim 42, wherein the received spread spectrum signal is non-coherently correlated using one of the sequences from a pair of Golay complementary sequences.

45. The method in claim 42, wherein the received spread spectrum signal is coherently correlated using one of the sequences from a pair of Golay complementary sequences, further comprising:

multiplying the second correlator output and the outputs of the second group of delay lines by respective elements of a reverted version of the signature sequence.

46. A correlator for use in a base station to correlate a received signal transmitted by a mobile station, comprising:

a matched filter corresponding to a complementary pair of sequences to correlate the received signal with one of the complementary pair of sequences, where each of the complementary sequences has minimal aperiodic autocorrelation sidelobe values for all non-zero delays of that complementary sequence and a maximal autocorrelation main lobe value for a zero delay of that complementary sequence;

a detector detecting a peak output from the matched filter; and

timing circuitry using the detected peak output to generate a timing estimate for synchronizing transmissions between the base station and mobile station;

wherein the complementary sequences include a primary synchronization sequence and plural secondary synchronization sequences transmitted by the base station and used by the mobile station to attain primary and secondary synchronization with the base station.

47. The correlator in claim 46, wherein the one sequence is used as a preamble portion of a random access message transmitted by the mobile station to the base station over a random access channel.

48. The correlator in claim 46, wherein the one sequence is used as a synchronization sequence transmitted by the base station over a broadcast channel.

49. The correlator in claim 46, wherein the complementary sequences are Golay sequences of a length 2 or longer.

50. The correlator in claim 46, wherein the complementary pair of sequences is a Golay pair and the matched filter is an efficient Golay correlator using a reduced amount of memory or a reduced amount of data processing operations.

51. A bank of correlators for use in a mobile transceiver to correlate received signals transmitted by a base station with a plurality of Golay complementary sequences, each correlator comprising:

a matched filter corresponding to a Golay complementary pair of sequences to correlate the received signal with one of the Golay complementary pair of sequences;

a detector detecting a peak output from the matched filter; and

timing circuitry using the detected peak output to generate a timing estimate for synchronizing transmissions between the mobile transceiver and the base station.

52. The bank of correlators in claim 51, wherein each of the Golay complementary sequences has minimal aperiodic autocorrelation sidelobe values for non-zero delays of that Golay complementary sequence and a maximal autocorrelation main lobe value for a zero delay of that Golay complementary sequence.

53. The bank of correlators in claim 51, wherein the Golay complementary sequences include a primary synchronization sequence and plural secondary synchronization sequences transmitted by the base station and used by the mobile station to attain primary and secondary synchronization with the base station.

54. The bank of correlators in claim 53, wherein the matched filter in each correlator is an efficient Golay correlator using a reduced amount of memory or a reduced amount of data processing operations.

56. A method for use in synchronizing mobile stations with a base station using a primary synchronization code and plural secondary synchronization codes transmitted by the base station, comprising:

providing one of a set of Golay complementary sequences as the primary synchronization code;

providing plural other ones of the set of Golay complementary sequences with minimal cross-correlation with the primary synchronization code as the secondary synchronization codes; and

obtaining a primary synchronization and a secondary synchronization with the base station using the provided primary and secondary synchronization codes.

57. The method in claim 56, wherein the primary synchronization code and 17 secondary synchronization codes are selected from a set of 256 orthogonal Golay sequences.

58. The method in claim 56, wherein the minimal cross-correlation is determined using a threshold.

59. The method in claim 58, wherein the minimal cross-correlation is minimal aperiodic cross-correlation.

60. The method in claim 56, further comprising:

using a bank of efficient Golay correlators to correlate a signal received by the mobile station with the primary and secondary synchronization codes.