Quadriphase spreading codes in code division multiple access communications

• US 6,393,047 B1
• Filed: 06/16/1998
• Issued: 05/21/2002
• Est. Priority Date: 06/16/1998
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Popovicʼ, Branislav M.
• Primary Examiner(s): Bocure, Tesfaldet
• Assistant Examiner(s): PHU, PHUONG M
• Application Number:
  US09/097,916
• Time in Patent Office: 1,435 Days
• Field of Search: 375/146, 375/140, 370/320, 370/335, 370/342, 370/441
• US Class Current: 375/140
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 7/2628 using code-division multiple access [CDMA] or spread spectrum multiple access [SSMA] H04B7/2618 takes precedence
• CPC Class Codes: H04J 13/105 by extending
• CPC Class Codes: H04J 2013/0037 Multilevel codes

Abstract

Optimal code sequences are generated for use in spreading and de-spreading functions in a code division multiple access (CDMA) communications system. In particular, a family of quadriphase spreading codes is employed that provides a maximal number of spreading codes to achieve a high capacity in the CDMA communications system while at the same time having a minimal peak cross-correlation between any two spreading codes within that family to ensure cross-correlation interference is kept at or below acceptable levels. That optimal quadriphase spreading code family is the S(2) family of four phase code sequences of length L=2^m−1, where m is an integer greater than or equal to 5. The size of the S(2) family of quaternary spreading codes is (L+2)(L+1)², and the maximum cross-correlation is 1+4{square root over ((L+1))}. The spreading codes are preferably allocated to base stations using specific code subsets of the S(2) family having the same cross-correlation properties of the S(0) and/or S(1) family of codes. Spreading codes are advantageously extended by one or more code symbols as necessary or otherwise desirable. For example, to support variable transmission rate services, it is desirable to employ spreading codes whose length may be expressed as an integer multiple of each spreading factor in the mobile communications system. Since individual spreading codes have a length of 2^m−1, one code symbol is added to the generated spreading code.

Claims

1. In a direct sequence spread spectrum (DSSS) mobile communications system in which a plurality of mobile radio stations communicate with one or more radio base stations located in corresponding geographic areas over a radio channel, each radio channel corresponding to one of a set of spreading codes, one or more of the radio stations, comprising:

a spreading code generator configured to provide quaternary spreading codes from a family of quaternary sequences of length L=2^m−

1, where m is an integer greater than or equal to 5, having code elements from an alphabet {0, 1, 2, 3}, generated by summing modulo-4 three component sequences including a first component quaternary sequence a, a second component binary sequence b, and a third component binary sequence c, where the component binary sequences b and c are multiplied by 2 before the modulo-4 summing;

a spreader configured to spread an information signal to be transmit by the mobile radio using one of the quaternary spreading codes to provide a spread signal; and

a de-spreader configured to de-spread a received signal using one of the quaternary spreading codes.

2. The radio station in claim 1, further comprising:

a modulator configured to modulate the spread signal onto a radio carrier, and

a demodulator configured to demodulate a received radio signal and providing the demodulated signal to the de-spreader.

3. The radio station in claim 1, wherein the family of quaternary spreading codes provides a maximal number of spreading codes of a particular length that have a minimal cross-correlation.

4. The radio station in claim 3, wherein the size of the family of quaternary spreading codes is (L+2)(L+1)² and the maximum absolute cross-correlation value for the family of quaternary spreading codes is 1+4{square root over ((L+1+L ))}.

5. The radio station in claim 1, wherein the first component quaternary sequence is defined by:

6. The radio station in claim 5, wherein the code generator includes first, second, and third feedback shift registers that are configured to generate the first, second, and third component quaternary sequences, respectively, and wherein the corresponding initial states of the first, second, and third shift registers are determined for each of the spreading codes in the family of quaternary spreading codes.

7. The radio station in claim 1, wherein the radio station is a mobile radio station and one of the base stations assigns one of the spreading codes to the mobile radio station in response to a request to a communication involving the mobile radio station.

8. The radio station in claim 1, wherein spreading code generator is configured to map the quaternary spreading code to a quadriphase spreading code.

9. The radio station in claim 1, wherein the code generator is configured to extend the length of the generated quaternary spreading codes.

10. The radio station in claim 9, wherein the generated quaternary spreading codes are extended by one quaternary symbol so that the quaternary spreading code length is a power of 2.

11. In a code division multiple access (CDMA) mobile communications system in which a plurality of mobile radio stations communicate with one or more radio base stations located in corresponding geographic areas over a radio channel, each radio channel corresponding to one of a set of spreading codes, one or more of the radio stations, comprising:

a code generator configured to provide quadriphase spreading codes determined from an S(2) set of quaternary spreading codes having a maximal number of quaternary spreading codes with a minimal cross-correlation and to extend a length of the S(2) quaternary spreading codes to support multi-rate communications in the CDMA mobile communications system;

a spreader configured to spread an information signal to be transmit by the radio station using one of the quadriphase spreading codes allocated to the radio station to provide a spread signal; and

a modulator configured to modulate the spread signal onto a radio carrier.

12. The radio station in claim 11, further comprising:

a demodulator configured to demodulate a received CDMA signal from the radio carrier, and

a de-spreader configured to de-spread the received CDMA signal using the one quadriphase spreading code to provide a received information signal.

13. The radio station in claim 11, wherein information signals to be transmit are allocated into a real data stream and an imaginary data stream, the radio station further comprising:

a real channel spreader configured to spread the real data stream using a real channelization code;

an imaginary channel spreader configured to spread the imaginary data stream using an imaginary channelization code; and

a combiner configured to combine outputs from the real and imaginary channel spreaders to generate a complex signal,

wherein the spreader is configured to spread the complex signal using the one quadriphase spreading code allocated to the mobile radio.

14. The radio station in claim 13, wherein the modulator is a quadrature phase shift keying (QPSK) modulator and wherein the complex signal is split into real and imaginary components which are then input to real and imaginary inputs of the QPSK modulator.

15. The radio station in claim 14, wherein the code generator is configured to map the quaternary spreading code to a quadriphase spreading code.

16. The radio in claim 11, wherein the code generator is configured to extend the length of the S(2) quaternary spreading codes periodically.

17. The radio station in claim 11, wherein the S(2) quaternary spreading codes are extended by one quaternary symbol so that the S(2) quaternary spreading code length is a multiple of each of multiple spreading factors used in the CDMA mobile communications system.

18. In a code division multiple access (CDMA) communications system in which a plurality of communications devices communicate using allocated communications channels, each channel corresponding to one of a set of CDMA spreading codes, a method comprising:

generating a family of original codes, each original code having a predetermined length, and

extending the length of original codes from the family of spreading codes by a code symbol to generate a family of CDMA spreading codes by detecting the end of one of the original codes, and adding the code symbol to the end of one of the original codes.

19. The method in claim 18, wherein the code symbol is the same for all of the original codes in the family.

20. The method of claim 18, wherein the code symbol is the same as a first code symbol in the one original code.

22. The method in claim 18, wherein the family of original codes are quadriphase codes, and the code symbol used to extend the original code has four possible values.

46. The method in claim 18, wherein the length of the original codes are extended without having to increase the number of the original codes from the family.

21. In a code division multiple access (CDMA) communications system in which a plurality of communications devices communicate using allocated communications channels, each channel corresponding to one of a set of CDMA spreading codes, a method comprising:

generating a family of original codes, each original code having a predetermined length, and

extending the length of original codes from the family of spreading codes by a code symbol to generate a family of CDMA spreading codes by detecting the end of one of the original codes, and adding the code symbol to the end of one of the original codes,

wherein the family of original codes has a length of L=2^m−

1, where m is an integer, and a single code symbol is added to the end of the original code periodically.

23. In a code division multiple access (CDMA) communications system in which a plurality of communications devices communicate using allocated communications channels, each channel corresponding to one of a set of CDMA spreading codes, a method comprising:

generating a family of original codes, each original code having a predetermined length, and

extending the length of original codes from the family of spreading codes by a code symbol to generate a family of CDMA spreading codes by detecting the end of one of the original codes, and adding the code symbol to the end of one of the original codes,

wherein the family of original codes are an S(2) family of codes and the code symbol is selected to minimize cross-correlation between CDMA spreading codes.

24. A CDMA code generator providing CDMA spreading codes, comprising:

one or more feedback shift registers having m stages, where m is an integer, where an output of a last stage is fed back to an input of a first stage, the output of the one or more feedback shift registers corresponding to one of a family of codes, and

electronic circuitry for adding an additional code symbol to the one code to provide an extended code corresponding to one of the CDMA spreading codes.

25. The CDMA code generator in claim 24, further comprising:

a comparator for detecting an end of the one code and signaling the electronic circuitry to add to the additional code symbol to the end of the one code.

26. The CDMA code generator in claim 24, further comprising:

a counter for generating a counter output at the end of an extended code period which causes the shift registers to be set respective initial states.

27. The CDMA code generator in claim 24, wherein the added code symbol is the same for all of the codes in the family.

28. The CDMA code generator in claim 24, wherein the added code symbol is the same as the first code symbol in the one original code.

29. The CDMA code generator in claim 26, wherein the family of original codes has a length L=2^m−

1, where m is a positive integer, and a single code symbol added to the end of one of the original codes is selected to reduce cross-correlation between the CDMA spreading codes.

30. The CDMA code generator in claim 24, wherein the family of codes correspond to a family of quaternary sequences of length L=2^m−

1, where m is an integer greater than or equal to 5, having code elements from an alphabet {0, 1, 2, 3}, generated by summing modulo-4 three component sequences including a first component quaternary sequence a, a second component binary sequence b, and a third component binary sequence c, wherein the one or more feedback shift registers includes first, second, and third feedback shift registers that generate the first, second, and third component quaternary sequences, respectively, with the output of the second and third shift registers being multiplied by 2 before the first, second, and third component quaternary sequences are modulo-4 summed.

31. The CDMA code generator in claim 30, wherein the code generator is configured to map the extended quaternary sequence to complex quadriphase CDMA spreading codes.

32. In a mobile communications system including plural base stations for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base stations, a method comprising:

allocating a first subset of the particular spreading code family to a first base station, and

allocating a second subset of the particular spreading code family to a second base station,

wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.

33. The method in claim 32, wherein the particular spreading code family corresponds to an S(2) code family and the first and second subsets are associated with one or both of an S(0) code family and an S(1) code family.

34. The method in claim 33, wherein each of the first and second spreading code subsets is defined by three component sequences such that a first component sequence includes an index of r=0, 1, 2 . . . , L+1, where L is the spreading code length, and one or more of indices for the second and third component sequences are different for the first and second base stations.

35. The method in claim 32, further comprising:

for a particular call, assigning a mobile station associated with the first base station an assigned code from the first subset of spreading codes;

associating a flag with the assigned code;

setting the flag to a first value when the mobile station is involved in the call;

setting the flag to a second value after a prescribed time expires; and

prohibiting assignment of the assigned code to another mobile station until after the prescribed time expires.

36. The method in claim 35, further comprising:

setting the flag to the first value at the beginning of the call, and

measuring the prescribed time from the beginning of the call.

37. The method in claim 35, wherein the mobile station employs the assigned code for the duration of the call even when the call is handedover to the second base station.

38. The method in claim 37, further comprising:

setting the flag to the first value when the mobile is involved in the call;

measuring the prescribed time from a time associated with the handover; and

if the mobile station has not been in a handover during the call, the flag is set to the second value at the end of call.

39. In a mobile communications system including base station having plural sectors for communicating with mobile stations and employing spreading codes from a particular spreading code family for radio communications between the mobile stations and the base station, a method comprising:

allocating a first subset of the particular spreading code family to a first base station sector, and

allocating a second subset of the particular spreading code family to a second base station sector,

wherein the spreading codes in the first and second subsets have lower cross-correlation than spreading codes in the particular spreading code family.

40. The method in claim 39, wherein the particular spreading code family corresponds to an S(2) code family and the first and second subsets are associated with one or both of an S(0) code family and an S(1) code family.

41. The method in claim 40, wherein each of the first and second spreading code subsets is defined by three component sequences such that a first component sequence includes an index of r=0, 1, 2 . . . , L+1, where L is the spreading code length, and one or more of indices for the second and third component sequences are different for the first and second base station sectors.

42. The method in claim 39, further comprising:

for a particular call, assigning a mobile station associated with the first base station sector an assigned code from the first subset of spreading codes;

associating a flag with the assigned code;

setting the flag to a first value when the mobile station is involved in the call;

setting the flag to a second value after a prescribed time expires; and

prohibiting assignment of the assigned code to another mobile station until after the prescribed time expires.

43. The method in claim 42, further comprising:

setting the flag to the first value at the beginning of the call, and

measuring the prescribed time from the beginning of the call.

44. The method in claim 42, wherein the mobile station employs the assigned code for the duration of the call even when the call is handedover to the second base station sector.

45. The method in claim 44, further comprising:

setting the flag to the first value when the mobile station is involved in the call;

measuring the prescribed time from a time associated with the handover; and

if the mobile station has not been in a handover during the call, the flag is set to the second value at the end of call.

47. In a code division multiple access (CDMA) communications system in which a plurality of communications devices communicate using allocated communications channels, each channel corresponding to one of a set of CDMA spreading codes, a method comprising:

generating a family of original S(2) quaternary spreading codes, each original code having a predetermined length, and

extending the length of original codes from the family of original S(2) quaternary spreading codes by a code symbol to generate a family of CDMA spreading codes without having to increase the number of the original codes from the family.

48. The method in claim 47, wherein the extending further comprises:

detecting the end of one of the original codes, and

adding the code symbol to the end of one of the original codes.

49. The method in claim 47, wherein the code symbol is the same for all of the original codes in the family.

50. The method in claim 47, wherein the code symbol is the same as the first code symbol in the one original code.

51. The method in claim 47, wherein the code symbol is added to the end of one of the original codes periodically.

52. The method in claim 47, wherein the family of original codes are quadriphase codes, and the code symbol used to extend the original code has four possible values.

53. The method in claim 47, wherein the code symbol is selected to minimize cross-correlation between CDMA spreading codes.

54. A CDMA code generator providing CDMA spreading codes, comprising:

one or more feedback shift registers having m stages, where m is an integer, where an output of a last stage is fed back to an input of a first stage, the output of the one or more feedback shift registers corresponding to one of a family of S(2) quaternary spreading codes of length L=2^m−

1, and

electronic circuitry for adding an additional code symbol to the one S(2) quaternary spreading code to provide an extended S(2) quaternary spreading code corresponding to one of the CDMA spreading codes.

55. The CDMA code generator in claim 54, further comprising:

a comparator for detecting an end of the one S(2) quaternary spreading code and signaling the electronic circuitry to add to the additional code symbol to the end of the one S(2) quaternary spreading code.

56. The CDMA code generator in claim 54, further comprising:

a counter for generating a counter output at the end of an extended code period which causes the shift registers to be set respective initial states.

57. The CDMA code generator in claim 54, wherein the added code symbol is the same for all of the codes in the family.

58. The CDMA code generator in claim 54, wherein the added code symbol is the same as the first code symbol in the one original code.

59. The CDMA code generator in claim 54, wherein a value of the additional code symbol is selected to reduce cross-correlation between the CDMA spreading codes.