Method and apparatus for near-optimal scaling of log-likelihood ratio (LLR) computation in turbo decoding for hybrid automatic repeat request (ARQ)

• US 7,376,209 B2
• Filed: 06/06/2003
• Issued: 05/20/2008
• Est. Priority Date: 06/06/2003
• Status: Expired due to Fees

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Bhushan, Naga, Ma, Jun, Namgoong, June, Black, Peter, Fan, Mingxi
• Primary Examiner(s): Odom; Curtis B.
• Application Number:
  US10/456,148
• Publication Number:
  US 20040258139A1
• Time in Patent Office: 1,810 Days
• Field of Search: 714/791, 714/792, 714/793, 714/794, 714/795, 375/341, 370/912
• US Class Current: 375/341
• CPC Class Codes: H03M 13/2957 Turbo codes and decoding
• CPC Class Codes: H03M 13/3927 Log-Likelihood Ratio [LLR] computation by combination of forward and backward metrics into LLRs
• CPC Class Codes: H03M 13/658 Scaling by multiplication or division
• CPC Class Codes: H04L 1/0045 Arrangements at the receiver end
• CPC Class Codes: H04L 1/0066 Parallel concatenated codes
• CPC Class Codes: H04L 1/1812 Hybrid protocols; Hybrid automatic repeat request [HARQ]
• CPC Class Codes: H04L 1/1825 Adaptation of specific ARQ protocol parameters according to transmission conditions
• CPC Class Codes: Y10S 370/912 Packet communications

Abstract

A method and an apparatus for scaling demodulated data symbols contained in a packet to generate scaled log-likelihood ratios for Turbo decoding are disclosed. A packet consists of one or more subpackets depending on the type of packet. Each subpacket is identified by a subpacket identification number. The payload size of the packet and the subpacket identification number may be determined by decoding a reverse rate indicator (RRI) channel. A scale factor which is associated with a specific subpacket identification number and a specific payload size results in a performance measure that is closest to an expected performance measure. The scale factor is used for scaling the demodulated data symbols to generate scaled log-likelihood ratios for Turbo decoding.

Claims

1. A method for decoding data in a packet comprising one or more subpackets, the method comprising:

determining a subpacket identification number for a received subpacket of the packet, the subpacket identification number associated with a scale factor which results in a performance measure that is closest to an expected performance measure;

applying the scale factor to demodulated data symbols in the packet to generate scaled log-likelihood ratios; and

decoding the scaled log-likelihood ratios.

2. The method of claim 1, wherein the performance measure comprises a normalized ratio of received energy to noise power spectral density, and wherein the expected performance measure comprises a normalized expected ratio of received energy to noise power spectral density.

3. The method of claim 1, wherein the performance measure comprises a normalized data rate, and wherein the expected performance measure comprises a normalized expected data rate.

4. The method of claim 1, wherein the step of determining the subpacket identification number for the received subpacket comprises decoding a reverse rate indicator (RRI) channel.

5. The method of claim 4, further comprising demodulating received data in the packet by maximal ratio combining (MRC) to generate the demodulated data symbols.

6. The method of claim 5, wherein the step of applying the scale factor comprises scaling the demodulated data symbols by the square root of the scale factor to obtain the scaled log-likelihood ratios.

7. The method of claim 6, wherein said decoding the scaled log-likelihood ratios comprises Turbo decoding the scaled log-likelihood ratios.

8. The method of claim 1, wherein the packet comprises one or more subpackets depending on whether the packet belongs to one of a plurality of predetermined packet types, the method further comprising:

determining a traffic-to-pilot ratio to achieve a predetermined fixed packet error rate for each of the predetermined packet types; and

computing the expected performance measure based on the traffic-to-pilot ratio for each of the predetermined packet types.

9. The method of claim 1, wherein:

the packet comprises one or more subpackets depending on whether the packet belongs to one of a plurality of predetermined packet types; and

the predetermined packet types consist of first, second, third and fourth packet types, a packet of the first packet type consisting of one subpacket, a packet of the second packet type consisting of two subpackets, a packet of the third packet type consisting of three subpackets, and a packet of the fourth packet type consisting of four subpackets.

10. The method of claim 9, wherein each subpacket consists of four slots.

11. The method of claim 9, wherein a first one of the subpackets in a packet selected from the group consisting of the first, second, third and fourth packet types is assigned subpacket identification number 0, wherein a second one of the subpackets succeeding the first subpacket in a packet selected from the group consisting of the second, third and fourth packet types is assigned subpacket identification number 1, wherein a third one of the subpackets succeeding the second subpacket in a packet selected from the group consisting of the third and fourth packet types is assigned subpacket identification number 2, and wherein a fourth one of the subpackets succeeding the third subpacket in a packet of the fourth packet type is assigned subpacket identification number 3.

12. The method of claim 11, wherein four predetermined scale factors λ

₀, λ

₁, λ

₂ and λ

₃ are associated with the subpacket identification numbers 0, 1, 2 and 3, respectively.

13. The method of claim 12, further comprising storing, at a base station receiving the subpacket in a reverse link, traffic-to-pilot ratios P₄, P₈, P₁₂ and P₁₆ for the first, second, third and fourth packet types, respectively.

14. The method of claim 13, further comprising storing the traffic-to-pilot ratios P₁₆, P₁₂, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively, prior to said determining the subpacket identification number.

15. The method of claim 13, further comprising storing the traffic-to-pilot ratios P₁₆, P₁₆, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively, prior to said determining the subpacket identification number.

16. The method of claim 1, wherein the scale factor is associated with the subpacket identification number and a payload size of the packet obtained by decoding a reverse rate indicator (RRI) channel.

17. A base station apparatus, comprising:

means for receiving data in a packet consisting of one or more subpackets depending on whether the packet belongs to one of a plurality of predetermined packet types; and

means for scaling demodulated data symbols in the packet to compute scaled log-likelihood ratios for decoding the data, the means for scaling the demodulated data symbols comprising;

means for determining a subpacket identification number for a most recently received subpacket of the packet, the subpacket identification number associated with a scale factor which results in a performance measure that is closest to an expected performance measure, and

means for applying the scale factor to the demodulated data symbols to generate scaled log-likelihood ratios.

18. The apparatus of claim 17, wherein the performance measure comprises a normalized ratio of received energy to noise power spectral density, and wherein the expected performance measure comprises a normalized expected ratio of received energy to noise power spectral density.

19. The apparatus of claim 17, wherein the performance measure comprises a normalized data rate, and wherein the expected performance measure comprises a normalized expected data rate.

20. The apparatus of claim 17, wherein the means for determining the subpacket identification number for the most recently received subpacket comprises means for decoding a reverse rate indicator (RRI) channel.

21. The apparatus of claim 17, further comprising means for demodulating the received data by maximal ratio combining (MRC) to generate the demodulated data symbols.

22. The apparatus of claim 21, wherein the means for applying the scale factor comprises means for scaling the demodulated data symbols by the square root of the scale factor to obtain the scaled log-likelihood ratios.

23. The apparatus of claim 17, farther comprising a Turbo decoder, connected to the means for scaling the demodulated data symbols, to decode the scaled log-likelihood ratios.

24. The apparatus of claim 17, farther comprising:

means for determining a traffic-to-pilot ratio to achieve a predetermined fixed packet error rate for each of the predetermined packet types; and

means for computing the expected performance measure based on the traffic-to-pilot ratio for each of the predetermined packet types.

25. The apparatus of claim 17, wherein the predetermined packet types consist of first, second, third and fourth packet types, a packet of the first packet type consisting of one subpacket, a packet of the second packet type consisting of two subpackets, a packet of the third packet type consisting of three subpackets, and a packet of the fourth packet type consisting of four subpackets.

26. The apparatus of claim 25, wherein each subpacket consists of four slots.

27. The apparatus of claim 25, wherein a first one of the subpackets in a packet selected from the group consisting of the first, second, third and fourth packet types is assigned subpacket identification number 0, wherein a second one of the subpackets succeeding the first subpacket in a packet selected from the group consisting of the second, third and fourth packet types is assigned subpacket identification number 1, wherein a third one of the subpackets succeeding the second subpacket in a packet selected from the group consisting of the third and fourth packet types is assigned subpacket identification number 2, and wherein a fourth one of the subpackets succeeding the third subpacket in a packet of the fourth packet type is assigned subpacket identification number 3.

28. The apparatus of claim 27, wherein four predetermined scale factors λ

₀, λ

₁, λ

₂ and λ

₃ are associated with the subpacket identification numbers 0, 1, 2 and 3, respectively.

29. The apparatus of claim 28, further comprising means for storing traffic-to-pilot ratios P₄, P₈, P₁₂ and P₁₆ for the first, second, third and fourth packet types, respectively.

30. The apparatus of claim 29, further comprising means for storing the traffic-to-pilot ratios P₁₆, P₁₂, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively.

31. The apparatus of claim 29, further comprising means for storing the traffic-to-pilot ratios P₁₆, P₁₆, P₈ and P₈ as the predetermined scale factors λ

<sub>3, λ

2</sub>, λ

₁ and λ

₀, respectively.

32. The apparatus of claim 17, wherein the scale factor is associated with the subpacket identification number and a payload size of the packet obtained by decoding a reverse rate indicator (RRI) channel.

33. The apparatus of claim 17, wherein the means for scaling demodulated data symbols comprises a log-likelihood ratio scale factor look-up table.

34. A computer readable medium having instructions stored thereon that, when executed, decode data in a packet comprising one or more subpackets, the computer readable medium comprising sets of instructions for:

determining a subpacket identification number for a received subpacket of the packet, the subpacket identification number associated with a scale factor which results in a performance measure that is closest to an expected performance measure;

applying the scale factor to demodulated data symbols in the packet to generate scaled log-likelihood ratios; and

decoding the scaled log-likelihood ratios.

35. The computer readable medium of claim 34, wherein the performance measure comprises a normalized ratio of received energy to noise power spectral density, and wherein the expected performance measure comprises a normalized expected ratio of received energy to noise power spectral density.

36. The computer readable medium of claim 34, wherein the performance measure comprises a normalized data rate, and wherein the expected performance measure comprises a normalized expected data rate.

37. The computer readable medium of claim 34, wherein the set of instructions for determining the subpacket identification number for the received subpacket comprises a set of instructions for decoding a reverse rate indicator (RRI) channel.

38. The computer readable medium of claim 37, further comprising a set of instructions for demodulating data in the packet received at the base station by maximal ratio combining (MRC) to generate the demodulated data symbols.

39. The computer readable medium of claim 38, wherein the set of instructions for applying the scale factor comprises a set of instructions for scaling the demodulated data symbols by the square root of the scale factor to obtain the scaled log-likelihood ratios.

40. The computer readable medium of claim 39, wherein the set of instructions for decoding the scaled log-likelihood ratios comprises a set of instructions for Turbo decoding the scaled log-likelihood ratios.

41. The computer readable medium of claim 34, wherein the packet comprises one or more subpackets depending on whether the packet belongs to one of a plurality of predetermined packet types, the computer readable medium further comprising sets of instructions for:

determining a traffic-to-pilot ratio to achieve a predetermined fixed packet error rate for each of the predetermined packet types; and

computing the expected performance measure based on the traffic-to-pilot ratio for each of the predetermined packet types.

42. The computer readable medium of claim 34, wherein:

the packet comprises one or more subpackets depending on whether the packet belongs to one of a plurality of predetermined packet types; and

the predetermined packet types consist of first, second, third and fourth packet types, a packet of the first packet type consisting of one subpacket, a packet of the second packet type consisting of two subpackets, a packet of the third packet type consisting of three subpackets, and a packet of the fourth packet type consisting of four subpackets.

43. The computer readable medium of claim 42, wherein each subpacket consists of four slots.

44. The computer readable medium of claim 42, wherein a first one of the subpackets in a packet selected from the group consisting of the first, second, third and fourth packet types is assigned subpacket identification number 0, wherein a second one of the subpackets succeeding the first subpacket in a packet selected from the group consisting of the second, third and fourth packet types is assigned subpacket identification number 1, wherein a third one of the subpackets succeeding the second subpacket in a packet selected from the group consisting of the third and fourth packet types is assigned subpacket identification number 2, and wherein a fourth one of the subpackets succeeding the third subpacket in a packet of the fourth packet type is assigned subpacket identification number 3.

45. The computer readable medium of claim 44, wherein four predetermined scale factors λ

₀, λ

₁, λ

₂ and λ

₃ are associated with the subpacket identification numbers 0, 1, 2 and 3, respectively.

46. The computer readable medium of claim 45, further comprising a set of instructions for storing traffic-to-pilot ratios P₄, P₈, P₁₂ and P₁₆ for the first, second, third and fourth packet types, respectively.

47. The computer readable medium of claim 46, further comprising a set of instructions for storing the traffic-to-pilot ratios P₁₆, P₁₂, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively, prior to determining the subpacket identification number.

48. The computer readable medium of claim 46, further comprising a set of instructions for storing the traffic-to-pilot ratios P₁₆, P₁₆, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively, prior to determining the subpacket identification number.

49. The computer readable medium of claim 34, wherein the scale factor is associated with the subpacket identification number and a payload size of the packet obtained by decoding a reverse rate indicator (RRI) channel.

50. A base station apparatus comprising:

an antenna coupled to a receiver, the receiver configured to receive data in a packet consisting of one or more subpackets depending on whether the packet belongs to one of a plurality of predetermined packet types;

a demodulator configured to demodulate received data and generate demodulated data symbols;

a log-likelihood ratio (LLR) computer configured to;

determine a subject identification number for a most recently received subpacket of the packet, the subpacket identification number associated with a scale factor which results in a performance measure that is closest to an expected performance measure, and

apply the scale factor to the demodulated data symbols to generate scaled log-likelihood ratios.

51. The apparatus of claim 50, wherein the performance measure comprises a normalized ratio of received energy to noise power spectral density, and wherein the expected performance measure comprises a normalized expected ratio of received energy to noise power spectral density.

52. The apparatus of claim 50, wherein the performance measure comprises a normalized data rate, and wherein the expected performance measure comprises a normalized expected data rate.

53. The apparatus of claim 50, further comprising a decoder configured to decode a reverse rate indicator (RRI) channel for determining the subpacket identification number for the most recently received subpacket.

54. The apparatus of claim 50, wherein the demodulator performs maximal ratio combining (MRC) to generate the demodulated data symbols.

55. The apparatus of claim 54, wherein the LLR computer scales the demodulated data symbols by the square root of the scale factor to obtain the scaled log-likelihood ratios.

56. The apparatus of claim 50, further comprising a Turbo decoder, connected to the LLR computer to decode the scaled log-likelihood ratios.

57. The apparatus of claim 50, wherein the LLR computer further configured to:

determine a traffic-to-pilot ratio to achieve a predetermined fixed packet error rate for each of the predetermined packet types; and

compute the expected performance measure based on the traffic-to-pilot ratio for each of the predetermined packet types.

58. The apparatus of claim 50, wherein the predetermined packet types consist of first, second, third and fourth packet types, a packet of the first packet type consisting of one subpacket, a packet of the second packet type consisting of two subpackets, a packet of the third packet type consisting of three subpackets, and a packet of the fourth packet type consisting of four subpackets.

59. The apparatus of claim 58, wherein each subpacket consists of four slots.

60. The apparatus of claim 58, wherein a first one of the subpackets in a packet selected from the group consisting of the first, second, third and fourth packet types is assigned subpacket identification number 0, wherein a second one of the subpackets succeeding the first subpacket in a packet selected from the group consisting of the second, third and fourth packet types is assigned subpacket identification number 1, wherein a third one of the subpackets succeeding the second subpacket in a packet selected from the group consisting of the third and fourth packet types is assigned subpacket identification number 2, and wherein a fourth one of the subpackets succeeding the third subpacket in a packet of the fourth packet type is assigned subpacket identification number 3.

61. The apparatus of claim 60, wherein four predetermined scale factors λ

₀, λ

₁, λ

₂ and λ

₃ are associated with the subpacket identification numbers 0, 1, 2 and 3, respectively.

62. The apparatus of claim 61, further comprising memory for storing traffic-to-pilot ratios P₄, P₈, P₁₂ and P₁₆ for the first, second, third and fourth packet types, respectively.

63. The apparatus of claim 62, further comprising memory for storing the traffic-to-pilot ratios P₁₆, P₁₂, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively.

64. The apparatus of claim 62, further comprising memory for storing the traffic-to-pilot ratios P₁₆, P₁₆, P₈ and P₈ as the predetermined scale factors λ

₃, λ

₂, λ

₁ and λ

₀, respectively.

65. The apparatus of claim 50, wherein the scale factor is associated with the subpacket identification number and a payload size of the packet obtained by decoding a reverse rate indicator (RRI) channel.

66. The apparatus of claim 50, further comprising a log-likelihood ratio scale factor look-up table connected to the LLR computer.

67. A wireless communication system comprising:

an access terminal;

a base station in communication with the access terminal, the base station comprising;

an antenna coupled to a receiver, the receiver configured to receive data in a packet consisting of one or more subpackets transmitted by the access terminal depending on whether the packet belongs to one of a plurality of predetermined packet types;

a demodulator configured to demodulate received data and generate demodulated data symbols;

a log-likelihood ratio (LLR) computer configured to;

determine a subpacket identification number for a most recently received subpacket of the packet, the subpacket identification number associated with a scale factor which results in a performance measure that is closest to an expected performance measure, and

apply the scale factor to the demodulated data symbols to generate scaled log-likelihood ratios.