Soft information scaling for iterative decoding

• US 7,231,577 B2
• Filed: 08/07/2003
• Issued: 06/12/2007
• Est. Priority Date: 02/26/2003
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Jin, Hui, Richardson, Tom, Novichkov, Vladimir
• Primary Examiner(s): Decady; Albert
• Assistant Examiner(s): Alphonse; Fritz
• Application Number:
  US10/635,942
• Publication Number:
  US 20040168114A1
• Time in Patent Office: 1,405 Days
• Field of Search: 714/758, 714/752, 714/747, 714/748, 714/750, 714/754, 714/766, 714/768, 714/795
• US Class Current: 714/758
• CPC Class Codes: H03M 13/00 Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes error detection or error correction for analogue/digital, digital/analogue or code conversion H03M1/00 – H03M11/00; specially adapted for digital computers G06F11/08; for information storage based on relative movement between record carrier and transducer G11B, e.g. G11B20/18; for static stores G11C
• CPC Class Codes: H03M 13/1102 Codes on graphs and decoding on graphs, e.g. low-density parity check [LDPC] codes
• CPC Class Codes: H03M 13/296 Particular turbo code structure
• CPC Class Codes: H03M 13/3707 Adaptive decoding and hybrid decoding, e.g. decoding methods or techniques providing more than one decoding algorithm for one code
• CPC Class Codes: H03M 13/6325 Error control coding in combination with demodulation
• CPC Class Codes: H03M 13/658 Scaling by multiplication or division
• CPC Class Codes: H04L 1/005 Iterative decoding, including iteration between signal detection and decoding operation

Abstract

Methods and apparatus for scaling soft values as part of an error correction decoding process are described. Accurate decoding depends on use of the appropriate scale factor. Selection and use of the scale factor to scale soft values is designed to improve and/or optimize decoder performance without the need for prior knowledge of the correct scale factor or the actual channel conditions at the time the signal from which the soft values were obtained was transmitted through a communications channel. The techniques of the present invention assume that the soft values to be processed were transmitted through a communications channel having a quality that can be accurately described by a channel quality value. A scale factor is determined from the distribution of soft values to be scaled and an assumption that the channel through which they were transmitted was of the quality corresponding to a preselected channel quality value.

Claims

1. A method of operating an apparatus to scale soft input values obtained, from a signal transmitted through a communications channel, as part of a decoding process, the method comprising:

computing a current scaling factor as a function of a preselected channel quality value and at least one of said soft values, said preselected channel quality value being independent of actual channel conditions at the time said signal was transmitted; and

scaling one of said soft values using said computed current scaling factor to produce a scaled soft value.

2. The method of claim 1, wherein a plurality of soft value distributions are possible, a subset of possible soft value distributions corresponding to said preselected channel quality value while other possible distributions correspond to other channel quality values, said step of computing a current scaling factor including:

determining a scaling factor which, when applied to said received soft values, produces a soft value distribution in said subset of soft value distributions corresponding to said preselected channel quality value.

3. The method of claim 2, wherein determining a scaling factor includes:

computing from at least some of said soft input values a plurality of channel quality values, each channel quality value corresponding to a different scale factor.

4. The method of claim 3, further comprising:

interpolating between at least two of said plurality of channel quality values to produce an interpolated value; and

determining said current scale factor as a function of the interpolated quality value.

5. The method of claim 2, wherein determining the current scale factor is part of a iterative process that includes:

updating the current scale factor as a function of a soft value scaled by the current scale factor being updated.

6. The method of claim 5, wherein said updating includes:

comparing a channel quality value corresponding to the scaled soft value to the target quality value to determine a difference between the target quality value and the corresponding quality value; and

adjusting the scaling factor as a function of said determined difference.

7. The method of claim 6, wherein said scale factor is adjusted in a direction which reduces subsequent differences between the channel quality value corresponding to a subsequently processed soft value and said target channel quality value.

8. The method of claim 6, wherein scale factor adjustments are made within a range extending between a maximum permitted scaling value and a minimum permitted scaling value, individual scale factor adjustments being no larger than a maximum adjustment step size of 2% of the maximum permitted scaling value.

9. The method of claim 3, wherein computing said scaling factor includes;

determining a current channel quality function from a first scale factor.

10. The method of claim 9, further comprising:

solving said function to determine a scale factor which,

when applied to said function given said at least some soft input values, produces said target channel quality,

said determined scale factor being used as said current scale factor.

11. The method of claim 1, wherein said preselected channel quality value is a channel capacity value.

12. The method of claim 1, wherein said preselected channel quality value is a value corresponding to a quality region that is within but near the edge of an acceptable channel quality region.

13. The method of claim 1, wherein said decoding process includes at least one of a low density parity check decoding operation and a turbo code decoding operation.

14. An apparatus for determining a factor to be used to scale soft input values obtained, from a signal transmitted through a communications channel, comprising:

a receiver for receiving a signal transmitted through a communications channel;

means for generating soft input values from said received signal;

memory for storing a preselected channel quality value, said preselected channel quality value being independent of actual channel conditions at the time said signal was transmitted; and

means for computing a scaling factor as a function of said preselected channel quality value and at least one of soft input values.

15. The apparatus of claim 14, wherein said preselected channel quality value is a value corresponding to a quality region that is near the edge of an acceptable channel quality region.

16. The apparatus of claim 15, wherein said preselected channel quality value is a channel capacity value.

17. The apparatus of claim 14, wherein a plurality of soft value distributions are possible, a subset of possible soft value distributions corresponding to said preselected channel quality value while other possible distributions correspond to other channel quality values, said means for computing a scaling factor including:

means for determining a scaling factor which, when applied to said received soft values, produces a soft value distribution in said subset of soft value distributions corresponding to said preselected channel quality value.

18. The apparatus of claim 17, wherein said means for determining a scaling factor includes:

means for computing from at least some of said soft input values a plurality of channel quality values, each channel quality value corresponding to a different scale factor.

19. The apparatus of claim 18, further comprising:

means for interpolating between at least two of said plurality of channel quality values to produce an interpolated value; and

means for determining said scale factor as a function of the interpolated quality value.

20. The apparatus of claim 19, wherein said means for computing and means for interpolating each include computer instructions for controlling a processor to implement at least a portion of said computing and interpolating operations.

21. The apparatus of claim 17, wherein said means for determining the current scale factor performs an interactive process, said apparatus further including:

a control loop for updating the current scale factor as a function of a soft value scaled by the current scale factor being updated.

22. The apparatus of claim 21, wherein said control loop includes:

a comparator for comparing a channel quality value corresponding to the scaled soft value to the target quality value to determine a difference between the target quality value and the corresponding quality value; and

means for adjusting the scaling factor as a function of said determined difference.

23. The apparatus of claim 18, wherein said means for computing said scaling factor includes:

means for determining a channel quality function from a first scale factor; and

means for solving said function to determine a scale factor which, when applied to said function given said at least some soft input values, produces said target channel quality, said determined scale factor being used as said current scale factor.