Adjusting soft values to reduce bias
A method is described for adjusting soft values in a code block prior to decoding. A count of saturated soft values in a code block is determined and one or more of the soft values in the code block are adjusted based on the count of saturated soft values. The count may be determined by incrementing the count for each positive saturated soft value and decrementing the count of each negative saturated soft value. The soft values may be adjusted by adding an offset computed based on the count to the soft values.
1. A method of adjusting soft values input to a decoder in a wireless receiver, said method comprising:
determining a count of saturated soft values in a code block containing a plurality of soft values output by a demodulator; and
adjusting said soft values in said code block based on said count of saturated soft values to compensate for bias attributable to bit width limitations of the decoder.
2. The method of claim 1 wherein determining a count of saturated soft values comprises incrementing said count for each positive saturated soft value in said code block, and decrementing said count for each negative saturated soft value in said code block.
3. The method of claim 1 wherein adjusting said soft values in said code block comprises computing an offset based on said count, and adding said offset to one or more soft values in said code block.
4. The method of claim 3 wherein adjusting said soft values in said code block further comprises limiting the adjusted soft values to a predetermined maximum value.
5. The method of claim 3 wherein computing the offset comprises determining a saturation ratio based on said count, and selecting one of two or more predetermined offsets based on said saturation ratio.
6. The method of claim 5 wherein determining the saturation ratio comprises dividing said count by a number of soft values in said code block.
7. The method of claim 5 wherein selecting one of two or more predetermined offsets based on said saturation ratio comprises mapping said saturation ratio to a corresponding offset.
8. The method of claim 7 wherein mapping said saturation ratio to a corresponding offset comprises comparing said saturation ratios to one or more threshold values and selecting an offset based on an outcome of said comparisons.
9. A device for adjusting soft values input to a decoder in a wireless receiver, said device comprising:
a saturation counter to determine a count of saturated soft values in a code block containing a plurality of soft values output by a demodulator; and
an adjustment unit for adjusting one or more soft values in said code block based on said count to compensate for bias attributable to bit width limitations of the decoder.
10. The device of claim 9 wherein said saturation counter is configured to increment said count for each positive saturated soft value in said code block, and decrement said count for each negative saturated soft value in said code block.
11. The device of claim 9 wherein the adjustment unit comprises an offset calculator to compute an offset based on said count, and a modification unit to add said offset to one or more soft values in said code block.
12. The device of claim 11 wherein the modification unit is further configured to limit the adjusted soft values to a predetermined maximum value.
13. The device of claim 11 wherein said offset calculator is configured to compute a saturation ratio based on said count and to select one of two or more predetermined offsets based on said saturation ratio.
14. The device of claim 13 wherein said offset calculator determines said saturation ratio by dividing said count by a number of soft values in said code block.
15. The device of claim 13 wherein said offset calculator is further configured to map said saturation ratio to a corresponding offset.
16. The device of claim 15 wherein said offset calculator maps said saturation ratio to a corresponding offset by comparing said saturation ratios to one or more threshold values and selecting an offset based on an outcome of said comparisons.