Detection, avoidance and/or correction of problematic puncturing patterns in parity bit streams used when implementing turbo codes
Detecting, avoiding and/or correcting problematic puncturing patterns in parity bit streams used when implementing punctured Turbo codes is achieved without having to avoid desirable code rates. This enables identification/avoidance of regions of relatively poor Turbo code performance. Forward error correction comprising Turbo coding and puncturing achieves a smooth functional relationship between any measure of performance and the effective coding rate resulting from combining the lower rate code generated by the Turbo encoder with puncturing of the parity bits. In one embodiment, methods to correct/avoid degradations due to Turbo coding are implemented by puncturing interactions when two or more stages of rate matching are employed.
1. An apparatus configured to perform rate matching for turbo coded transport channels in wireless communications, the apparatus comprising:
a bit separation unit configured to separate bits in a transport block into first, second and third code blocks;
a first interleaver configured to interleave turbo encoded systematic bits arranged in the first code block;
a second interleaver configured to interleave a first turbo encoded parity bit stream arranged in the second code block;
a third interleaver configured to interleave a second turbo encoded parity bit stream arranged in the third code block;
a first rate matching unit configured to perform a first stage of rate matching, wherein the performing a first stage of rate matching includes testing whether puncturing will cause a problematic puncturing pattern;
on a condition that the puncturing will cause a problematic puncturing pattern, the first rate matching unit configured to adjust the first stage rate matching; and
a second rate matching unit configured to perform a second stage of rate matching.
2. The apparatus of claim 1 wherein the first rate matching unit is configured to test whether puncturing will cause a problematic puncturing pattern by evaluating an implicit duration function which predicts a length of periodicity in a puncturing pattern.
3. The apparatus of claim 1 wherein the first rate matching unit is configured to adjust the first stage of rate matching by increasing a number of bits punctured in the interleaved first parity bit stream.
4. The apparatus of claim 1 wherein the first rate matching unit is configured to adjust the first stage of rate matching by decreasing a number of bits punctured in the interleaved second parity bit stream.
5. The apparatus of claim 1 wherein the first rate matching unit is configured to adjust the first stage of rate matching by keeping a total number of punctured bits the same.
6. The apparatus of claim 1 wherein the second rate matching unit is further configured to:
calculate a composite puncturing rate for each of the interleaved systematic bits, the interleaved first parity bit stream and the interleaved second parity bit stream;
compare the composite puncturing rate with a look up table to determine whether the composite puncturing rate is a problematic rate; and
on a condition that the composite puncturing rate is a problematic rate, apply a dithering algorithm.
7. The apparatus of claim 6 wherein the look up table predicts rates at which modulo-7 periodic patterns arise.
8. The apparatus of claim 6 wherein the second rate matching unit is configured apply the dithering algorithm by puncturing by switching between a first puncturing rate and a second puncturing rate to an interleaved parity bit stream, wherein the first puncturing rate is higher than an original puncturing rate, and the second puncturing rate is lower than the original puncturing rate.