Assessing interleaver suitability for turbo codes

The two major purposes of the interleaver in a turbo coding scheme are, firstly, to improve the weight distribution of the turbo code, and, secondly, to make the performance of iterative decoding as close to maximum likelihood as possible. Both these purposes should be considered when ranking and designing interleavers for turbo codes. In lack of a measure of how well the second purpose is carried out we investigate a method to assess the iterative decoding suitability of interleavers. This suitability measure is used as a complement to the weight distribution, which is an insufficient base for ranking interleavers when iterative decoding is employed. It is exemplified that with proper consideration of both interleaver tasks it is possible to single out interleaver designs which yield turbo codes with improved bit and frame error rate performances, compared to a number of other interleaver designs.