Approximate performance analysis of turbo codes with fixed interleavers

The weight spectrum of a turbo code is useful in deriving its performance bounds. Due to the randomness and large size of the interleaver, it is extremely difficult to obtain the exact weight spectrum. In the past, the average weight spectrum, averaged over all interleavers, was used in deriving the bounds. By introducing several limiting factors, we are able to derive an approximate weight spectrum for turbo codes with fixed interleavers. The complexity of the algorithm grows only linearly with the size of the interleaver.