Modified interleaver design to reduce error floor in turbo codes for wireless communication

In this paper an improved and modified interleaver design based on the code distance spectrum is proposed. The low weight paths in the code trellis which give large contributions to the error probability in the signal-to-noise ratio region of interest for practical communication systems are eliminated, so that they do not appear in the overall code trellis after interleaving. The proposed interleaver improves the code error performance at moderate to high SNR and considerably increases the asymptotic slope of the error probability curves. As a result, the bit error rate and frame error rate performance of turbo codes are improved, and the “error floor” is reduced considerably. This paper analyzes the impact of weight-2, 3, 4 and 5 input sequences on the codewords. Considering all these low weight inputs, a set of condition is developed and this constraint set is suggested to be used with the S-random interleaving algorithm. This improved algorithm can eliminate or decrease the low free distance of codewords.