Design of fast-prunable S-random interleavers

In this paper, a technique to obtain good prunable S-random interleavers to be used in parallel and serially concatenated codes with interleavers is proposed: The new design is based on an algorithm to extend the interleaver size in a wide interval of values, while keeping good spreading properties in the whole range. Then, a modified pruning technique to be used for this class of interleavers is proposed: It is shown that it produces good interleavers also when applied to nonprunable S-random permutations. Results are given in terms of spreading parameters, obtainable free distance of the actual codes, and simulated frame-error probabilities. They show that the designed interleavers behave in the whole size range as optimally designed interleavers for each size. Different pruning algorithms are described and their complexity, crucial for their feasibility, is analyzed. Finally, the tradeoffs between area savings and complexity are discussed in a realistic example.