Coprime Interleavers Revisited

We investigate good coprime interleavers that perform well in turbo codes and allow for memory-efficient implementation. We first evaluate the spread and the randomness properties of these interleavers using the cycle correlation sum (CCS) and the variance of the second order spread spectrum (VSSS) criteria, respectively. A critical parameter cpower is introduced for interleavers whose lengths are power of 2. It is found that a large cpower typically associates with a small CCS value, indicating a good minimum spread of the coprime interleaver, but the degree of randomness decreases with growing cpower . We next develop rules on choosing good parameters that strike a good balance between minimum spread and degree of randomness. Simulation results confirm that the coprime interleavers designed from our rules perform as well as or better than S-random interleavers.