Two methods to increase the minimum distance for turbo codes with QPP interleavers

Two search methods of quadratic permutation polynomials (QPP) for interleavers used in turbo codes are proposed. These methods lead to larger minimum distances and smaller multiplicities compared to interleavers proposed by Takeshita. The search is accomplished in a limited set of polynomials, that is, those for which the spreading factor and Omega´ metric are maximum. The minimum distance is computed by means of Garello algorithm in which the maximum weight of information sequence is 3 or 4, reducing the search time. The results obtained for various lengths show the efficiency of the proposed methods.