Improved Method for Searching of Interleavers Using Garello's Method
  In this paper an improved method for searching good interleavers from a certain set is proposed. The first few terms, corresponding to maximum distance of approximately 40 of the distance spectra, for turbo codes using these interleavers are determined by means of Garello's method. The method is applied to find quadratic permutation polynomials (QPP) based interleavers. Compared to previous methods for founding QPP based interleavers, the search complexity is reduced, allowing to find interleavers of higher length. This method has been applied for QPP interleavers with lengths from the LTE (Long Term Evolution) standard up to 1504. The analyzed classes are those with the largest spread QPP (LS-QPP), with the D parameter equal to that of LTE interleaver (D_L_T_E-QPP), and the class consisting of all QPP interleavers for lengths up to 1008. The distance spectrum optimization is made for all classes. For the class of LS-QPP interleavers of small lengths, the search led to superior or at least equal performances with those of the LTE standard. For larger lengths the search in the class of D_L_T_E-QPP interleavers is preferred. The interleavers from the entire class of QPPs lead, in general, to weaker FER (Frame Error Rate) performance. 
Lucian Trifina and Daniela Tarniceriu
cs.IT math.IT