Explicit interleavers for a Repeat Accumulate Accumulate (RAA) code construction

Repeat accumulate accumulate (RAA) codes are turbo-like codes where the message is first repeated k ges 2 times, passed through a first permutation (called interleaver), then an accumulator, then a second permutation, and finally a second accumulator. Bazzi, Mahdian, and Spielman (2003) prove that RAA codes are asymptotically good with high probability when the two permutations are chosen at random. RAA codes admit linear-time encoding algorithms, and are perhaps the simplest known family of linear-time encodable asymptotically good codes. An explicit construction of an asymptotically good RAA code is thus a very interesting goal. We focus on the case when k = 2 and we consider a variation of RAA codes where the inner repeat accumulate code is systematic. We give an explicit construction of the first permutation for which we show that the resulting code is asymptotically good with high probability when the second permutation is chosen at random. The explicit construction uses a cubic Hamiltonian graph with logarithmic girth.