In this paper the impact of S-type permutors, i.e., permutors that provide a non-trivial separation, on the minimum distance of double serially concatenated convolutional codes is considered. A joint design of the outer and inner permutor is presented on the basis of a newly introduced permutor parameter - the so-called symbol span. This design guarantees that the minimum distance of the overall code is lower bounded by the product of the free distances of all three component codes. Simulation results are presented and comparisons with single serially concatenated and double/generalized repeat accumulate codes are briefly sketched.
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