Turbo Codes with Internal Pilot Insertion

In this paper we present a simple modification of a classical turbo code allowing for improved distance properties and rate adaptation. The proposed scheme suggests the insertion of pilot bits in the data sequence in such a manner that low weight codewords are eliminated or their weight multiplicity being reduced. In this way, pilots are utilized to increase the minimum distance of the code effectively. Furthermore, the rate can be made adaptive, almost arbitrarily according to the channel conditions under fixed interleaver length constraint. The resulting code uses fixed length codeword, can support a wide range of rates and even outperform the original system that uses external pilot insertion, in terms of error performance specifically at high signal to noise ratio. The proposed scheme does not add any complexity to the encoder neither to the decoder. Two recursive systematic convolutional (RSC) encoders together with S-random interleaver are used at the encoder side. At the decoder, soft-input soft-output Viterbi algorithm (SOVA) to accomplish iterative decoding with a maximum of five iterations is used.