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, softinput
soft-output Viterbi algorithm (SOVA) to accomplish
iterative decoding with a maximum of five iterations is used.