On in-line bit interleaving for serially concatenated systems

In-line bit interleaving has over the last decade been used in a number of papers, where it has proven to he very effective. However, little or nothing has been said about its advantages and why it should be used in favor of conventional bit interleaving. In this paper, we offer an analysis of the inline bit interleaver and elucidate the advantages of using it. The mutual information (MI) constellation is introduced as a useful tool, both for the analysis and the code design. With its assistance, we show how some common code design principles rely on conventional bit interleaving. The system considered is serially concatenated trellis coded modulation (SCTCM), consisting of a serially concatenated convolutional code (SCCC) of rate 2/3 and an 8PSK mapper. Simulation results are given for the additive white Gaussian noise (AWGN) channel. The results show a clear improvement in the bit error rate (BER) performance for systems using in-line bit interleaving.