Improved iterative decoder for distributed turbo trellis coded modulation decode-and-forward relay channels

An improved iterative decoder (IID) for 3-node decode-and-forward (DF) relay channels employing distributed turbo trellis coded modulation (DTTCM) schemes which takes into account the decoding errors at the relay is proposed. The principle idea is to modify the extrinsic information between the component decoders to eliminate the effect of error propagation between the decoders, depending on the channel conditions between the source, the relay and the destination nodes in terms of signal-to-noise ratio (SNR) per bit. The performance of this decoder is compared with the performance of an iterative decoder proposed in the literature which is called normal iterative decoder (NID) in this paper and does not take into account the decoding errors at the relay, by simulation. The comparison shows that the IID outperforms the NID when the source-relay channel is bad with low SNR, thus yielding lots of decoding errors at the relay.