We present a low complexity iterative decoding method for a multiplexed Alamouti space-time code serially concatenated by a convolutional code. The space-time signalling code consists of a Gray coded 16-QAM mapper and two spatially overlaid Alamouti signalling blocks. A linear complexity decoder is used for decoding the inner space-time code. Ordinarily, an iterative decoding between the inner and outer decoders at the receiver does not improve performance when using a Gray coded modulation scheme. However we show that turbo gains are possible when a multiplexed Alamouti code is used. The low complexity inner decoder is based on Bayesian interference cancelling approach. We present a method of using both the extrinsic and full a posteriori values from the outer decoder to facilitate gains from iterations. The modified iterative decoder presented here favourably compares to a space-time bit inter-leaved coded modulation (ST-BICM) system providing the same spectral efficiency in terms of complexity-performance tradeoff.
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