Layered encoding for 16- and 64-QAM iterative MIMO receivers

In a previous work the authors demonstrated that, for MIMO links using 4-QAM, receivers which involve iterations between a successive interference cancellation (SIC) detector and a convolutional decoder are able to outperform receivers based on the a posteriori probability (APP) detector and a turbo decoder. This is achieved at significantly lower complexity and without the need for matrix channel equalization. However for higher-order modulation schemes such as 16- and 64-QAM, the iterative receiver is unable to converge appropriately, while the APP-based receiver performs adequately but at the expense of prohibitive computational complexity. A novel layered encoding scheme is proposed at the transmitter which allows the iterative receiver to operate with higher-order modulations. Here, iterations are performed between the detector and a turbo-decoder. In addition, soft output combining, inherent to the iterative receiver architecture, is optimized so as to maximize the mutual information transfer at each iteration. It is shown that, in conjunction with the layered encoding scheme, the iterative receiver again outperforms the APP-based receiver, at a lower computational complexity.