A Low-Complexity EP Detector for IDD in Massive-MIMO Transmission

Based on the framework of iterative detection and decoding(IDD), a low complexity successive over relaxation(SOR) expectation propagation(EP) is proposed for massive MIMO. When calculating the posterior distribution of the received signal in EP iteration, matrix inversion with high complexity can be replaced by a linear equation, which can be solved by SOR. In IDD structure, the feedback of the decoder is used as a priori information of EP iteration. The feedback symbols generated by the high reliability decoder have small variance and make the coefficient matrix of linear equation diagonally dominant. Therefore, SOR can converge quickly under this scheme. The above SOR method reduces the computational complexity from $\mathcal{O}(N^{3})$ to $\mathcal{O}(N^{2})$. Furthermore, the log likelihood ratio(LLR) generated by the decoder is multiplied by a scale-factor to improve decoding accuracy. Numerical results show that SOR-EP maintains almost as good performance as traditional EP. The proposed method converges in various loaded scenarios, especially in the heavily-loaded case. This is superior to other low-complexity EP methods.