EXIT Chart Evaluation of a Receiver Structure for Multi-User Multi-Antenna OFDM Systems

In this paper we evaluate, by means of Extrinsic Information Transfer (EXIT) charts, an iterative receiver that has emerged as a promising candidate for non-coherent multi-user multi-antenna OFDM systems. The receiver performs parallel interference cancellation (followed by linear filtering) and channel estimation, using soft symbols obtained from a bank of single-user decoders. For the sake of conceptual clarity we study a system with two single antenna users and a receiver with two antennas, and we demonstrate how the convergence behavior of the receiver can be visualized using paired three dimensional EXIT surfaces. Our results show that the actual decoder trajectories obtained through simulations are well predicted from the EXIT charts. For the iterative receiver under investigation we identify a very specific problem with EXIT chart generation; the EXIT curve for the inner component decoder depends on the outer encoder. To handle this problem we propose a modification to the iterative receiver which solves the aforementioned problem; the performance degradation is demonstrated to be small.