MIMO two-way relay channel with superposition coding and imperfect channel estimation

This paper deals with the superposition coding (SPC) scheme in multiple-input multiple-output two-way relay channels subject to imperfect channel estimation. We determine the achievable rate region and the related channel estimation error degradation and we show how the available power is split between the two bi-directional data flows in order to maximize the system performance and to support fairness. Two main SPC techniques are investigated: (a) SPC without channel state information (CSI) at the users, (b) SPC with CSI at the users where a waterfilling power allocation is employed. We demonstrate that CSI significantly improves the achievable rate at low signal-to-noise ratios (SNRs) while it becomes less critical at high SNRs. Furthermore, a SPC power allocation that incorporates the average channel statistics and is independent on the instantaneous channel conditions as well as on the CSI is also proposed.