Joint source and relay optimization for non-regenerative MIMO two-way relay systems with imperfect CSI

In this paper, we consider a non-regenerative MIMO two-way relay system with imperfect channel state information (CSI). We employ a stochastic approach to model the channel uncertainties and address the robust joint source and relay optimization problem based on the minimum mean squared error (MMSE) criterion. With imperfect CSI, the self-interference (SI) cannot be completely canceled at destination nodes. Hence, both channel uncertainties and residual self-interference should be considered. We develop an optimization framework that unifies both frequency-division duplex (FDD) and time-division duplex (TDD) systems despite their different channel statistical properties. Two robust algorithms are proposed to minimize the sum mean squared error (MSE) averaged over channel uncertainties. The first algorithm adopts alternating optimization to update the source precoders, relay precoder and destination receivers iteratively with guaranteed convergence. In the second algorithm, only the relay precoder with certain structure is considered. Then the relay precoder design is reduced to the simple power allocation problem. Simulation results show that the proposed algorithms provide robustness against channel uncertainties, especially when the signal-to-noise ratio is high