Relay precoding in multiuser MIMO channels for physical layer security

In this paper, we investigate precoding and limited feedback schemes for multiuser multi-input multi-output (MIMO) relay systems providing physical layer security. Thereby, we take the effect of the quantized channel state information (CSI) into account for relay precoder optimization. With the quantized CSI available, the relay station (RS) optimizes its precoder by maximizing the transmit power of the artificial noise, which is radiated isotropically to mask the information signals of the intended users, while meeting minimum mean square error (MMSE) constraints of the intended user links under CSI mismatch. An optimal solution for the relay design is derived based on the Karush-Kuhn-Tucker (KKT) conditions, having a comparable computational complexity as the conventional strategy. Numerical results verify the effectiveness of the proposes strategy.