Secure Constructive Interference Precoding for Downlink MIMO Relay System

Constructive Interference (CI) has shown great advantages in improving security and reliability of communication systems, which utilizes channel state information (CSI) and knowledge of the instantaneous information data on symbol level. In this paper, we explore the CI-based secure precoding problem under a dual-hop and half-duplex downlink transmission relay system in the presence of an eavesdropper. We propose to jointly optimize the precoding strategy at the source and at the relay through an alternating optimization process. To alleviate the high computational costs and circumvent the difficulty of practical implementation, we propose a low-complexity iterative algorithm for the optimization, where we use Karush-Kuhn-Tucker (KKT) conditions to analyze and simplify the previous optimization problem at the relay. Numerical results show that the proposed algorithm can achieve an improved performance compared with traditional zero-forcing/regularized zero-forcing (ZF/RZF) methods and significantly degrade the eavesdropper's performance.