Active Reconfigurable Intelligent Surface (RIS) Aided Secure Wireless Transmission Under a Shared Power Source Between Transmitter and RIS

In this paper, an active RIS assisted multiple-input single-output system is studied, and we focus on enhancing the achievable secrecy rate of legitimate user in the presence of an eavesdropper (Eve). Different from the most existing studies that the active RIS is equipped with an independent power source, we consider a more fairness setting that the transmitter (Tx) and RIS share the same power source. And given imperfect channel state information (CSI) between RIS and Eve with bounded estimation error, a complicate non-convex robust secrecy rate optimization problem is formulated. To solve this problem, we adopt an efficient alternating optimization algorithm in combination with successive approximation and line search algorithm to jointly optimize the optimal beamformer at Tx, the optimal amplification and phase shift coefficient at RIS. Numerical examples show that the secrecy rate performance returned by the proposed algorithm for the active RIS case outperforms the benchmark scheme for the passive RIS case. Furthermore, the performance gain is significant by allocating only a small fraction of the total power to active RIS.