Secure Communication in RIS-assisted MIMO systems: Active RIS or Passive RIS?

Abstract

As 6G technology continues to evolve, RIS is expected to play a significant role in addressing the challenges of future communication systems. Recently, to combat the "double fading" effect caused by the signal propagating over the reflected link in RIS-assisted systems, active RIS has been proposed and received significant attention. This paper focuses on addressing the issue of secure transmission at the physical layer in a multiple-input multiple-output (MIMO) system aided by active RIS. In this system, the user (UE) receives multiple parallel streams of data sent from the base station (BS). Specifically, to enhance the anti-eavesdropping capability, we propose a formulation for maximizing the secrecy rate, which involves a non-convex problem. To solve this problem, we transform it into an equivalent form by exploiting the equivalence between the data rate and the minimum mean-square error (MMSE), and propose an efficient alternating optimization algorithm to optimize the BS precoder and RIS beamformer jointly. Simulation results validate that with the same power consumption, the secrecy rate is obviously improved in the active RIS assisted MIMO systems compared with the passive RIS case.