Two-Timescale Transmission Design for Wireless Communication Systems Aided by Active RIS

This paper considers an active reconfigurable intelligent surface (RIS)-aided communication system, where an <tex-math notation="LaTeX">M</tex-math> -antenna base station (BS) transmits data symbols to a single-antenna user via an <tex-math notation="LaTeX">N</tex-math> -element active RIS. We use two-timescale channel state information (CSI) in our system, so that the channel estimation overhead and feedback overhead can be decreased dramatically. For both the uplink and downlink systems, we derive the closed-form approximate expressions of the achievable rates (ARs) and propose the phase shift optimization schemes. In addition, we compare the performance of the active RIS system with that of the passive RIS system. The numerical results show that the active RIS system achieves a larger AR than the passive RIS system.