Statistical CSI-based dual-hop hybrid RIS-assisted wireless communication

Abstract

Statistical channel state information (S-CSI) based ergodic achievable rate maximization is investigated for dual-hop hybrid reconfigurable intelligent surfaces (D-HRIS) assisted single-user wireless communication systems. A communication model, in which the transmitted signal is reflected through passive RIS (pRIS) to the active RIS (aRIS) and then reflected to the receiver via aRIS, is regarded as D-HRIS-assisted communication. The ergodic achievable rate is analyzed and its approximate expression is derived for this system. Based on the S-CSI, a low-complexity iterative updating scheme is proposed to design the precoding of the base station (BS) and the reflecting matrices of hybrid RISs (HRIS) to maximize the ergodic achievable rate. Specifically, the ergodic achievable rates of the proposal are 1.32 and 1.16 times larger than those of the dual-hop pRIS-aided scheme when the number of elements at the second-hop RIS is 20 and 100, respectively. When the Rician-K factor is larger than 0 dB, the ergodic achievable rate of the proposal is close to that of the instantaneous channel state information (I-CSI) based D-HRIS-assisted communication. And the performance of the system can still be guaranteed when RIS employs discrete phase shifters.