Active Reconfigurable Intelligent Surface Aided Communication with Partial CSI

Active reconfigurable intelligent surface (RIS) technique has recently been proposed to compensate for the severe "multiplicative fading" effect in the traditional passive RIS-aided systems. This work explores a practical issue in the active RIS-aided systems that system rate and RIS transmit power expressions need the knowledge of independent CSI of RIS reflection channels, however, which cannot be estimated due to the lack of signal processing ability of active RIS. Under this practical limitation, we exploit a rate outage constrained beamforming problem, which is solved by using the Bernstein-Type inequality (BTI) and semidefinite relaxation (SDR) techniques. Numerical results show that, compared with the scenario without RIS, active RIS can bring hundreds of times performance gain, while passive RIS can only improve ten to dozens of times performance gain.