Spectral Efficiency Maximization for Double-Faced Active Reconfigurable Intelligent Surface

Although the recently emerging reconfigurable intelligent surface (RIS) has exhibited great potentials to enhance wireless communications, it has intrinsic defects as well, i.e., the double fading effect severely restricts its coverage and the conventional single-faced structure restrains its service within only half-space. To overcome these shortcomings, this paper proposes a novel double-faced active (DFA)-RIS structure. This new design can effectively magnify the impinging signal and refract it to fulfill
$360^\circ$
full-space coverage. Moreover, we study the joint design of transmit beamforming and the DFA-RIS configuration to maximize spectral efficiency (SE) in a multi-user multi-input-single-output system. Per-element power constraints (PEPCs) are considered to precisely reflect the limited magnifying capability of each RIS element's amplifier, which makes our problem have great number of constraints and hence become highly difficult. We successfully develop an analytic-based, highly computation-parallelizable and convergence guaranteed algorithm to resolve this challenge. Interestingly, our considered problem subsumes other state-of-the-art RIS structures, e.g. the single-faced active RIS and the passive reflective-transitive RIS, as special cases and therefore our proposal indeed provides a unifying solution. Extensive numerical results are presented to verify the efficiency of our solution and the advantageous performance of DFA-RIS.