To Reflect or Not to Reflect: On–Off Control and Number Configuration for Reflecting Elements in RIS-Aided Wireless Systems

Reconfigurable intelligent surface (RIS) has been regarded as a promising technique due to its high array gain, low cost, and low power. However, the traditional passive RIS suffers from the "double fading" effect, which has become a major bottleneck in restricting the performance of passive RIS-aided communications. Fortunately, active RIS can alleviate this problem by adjusting the phase shift and amplifying the received signal simultaneously. Nevertheless, a high beamforming gain often requires a large number of reflecting elements, which leads to non-negligible power consumption, especially for the active RIS. Thus, one challenge is how to improve the scalability of the RIS and the energy efficiency. Different from the existing works where all reflecting elements are activated, we propose a novel element on-off mechanism where reflecting elements can be flexibly activated and deactivated. To achieve a tradeoff between the transmission rate and energy consumption, two different optimization problems for passive RIS and active RIS are formulated by maximizing the total energy efficiency, where the constraints of the maximum power of users and the RIS, the minimum transmission rate, the element on-off factor, and the unit moduli of passive elements are taken into account. In light of the intractability of the formulated problems, we develop two different alternating optimization-based iterative algorithms by combining quadratic transform, variable substitution, and the successive convex approximation method to obtain sub-optimal solutions. Furthermore, in order to gain more insight into problems, we consider special cases involving transmission rate maximization problems for given the same total power budget, and respectively analyze the number configuration for passive RIS and active RIS. Simulation results verify that the proposed algorithms outperform existing algorithms, and reflecting elements under the proposed algorithms can be flexibly activated and deactivated.