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Reconfigurable intelligent surface (RIS) has been regarded as a promisingtechnique due to its high array gain and low power. However, the traditionalpassive RIS suffers from the ``double fading'' effect, which has restricted theperformance of passive RIS-aided communications. Fortunately, active RIS canalleviate this problem since it can adjust the phase shift and amplify thereceived signal simultaneously. Nevertheless, a high beamforming gain oftenrequires a number of reflecting elements, which leads to non-negligible powerconsumption, especially for the active RIS. Thus, one challenge is how toimprove the scalability of the RIS and the energy efficiency. Different fromthe existing works where all reflecting elements are activated, we propose anovel element on-off mechanism where reflecting elements can be flexiblyactivated and deactivated. Two different optimization problems for passive RISand active RIS are formulated by maximizing the total energy efficiency. Wedevelop two different alternating optimization-based iterative algorithms toobtain sub-optimal solutions. Furthermore, we consider special cases involvingrate maximization problems for given the same total power budget, andrespectively analyze the number configuration for passive RIS and active RIS.Simulation results verify that reflecting elements under the proposedalgorithms can be flexibly activated and deactivated.

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