  Reconfigurable intelligent surface (RIS) has been regarded as a promising
technique due to its high array gain and low power. However, the traditional
passive RIS suffers from the ``double fading'' effect, which has restricted the
performance of passive RIS-aided communications. Fortunately, active RIS can
alleviate this problem since it can adjust the phase shift and amplify the
received signal simultaneously. Nevertheless, a high beamforming gain often
requires a 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. Two different optimization problems for passive RIS
and active RIS are formulated by maximizing the total energy efficiency. We
develop two different alternating optimization-based iterative algorithms to
obtain sub-optimal solutions. Furthermore, we consider special cases involving
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 reflecting elements under the proposed
algorithms can be flexibly activated and deactivated.
