  Although the emerging reconfigurable intelligent surface (RIS) paves a new
way for next-generation wireless communications, it suffers from inherent
flaws, i.e., double-fading attenuation effects and half-space coverage
limitations. The state-of-the-art double-face active (DFA)-RIS architecture is
proposed for significantly amplifying and transmitting incident signals in
full-space. Despite the efficacy of DFA-RIS in mitigating the aforementioned
flaws, its potential drawback is that the complex active hardware also incurs
intolerable energy consumption. To overcome this drawback, in this paper we
propose a novel dynamic energy-saving design for the DFA-RIS, called the
sub-array based DFA-RIS architecture. This architecture divides the DFA-RIS
into multiple sub-arrays, where the signal amplification function in each
sub-array can be activated/deactivated dynamically and flexibly. Utilizing the
above architecture, we develop the joint optimization scheme based on transmit
beamforming, DFA-RIS configuration, and reflection amplifier (RA) operating
pattern to maximize the energy efficiency (EE) of the DFA-RIS assisted
multiuser MISO system considering the perfect/imperfect channel state
information (CSI) case. Then, the penalty dual decomposition (PDD) based
alternating optimization (AO) algorithm and the constrained stochastic
majorization-minimization (CSMM) based AO algorithm address non-convex problems
in the perfect/imperfect CSI case, respectively. Simulation results verified
that our proposed sub-array based DFA-RIS architecture can benefit the EE of
the system more than other RIS architectures.
