  Deploying active reflecting elements at the intelligent reflecting surface
(IRS) increases signal amplification capability but incurs higher power
consumption. Therefore, it remains a challenging and open problem to determine
the optimal number of active/passive elements for maximizing energy efficiency
(EE). To answer this question, we consider a hybrid active-passive IRS (H-IRS)
assisted wireless communication system, where the H-IRS consists of both active
and passive reflecting elements.Specifically, we study the optimization of the
number of active/passive elements at the H-IRS to maximize EE. To this end, we
first derive the closed-form expression for a near-optimal solution under the
line-of-sight (LoS) channel case and obtain its optimal solution under the
Rayleigh fading channel case. Then, an efficient algorithm is employed to
obtain a high-quality sub-optimal solution for the EE maximization under the
general Rician channel case. Simulation results demonstrate the effectiveness
of the H-IRS for maximizing EE under different Rician factors and IRS
locations.
