  This chapter delves into the critical aspects of optimizing energy efficiency
(EE) in active reconfigurable intelligent surface (RIS)-assisted massive MIMO
(M-MIMO) wireless communication systems. We develop a comprehensive and unified
theoretical framework to analyze the boundaries of EE within M-MIMO systems
integrated with active RIS while adhering to practical constraints. Our
research focuses on a formulated EE optimization problem aiming to maximize the
EE for active RIS-assisted M-MIMO communication systems. Our goal is to
strategically find the number of active RIS elements for outperforming the EE
attainable by an entirely passive RIS. Besides, the proposed novel solution has
been tailored to the innovative problem. The formulation and solution design
consider analytical optimization techniques, such as lagrangian dual transform
(LDT) and fractional programming (FP) optimization, facilitating the effective
implementation of RIS-aided M-MIMO applications in real-world settings. In
particular, our results show that the proposed algorithm can provide up to 120%
higher EE than the entirely passive RIS. Besides, we found that the active RIS
can operate with less than half of the reflecting elements for the entirely
passive RIS. Finally, in view of active RIS achieving the complete utilization
of amplification power available, it should be equipped with a reasonable
number of reflecting elements above N = 49.
