# Energy Efficiency for 5G and Beyond 5G: Potential, Limitations, and Future Directions
## 2 Energy Efficiency Solutions Available in the Market
in [23] introduced two novel and robust beamforming (BF) schemes to improve the secrecy energy efficiency (SEE) of Satellite-Terrestrial Integrated Networks (STINs) in mmWave band shared with cellular systems. The SEE metric is evaluated to gauge performance and is defined as the ratio of the achievable sum rate to the total power consumption. The element of novelty of the proposal is achieved through the following: hybrid analog-digital beamforming, robust design for the imperfect knowledge of the AoDs (angles of departure) for wiretap channels, leveraging interference from BSs to enhance the SEE performance of ESs (earth stations), optimization algorithms such as the Charnes-Cooper approach and the SCA (sequential convex approximation) method to solve the nonconvex optimization problem associated with hybrid beamforming design, and joint optimization of the BF weight vectors at the satellite. While the authors acknowledge the computational complexity, they highlight the importance of selecting feasible initial points for the iterative optimization algorithms. One key aspect of the solution is that it takes into consideration realistic scenarios involving both single and multiple ESs. For a single ES, the authors propose using an iterative optimization algorithm that involves two stages: first obtaining the digital beamformer using the Charnes-Cooper approach, with auxiliary variables converted into second-order cone (SOC) constrains solved using semi-definite relaxation (SDR) and randomization techniques, and, second, using bisection search and SDR to obtain the analog beamformer. To further improve the solution, the authors utilize the iterative penalty function approach to minimize the difference between the trace and maximum eigenvalue of the beamforming matrices, ensuring rank-1 constraint. For more complex scenarios with multiple ESs, the authors adopt the SCA method, approximating original non-convex problems with a series of convex problems. The algorithm iteratively solves the convex problems, refining the solution until convergence. Similarly to the case of a single ES, the authors optimize digital and analog beamformers by adding steps to handle multi-user scenarios, including auxiliary variables and first-order Taylor series expansions to approximate non-convex constraints. This solution takes into consideration practical constraints like imperfect channel state information (CSI), the use of massive antenna arrays due SNIR requirements, and RF chain limitation at the base station. Simulation results showed that the proposed hybrid beamforming approach consistently outperforms digital beamforming, and interleaved and localized hybrid architecture has better performance compared to fully connected architecture. A similar approach involving beamforming techniques, which also takes into consideration jamming and eavesdropping attacks, is provided by [PERSON] et al. in [24]. They propose a block coordinate descent (BCD) framework that jointly optimizes the user'sreceived decoder, the terrestrial and aerial digital precoders, and the multilayer RIS analog precoder. The BCD framework includes techniques like the heuristic beamforming scheme, SCA approach, and a monotonic vertex-update algorithm with a penalty convex-concave procedure (P-CCP), to address the non-convexity of the optimization problem. This approach also takes into consideration the imperfection of CSI. Simulation results have shown an enhancement of 0.0814 and 0.1190 bps/Hz/Joule in comparison with the SDR-based BF scheme and SCA-based BF scheme, respectively. Although energy efficiency is not their main focus, [PERSON] et al. in [25] indirectly contribute to improving it by considering the use of artificial noise (AN) and phase shifters. These techniques can reduce the power consumption of intelligent reflecting surface (IRS)-assisted simultaneous wireless information and power transfer SWIPT networks, maintaining a secrecy performance that is the prime objective.