The Energy Footprint of 5G Multi-RAT Cellular Architectures

While environmental and financial reasons point to the minimization of the number of radio sites to contain energy use, emissions, and operating expenses of cellular networks, the industry has, in retrospect, partly headed in the opposite direction. The reasons are the deployment of new technologies alongside the existing ones, the use of higher frequencies, and the expansion of radio coverage. In some cases, the issue has been addressed by replacing legacy access sites with Multi-RAT infrastructures, composed of individual radio elements that run multiple technologies concurrently. Now, the transition to 5G poses additional challenges. This paper reviews Multi-RAT architectures, outlines their benefits, discusses how 5G can be integrated, and provides guidance in terms of architectural recommendations, all from an energy consumption standpoint. Specifically, we firstly summarize the transition of monolithic base stations into modern radio elements, exposing the energy rationale and discussing the impact of each main network component. From this basis, we survey different deployment strategies and evaluate their energy implications in light of the constraints and opportunities given by the 5G New Radio standard, its flagship applications, and its transport requirements. Then, we lay down energy-saving estimates quantifying the contribution of each main network segment, revealing the most promising architectures, and identifying the main challenges and the research directions ahead.