Toward green 6G RAN: A survey of energy-efficient mechanisms and 3GPP standardization efforts

Energy efficiency has emerged as a critical design objective for sixth-generation (6G) radio access networks (RAN) due to their significant environmental and economic impact. According to multiple industry and academic projections, 6G networks could consume two to three times more energy than fifth-generation (5G) networks, particularly since the RAN accounts for approximately 73% of total network energy consumption (within a commonly reported range of 70%–80%). This survey provides a comprehensive overview of energy-efficient mechanisms for 6G RAN across four key optimization domains: time-domain techniques (e.g., discontinuous reception/transmission and adaptive synchronization signaling), frequency-domain methods (e.g., bandwidth part operation and carrier activation), spatial-domain approaches (e.g., dynamic antenna adaptation and beamforming), and power-domain optimizations (e.g., power amplifier efficiency and dynamic power control). Beyond conventional mechanisms, the survey reviews emerging artificial intelligence (AI)/machine learning (ML)-driven optimization approaches, including traffic prediction, reinforcement learning, and federated learning frameworks, highlighting how 6G networks can achieve AI-native energy awareness from the design stage. As third generation partnership project (3GPP) standardization evolves from Release 15 through Release 21, with particular focus on the energy-native directions of Releases 19–21, energy-saving technologies are increasingly aligned with quantitative and measurable energy-efficiency requirements for 6G systems. Additionally, the survey examines several emerging enabling technologies expected to influence the energy efficiency of future networks, including reconfigurable intelligent surfaces (RIS), terahertz communications, and holographic multiple-input and multiple-output (MIMO). It also identifies several critical standardization challenges, such as AI/ML framework interoperability, the lack of consensus on energy measurement methodologies, and ongoing debates regarding unified energy-efficiency metrics. Our findings suggest that achieving the targeted 50% reduction in overall energy consumption for 6G will require coordinated advancements across multiple dimensions, including hardware efficiency, architectural design, and intelligent control mechanisms. These developments are expected to align with the 3GPP Release 21 specifications anticipated for 2027–2028.