Table 1.

Summary of surveyed works.

Optimization Scope	Problem Addressed	Citation
EE at the BS level	Dissection of a BS and figures for energy consumption	[1]
	Downlink Massive MIMO Systems: Achievable Sum Rates and Energy Efficiency Perspective for Future 5G Systems	[16]
	Energy Efficiency in massive MIMO based 5G networks: Opportunities and Challenges	[17]
	EE improvement by a Centralized BB processing design	[18]
	Analytical modelling of EE for a heterogeneous network	[19]
	Energy Efficiency Metrics for Heterogeneous Wireless Cellular Networks	[20]
	Incentive based sleeping mechanism for densely deployed femto cells	[21]
	Sector based switching technique	[22]
	On interdependence among transmit and consumed power of macro base station technologies	[23]
	Utilization of Nash product for maximizing cooperative EE	[24]
	Energy Efficiency in Wireless Networks via Fractional Programming Theory	[25]
	Energy efficiency maximization oriented resource allocation in 5G ultra-dense network: Centralized and distributed algorithms	[26]
	Comparison of Spectral and Energy Efficiency Metrics Using Measurements in a LTE-A Network	[27]
	Energy Management in LTE Networks	[28]
	Energy-efficient resource allocation scheduler with QoS aware supports for green LTE network	[29]
	Interference-area-based resource allocation for full-duplex communications	[30]
	A resource allocation method for D2D and small cellular users in HetNet	[31]
	Highly Energy-Efficient Resource Allocation in Power Telecommunication Network	[32]
	EE enhancement with RRC Connection Control for 5G New Radio (NR)	[6]
	Proactive caching based on the content popularity on small cells	[7]
	Cooperative Online Caching in Small Cell Networks with Limited Cache Size and Unknown Content Popularity	[33]
	Economical Energy Efficiency: An Advanced Performance Metric for 5G Systems	[34]
	Energy-efficient design for edge-caching wireless networks: When is coded-caching beneficial?	[35]
	Content caching in small cells with optimized UL and caching power	[36]
	An effective cooperative caching scheme for mobile P2P networks	[37]
	EE analysis of heterogeneous cache enabled 5G hyper cellular networks	[8]
EE at the network level	Motivation for infrastructure sharing based on current energy consumption figures	[2,38]
	Energy efficiency in 5G access networks: Small cell densification and high order sectorisation	[39]
	Energy-Efficient User Association and Beamforming for 5G Fog Radio Access Networks	[40]
	Global energy and spectral efficiency maximization in a shared noise-limited environment	[9]
	EE Resource Allocation in NOMA	[41]
	Concept and practical considerations of non-orthogonal multiple access (NOMA) for future radio access	[42]
	Optimum received power levels of UL NOMA signals for EE improvement	[43]
	Spectral efficient nonorthogonal multiple access schemes (NOMA vs RAMA)	[44]
	Non-Orthogonal Multiple Access: Achieving Sustainable Future Radio Access	[45]
	Mode Selection Between Index Coding and Superposition Coding in Cache-based NOMA Networks	[46]
	Use case of shared UE side distributed antenna System for indoor usage	[47]
	Optimized Energy Aware 5G Network Function Virtualization	[48]
	Energy Efficient Network Function Virtualization in 5G Networks	[49]
	Network Function Virtualization in 5G	[50]
	A Framework for Energy Efficient NFV in 5G Networks	[51]
	Energy efficient Placement of Baseband Functions and Mobile Edge Computing in 5G Networks	[52]
	Energy Efficiency Benefits of RAN-as-a-Service Concept for a Cloud-Based 5G Mobile Network Infrastructure	[53]
	Dynamic Auto Scaling Algorithm (DASA) for 5G Mobile Networks	[54]
	Design and Analysis of Deadline and Budget Constrained Autoscaling (DBCA) Algorithm for 5G Mobile Networks	[55]
EE using SDN technology	Impact of software defined networking (SDN) paradigm on EE	[56]
	EE gains from the separated control and data planes in a heterogeneous network	[57]
EE using ML techniques	Machine Learning Paradigms for Next-Generation Wireless Networks	[58]
	Switch-on/off policies for energy harvesting small cells through distributed Q-learning	[59]
	Duty cycle control with joint optimization of delay and energy efficiency for capillary machine-to-machine networks in 5G communication system	[60]
	Distributed power control for two tier femtocell networks with QoS provisioning based on Q-learning	[61]
	Spectrum sensing techniques using both hard and soft decisions	[62]
	EE resource allocation in 5G heterogeneous cloud radio access network	[63]