I. INTRODUCTION  

II. 3GPP NR ENERGY EFFICIENCY RELATED ENABLING TECHNOLOGIES  

III. POWER CONSUMPTION MODELS  

IV. ENERGY EFFICIENCY METRICS  

V. THEORETICAL UNDERSTANDING OF ENERGY EFFICIENCY: MASSIVE MIMO  

VI. TIME-DOMAIN ENERGY SAVING-BASED SOLUTIONS  

VII. CARRIER-DOMAIN ENERGY SAVING-BASED SOLUTIONS  

VIII. ANTENNA-DOMAIN ENERGY SAVING-BASED SOLUTIONS  

IX. MACHINE LEARNING AND DATA-DRIVEN ENERGY EFFICIENCY OPTIMIZATION  

X. OPEN RESEARCH DIRECTIONS  

XI. CONCLUSIONS