## 1 Introduction
### Related Work
ES improvement with a focus on mobility management and resource allocation has been studied more extensively despite its relatively small gain compared to switching On-Off underloaded Base Stations (BSs) [24, 25, 22, 26]. ES gains by switching On-Off operation would improve the situation to only a limited degree for a given throughput until they are further enriched with the proactive and autonomous approaches by intelligent switching methods. In this direction of research, some recent works show promising results in terms of potential ES [1, 12, 13, 14, 15, 16, 25, 26]. However, they fall short for 5G demands, to the best of our knowledge, due to the following five limitations:  
1. Sensitive mode of operation: Typical ES SON algorithms are susceptible to reaction that achieves ES at the expense of QoS after an event has been completed. Given the well-populated city dynamics with bus passenger ridership in relation to deployed cellular environment, by the time SCs overloading or underloading is detected and a realistic algorithm is opted to solve the known issue, the conditions may already change [7]. In the 5G environment, this problem can further escalate when disparate passenger ridership and plethora of cell types responsible to support smart city eco-system are not in harmony. 2. SCs wake up time: Sleeping SCs require a specific amount of time to wake up [27]. Any passenger entering a SC footprint that is still in a sleeping state would add high latency experience. Thus, there is a need to modernise conventional paradigms pro-actively to maintain low latency requirements of 5G in a more agile fashion, i.e., pro-active ES by passenger's mobility management. 3. User Association to sleeping SCs: A key challenge in the HetNet cell On-Off switching strategy is to establish user associations (bus passengers ridership association) to the correct serving SCs that are switched ON while passengers are within its coverage footprint [17], thus contributing to overhead challenges. Existing ES schemes have not apparently provided evidence to address this challenge where 5G QoS demands low-overhead, low-costs and highly efficient architectures. SON upright design: Conventional ES solutions when implemented together in a HetNet environment are susceptible to conflicts [5] that require intelligence to resolve. SON use-cases that are liable to be conflicted are: traffic offloading while SC switching [3,13] and prediction of passengers to neighbouring cells [6]. For the first conflict, Cell Individual Offsets (CIOs) along with transmission power settings play a major role, whereas a correct distancing metric for the classification of mobility predictions is used for the second one. Furthermore, traffic offloading through vertical, horizontal or both is an important method when BS transmit power is concerned [25]. In horizontal offloading, SCs have low transmit powers within the certain cell range to offload the traffic of neighbouring cells.Therefore, between SCs, horizontal offloading cannot always be realised. Consequently, vertical offloading often becomes the only choice for some SCs to go into sleep mode if its neighbouring SCs are not in the proximity. 5. CO\({}_{2}\) Emissions: To the best of our knowledge, this challenge coupled with ES is either overlooked by researchers or lacks comprehensive quantisation [2,3] to the extent of ES saving impact on CO\({}_{2}\) emissions.