## 1 Introduction  
Massive growth in the number of mobile users and mobile user equipment (UE) has sown the seeds of data hunger and severely high traffic congestion, especially in cellular networks. This problem has been aggravated by the emerging 5G technologies, which leads to increasingly dense network deployments to cater for growing user demands. The densification of base stations (BSs) within a geographical area has led to the emergence of a new network architecture known as a heterogeneous network (HetNet), which employs multiple BSs with different transmit power capabilities [1]. The unprecedented network densification through HetNets deployment has resulted in a scarcity of radio resources (particularly spectrum) and a tremendous increase in energy consumption. Over the years, many radio resource management techniques have been proposed to better utilise radio resources, which has cost the mobile service providers (MSPs) billions of dollars yearly. Recently, it has been reported that, other than the costly radio resources, the MSPs have also spent as much as half of their annual operating expense on energy costs [2]. Hence,making HetNets spectrally and energy-efficient not only can result in a tangible positive environmental impact, but also help MSPs in attaining long-term profitability. Undoubtedly, the energy consumption of cellular systems is highly dependent on the mobile users' characteristics. The user activities and data usage patterns in a large cellular network can be modelled statistically using a Markov model with tri-non-negative matrix factorisation [3]. This work has demonstrated that data usage across mobile users are severely uneven for a cellular network. From this report, we can infer that the data usage and user activities for a HetNet will follow this pattern and may demonstrate a more uneven and unpredictable trend due to irregular cell sizes in HetNet. One observation from this work implies that around-the-clock, not every hour is considered a peak hour in terms of data usage. Indeed, during non-peak hours, the excess radio resources are being wasted together with the energy used to power the various networking devices in a HetNet [4]. Studies suggest that most of the power consumption (about 58%) of a communication system is mainly contributed by the base station (BS) [5]. Thus, to reduce wastage of radio resources and save energy, certain small-cell BSs with low spectrum utilisation in a HetNet can be switched off; however, it is noteworthy that it does not mean there are no mobile users at all during off-peak hours. On the other hand, there might be a small number of users who still need to access the services of the BSs; therefore, shutting down any small-cell BS in this scenario is unwise and it may lead to a coverage hole where the affected small-cell users could lose connection to any BS services permanently. Fortunately, HetNet architecture can fill the coverage hole by allowing macro BS (MBS) to serve the affected small-cell users [6]. Nevertheless, deployment of multiple cells within a HetNet leads to inter-cell interference (ICI) since the coverage area of small-cell BSs overlaps with the coverage of MBS. The ICI effect can be mitigated by using a coordinated multi-point joint-processing (CoMP-JP) technique [7]. In a HetNet equipped with a CoMP-JP mechanism, the MBS and small-cell BSs share their channel state information (CSI) and data to coordinate the transmission process and hence avoiding interference. As specified in the CoMP standards of 3 GPP Release 9, 10 and Beyond [8], apart from increasing a particular UE's date rate, the appropriate coordination of signals received from multiple interfering cells also improves the performance of the overall network with enhanced coverage area. Anyhow, the BSs that are isolated geographically should have proper synchronisation to avoid performance collision.