## 2 Related Work and Contributions
Energy exchange is enabled via the smart grid among different BSs so that BSs with excess harvested energy can channel out the energy to other BSs demanding for extra energy. To the best of our knowledge, this is the first work to incorporate base station switching (BS-Sw) into a CoMP-enabled hybrid-powered BS connected to a smart grid. Unlike the work in [12], which proposes a joint power allocation and energy cooperation for a CoMP-enabled HetNet, this work extends the previous works further by considering the BS-Sw and power allocation in a CoMP-enabled HetNet, which employs smart grid for energy cooperation. The problem formulated is more challenging by taking BS-Sw into account because the strategy to switch off the BS has a direct impact on the power allocation and energy cooperation. In short, the main contributions of this paper are summarised as follows:  
* Develop a HetNet infrastructure with CoMP-enabled hybrid-powered BSs framework by incorporating BS-Sw technique to improve the EE of the smart grid based HetNet and hence reducing the GPC and mitigating interference among the cells. * Formulate an EE optimisation problem for a CoMP-enabled hybrid-powered HetNet that allows BSs to cooperatively allocate their transmit powers and optimally switch off subject to the minimum QoS requirement. * Design an optimal binomial and two sub-optimal heuristic optimisation methods to solve BS-Sw problem in order to maximise EE and simultaneously reduce the total GPC by utilising the harvested renewable energy (RE). * Propose an energy-efficient joint power allocation and BS-Sw algorithm by incorporating the sub-gradient method into the proposed binomial or heuristic methods to reduce computational complexity. Along with it, [PERSON]'s algorithm is adopted additionally to ensure faster convergence speed while solving the EE problem. \begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline
**Related Work** & **Problem** & **Optimisation Technique** & **Main Objective** & **Data Cooperation** & **RE** & **Power Allocation** & **Energy Sharing** \\ \hline \multirow{2}{*}{[23]} & Resource allocation, inter-cell interference, BS-Sw, and energy consumption & \multirow{2}{*}{Utility maximisation approach} & Offload macrocell traffic and optimise energy performance & \multirow{2}{*}{Yes} & \multirow{2}{*}{Yes} & \multirow{2}{*}{No} & \multirow{2}{*}{No} \\ \cline{2-2} \cline{5-8}  & BS-Sw, user association, and power control & & Iterative framework and game theory & Maximise the system EE & \multirow{2}{*}{Yes} & \multirow{2}{*}{No} & \multirow{2}{*}{Yes} & \multirow{2}{*}{No} \\ \hline \multirow{2}{*}{[24]} & Radio resource and power management combined with BS-Sw & Dual decomposition-based method and low complexity iterative algorithm & Minimise the total power consumption & No & Yes & Yes & Yes \\ \hline \multirow{2}{*}{[25]} & BS sleeping strategy & Coalitional games & Maximise on-grid energy saving & No & Yes & No & No \\ \hline \multirow{2}{*}{[26]} & Utilization of the green energy and grid energy consumption & JT CoMP & Enhance network EE & Yes & Yes & No & No \\ \hline \multirow{2}{*}{[27]} & BS sleeping strategy and power control & Binomial, heuristic and Lagrange dual decomposition & Maximise EE & Yes & Yes & Yes & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the relevant work that employs BS-Sw.