## 5 Conclusions and Future Work  
Ever-increasing energy consumption is one of the most important issues related to the problem of global warming and thus the energy consumption of mobile communication networks has drawn much attention since it takes a significant part of the total energy consumption of ICT. In this paper, we considered 5G networks with heterogeneous macro cells and small cells, where data and control planes are separated, and an MBS manages the control and low rate data traffic, while an SBS manages high rate data traffic, in contrast to the basic separation architecture where an MBS manages the control while an SBS manages data traffic. Then, an energy saving scheme for BSs was proposed, based on the state management of BSs and session management of UEs, where the state of BS is determined depending on the number of UEs that request high rate data traffic and the number of UEs that exist under the overlapping areas commonly covered by the considered BS and the neighbor BSs.  
In the proposed energy saving scheme, if a BS has a high probability of activation due to higher number of UEs that request high rate data under the coverage of the BS or higher number of UEs in the overlapping area commonly covered by neighbor BSs and the considered BS, the BS moves to _sleep_ state. Otherwise, the BS moves to _off_ state in order to save more energy. We formulated an optimization problem for the proposed energy saving scheme, where the objective function is the sum of power consumption by an MBS, power consumption by SBSs under the coverage of a BS, and energy consumption due to switching between different states of BSs. Since the developed optimization problem is a NP-hard problem, the solution was obtained using particle swarm optimization. Numerical results showed that the proposed energy saving scheme in the modified separation architecture had better energy efficiency compared to the basic and modified separation schemes. Also, the proposed energy saving scheme had lower aggregate delay than other schemes. These results show that the proposed scheme is energy-efficient with less aggregate delay.  
In our future work, we will consider the energy consumption of UEs together in a multi-macro cell environment. Since UEs consume more energy when they communicate with an MBS, since the distance between a UE and an MBS is generally much longer than that between a UE and a nearby SBS, the proposed energy saving scheme has more energy consumption of UEs. This may degrade the quality of experience (QoE) of UEs. Therefore, we will consider the energy consumption of BSs  
Figure 20: Aggregate delay.  
and UEs together and draw an optimal scheme from the aspect of the sum of energy consumption of BSs and UEs, while satisfying the QoE of UEs. Also, work on how to apply the concept of separated architecture to heterogeneous networks owned by multi-operator mobile networks will be carried out in our future works.  
The authors would like to thank anonymous reviewers, who greatly improved the quality of this paper. This research was supported in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1037728).  
[PERSON] and [PERSON] conceived and designed the proposed scheme. [PERSON] and [PERSON] conceived and designed the simulations; [PERSON] performed the simulations; [PERSON] and [PERSON] analyzed the data; [PERSON] wrote the paper. Both authors have read and approved the final manuscript.  
The authors declare no conflict of interest.