## 2 Proposed Energy Saving Scheme  
In this paper, we consider 5G networks with heterogeneous macro cells and small cells, where data and control planes are separated, and we consider two types of data traffic, i.e., low rate data traffic and high rate data traffic. In basic separation architecture, an MBS manages the control signal and an SBS manages both low rate data traffic and high rate data traffic. In the considered modified separation architecture, which is similar to the work in [29], an MBS manages the control signal and low rate data traffic and an SBS manages high rate data traffic. Table 1 summarizes the basic and modified separation architectures. We note that data traffic of a UE which is not within the coverage of any SBS should be accommodated by an MBS, which is common to both basic and modified separation architectures. In the modified separation architecture, the state of a BS is defined as the _on_, _off_, and _sleep_ states, and a BS in _sleep_ state operates as in low power mode, as defined in [26]. If a BS is in _sleep_ state, it consumes significantly lower power than that in _on_ state. A BS in _sleep_ state has smaller state transition time to/from _on_ state than that between _on_ and _off_ states. In the basic separation architecture, since both low rate data traffic and high rate data traffic are managed by an SBS only, if a UE is within the coverage of an SBS, the SBS can move to _off_ state, only if there is no UE requiring either low or high rate data traffic. On the other hand, in the modifiedseparation architecture, since an SBS only manages high rate data traffic, it has a higher chance of moving into _off_ state than that in the basic separation architecture. In basic separation architecture and modified separation architecture, energy saving of BSs can be performed by turning off unused BSs and this energy saving scheme is denoted as a conventional energy saving scheme in this paper. In the proposed energy saving scheme, the state of a 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. If all the UEs that request high rate data traffic under the coverage of a BS can be covered by neighbor BSs, the considered BS can be put into either sleep state or off state to save energy. In this paper, if a BS has a high probability of activation due to a higher number of UEs requiring high rate data traffic under the coverage of the BS or a higher number of UEs in the overlapping area commonly covered by neighbor BSs and the considered BS, the BS can be put into _sleep_ state for fast transition to _on_ state when needed. Otherwise, the BS moves to off state in order to save more energy. Figure 4 shows the conventional energy saving operation of BSs in the modified separation architecture, where the state of an SBS can be either _on_ or _off_ state. The UE(HD) represents a UE with high rate data (HD) traffic and the UE(LD) represents a UE with either low rate data (LD) traffic or no traffic. Since an MBS can manage low rate data traffic, only SBSs with UEs requiring high rate data traffic needs to be awake and other SBSs can be turned off to save energy. In the left hand figure of Figure 4, the SBSs 2, 3, and 4 need to be in _on_ state to serve UEs with high rate data traffic. On the other hand, the SBS 2 can be turned off if all the UE(HD)s covered only by the SBS2 finishes their high rate data traffic, as shown in the right hand figure of Figure 4.  
Figure 5 shows the operation of the proposed energy saving operation of BSs in the modified separation architecture, where the state of an SBS can be either _on_, _sleep_, or _off_ state. In the left hand figure of Figure 5, the SBSs 2, 3, and 4 need to be in _on_ state to serve UEs with high rate data traffic.