## 2 An Optimized Handover Based on Context Information
### An Optimized Handover Scheme
Then, if a handover is needed, the UE finds neighbor BSs satisfying RSRP condition. For high speed UE, the service of a UE is handed over to a macro cell to reduce the number of handovers, since frequent handovers between small cells and between a small cell and a macro cell, due to high speed UEs, degrade the quality of experience of UEs. Therefore, the UE makes a candidate list of macro cell and finds a target macro cell by solving optimization problem using PSO algorithm. For low speed UE, different criterion is applied for selecting a target cell depending on the type of data traffic. If a UE has high rate data traffic, it is handed over to a small cell BS, if there is any available small cell BS in order to offload more traffic from a macro cell BS. Therefore, the UE makes a candidate list of small cell and finds a target small cell by solving optimization problem using PSO algorithm. If a UE has low rate data traffic, it can be handed over to either a small cell BS or a macro cell BS. If there is any neighboring small cell BS which has higher traffic load than a threshold, it is handed over to a small cell BS. Therefore, the UE makes a candidate list of small cell after removing small cell with very low traffic load, since it is better to turn off the small cell with very low traffic load to save energy, by removing it from a candidate list of small cell. Then, it finds a target small cell by solving optimization problem using PSO algorithm. If there is no neighboring small cell BS which has higher traffic load than a threshold, it is handed over to a macro cell BS. Therefore, the UE makes a candidate list of macro cell and finds a target macro cell by solving optimization problem using PSO algorithm. Figure 4: Flowchart of the proposed handover. Although the idea of using traffic type and speed to associate a UE with a macro cell BS or a small cell BS in separated network architecture is similarly presented in [34], where only the UE with low speed and high rate data traffic is associated with a small cell BS and other UEs are associated with a macro cell BS, our proposed idea is different from the work in [34], since a UE with low speed and low rate data traffic can be handed over to a small cell BS, depending on the traffic load condition of neighboring small cell BSs. Also, although it is mentioned that the speed of UE is considered to select a target cell for association in [34], the mobility of UE is not considered actually at the analysis of the proposed sleeping schemes. On the other hand, our proposed scheme considers both data traffic type and mobility together to make a handover decision. Figures 5 and 6 show examples of a CAC-based handover scheme and the proposed handover scheme, respectively. In Figure 5, a data type, i.e., low rate data traffic or high rate data traffic, is not considered for a handover decision. It is assumed that cars, which are denoted as UE_C, move at high speed and connect to a macro cell BS. Pedestrians that are denoted as UE, move at a low speed and connect to small cell BSs. Low speed UEs 1, 2, 5, 6, and 7 move into new small cells along the directions of arrows and handed over to new small cell BSs. A high-speed UE moves from small cell 1 to small cell 2, but it is not handed over to small cell BS2, and therefore small cell BS2 can be turned off to save power. In Figure 6, handover is carried out based on the data type and speed of UE. Thus small cell BS 6 is turned off before handover since UE 8 has low rate data traffic and low speed and it can be connected to a macro cell BS if SBS(6) has very low traffic load. Low speed UEs 1, 2, 5, 6, and 7 move into new small cells along the direction of arrows and handed over to new small cell BSs. Small cell BS 4 can be turned off since high speed UE 6 and UE 7 move out of the small cell and UE 5 moving into the small cell has low rate data traffic and low speed and it is handed over to a macro cell BS since the traffic load of SBS(4) is very low.