# Handover Management for D2D Communication in 5G Networks
## 4 Proposed Approach
### Choosing Handover Target
For the comparison of the two mobility models, the FD model has the larger difference in RSRPs between both DUEs of the same pair, which induces more handovers due to relatively unstable received signal conditions for each D2D pair but it does not really affect the average utility value of the DTDP because, for the utility function, both DUEs' RSRPs are averaged. Similarly, since the satisfied ratio mainly pertains to the average utility, it would not be affected by different mobility models. Figure 6a illustrates the impacts of the distance between the two DUEs of the same pair on the average handover time and the average throughput. The handover time is calculated as the total time for both DUEs of the same pair to handover to their targets. Note that the time spent for each user to conduct its handover process is set to a fixed value (40 ms). The delay caused by other procedures include waiting time for a DUE to complete its handover process when the other has done, the transmission time for the original serving eNB to transmit the handover commands to both DUEs, and the transmission time for both DUEs to send the successful-handover messages to the target eNB. Thus, for Figure 6a, as we can see that the average handover time increases with the increase of the distance between the two DUEs of a D2D pair due to the increased waiting time for both DUEs to complete their handover procedures and the increased transmission delay. Also, we can see that the average handover time has a negative effect on the average throughput. This is because, in addition to the decreasing of throughput due to the lower channel gain caused by the increasing distance between two DUEs, the blocking of data transmission between two DUEs during their handover processes would also degrade the throughput. For the comparison of different methods, the conventional standard has the highest probability of letting both DUEs handover to their targets in a non-simultaneous way. Thus, the time consumption in waiting for both DUEs to complete their handover procedures becomes the largest among all methods. As a result, it has the highest average handover time. For D-A3, it is better than the conventional standard in terms of average handover time owing to the choosing of the same target eNB for both DUEs. Since our proposed method chooses the same target for two DUEs and also jointly conducts their handover procedures, DTDP has the lowest average handover time. Figure 6b shows the cumulative distribution function (CDF) of the control signal transmission delay and the average throughput. For the control signal transmission, we consider two different cases. The first one is that both DUEs of the same pair connect to the same eNB for their control signals transmission. The other one is that DUEs connect to different eNBs, which is possible when such a D2D pair is moving around at the cell boundary. As mentioned before, the latter has additional latency due to the information exchanges on the X2 interface among different eNBs and among eNBs and D2D devices. Therefore, in this aspect, we can observe that there is higher transmission latency for both DUEs of the same pair connected to different eNBs than connected to the same eNB. Furthermore, as we can see from the average throughput when the two DUEs connect to different eNBs, their control channels have higher transmission latency. This also induces lower throughput. Another issue that deserves further investigation is the number of D2D pairs having both parties connected to the same eNB. To reduce the number of unnecessary handovers, DP and DTDP are designed to let both parties of a D2D pair conduct handover together. As a result, the signaling cost can be minimized. In the next experiment, non-simultaneous handover is allowed for D-A3, denoted by ID-A3. Figure 7 reports the ratio of D2D pairs having both users connected to the same eNB. Both DP and DTDP have a ratio of one. On the other hand, ID-A3 has a ratio of around 70%.