# Handover Management for D2D Communication in 5G Networks
## 2 Related Works  
Many previous works in the literature have considered the problem of handover. According to the conventional LTE-A standard, a single cellular user can hand over from the serving eNB to the target eNB if the RSRP from the target eNB is higher than the RSRP from the serving eNB plus a hysteresis margin for a certain period, known as the time to trigger (TTT). This procedure is the so-called A3 event [6; 7; 8]. Therefore, some of the related works mainly focus on the configuration of the hysteresis margin and TTT. [PERSON] et al. [9] employed different combinations of eNB coverage, hysteresis margin, and TTT in the urban and suburban areas to determine a better parameter setting. Likewise, [PERSON] et al. [10] observed the different failure rates in handover and connectivity for different TTTs to decide an optimal TTT value. Numerous studies concentrate on the reduction of the number of handovers in order to decrease the latency. They first choose the target eNB based on the movements of devices and then decide whether or not to handover to the target based on the received signal condition. [PERSON] et al. [11] proposed a handover mechanism to decide the best handover target for each user according to its historical movement records. In [PERSON] et al. [12], a weighted-based handover scheme was proposed. The authors set the weights of candidate eNBs according to UE's bearing of movement. An eNB with the highest weight was picked up as the target. However, when a user makes a U-turn, the weights of some neighbor eNBs will still increase even though the user is moving away from such eNBs, which would result in additional unnecessary handovers. In [PERSON] et al. [13], the Markov Chain is used to decide the connection probabilities to different eNBs based on the distances between a device and the neighbor eNBs. However, all the above works only consider the handover for a single cellular user. Various works are devoted to D2D communication from different aspects. [PERSON] et al. [14] presents a solid study on the improvement of user SINR in LTE HetNets using D2D relay concept. There also exist studies focusing on the D2D handover management. [PERSON] et al. [15] provides a valuable insight from an analytic perspective into the D2D management problem. The D2D handover management problem with reference point group mobility (RPGM) model is modeled as a two-stage queuing network and a multi-dimensional Makov chain and used to evaluate the blocking and successful handover probability. [PERSON] et al. [16] reports a comprehensively comparative study on different D2D handover management techniques. They mathematically analyze the performance difference of selected solutions and conclude that a software defined network (SDN)-based solution outperforms others. In upholding the same belief that SDN-based solution is preferred, [PERSON] et al. [17] details the architecture and procedure deign, and demonstrates its superiority. Although an SDN-based approach is promising, a potential barrier is that SDN in not widely deployed. [PERSON] et al. [18] focuses on the handover of D2D clusters in fast-moving environment, such as highways. Their design takes advantage of D2D multicast in signaling and resource allocation. Simulation shows that the proposed approach can reduce communication interruption probability and handover latency. This works can be more complete if directional change can be considered. As mentioned before, two devices of the same D2D pair are possibly connected to different eNBs at the cell boundary, which brings about additional latency. Therefore, some approaches have been proposed to cope with this problem. In [PERSON] et al. [19], in order to let the users of the same D2D pair link to the same eNB as much as possible, the authors proposed a D2D handover mechanism. Specifically, for a D2D pair, if one of the devices has met the handover condition while the other has not, then it is allowed to postpone the handover decision until both devices have satisfied the handover condition.