# Handover Management for D2D Communication in 5G Networks
## 1 Introduction  
Due to the rapid evolution of technologies in recent years, the popularization of mobile devices such as smartphones, laptops, and tablets has resulted in immense changes in Internet connection scenarios. The wireless communication system enables ubiquitous connectivity for mobile users anytime and anywhere. More and more new mobile applications, such as high-definition audio, multimedia streaming, and augmented reality, have emerged, which has led to a tremendous demandfor mobile data traffic. However, due to the scarce radio resources in wireless networks, an effective way to allocate radio resources while considering the throughput of the entire network and the quality of service (QoS) demand of each user equipment (UE) has become an important issue. As a promising technique for fifth-generation (5G) cellular networks to solve the problem mentioned above, D2D communication has been first introduced in the Third Generation Partnership Project (3 GPP) in Release 12, which adopts the concept of proximity services (ProSe) [1]. By allowing direct communication between two mobile devices without the involvement of an E-UTRAN Node B (eNB), device-to-device (D2D) communication can reduce loads of eNBs and the core network. Moreover, the transmission quality of UEs and the performance of the entire network would be improved as a result of a shorter transmission distance between two mobile devices. However, expecting the mobility of users, seamless handover is necessary in order to provide ubiquitous communication when a user moves across a cell boundary. Most of the studies so far focus on the conventional Long Term Evolution-Advanced (LTE-A) handover procedure for a single cellular user [2; 3; 4; 5]. The handover decision is mainly based on UEs' reports of received signal strength and quality, such as reference signal received power (RSRP), received signal strength indicator (RSSI), and reference signal received quality (RSRQ). In D2D communication, on the other hand, although the data can be directly transmitted among D2D devices, the connections between D2D UEs and eNBs are still needed in order to transmit the control signals so that the base stations can adjust the power of devices and allocate required resources to UEs. Thus, there are two essential issues to be resolved in D2D handover management. First, it is possible to see that both devices of the same D2D pair are connected to different eNBs at the cell boundary. Such a configuration will generate additional latency due to the signal exchanges between the two eNBs using the X2 interface. Therefore, allowing a D2D pair to be associated with the same eNB bears a great advantage of scaling down signaling complexity and overhead among eNBs and D2D devices. Second, seeing UE movements and the possible shadow fading in a real environment, unneeded handovers would take place between two eNBs within a short period of time. The so-called ping-pong effect may happen when a D2D user moves around in the overlapped coverages of multiple eNBs or is obscured by trees, buildings, or other obstructions. In this aspect, the number of handovers incurred by the ping-pong effect also generates additional latency and degrades the system performance due to the reallocation of resources to the D2D users. To jointly address these two issues, in this paper, we propose a novel D2D handover mechanism for every pair of D2D users to choose their handover target eNB based on their movements and make a handover decision based on the stability of the connections. In the proposed method, for the sake of reducing unnecessary handovers incurred by the shadow fading, a judicious selection of the handover target is indeed critical. In this aspect, we consider users' movements, including directional changes and distances to its neighbor eNBs, along with the signal-to-noise ratio (SNR) measured by each D2D UE. We first choose the most appropriate eNB as the potential handover target for each D2D pair from the candidate set of neighbor eNBs.