Part of this work has been performed in the framework of the FP7 project ICT-317669 METIS, which is partly funded by the European Union. The authors would like to acknowledge the contributions of their colleagues in METIS, although the views expressed are those of the authors and do not necessarily represent the project. 978-1-4799-3086-9/14/$31.00 ©2014 IEEE Smart Mobility Management for D2D Communications in 5G Networks Osman N. C. Yilmaz, Zexian Li, Kimmo Valkealahti, Mikko A. Uusitalo, Martti Moisio, Petteri Lundén, Carl Wijting Nokia Research Center Nokia Corp. Espoo, Finland E-mail: firstname.middleinitials.lastname@nokia.com Abstract—Direct device-to-device (D2D) communications is regarded as a promising technology to provide low-power, high- data rate and low-latency services between end-users in the future 5G networks. However, it may not always be feasible to provide low-latency reliable communication between end-users due to the nature of mobility. For instance, the latency could be increased when several controlling nodes have to exchange D2D related information among each other. Moreover, the introduced signaling overhead due to D2D operation need to be minimized. Therefore, in this paper, we propose several mobility management solutions with their technical challenges and expected gains under the assumptions of 5G small cell networks. Keywords— D2D, 5G, Mobility, Small cells, Context-aware handover, Self-organization I. INTRODUCTION The more the world becomes connected, the more the wireless devices appear in our proximity. Due to the rapid increase in the number of connected devices and traffic volumes, the fifth generation (5G) networks are expected to be much more dynamic and densely deployed than today’s networks [1] as depicted in Figure 1. In addition to the current cellular services, wireless devices in the future are expected to be constantly interacting with each other as well as with their environment (e.g., data communications from wireless sensors to device or vice versa). Besides the human-centric device-to- device (D2D) communications, one very important use case for D2D is vehicle-to-vehicle (V2V) communications [2] where the mobility plays a very important role. Fig. 1. 5G communications scenario where small cells, nomadic cells and D2D are expected to be essential technical enablers for capacity extension and traffic offloading in the future networking. D2D communications has already been of the attention of wireless communications community for many years [3]. Most recently more and more people believe that D2D communications will be a corner stone in the future 5G networks. For instance, the third generation partnership project (3GPP) agreed that device-to-device (D2D) discovery and communication will become one of the new features to be studied during 3GPP Rel-12 and Rel-13 timeframes under the LTE Proximity Services (ProSe) study item [4]. In 3GPP, two new types of ProSe communication scenarios are defined [5]: (1) direct data path where two devices are exchanging packet data without involvement of any network element in the data plane; and (2) locally-routed data path where D2D user equipment (UE) exchanges the data locally by relaying through the controlling node without the involvement of core network elements. However, due to the short-time frame, the outcome of 3GPP work on D2D will be reduced at least in Rel-12 time frame. In the current standardization there has not been much emphasis on the commercial use cases which are mainly considered for the future 5G scenarios [2]. To satisfy the needs of 2020 wireless communications society, 5G communication system has to be significantly more efficient and scalable in terms of energy, cost and spectral efficiency [6]. Efficiency and scalability will be vital in order to reach the specified targets, i.e., 1000 times higher mobile data volume per area, 10 to 100 times higher number of connected devices and 5 times reduced end-to-end latency as discussed in EU FP7 METIS project [6]. In addition, next generation networks have to support a significant diversity of use cases, such as the different requirements of services in mobility management. These requirements also apply to D2D communications ranging from device discovery to interference management. In this paper we focus on the need of reduced control signaling and improved end-to-end (E2E) latency in network-assisted D2D communications by presenting two smart mobility management solutions as the support for ultra- reliable communications (e.g., V2V communications) and low- latency services in future ultra-dense networks is a requirement to be realized by beyond-2020 (5G) communication systems.