To satisfy the high data demands in future cellular networks, an ultra-densification approach is introduced to shrink the coverage of base station (BS) and improve the frequency reuse. The gain in capacity is expected but at the expense of increased interference, frequent handovers (HOs), increased HO failure (HOF) rates, increased HO delays, increase in ping pong rate, high energy consumption, increased overheads due to frequent HO, high packet losses and bad user experience mostly in high-speed user equipment (UE) scenarios. This paper presents the general concepts of radio access mobility in cellular networks with possible challenges and current research focus. In this article, we provide an overview of HO management in longterm evolution (LTE) and 5G new radio (NR) to highlight the main differences in basic HO scenarios. A detailed literature survey on radio access mobility in LTE, heterogeneous networks (HetNets) and NR is provided. In addition, this paper suggests HO management challenges and enhancing techniques with a discussion on the key points that need to be considered in formulating an efficient HO scheme. INDEX TERMS Radio access mobility, cell selection, handover, LTE, 5G, NR, mobility enhancers. Ph.D. degree in electrical engineering with Aalto University, Finland. He has more than two years, from 2013 to 2015, of professional experience as an RF Planning and Optimization Executive at Wi-Tribe Pakistan Ltd. (Ooredoo Group). During his stay at KFUPM, he was associated with the King Abdullah University of Science and Technology (KAUST), Saudi Arabia, as a Visiting Student and a KFUPM-KAUST Joint Research Initiative. He has been a Researcher with the Helsinki Research Center, Huawei Technologies Finland Oy, since February 2018. His current research interest includes energy-efficient mobility for small-cell overlaid cellular networks. He received the Gold Medal Award for obtaining the first position in the B.Sc. degree. XAVIER GELABERT received the M.Sc. degree in electrical engineering from the KTH Royal Institute of Technology, Stockholm, in 2003, and the joint B.Sc. and M.Sc. degrees in telecom engineering and the Ph.D. degree (Hons.

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SECRET is a collaborative European Training Network (ETN) committed to create an "excellent" educational training platform for Early Stage Researchers (ESRs) in the field of wireless communications and networking for 5G. The project is recently funded by the European Commission under the H2020 research and innovation program, through the Marie Curie People Program.This proposal targets to narrow the gap between current networking technologies and the foreseen requirements of future 2020 networking, through the recruitment and training of 17 ESRs. SECRET aims to strike a note by delivering higher capacity, ability to support more users, and lowering the cost per bit by adopting technology trends widely accepted to form part of the 5G roadmap, through the deployment of new disruptive "femtocell" type cells on demand, to what we refer to as mobile small cells. This will be complemented by a wireless high-speed fronthaul to bridge the small cell network to the core. Moreover, novel techniques will be investigated, including "network coding", "cooperation", and "energy-aware smart front-end". Additionally, due to the confidential information that will be communicated over in future networks, a lightweight security framework built on secure network coding will be proposed.

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Abstract-Beyond 3G (B3G) networks will encompass, among other features, a wide range of radio access technologies (RATs) providing users with a flexible and efficient access to the increasing pool of demanding services. This will allow users to get connected using the access technology that is most suitable according to some specified criteria. Consequently, to take full advantage of B3G networks, mobile terminals will need to support a larger set of capabilities. Among those, the support of different RATs, i.e. multi-mode capacity, is a must. This paper addresses the impact of multi-mode terminals in an EDGE/UMTS heterogeneous network with multi-service provisioning. Results indicate that multi-mode terminal availability should be considered when designing common radio resource management strategies in heterogeneous wireless access networks. Specifically, a service-based initial RAT selection policy is evaluated, revealing different behaviors for different multi-mode terminal availabilities and service-class mixings. In order to compensate the limitations imposed by non-multi-mode terminals, it is suggested to actuate over GERAN by using a resource reservation scheme for interactive users. By doing so, we tradeoff the QoS between multi-service/multi-mode users.

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In this paper we address the impact of cell size on the handover procedure in a Long Term Evolution (LTE) network. In particular, we highlight the potential problems that may occur when small cell densification is applied. In addition, the impact of the User Equipment (UE) speed is also analyzed. System level simulations are provided using a detailed LTE network simulator accounting for multiple points-of-failure and channel modeling compliant with LTE standards. We conclude that a certain cell size can be found around which any increase or decrease of the cell size brings performance degradations due to different limitations in the uplink. The performance is also degraded as UE speed increases, especially for small cell sizes. And for large cell sizes, we note that low speed UEs handover failures may rise due to the inability to "escape" from a poor radio condition area.