Are Mobility Management Solutions Ready for 5G and Beyond?I Akshay Jain1,∗, Elena Lopez-Aguilera1, Ilker Demirkol1 1Department of Network Engineering, Universitat Polit`ecnica de Catalunya BarcelonaTECH, Barcelona 08034, Spain. Abstract Enabling users to move to different geographical locations within a network and still be able to maintain their connectivity and most essentially, continuity of service, is what makes any wireless network ubiquitous. Whilst challenging, modern day wireless networks, such as 3GPP-LTE, provision satisfactory mobility management (MM) performance. However, it is estimated that the number of mobile subscriptions will approximately touch 9 billion and the amount of data traffic will expand by 5 times in 2024 as compared to 2018. Further, it is expected that this trend of exponential growth will be maintained well into the future. To cope with such an exponential increase in cellular traffic and users alongside a burgeoning demand for higher Quality of Service (QoS), the future networks are expected to be highly dense and heterogeneous. This will severely challenge the existing MM solutions and ultimately render them ineffective as they will not be able to provide the required reliability, flexibility, and scalability. Consequently, to serve the 5G and beyond 5G networks, a new perspective to MM is required. Hence, in this article we present a novel discussion of the functional requirements from MM strategies for these networks. We then provide a detailed discussion on whether the existing mechanisms conceived by standardization bodies such as IEEE, IETF, 3GPP (including the newly defined 5G standards) and ITU, and other academic and industrial research efforts meet these requirements. We accomplish this via a novel qualitative assessment, wherein we evaluate each of the discussed mechanisms on their ability to satisfy the reliability, flexibility and scalability criteria for future MM strategies. We then present a study detailing the research challenges that exist in the design and implementation of MM strategies for 5G and beyond networks. Further, we chart out the potential MM solutions and the associated capabilities they offer to tackle the persistent challenges. We conclude this paper with a vision for the 5G and beyond MM mechanisms. Keywords: 5G, Beyond 5G, 6G, Mobility Management, SDN, Meta-Surfaces. 1. Introduction nal to Interference and Noise Ratio (SINR), Reference Sig- nal Received Quality (RSRQ), Reference Signal Received Future wireless networks define a very challenging envi- Power (RSRP), etc. However, in addition to the signal ronment for mobility management (MM) solutions, due to quality parameter centric handovers, modern day appli- the significant increase in density (in terms of both users cations necessitate that other parameters such as avail- and deployed access points), in heterogeneity (given the able core network bandwidth, End-to-End (E2E) latency, various radio access technologies (RATs) supported), as backhaul bandwidth and backhaul reliability [1] are also well as in programmability (the network as well as the taken into consideration. Moreover, maintaining Quality environment can be programmable). To achieve an ubiq- of Service (QoS), e.g., provisioning service continuity, link uitous network service in such challenging environments, it continuity, required bit-rate and latency, during mobility is critical to devise effective MM strategies that facilitate arXiv:1902.02679v3 [cs.NI] 21 May 2020 scenarios has been one of the primary objectives for novel seamless mobility by allowing users to traverse through the MM mechanisms. Multiple strategies to satisfy such QoS network without losing connectivity and service continuity. criteria such as service migration [2], service replication One of the traditional approaches for allowing applica- [3], path reconfiguration [4], etc., have been proposed by tions to serve a user in mobile scenarios has been to main- the research community. MM solutions for 5G and beyond tain network connectivity through handovers based on cri- networks are also expected to ensure E2E connectivity and teria such as Radio Signal Strength Indicator (RSSI), Sig- session continuity through the maintenance/preservation of IP address of the user towards the core network entity that provisions the service for the corresponding user. IThis work has been supported in part by the EU Horizon 2020 re- search and innovation programme under grant agreement No. 675806 To motivate further, we consider an illustrative exam- (5GAuRA), and by the ERDF and the Spanish Government through ple of the future mobility scenario as presented in Fig- project RYC-2013-13029. ∗Corresponding author ure 1. It shows the extraordinary nature of complexity Email address: [email protected] (Akshay Jain ) that the future networks will present for MM. As shown in Preprint submitted to Elsevier April 2020 (a) (b) Core Network Core Network Heterogeneous Heterogeneous Access Network Access Network UE UE Device-to-Device Device-to-Device Cluster Cluster (d) (c) Core Network Core Network Heterogeneous Access Network Heterogeneous Access Network UE UE Device-to-Device Device-to-Device Cluster Cluster LTE/VLC/ LTE-eNB/5G Wi-Fi Access 2G/3G Access Drone Good wireless LiFi small cell Access Points Point Points Access Point link Core Network Degrading IoT Devices IP Core Prospective good Element wireless link wireless link Delay tolerant Delay sensitive New delay Device-to-Device Meta-Surface flow flow sensitive flow cluster flow Figure 1: An illustrative 5G and beyond network mobility scenario. 2 Figure 1(a), a mobile user equipment (UE) is connected D2D cluster is firstly serviced by a drone AP, which then to multiple RATs (5G Access Point (AP)/ Long Term relays information to/from the ground based APs. These Evolution (LTE) eNode B (eNB)/visible light communi- ground based APs assist in serving the data flows gener- cations (VLC) and Light Fidelity (LiFi) small cells [5{7], ated from the devices in the D2D cluster by relaying the etc.), while having a delay tolerant and a delay sensitive data to the relevant servers in the core network. application datastream (flows) with distinct QoS profiles. Given the complexity of the scenario presented in Fig- Also, the AP through which the delay tolerant flow is being ure 1, it is evident that no single MM mechanism will served to the user has a good wireless link with a meta- form the solution to all the possible situations and sce- surface in the vicinity. While, traditionally the environ- narios that will be prevalent. And, although current MM ment between the user and an AP is considered as an ad- mechanisms propose methods for careful RAT and AP se- versary in all the generations of mobile communications, lection, IP packet forwarding, route optimization, and ses- including 5G, in beyond 5G (B5G) networks the environ- sion management, a more than 10-fold increase in user den- ment will be programmable and hence, will be an ally by sity coupled with the heterogeneity in flow types and net- provisioning favorable transmission channels [8{10]. These work will extremely limit their capabilities, as explained in favorable channels will essentially consist of reflected sig- the subsequent sections in detail. New user applications nals, the phases and polarizations of which will be adjusted such as Augmented Reality, Virtual Reality, Vehicle-to- by thin (but electrically significant) surfaces, also known Everything (V2X), etc., will present very restrictive delay as meta-surfaces, so that they interfere constructively at requirements, exceptionally high reliability and bandwidth the receiver [8, 9]. In addition to the meta-surfaces, future requirements [11], that will consequently severely challenge networks will also consist of mobile APs such as drones, as the capabilities of current MM strategies. Further, the ra- shown in Figure 1(a). Note that, the density of meta- dio access network (RAN) technologies themselves are ex- surfaces and drone APs will also be extremely high in pected to undergo important transformation in the future future networks. Further, in the scenario illustrated, we networks given the significant interest in VLC, LiFi, etc., consider the use case wherein the drone AP is servicing a [5, 6]. Whilst both LiFi and VLC, being TeraHertz (THz) device-to-device (D2D) cluster, and connecting it to the bandwidth technologies, enable near Terabits per second core network through one of the ground based APs. The (Tbps) speeds, they are significantly impaired by the envi- D2D cluster over the course of its existence does not gen- ronment. This consequently has significantly more detri- erate packets as frequently as the other users, since the mental effects on the user QoS during mobility scenarios, cluster devices mainly host Internet of Things (IoT) appli- which we will discuss in further detail in the later sections. cations. Also, owing to the telecom operators' desire to serve Next, in Figure 1(b), as the user moves, it starts to reg- more industry verticals, a new set of mobility patterns ister wireless links with better signal quality from other will emerge. For example, a platoon of vehicles moving APs as compared to those it is already associated to. It coherently together, vehicles disbanding from one platoon is imperative to state here that, the APs can be from the to join another, ultra-fast moving users (in excess of 500 same or different network operators. Henceforth, a care- km/h), moving access points (such as those on drones [12]), ful and efficient RAT and AP selection for each flow will etc., thus introducing another dimension to the MM prob- be necessary as part of the future MM mechanisms. It lem. Henceforth, the ability to serve devices with mo- is interesting to observe that while the AP used for serv- bility patterns that will be more diverse and challenging ing the delay tolerant flow in Figure 1(a) no longer has as compared to current day network scenarios, will be a a good link quality, through the meta-surfaces and their significant challenge towards the design, development and programmable nature it still has a good wireless link to deployment of 5G and beyond MM mechanisms.