# The Challenges and Compatibility of Mobility Management Solutions for Future Networks
## 1 Introduction  
The meaning of handoff in a telecommunication network is to shift a continuing data or call session from one channel to another; this handoff can be attained with the help of MM mechanisms used in 3G, 4G, LTE, or LTE-A networks. These mechanisms (3G, 4G, or LTE-A MM mechanisms) guarantee the continuity of service and seamless connectivity for the devices when transported from the physical location where they were originally attached. However, in 5G network scenarios, we cannot utilize these inherited MM mechanisms due to the inefficiency, non-compatibility, or even un-usability of these mechanisms. The maturity of 4G and its standards, such as ETSI, IETF, and 3 GPP, has provided methods and techniques that can be used for both the flexibility and dependability of wireless networks. Still, these techniques and procedures are not so feasible for (i) heterogeneity-type networks,(ii) generation of revenue for the industry, and the (iii) softwarization deployment approach. In this respect, researchers [1] also used MM techniques (multi radio access technology (RAT) and software-defined networking (SDN)), but these, too, cannot tackle the MM challenges of 5G and future networks due to the volume of devices, complexity, etc.  
Similarly, as noted in the literature [2; 3], these MM techniques are only feasible for current architectures and unable to cope with 5G and future networks. Next, the MM techniques of group handover, progressive cell association [4; 5; 6], etc., are also not suitable for tackling the issue of complexity and core network signaling in 5G and future networks. Consequently, such unfeasible aspects have forced researchers to initiate a great revolution in MM mechanisms for 5G and future networks. Researchers face issues in molding existing MM to the 5G network. These issues are encountered due to the architectural nature of the 5G network, in which the provision of diverse types of services and satisfying the expectations of users (120\(\times\) increase in data rates) are a must compared to the LTE network [6; 7]. Moreover, the theme of 5G is to ensure minimum latency with required reliability for delay-sensitive applications, such as emergency services, augmented reality, multimedia applications, real-time applications, etc. [6].  
Furthermore, the ITU defined the detailed requirements that 5G networks will have to fulfill [8]. Specifically, the central challenge of the 5G network is to provide services to users at the time of mobility, when the data rate is almost more than 1 Gbps, with heterogeneous applications and mobility profiles and extreme dependency on the network. However, the literature and understanding to date suggest there has been no thorough assessment of the functional requirements, possible solutions, and challenges concerning MM mechanisms. Hence, we discuss the MM mechanisms of 3G, 4G, LTE, LTE-A, etc. and their issues in the 5G network, followed by state-of-the-art MM mechanisms for 5G and future networks. We divide the article into the following sections. Section 2 presents the functional requirements and design principles for 5G and future MM mechanisms. Section 3 lays out the conditions for qualitative analysis and the criteria that must be met for the satisfaction of these conditions. Section 4 presents a qualitative analysis of legacy mechanisms and compares the advantages and drawbacks of 5G and future MM solutions. Next, in Section 5, we analyze current state-of-the-art mechanisms and their usability, challenges, and possible compatibility as MM solutions with 5 G and future networks. Further, in Section 6, we present possible strategies and existing challenges that will help to resolve the issues presented in Sections 5 and 6 with the requirements stated in Section 2. Furthermore, Section 6 also proposes a framework for 6G MM solutions. Lastly, Section 7 concludes with possible future directions.