Release Date：2021-11-30  Author：By Guo Xuefeng  Click：

While 3G and 4G use centralized mobility management to provide consistent service continuity for all users, 5G defines three types of service and session continuity modes to meet differentiated continuity requirements of different services. Under 5G-Advanced, the network and application scenarios will be greatly changed, posing new challenges to the mobility management technology. 

Mobility Management Requirement Analysis
From the perspective of mobility and handover scenarios, mobile subjects are becoming more ubiquitous, horizontal handover more frequent and vertical handover more common. With cloud-network synergy and computing-network integration, network connections evolve from physical entity connections to virtual connections with intangible contents, services, and computing power, resulting in increasing ubiquity of mobile scenarios and entities (including terminals, servers, and networks). The flattening network and high-density networking at the wireless side make horizontal handover more frequent. On the other hand, cross-network vertical handover becomes normal since coordinated heterogeneous networks provide full-scenario coverage and IPv6 and multi-host terminals get wide adoption. When it comes to service continuity requirements, different applications have different requirements for service continuity. ToC services like web browsing and video services are not sensitive to connection interruption caused by mobile handover. ToB applications such as the Internet of Vehicles, UAV and industrial Internet require seamless handover management and deterministic network performance guarantee. 
It can be seen that in the 5G-Advanced network, the mobility management technology needs to solve at least the following problems:
—Provide differentiated service continuity services for different scenarios.
—Provide zero-interruption and zero-packet-loss network connections to meet seamless handover requirements.
—Guarantee consistent network performance before and after a handover to achieve fast handover with deterministic network QoS.

Key Technologies of Enhanced Mobility Management
In line with ubiquitous mobile scenarios and differentiated service continuity requirements, mobility management also needs to evolve. This can be started from multi-connection management, bidirectional perception of application and network, AI enablement, and service-based architecture design (Fig. 1).

Multi-Connection Management in Heterogeneous Networks 
Centralized mobility management using fixed anchors introduces an increase of transmission delay and reduced network performance. In distributed mobility management scheme, when the data plane mobility anchors are relocated to complete a handover, network interruption occurs. 
For ultra-reliable communication, 3GPP has defined the redundant user plane transmission solution based on dual connections. Two redundant PDU sessions are used to transmit data, enabling a reliability of at least 99.9999%. 
Combining the dual connectivity solution with distributed mobility management can effectively solve the connection interruption problem with the mobility anchors and ensure that the network performance is consistent before and after the handover. That is, the data plane uses a distributed deployment of mobility anchors, and mobile terminals use dual connectivity to maintain communication connections with the network. When a UE is on the move, the dual-connection handover mechanism allows the handover of only one connection at a time while the other connection is still available. During the moving process, the network is always available, thus avoiding connection interruption. Data-plane anchors closer to mobile terminals are selected. This, together with technologies such as explicit path, ensures consistent network performance before and after handover. 
Bidirectional Perception of Application and Network
The future network is moving towards collaboration among the cloud, network, edge, terminal and application. By perceiving the applications' requirements, the network selects the corresponding mobility management policy for targeted network and path selection, handover triggering, and policy execution. On the other hand, applications can perceive information such as delay, bandwidth and congestion from the network in real time, and adjust data sending and receiving policies to enable a better experience. 
The deep packet inspection (DPI) technology is used to detect information about applications and packet contents. In the application-aware IPv6 networking (APN6) scheme, an IPv6 packet carries the application-related information including their requirements via its extension headers, and the network layer schedules the network resources and provisions the service accordingly.
An application can subscribe to information from the network capability exposure function such as SCEF/NEF, or implement network quality detection through technologies like STAMP, TWAMP and in-band OAM to obtain network information.
AI-Enabled Mobility Management
AI, as a basic element, will be deeply integrated into networks to achieve all-round intelligence covering NEs, networks, services, O&M and operations, thus improving network efficiency and reducing O&M costs. 
AI can also be used in mobility management. The research on AI-based mobility optimization is carried out in 3GPP R17 to predict and manage the location and track of the terminals and optimize the AMF paging process. 
Powered by AI, the system collects and analyzes the information from terminals, networks, and applications and trains a model for the prediction of terminal tracks and handovers, thus achieving active mobility management, decreased handover delay, optimal transmission paths and customized mobility management processes. 
Servitization of Mobility Management
To meet the service continuity requirements of multiple scenarios and differences, 5G-Advanced needs to provide a general mobility solution, and servitization is one of the feasible solutions. 
First, the mobility management functions are abstracted and encapsulated. Service-based functions can include dual connectivity in heterogeneous networks, replication redundancy elimination, bidirectional perception of application and network, AI-based active handover, caching and forwarding based on inter-access gateway tunnel, and identity and location separation. Second, the functions can be orchestrated. For different scenarios, function chains are formed through flexible orchestration, and function chain identifiers are provided for upper-layer selection. Finally, the new functions can be smoothly introduced. The overall framework remains unchanged, while functional components are updated with the evolution of technologies and scenarios. 
The research on mobility management for service continuity has been going through the entire development of wireless network, and is constantly facing new challenges. Integration with new technologies and protocols is one of the effective ways to continuously improve mobility management performance, and requires joint efforts of the industry.