Handover

Description

Handover (HO) is a core mobility procedure in cellular networks that ensures a User Equipment (UE) can move between cells, radio access modes, or even radio systems without losing its connection. In 3GPP terms, as defined in Release 19, it is 'the process in which the radio access network changes the radio transmitters or radio access mode or radio system used to provide the bearer services, while maintaining a defined bearer service QoS.' This broad definition encompasses intra-frequency, inter-frequency, and inter-RAT handovers. The handover process involves measurement reporting from the UE, decision-making by the network, preparation of target resources, execution (including synchronization and random access), and completion. It is critical for maintaining call continuity, data session persistence, and user experience during mobility.

Handovers can be categorized in several ways: based on the interface (Xn-based or NG-based), based on the number of connections (single or dual connectivity), and based on the trigger (e.g., coverage, load, service). In 5G NR, handovers are typically executed via the Xn interface between gNBs, or via the NG interface (N2) through the core network if Xn is not available. The procedure starts with the UE sending measurement reports to the serving cell. The network (source gNB) makes a handover decision based on these reports, radio resource management (RRM) policies, and QoS requirements. The source gNB then sends a Handover Request to the target gNB over Xn, which performs admission control and prepares the necessary resources. Upon acceptance, the source gNB sends an RRC Reconfiguration message to the UE with the target cell configuration. The UE then detaches from the source cell, synchronizes with the target cell, performs random access, and sends an RRC Reconfiguration Complete. Data forwarding may occur to minimize packet loss.

Key components of handover include measurement configuration (e.g., event A3 for intra-frequency, A4/A5 for inter-frequency), handover preparation (signaling between source and target), handover execution (UE actions), and handover completion (path switch in core network). The handover procedure is standardized in 3GPP TS 23.067 and TS 21.905, with detailed RAN procedures in TS 38.300 and TS 36.300. Handover performance is measured by interruption time, which in 5G NR can be as low as 0 ms for certain scenarios with techniques like Conditional Handover (CHO) and Dual Active Protocol Stack (DAPS).

In multi-connectivity scenarios (EN-DC, NR-DC), handover can involve the Master Node (MN) or Secondary Node (SN) independently. For example, a PSCell change is a type of handover for the secondary cell group. Handovers also play a role in network slicing, where the target cell must support the slice of the UE. The process must ensure that QoS flows are maintained; thus, the target cell must have the necessary resources and policies. Handover failures can lead to radio link failure (RLF) and re-establishment, which is why robust handover mechanisms are crucial.

Handover has evolved from simple voice-centric transfers in 2G to complex, multi-RAT, multi-connectivity procedures in 5G. Advanced features like Conditional Handover (CHO) and Make-Before-Break (MBB) aim to reduce interruption and improve reliability. As networks move towards 6G, handover will continue to be a key enabler for seamless, ubiquitous connectivity, integrating terrestrial and non-terrestrial networks.

Purpose & Motivation

Handover exists to ensure uninterrupted service as a mobile user moves across the coverage area of a cellular network. Without handover, a call or data session would drop when the UE leaves the coverage of one cell and enters another. The primary motivation is to maintain the bearer service QoS, which includes parameters like latency, throughput, and reliability. Early mobile networks (1G) had limited handover capabilities, often resulting in dropped calls. The introduction of handover in 2G (GSM) allowed seamless voice calls, and subsequent generations have enhanced it for data and advanced services.

The need for handover arises from the fundamental limitation of radio coverage: a single base station cannot cover an entire country. By dividing the coverage into cells, networks can reuse spectrum and increase capacity. However, this requires the ability to transfer ongoing connections between cells. Handover solves this by coordinating between the source and target cells, ensuring that resources are reserved and the UE is seamlessly transitioned. It also enables load balancing, where the network can move UEs from congested cells to less loaded ones to optimize performance.

In modern networks, handover is not just about coverage but also about QoS and service continuity. For example, a URLLC session may require a handover that maintains ultra-low latency, while an eMBB session may prioritize throughput. The evolution of handover from simple hard handovers to soft handovers (in CDMA) and now to conditional handovers (in 5G) reflects the growing demand for reliability and efficiency. Handover also facilitates inter-RAT mobility, allowing users to move between 4G and 5G seamlessly, which is essential for the gradual rollout of 5G. Overall, handover is a cornerstone of mobile network design, enabling the 'mobile' in mobile communications.