Inter-system handover

Description

Inter-system handover (ISHO) is a mobility procedure that transfers an active communication session from one radio access technology (RAT) to another. Unlike intra-system handover, which occurs within the same RAT (e.g., between two UMTS cells), ISHO involves a change of the underlying radio interface, core network anchor, or both. The primary motivation is to maintain service continuity as a mobile station moves out of coverage of one system and into another, or to optimise resource usage across overlapping networks.

The procedure involves several phases: measurement, decision, preparation, execution, and completion. The user equipment (UE) performs measurements on neighbouring cells of different RATs, as configured by the serving network. These measurements are reported to the serving radio network controller (RNC) or base station, which evaluates whether an inter-system handover is necessary. The decision may be triggered by coverage, load balancing, or service requirements. Once decided, the source network initiates handover preparation with the target network via core network interfaces (e.g., between SGSN and MSC or between MME and SGSN).

Architecturally, ISHO can be categorised as either a handover between UMTS and GSM/EDGE (UTRAN–GERAN), between LTE and UMTS (E-UTRAN–UTRAN), between LTE and GSM, or between 5G NR and LTE. The core network elements involved depend on the direction and the systems. For example, a handover from UTRAN to GERAN may involve the 3G SGSN and the 2G SGSN, with the MSC handling circuit-switched calls. In packet-switched domain, the SGSN may be relocated or retained depending on the scenario. In EPS, inter-system handover between E-UTRAN and UTRAN/GERAN is supported via the S3, S4, and S5/S8 interfaces, with the MME and SGSN coordinating.

Key technical challenges include differing security contexts, QoS mappings, and session management. The target system must be able to support the ongoing service; otherwise, the handover may be rejected or the service may be downgraded. For real-time services, seamless handover requires tight synchronisation and low latency. 3GPP has specified various procedures: PS handover, SRVCC (Single Radio Voice Call Continuity) for voice over LTE to UTRAN/GERAN, and CS fallback for LTE to UTRAN/GERAN. These mechanisms ensure that voice and data sessions survive the transition.

Inter-system handover is a cornerstone of multi-RAT networks, enabling operators to leverage existing 2G/3G investments while rolling out 4G and 5G. It allows load balancing across layers and ensures that users experience uninterrupted service even at the edge of coverage. As networks evolve towards 5G, inter-system handover between NR and LTE remains essential for coverage and mobility.

Purpose & Motivation

Inter-system handover exists to provide service continuity across heterogeneous radio access networks. Before its introduction, a mobile station moving out of coverage of one system would drop the call or data session. With ISHO, operators can seamlessly transfer sessions between different generations of technology, such as UMTS and GSM, or LTE and UMTS. This is critical for operators who deploy multiple RATs in overlapping areas and need to ensure that users do not experience service interruption when crossing technology boundaries.

The primary problem it solves is the coverage and capacity limitation of a single RAT. Early 3G networks (UMTS) were deployed in hotspots, while 2G (GSM) provided ubiquitous coverage. Without ISHO, a user on a 3G call would lose the connection when moving out of 3G coverage. ISHO allows the call to be handed over to 2G, preserving the session. Similarly, as LTE was introduced, inter-system handover between LTE and 3G/2G allowed early LTE deployments to rely on legacy networks for coverage holes and voice services.

Historically, the concept was first standardised in 3GPP Release 99, with the initial definition focusing on UMTS–GSM handover. Over releases, the terminology evolved to include other radio systems, such as UTRAN–GERAN, and later E-UTRAN–UTRAN. The introduction of IP-based core networks and the need for voice continuity led to enhancements like SRVCC and CS fallback. Today, inter-system handover is a fundamental enabler for multi-RAT mobility, ensuring that users remain connected as they move across 2G, 3G, 4G, and 5G networks.