Legacy Handover

Description

Legacy Handover (LHO) is a concept and set of procedures defined within 3GPP's management specifications, particularly TS 28.552 for 5G network management. It refers to the handover processes that involve legacy Radio Access Technologies (RATs), such as GSM, UMTS, and LTE, when they interoperate with 5G New Radio (NR) as part of a multi-RAT network. LHO is not a single protocol but a managed capability within the Operation, Administration, and Maintenance (OAM) system, focusing on the configuration, monitoring, and optimization of handover performance between 5G and these legacy systems.

Architecturally, LHO functionality is implemented within the Network Management (NM) and Domain Management (DM) systems, such as the Network Resource Model (NRM) for 5G. It involves managed objects that represent the handover relationships and performance measurements between a 5G Next Generation NodeB (gNB) or ng-eNB and legacy base stations like eNBs (LTE), NodeBs (UMTS), or BTSs (GSM). The OAM system collects Key Performance Indicators (KPIs) related to these handovers, such as handover success rates, preparation failures, execution times, and radio link failures post-handover. These KPIs are defined as performance measurements in the NRM and are crucial for network operators to assess and ensure seamless mobility for UEs capable of multi-RAT operation.

How it works involves the OAM system provisioning handover policy parameters (e.g., thresholds, biases for A2/B2 events in LTE-NR interworking) to the RAN nodes via the Itf-N interface. The RAN nodes (gNB and legacy eNB/NodeB) then execute the actual radio-level handover procedures (e.g., X2-based or N2/N26-based handovers) based on UE measurements and these configured policies. The OAM system's LHO management function monitors the outcomes, triggering optimization actions if KPIs degrade. For instance, it might adjust cell individual offsets (CIOs) or hysteresis values for handover events between a 5G cell and a 3G cell to reduce ping-pong effects or call drops. Its role is to provide the management plane tools necessary to maintain robust mobility in a heterogeneous network during the transition period where 5G coverage is being rolled out alongside extensive 2G/3G/4G infrastructure.

Purpose & Motivation

LHO was introduced to address a critical challenge in the phased deployment of 5G networks: maintaining seamless mobility and service continuity for UEs as they move between new 5G coverage areas and existing legacy (2G, 3G, 4G) networks. Prior to 5G, inter-RAT handovers were managed within the RAN and core network protocols, but the management and optimization of these procedures from a unified OAM perspective for the 5G era required new standardization. The existing management models for LTE did not fully encompass the integration and performance management of handovers to/from 5G NR, especially with the increased complexity of network slicing and dual connectivity.

The primary problem it solves is providing network operators with the necessary management capabilities to monitor, configure, and optimize handover performance involving legacy RATs in a 5G network environment. Without standardized LHO management, operators would lack visibility into key mobility KPIs between 5G and older technologies, making it difficult to troubleshoot issues like handover failures, poor voice call continuity (e.g., EPS Fallback to LTE/UMTS), or inefficient resource utilization during network traffic steering. This is particularly important for coverage-centric 5G deployments (e.g., using low-band NR) where handovers to ubiquitous LTE or 3G for coverage fill-in are frequent.

Its creation in Release 17 was motivated by the practical reality that 5G NR would coexist with legacy networks for many years. The 3GPP SA5 working group (Management and Orchestration) recognized the need to extend the 5G NRM to include performance management for these essential legacy interactions. LHO enables operators to ensure Quality of Experience (QoE) during this transition, supporting use cases like voice service fallback, coverage extension, and load balancing across different RAT generations through managed and optimized handover policies, which is vital for a smooth customer experience and efficient network operation.