HandOver Failure

Description

HandOver Failure (HOF) is a specific type of Radio Link Failure (RLF) that occurs during the execution of a handover (HO) procedure. A handover is the process of transferring an ongoing call or data session from one cell (the source cell) to another (the target cell). HOF is declared when this transfer process does not complete successfully, leading to a disruption or drop of the User Equipment's (UE's) connection. The network and the UE perform extensive measurements, signaling, and resource preparation to execute a handover; a failure in any of these steps can result in a HOF. From a measurement perspective, HOF is a critical mobility Key Performance Indicator (KPI) that operators continuously monitor to assess network health and performance.

The HOF procedure and its detection mechanisms are deeply integrated into the Radio Resource Control (RRC) protocol. The process begins with the source base station (e.g., gNB in 5G, eNB in LTE) making a handover decision based on UE measurement reports. It then issues a Handover Command (RRCConnectionReconfiguration message with mobilityControllnfo) to the UE. A HOF can occur in several phases: failure to receive the Handover Command, failure to access the target cell (Random Access Channel failure), failure to complete the RRC reconfiguration with the target cell, or a timeout during the execution. Upon detecting failure conditions, the UE initiates Radio Link Failure procedures, which may involve cell selection and re-establishment attempts, often reporting the failure cause to the network for analysis.

In the broader network architecture, HOF management involves several network elements. The Radio Access Network (RAN) nodes are primarily responsible for executing handovers and detecting failures. Operations, Administration, and Maintenance (OAM) systems collect HOF statistics and other related measurements (like Handover Success Rate) from these nodes. These statistics are used by Self-Organizing Network (SON) functions, specifically Mobility Robustness Optimization (MRO). MRO algorithms analyze HOF patterns (e.g., too early handover, too late handover, handover to wrong cell) and automatically adjust handover control parameters like hysteresis, time-to-trigger, and cell individual offsets to minimize future failures, creating a self-healing and self-optimizing network.

Purpose & Motivation

HOF exists as a defined concept and KPI to quantitatively measure and manage the reliability of the mobility function, which is fundamental to cellular networks. The primary problem it addresses is service continuity. In a mobile network, users move, and their connections must be seamlessly transferred between cells to maintain service. Failures in this process directly lead to poor user experience in the form of dropped calls, frozen video streams, or broken data sessions. By defining and measuring HOF, operators can identify problematic areas, cell boundaries, or parameter settings that cause service degradation.

Historically, handover failures were analyzed reactively through drive tests and customer complaints. The standardization of HOF reporting, particularly with the introduction of SON in 3GPP Rel-9 and later, provided a proactive, automated mechanism for failure detection and root cause analysis. This was motivated by the increasing complexity of networks (heterogeneous networks with macrocells, small cells), which made manual optimization impractical. The MRO function uses HOF data, along with other measurements like 'Too Early Handover' and 'Too Late Handover', to automatically tune network parameters, solving the problem of suboptimal handover performance in dynamically changing radio environments. This ensures robust mobility is maintained as networks evolve from 4G to 5G and beyond.