Conditional Handover

Description

Conditional Handover (CHO) is an advanced mobility procedure introduced in 3GPP Release 16 to improve handover robustness, particularly in high-mobility and high-frequency (e.g., mmWave) scenarios prone to radio link failures. Unlike conventional handovers, which are network-commanded and executed immediately, CHO decouples the handover preparation phase from the execution phase. The serving gNB (or ng-eNB) prepares one or more candidate target cells in advance by performing admission control and reserving resources. It then provides the UE with a CHO configuration containing the identities of these candidate cells and a set of execution conditions, typically based on radio measurements (e.g., A3/A5 events with offsets and time-to-trigger). The UE stores this configuration and continuously monitors the radio conditions of the serving and candidate cells.

When the UE determines that the pre-configured execution condition for a specific candidate cell is satisfied—and while the connection to the serving cell is still viable—it autonomously initiates the handover execution to that target cell. The UE performs random access to the chosen target cell using the pre-allocated resources (like a dedicated RACH preamble) and sends an RRC Reconfiguration Complete message. This triggers the target cell to inform the serving cell of the successful handover via the Xn interface, initiating the path switch and release of the old UE context. The key architectural components involved are the UE (which evaluates conditions and autonomously executes), the serving RAN node (which prepares the CHO and provides the configuration), the candidate target RAN nodes (which perform admission control and resource reservation), and the core network, which is updated post-execution via the NG interface.

CHO's role in the network is to act as a proactive mobility safety net. By preparing fallback options before the radio link deteriorates critically, it significantly reduces the probability of handover failures (HOF) and radio link failures (RLF). This is especially critical for services requiring ultra-reliable low-latency communication (URLLC) and in deployments using high-frequency bands with rapid signal fluctuations. The procedure is managed via RRC signaling (RRCReconfiguration message carries the CHO configuration) and inter-node coordination over the Xn interface (for preparation and completion). CHO can be configured with multiple candidate cells, and the UE selects the first one whose conditions are met, adding a layer of diversity and redundancy to the mobility process.

Purpose & Motivation

CHO was created to address the limitations of conventional 'network-commanded' handovers in 5G and beyond networks, especially as deployments expanded into frequency ranges above 6 GHz (FR2). In these high-frequency bands, radio signals are more susceptible to blockage and rapid fading, making the time-critical window for a successful network-commanded handover very narrow. Traditional handovers rely on measurement reports from the UE, a decision by the source node, and a handover command—a process that can fail if the radio link degrades faster than this signaling loop can complete, leading to service interruption.

The primary problem CHO solves is the reduction of handover failures and subsequent radio link failures in challenging mobility conditions. This includes high-speed scenarios (e.g., high-speed rail, vehicular), cell-edge areas with overlapping coverage, and environments with high shadowing or intermittent blockage. By shifting the execution decision to the UE based on pre-configured local conditions, CHO eliminates the critical delay involved in the network's decision-making and signaling loop. This makes the handover trigger more responsive to the instantaneous radio environment as perceived by the UE. Historically, before CHO, enhancements like Early Handover or Dual Connectivity partially addressed robustness but added complexity. CHO provides a more streamlined, preparation-based approach that improves reliability for latency-sensitive and mission-critical services, which was a key motivation for its standardization as part of 5G's enhanced mobile broadband (eMBB) and URLLC support.