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.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (303 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Conditional Handover (CHO) function was newly introduced as a handover mechanism to improve mobility performance. The release included foundational specifications for inter-gNB conditional handovers, establishing it as an alternative to traditional handover procedures. This mechanism was later identified as a key option for handover optimization analytics alongside DAPS and RACH-less handover.
- CR on U-plane handling for handover TS 38.300CR0029
- CR on message content in inter-RAT handover TS 38.300CR0030
- Clarifying the behaviour of UE to SgNB connection at a MeNB handover TS 33.401CR0630
- KeNB derivation in 5GS to EPS handover TS 33.501CR0104
- Corrections and clarifications to Handover from EPS to 5GS over N26 TS 33.501CR0105
- Clause 6.7.3.2 - Modification on algorithm selection during N2 handover TS 33.501CR0239
+ 70 more changes
In Release 16, Conditional Handover (CHO) was formally introduced as a new mobility function, enabling a UE to pre-prepare handover to one or multiple candidate cells and execute it autonomously when specific radio conditions are met. The release standardized key derivation procedures for CHO in both LTE and NR to ensure security, and it integrated CHO into the framework for Management Data Analytics to enable optimization analysis alongside other mechanisms like DAPS handover. Furthermore, Release 16 added specific performance measurements for inter-gNB conditional handovers to facilitate network monitoring and mobility robustness optimization.
- Add inter-gNB handover related measurements TS 28.552CR0005
- Add measurements related to inter-AMF handover TS 28.552CR0068
- Add measurements related to inter gNB Handover TS 28.552CR0079
- Add measurements related to intra gNB Handover TS 28.552CR0080
- Add a description of Inter-gNB handover Execution time measurement TS 28.552CR0127
- Add measurements related to handover between 5GS and EPS TS 28.552CR0142
+ 92 more changes
In Release 17, the Conditional Handover (CHO) function was enhanced with new performance measurements and analytics capabilities for optimization. Specifically, the release introduced Management Data Analytics (MDA) support for CHO to analyze mobility performance and recommend optimal handover mechanisms. It also added the capability for CHO to operate while maintaining a Secondary Cell Group (SCG) configuration in dual connectivity scenarios.
- Add beam specific handover counters to MRO TS 28.313CR0044
- NRM for CHO TS 28.541CR0608
- NRM for CHO Stage 3 TS 28.541CR0609
- NRM for DAPS handover TS 28.541CR0610
- ADD EPS fallback handover related Measurement TS 28.552CR0255
- Add EPS fallback handover mean time measurement TS 28.552CR0258
+ 58 more changes
In Release 18, key enhancements to Conditional Handover (CHO) included its introduction for Secondary Cell Groups (CHO with SCGs), enabling more robust dual-connectivity mobility, and the generalization of RACH-less handover procedures. The release also introduced specific corrections and optimizations for IoT and NTN scenarios, such as timer-based CHO for NR NTN, and refined the handover cancel procedure for CHO with SCGs.
- Add beam specific inter-system handover counters related to MRO TS 28.552CR0379
- Generalization of RACH-less handover [RACH-lessHO] TS 38.331CR4636
- Introduction of CHO with SCG(s) TS 38.423CR1090
- Modify the criteria for incrementing counters of Intra-GNB Handover Preparation TS 28.552CR0397
- Handover for Reduced Capability TS 36.300CR1394
- Correction of timer-based conditional handover for IoT NTN TS 36.300CR1399
+ 33 more changes
In Release 19, the enhancements for Conditional Handover (CHO) primarily focused on integrating it with Management Data Analytics (MDA) for optimization. Specifically, new capabilities were introduced for MDA to analyze and recommend the most optimal handover mechanism, including CHO, based on current statistics and future predictions of target cell resources. Furthermore, the release added support for analyzing handover performance at the beam level to improve CHO success rates.
- Rel-19 CR TS 28.104 New use case on Handover and service data correlation analytics TS 28.104CR0186
- Rel-19 CR 28.313 Use case and requirements for Inter-CU LTM and conditional LTM TS 28.313CR0075
- Add measurements for MLB related handover TS 28.552CR0578
- Rel-19 CR TS 28.552 New measurements to monitor Sub-Optimal Handovers (Intra-5GS and Inter-System) TS 28.552CR0608
- Introduction of early CSI acquisition for L3 handover [EarlyCSI_L3HO] TS 38.331CR5587
- Support for conditional intra-CU LTM and intra-CU LTM TS 38.401CR0462
+ 17 more changes
In Release 20, the enhancements for Conditional Handover (CHO) primarily involved its integration with Management Data Analytics (MDA) for optimization purposes. Specifically, the release introduced support for conditional LTM (Load and Traffic Management) control, allowing MDA to provide analytics for selecting the optimal handover mechanism, including CHO, based on predicted resource consumption and target cell conditions. This enabled handover optimization by recommending actions to prepare target gNB resources and improve selection criteria before executing the conditional handover.
Explore further
Broader topics and technologies where CHO plays a role.
Defining Specifications
3GPP specifications that define or reference CHO, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 28.104 vj30 | Management Data Analytics (MDA) | Rel-19 |
| TS 28.313 vk00 | Management and orchestration; SON for 5G networks | Rel-20 |
| TS 28.541 vk00 | 5G Network Resource Model (NRM) Stage 2/3 | Rel-20 |
| TS 28.552 vk10 | 5G Performance Management Measurements | Rel-20 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TR 33.877 vi00 | Technical Report on Security Aspects of AI/ML in RAN | Rel-18 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 36.423 vj10 | X2 Application Protocol (X2AP) Specification | Rel-19 |
| TR 36.763 vh00 | NB-IoT/eMTC Support for Non-Terrestrial Networks | Rel-17 |
| TS 37.320 vj00 | Minimization of Drive Tests (MDT) Overview | Rel-19 |
| TS 37.340 vj00 | Multi-Connectivity Operation Overview | Rel-19 |
| TS 37.483 vj10 | E1 Application Protocol (E1AP) | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.463 vj00 | E1 Application Protocol (E1AP) | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |
| TR 38.864 vi10 | Technical Report on Network Energy Savings for NR | Rel-18 |