Operator Controlled Signal Threshold per Access Technology

Description

OCST (Operator Controlled Signal Threshold per Access Technology) is a 3GPP parameter introduced in Release 19 and specified in TS 31.102 and TS 31.121. It allows mobile network operators to define signal strength thresholds for each access technology (e.g., 5G NR, LTE, UMTS) that a User Equipment (UE) should consider when performing cell selection, reselection, or handover. By setting these thresholds, operators can influence the UE's mobility decisions to optimize network performance, load balancing, and user experience.

In traditional networks, cell selection and reselection are based on standardized rules that prioritize certain access technologies or frequency layers. However, these rules may not always align with an operator's specific deployment strategy or real-time network conditions. OCST provides a mechanism for operators to customize these thresholds, ensuring that UEs prefer or avoid certain access technologies based on signal quality. For example, an operator might set a higher threshold for 5G NR to encourage UEs to stay on 5G when signal quality is good, but allow fallback to LTE when 5G signal drops below a certain level.

The OCST parameter is stored in the UE's Universal Subscriber Identity Module (USIM) or embedded UICC (eUICC) and is managed via the SIM/USIM application toolkit. TS 31.102 defines the USIM application characteristics, including the storage and management of OCST. TS 31.121 specifies the USIM application test specification, ensuring that UEs correctly interpret and apply OCST values. The parameter is per access technology, meaning that separate thresholds can be defined for NR, LTE, and other RATs.

When a UE is in idle mode, it uses OCST to evaluate candidate cells during reselection. If the signal strength of a cell in a given access technology is below the OCST threshold, the UE may deprioritize that technology and seek alternatives. In connected mode, OCST can influence handover decisions, although the final decision rests with the network. The parameter thus complements network-controlled mobility by providing operator preferences directly to the UE.

OCST is particularly relevant in multi-RAT deployments where operators need to manage interworking between 5G and LTE. It allows for dynamic adjustments based on network load, coverage, and service requirements. By storing OCST in the USIM, operators can update thresholds remotely via over-the-air (OTA) mechanisms, ensuring flexibility without requiring UE firmware updates.

In Release 19, OCST was introduced to enhance operator control over UE mobility in heterogeneous networks. Release 20 continues to support OCST, with specifications updated to reflect new access technologies and deployment scenarios. The definition remains unchanged, but its application may be extended to new RATs as they are introduced.

Purpose & Motivation

OCST exists to give operators granular control over how UEs select and reselect cells across different access technologies. In multi-RAT networks, standardized cell selection rules may not align with an operator's strategic goals, such as prioritizing 5G coverage or offloading traffic from congested LTE cells. Without OCST, operators have limited ability to influence UE behavior beyond broadcast parameters, which apply uniformly to all UEs and cannot be customized per subscription.

The motivation for OCST stems from the increasing complexity of 5G deployments, where multiple frequency layers and RATs coexist. Operators need to ensure that UEs camp on the best cell according to their policies, which may consider factors like spectrum efficiency, load, and service quality. Previous approaches relied on network-controlled handover and broadcast thresholds, but these lack the flexibility to differentiate between UEs or to apply operator-specific preferences stored in the USIM.

OCST solves this by allowing operators to define per-RAT thresholds that are stored in the USIM and applied by the UE during mobility procedures. This enables personalized mobility management, where different subscribers or device types can have different thresholds. For example, IoT devices might be configured to prefer LTE for power efficiency, while smartphones prioritize 5G for higher throughput. OCST thus enhances network optimization and user experience by aligning UE behavior with operator strategy.