Inter-Radio Access Technology

Description

Inter-RAT encompasses the procedures, protocols, and network capabilities that allow a User Equipment (UE) to transition between disparate radio access technologies. Common examples include mobility between LTE (E-UTRAN) and WCDMA/HSPA (UTRAN), between 5G NR and LTE, or between a 3GPP network (e.g., 5G) and a non-3GPP network (e.g., Wi-Fi). The architecture for IRAT mobility involves coordination between the core network (e.g., 5GC, EPC) and the different RANs. Key network nodes include the source and target base stations (e.g., gNB, eNB, NodeB) and core network control plane functions like the AMF/SMF in 5GC or MME in EPC, which manage the session and mobility context.

How IRAT works involves several key procedures. First is IRAT measurement, where the UE, configured by the network, measures the signal quality of neighboring cells on a different RAT. The network provides measurement gaps—short periods where the UE tunes its receiver away from the serving cell—to perform these measurements. Based on measurement reports from the UE, the network decides to initiate an IRAT handover. The core network orchestrates the handover by preparing resources in the target RAT, transferring the UE context (including security context and QoS flows), and commanding the UE to switch. For packet-switched services, this involves forwarding data packets from the source to the target system to minimize loss. In cases of idle mode mobility, IRAT cell reselection procedures allow the UE to autonomously select a suitable cell on a different RAT based on broadcast parameters and its own measurements.

IRAT also covers interworking scenarios beyond simple handover, such as simultaneous connectivity via multiple RATs (e.g., LTE-WLAN Aggregation in Rel-13) or access traffic steering, switching, and splitting between 3GPP and non-3GPP accesses in 5G (ATSSS). The role of IRAT is fundamental to providing continuous service coverage and leveraging the best available network resource. It allows operators to manage their multi-RAN deployments efficiently, offload traffic, and ensure users maintain connectivity when moving out of the coverage area of one technology into another, which is critical for voice and data service continuity.

Purpose & Motivation

IRAT technology exists to solve the fundamental problem of providing seamless mobility and service continuity in a world of multiple, coexisting radio access technologies. No single RAT provides universal coverage or optimal performance for all services and locations. 2G/3G, 4G LTE, 5G NR, and Wi-Fi networks are often deployed in overlapping layers. The purpose of IRAT procedures is to allow a mobile device to move between these technologies without dropping active sessions (like a voice call or video stream) and to efficiently manage its connectivity in idle mode.

Historically, as new generations were introduced (e.g., 3G UMTS, then 4G LTE), operators needed a migration path. Early systems had limited or cumbersome inter-RAT capabilities. Standardized IRAT handover and reselection procedures, significantly enhanced in 3GPP Rel-8 with the introduction of LTE, were motivated by the need for smooth network evolution and a consistent user experience. They addressed limitations where devices would lose connectivity when leaving the coverage of a newer technology, forcing a manual reconnection or a disruptive drop and re-establishment of sessions.

IRAT solves critical operational and user experience problems. It enables operators to refarm spectrum (e.g., moving spectrum from 3G to 4G/5G) while maintaining coverage through fallback to older technologies. It allows for load balancing and traffic steering between RATs to optimize network resource utilization. For the user, it provides the perception of a single, always-best-connected network, even though the underlying technology may change multiple times during a journey. In 5G, with its emphasis on non-3GPP integration, IRAT principles are extended to enable secure, seamless mobility between NR and trusted/untrusted Wi-Fi networks, which is essential for fulfilling 5G's promise of ubiquitous, high-performance connectivity.

Key Features

• Supports handover of active mode UEs between different RATs (e.g., NR to LTE, LTE to WCDMA)
• Defines measurement procedures and gaps for UEs to assess neighboring RAT cells
• Enables idle mode cell reselection across RATs based on priority and signal strength
• Facilitates transfer of security context and QoS parameters during RAT change
• Supports data forwarding between systems to minimize packet loss during handover
• Extends to interworking with non-3GPP accesses like Wi-Fi in later releases

Evolution Across Releases

Defining Specifications