RAN Controlled LTE-WLAN Interworking

Description

RAN Controlled LTE-WLAN Interworking (RCLWI) is a 3GPP feature introduced in Release 13 that empowers the LTE eNodeB (eNB) to make decisions and execute traffic steering between LTE and a trusted WLAN access network for a UE. In this architecture, the eNB utilizes measurement reports from the UE regarding both LTE and WLAN radio conditions (e.g., LTE RSRP/RSRQ and WLAN RSSI, channel utilization, backhaul rate) to determine whether to offload data traffic to WLAN or keep it on LTE. The eNB then provides steering commands to the UE via RRC signaling, instructing it to route traffic for specific Access Point Names (APNs) or bearers over WLAN.

The procedure involves several key components. The UE must support both LTE and WLAN radios and the necessary measurement reporting extensions defined in 36.331. The eNB is enhanced with RCLWI logic and communicates with a trusted WLAN Access Network (TWAN) via the Xw interface (specified in 36.463). The core network entities, specifically the MME and PGW, are involved in session management but are not the decision points for steering. When the eNB decides to steer traffic, it can command the UE to establish a connection to a specific WLAN and route selected IP flows. The UE uses IPsec tunnels (over IKEv2) or similar mechanisms to connect its traffic to the PGW via the TWAN, ensuring seamless mobility and session continuity for the offloaded bearers.

RCLWI enables more granular and dynamic traffic management compared to earlier core-network-centric solutions like ANDSF or non-seamless WLAN offload. Because the eNB has real-time, cell-level radio knowledge, it can make faster and more context-aware steering decisions, optimizing overall radio resource utilization and user experience. It supports both uplink and downlink steering and can be used for load balancing or leveraging WLAN for capacity enhancement. The architecture maintains core network control for authentication and policy, while delegating the access selection to the RAN.

Purpose & Motivation

RCLWI was created to address the limitations of earlier LTE-WLAN interworking solutions, which were primarily controlled by the core network (e.g., via ANDSF policies) or the UE itself. These approaches often lacked real-time awareness of the local radio environment, leading to suboptimal steering decisions, such as offloading to a congested WLAN or staying on a weak LTE cell. The need for tighter, more intelligent integration grew as WLAN became ubiquitous and operators sought to use it as a complementary radio access technology.

The primary problem RCLWI solves is the inefficient utilization of combined LTE and WLAN resources. By placing the control in the RAN, it allows steering decisions to be based on instantaneous radio conditions, network load, and operator policies at the cell edge. This results in better load balancing, improved overall network capacity, and enhanced user throughput. It provides operators with direct control over traffic distribution in their radio footprint.

Historically, RCLWI was part of 3GPP's broader effort in Release 13 and beyond (under the LTE-WLAN Aggregation/LWA umbrella) to deeply integrate WLAN into the mobile network architecture. It responded to the explosion of data traffic and the need to leverage all available access technologies seamlessly. RCLWI offered a more responsive alternative to static ANDSF policies, enabling dynamic traffic steering that could adapt to changing conditions, ultimately improving network efficiency and user experience in heterogeneous networks.