Radio Resource Control-terminating Integrated Access and Backhaul Donor

Description

The RRC-terminating IAB-donor is a functional role within the Integrated Access and Backhaul (IAB) architecture defined in 3GPP Release 18 and beyond. In an IAB network, IAB-nodes provide access and backhaul via wireless links, and IAB-donors connect the IAB network to the core network. For a mobile IAB-node (e.g., mounted on a vehicle or moving platform), the RRC-terminating IAB-donor is the specific donor that terminates the Radio Resource Control (RRC) connection of that mobile node. This means it handles all RRC signaling, including connection establishment, reconfiguration, mobility management, and release for the mobile IAB-node. The RRC-terminating IAB-donor may also be the F1-terminating IAB-donor, meaning it terminates the F1 application protocol (F1AP) interface towards the mobile IAB-node's DU part. This co-location of RRC and F1 termination simplifies architecture and reduces latency for control plane procedures.

Architecturally, the IAB network consists of IAB-donors (which have wired backhaul to the core) and IAB-nodes (which relay traffic wirelessly). The mobile IAB-node is an IAB-node that can move, requiring enhanced mobility support. The RRC-terminating IAB-donor is responsible for the RRC connection between the mobile IAB-node's MT (Mobile Termination) and the donor's CU-CP (Central Unit Control Plane). This RRC connection is used for configuring the mobile IAB-node's access and backhaul links, managing its mobility (e.g., handover between parent nodes), and coordinating with the core network. The donor may also host the CU-UP (User Plane) and DU (Distributed Unit) functions, but the RRC termination is specifically in the CU-CP.

In Release 18, the concept of mobile IAB was introduced to support IAB-nodes that can move, such as those on buses or trains. The RRC-terminating IAB-donor plays a critical role in maintaining the RRC connection during mobility. When the mobile IAB-node moves, it may change its parent IAB-node or even its serving IAB-donor. The RRC-terminating IAB-donor handles the RRC reconfiguration required for these changes, ensuring service continuity. If the RRC-terminating IAB-donor also terminates F1, it manages the F1AP signaling for the mobile IAB-node's DU, including UE context management and resource allocation.

The specifications 38.401 (NG-RAN architecture), 38.420 (Xn interface), 38.423 (XnAP), and 38.473 (F1AP) define the procedures and interfaces involving the RRC-terminating IAB-donor. For example, XnAP is used for inter-donor coordination, and F1AP for donor-DU communication. The RRC-terminating IAB-donor may interact with other IAB-donors via Xn for mobility and load balancing.

In summary, the RRC-terminating IAB-donor is a key architectural entity for mobile IAB, ensuring reliable control plane connectivity for moving IAB-nodes. Its potential dual role as F1-terminating IAB-donor streamlines the protocol stack and enhances efficiency.

Purpose & Motivation

The RRC-terminating IAB-donor concept was introduced to address the unique challenges of mobile IAB-nodes in 5G networks. Traditional IAB-nodes are fixed, and their RRC connections are terminated at a single donor. However, when IAB-nodes become mobile, they may need to change their serving donor or parent node frequently. Without a clear definition of which donor terminates RRC, mobility procedures could become ambiguous, leading to connection drops or increased latency. The RRC-terminating IAB-donor provides a stable anchor for the RRC connection, enabling seamless mobility and service continuity.

Prior to Release 18, IAB mobility was limited to fixed nodes, and the RRC termination was implicitly at the donor that served the IAB-node. With mobile IAB, the network needs to support scenarios where the IAB-node moves across different donors. The RRC-terminating IAB-donor solves this by explicitly defining the node responsible for RRC, which may be different from the donor currently serving the node's data traffic. This separation allows for flexible deployment and optimized routing.

Furthermore, the possibility for the RRC-terminating IAB-donor to also be the F1-terminating IAB-donor simplifies the architecture by co-locating control functions. This reduces signaling overhead and latency, as both RRC and F1 procedures can be handled by the same entity. The concept also facilitates inter-donor coordination via Xn, ensuring that mobility events are handled efficiently. Overall, the RRC-terminating IAB-donor is essential for enabling mobile IAB in 5G-Advanced, supporting use cases like connected vehicles and high-speed trains.