Acknowledged Mode Multicast Radio Bearer

Description

The AM MRB (Acknowledged Mode Multicast Radio Bearer) is a Rel-17 concept defined in 3GPP TS 38.323 (PDCP specification) and TS 38.331 (RRC specification). It is part of the 5G Multicast/Broadcast Services (MBS) framework introduced to support efficient delivery of the same content to multiple UEs. An MRB is a radio bearer that can carry multicast or broadcast traffic, and it can be configured with different RLC modes depending on the transmission path: Point-to-Multipoint (PTM) or Point-to-Point (PTP).

An AM MRB is specifically an MRB that is associated with at least one Acknowledged Mode (AM) RLC bearer for PTP transmission. In the NR MBS architecture, an MRB can have a split configuration: a PTM leg (using UM or AM RLC, typically UM for broadcast-like delivery) and one or more PTP legs (using AM RLC for reliability). The AM RLC bearer provides acknowledged delivery with ARQ retransmissions, ensuring that lost or corrupted PDUs are retransmitted. This is essential for services that require high reliability, such as mission-critical push-to-talk, software updates, or group communications where every recipient must receive the data correctly.

The MRB is anchored at the PDCP layer. PDCP handles duplication, reordering, and (for AM) retransmission across the legs. When an MRB has both PTM and PTP legs, the network can dynamically switch between them based on radio conditions, UE distribution, and service requirements. The AM MRB designation indicates that at least one of the PTP legs uses AM RLC, giving the bearer acknowledged-mode reliability. The UE's RRC configuration (via TS 38.331) specifies the MRB setup, including the RLC mode for each leg, logical channel identities, and PDCP configuration.

In operation, downlink MBS data arrives at the gNB from the core network (via the MBS user plane, e.g., through the MB-UPF in 5GC). The gNB's PDCP entity for the MRB processes the data and delivers it to the configured RLC bearers. For the AM PTP leg, the RLC entity performs segmentation, ARQ, and retransmission as needed. The UE receives the data, and its RLC AM entity sends status reports (ACK/NACK) to trigger retransmissions. This closed-loop reliability is what distinguishes AM MRB from UM-based bearers used for less demanding broadcast services.

The AM MRB is particularly relevant for UEs in poor coverage or at cell edges, where PTM transmission may be unreliable. By switching to or supplementing with a PTP AM leg, the network ensures delivery. The concept also supports mobility: as a UE moves between cells, the PTP leg can be re-established with the new cell while the PTM leg continues, maintaining service continuity. The AM MRB thus combines the efficiency of multicast with the reliability of unicast acknowledged delivery.

Purpose & Motivation

5G MBS was introduced to deliver the same content efficiently to many users — for applications like live streaming, group communications, public safety, and IoT firmware updates. A pure PTM broadcast is spectrally efficient but unreliable: there is no feedback, so lost packets cannot be recovered. A pure PTP unicast is reliable but inefficient for large groups, consuming dedicated resources per UE.

The AM MRB exists to bridge these extremes. By associating an MRB with at least one AM RLC bearer for PTP transmission, the network gains a reliability mechanism (ARQ retransmissions) while still leveraging multicast where possible. This is critical for services that cannot tolerate packet loss, such as mission-critical communications or coordinated group operations.

Before MBS, LTE eMBMS provided broadcast/multicast but with limited reliability options and no dynamic switching between PTM and PTP within a single bearer. The NR MBS design, including AM MRB, allows dynamic adaptation: the network can use PTM for UEs in good conditions and PTP AM for those needing reliability, all under one MRB. This flexibility improves both spectral efficiency and service quality, and it aligns with 5G's goal of supporting diverse service requirements (eMBB, URLLC, mMTC) on a unified air interface.