Multicast Multicast Radio Bearer

Description

A Multicast MRB (Multicast Radio Bearer) is a radio bearer configured for MBS (Multicast/Broadcast Services) multicast delivery, as defined in 3GPP TS 38.323 and TS 38.331. It is the radio-level transport construct that carries multicast service data from the gNB to multiple UEs in a cell using point-to-multipoint transmission. Unlike a unicast data radio bearer (DRB), which is dedicated to a single UE, a Multicast MRB is shared by a group of UEs that have joined the same multicast session. This allows the network to deliver the same content—such as live video, software updates, or group communications—to many users simultaneously without duplicating transmissions per UE.

Architecturally, the Multicast MRB sits between the SDAP/PDCP layers and the RLC/MAC layers. In the NR MBS architecture, multicast delivery can use either a point-to-multipoint (PTM) radio bearer or a point-to-point (PTP) radio bearer, or a combination. The Multicast MRB is the PTM bearer used for multicast. At the PDCP layer (TS 38.323), the MRB has its own PDCP entity that handles ciphering, integrity protection (if applicable), and header compression for the multicast flow. The RLC layer may use UM (unacknowledged mode) for PTM transmission, since feedback from many UEs would be complex. The MAC layer schedules the MRB transmissions using a group-common RNTI (e.g., G-RNTI) so that all interested UEs can receive the same transmission.

How it works: when a multicast session is established, the gNB configures one or more Multicast MRBs for the session. UEs that join the multicast service are configured via RRC (TS 38.331) with the MRB configuration, including the PDCP/RLC/MAC parameters and the group-common RNTI. The gNB then transmits the multicast data on the MRB, and all UEs in the multicast group monitor the corresponding PDCCH/PDSCH resources. The UEs do not need individual unicast bearers for the multicast content, which saves air-interface resources. If a UE requires reliability enhancements, the network may also configure a PTP leg or use HARQ feedback mechanisms defined for MBS.

Key components include the Multicast MRB itself, the PDCP entity for the multicast flow, the RLC/MAC configuration with group-common scheduling, and the RRC configuration that ties the UE to the multicast session. The MRB is identified by a bearer identity and is associated with a multicast session (e.g., a Temporary Mobile Group Identity, TMGI). The gNB manages the MRB and can switch between PTM and PTP delivery depending on the number of UEs and their channel conditions. In Rel-17, MBS was introduced for NR, and the Multicast MRB was defined as the radio bearer for multicast delivery. Rel-18 and Rel-19 continued to refine MBS, including enhancements for broadcast and multicast reception in different RRC states.

Multicast MRB matters because it enables efficient group delivery of content over NR. Without it, the network would have to send separate unicast streams to each UE, consuming far more radio resources. By using a shared bearer, the network can serve large audiences—stadiums, campuses, public safety groups—with a single transmission. It also supports new services such as 5G multicast broadcast, live streaming, and mission-critical push-to-talk. The MRB is a foundational element of the MBS framework, bridging the core network's multicast session management with the radio interface's point-to-multipoint capabilities.

Purpose & Motivation

Multicast MRB exists to provide an efficient radio bearer for multicast services in NR. Before MBS, multicast delivery in cellular networks was primarily handled by eMBMS (LTE Broadcast) or by unicast duplication. eMBMS used MBSFN (Multicast-Broadcast Single-Frequency Network) subframes, which were not well suited to NR's flexible numerology and beam-based operation. Unicast duplication, on the other hand, wastes resources because the same content is sent separately to each UE. The Multicast MRB solves this by defining a shared radio bearer that can deliver the same content to many UEs in a cell with a single transmission, leveraging NR's scheduling and beamforming capabilities.

The motivation is to support high-demand group services—live sports, concerts, software updates, emergency alerts, and group communications—without overwhelming the network. In scenarios like a stadium, thousands of users may want the same video stream; a Multicast MRB allows the gNB to broadcast it once, freeing resources for other traffic. For public safety, multicast enables efficient group calls and situational awareness. The Rel-17 MBS work introduced the MRB as part of the NR MBS architecture, and later releases enhanced it for broader use.

Historically, LTE's eMBMS provided multicast/broadcast, but it had limitations: it required dedicated subframes, was not easily integrated with unicast, and did not support the flexibility of NR. The Multicast MRB in NR is designed to coexist with unicast, support dynamic switching between PTM and PTP, and operate in both RRC_CONNECTED and RRC_INACTIVE states (with Rel-18 enhancements). It addresses the need for a modern, efficient multicast bearer that can scale with NR's capabilities.