MBMS Master Radio Network Controller

Description

The MBMS Master Radio Network Controller (MRNC) is a functional role defined within the UMTS Terrestrial Radio Access Network (UTRAN) architecture for MBMS. In a typical MBMS deployment covering a large area, multiple RNCs (and their controlled Node Bs) will be involved in broadcasting the same content. To coordinate this transmission and enable features like Selective Combining for improved reception, one RNC is designated as the Master for that specific MBMS service and area. The MRNC is the anchor point for the Iu bearer from the Core Network (via the SGSN) for the MBMS data. It receives the MBMS data stream from the core and is responsible for its distribution over the Iur interface to other RNCs involved in the broadcast, which are termed Slave RNCs (SRNCs).

A key technical function of the MRNC is the handling of MBMS session control signaling. It terminates the MBMS control plane from the Core Network (Iu interface) and generates the necessary Radio Access Network Application Part (RANAP) messages for the UTRAN side. The MRNC manages the activation and deactivation of the radio resources for the MBMS bearer within its coordinated set of RNCs. It also plays a vital role in transmission synchronization. The MRNC ensures that the MBMS data frames are transmitted from all involved Node Bs (under the MRNC and all SRNCs) in a time-synchronized manner. This synchronization allows User Equipment (UE) at the cell edge, receiving signals from multiple cells, to perform macro-diversity combining (also called Selective Combining), significantly improving reception reliability and broadcast coverage.

The MRNC role is dynamic and service-specific. An RNC can be the MRNC for one MBMS service area and a Slave RNC for another. The determination of which RNC acts as the MRNC is based on operational configuration and potentially the service area topology. The MRNC interfaces with the Core Network via the Iu interface and with other RNCs via the Iur interface. Its responsibilities are detailed in UTRAN specifications, particularly those dealing with MBMS architecture (TS 25.346), Iur interface signaling (TS 25.423), and overall UTRAN functions (TS 25.401).

Purpose & Motivation

The MRNC concept was introduced to solve the coordination problem inherent in broadcasting over a cellular network built for unicast. In standard unicast UTRAN operation, each UE has a dedicated Serving RNC (SRNC). For a broadcast service, a single data stream needs to be delivered identically from many cells. Without a coordinator, each RNC would operate independently, leading to unsynchronized transmissions that a UE cannot combine, resulting in poor performance at cell edges and inefficient use of radio resources.

The MRNC provides this essential coordination layer. Its creation was motivated by the need for efficient MBMS delivery with macro-diversity gains. By centralizing session control and data distribution for a service area, the MRNC ensures synchronized transmission timing across multiple RNCs. This enables UEs to use receiver combining techniques, which are critical for achieving acceptable coverage and quality for broadcast services without excessively high transmission power. The MRNC architecture allows the reuse of existing Iur interfaces between RNCs for MBMS data forwarding, creating a logical point-to-multipoint distribution tree within the RAN that mirrors the service's broadcast nature.