Description
The Frequency Selection Area (FSA) is a logical concept and identifier introduced in 3GPP Release 17 as part of the enhanced Multimedia Broadcast Multicast Service (MBS) framework for 5G networks. An FSA defines a geographical or logical area within which specific frequency selection policies and configurations are applied for the delivery of broadcast and multicast services. It is a key component in managing radio resources efficiently for MBS, enabling the network to direct User Equipment (UE) to optimal frequencies for receiving multicast/broadcast content based on their location and network conditions.
Architecturally, an FSA is configured and managed by the network, typically by the 5GC Core Network and the NG-RAN. The FSA ID is a unique identifier that is broadcasted in system information or provided via dedicated signaling to UEs interested in MBS. The network maps FSAs to specific frequency layers, cell lists, or MBS session areas. When a UE enters an FSA, it uses the associated FSA ID to determine which frequency to monitor or camp on for MBS reception, potentially involving reselection or handover procedures optimized for multicast flows.
How FSA works involves several steps. First, the network plans and configures FSAs, assigning FSA IDs and associating them with MBS services and frequency resources. This information is communicated to RAN nodes (gNBs). For UEs, the FSA ID is acquired either from broadcast system information blocks (SIBs) or via dedicated RRC signaling when joining an MBS session. The UE then uses this ID, possibly in conjunction with a pre-configured or network-provided mapping, to select the appropriate carrier frequency for MBS reception. This mechanism allows for load balancing, interference mitigation, and efficient use of spectrum dedicated to broadcast/multicast. Key specifications like TS 23.247 (MBS architecture) and TS 38.300 (NR overall description) detail its integration into service and RAN procedures.
The role of FSA in the network is crucial for scalable and efficient MBS delivery in 5G. It enables frequency-domain resource management for broadcast services, separating MBS traffic from unicast traffic where beneficial. This supports use cases like public safety communications, live event streaming, and software updates over the air. By defining FSAs, the network can optimize coverage, capacity, and UE battery life for multicast reception, making 5G MBS a viable and efficient technology for group communications.
Purpose & Motivation
FSA was created in Release 17 to address specific challenges in deploying efficient Multimedia Broadcast Multicast Services (MBS) in 5G NR networks. Prior to Rel-17, MBS in 4G LTE (eMBMS) had mechanisms like MBSFN areas, but 5G introduced a more flexible architecture. The motivation for FSA stems from the need for dynamic and efficient frequency resource management for broadcast/multicast traffic, which can have different coverage and capacity requirements compared to unicast.
The historical context is the 3GPP's work on enhancing MBS for 5G to support new use cases like V2X group communications, IoT software updates, and TV broadcasting. A key problem was how to efficiently direct UEs to the correct frequency for MBS without excessive signaling overhead or suboptimal resource usage. FSA solves this by providing a logical identifier that encapsulates frequency selection policy for an area, simplifying UE procedures and enabling network-controlled optimization. It addresses limitations of static configurations by allowing network planning to adapt MBS frequency usage per area, improving spectrum efficiency and service reliability for multicast applications in diverse deployment scenarios.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (17 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the MBS Frequency Selection Area (FSA) ID was introduced for broadcast MBS sessions to guide UE frequency selection. The FSA ID identifies a preconfigured area where RAN nodes announce the ID and an associated frequency via SIBs, enabling a UE to select the correct frequency by comparing the FSA ID in the service announcement with those broadcast in the system information. For a broadcast session, the MB-SMF determines the MBS FSA ID(s), which are then communicated to the NG-RAN nodes via the AMF and included in the service announcement towards the UE.
- CR on the carrier selection for random access TS 38.300CR0134
In Release 16, the new Frequency Selection Area (FSA) function was introduced specifically for broadcast Multicast/Broadcast Service (MBS) sessions to guide UE frequency selection. The MBS FSA ID, configured via OAM and announced in SIBs by RAN nodes, allows the UE to compare IDs received in a service announcement with those broadcast by the network to select the correct frequency. For a broadcast session, the MB-SMF determines the MBS FSA ID(s) and provides them to the NG-RAN nodes via the AMF to determine which cells and frequencies should broadcast the session data.
- Correction of configuration of PC5 RAT selection and Tx profiles TS 24.587CR0039
- Correction of coding of configuration of PC5 RAT selection and Tx profiles TS 24.588CR0004
- Update RAT selection rule TS 24.587CR0127
- Update RAT selection rule TS 24.588CR0021
- Correction on CSI-RS Based Intra-frequency and Inter-frequency Measurement Definition TS 38.300CR0265
- Including SRS frequency information in Positioning Information Request TS 38.473CR0725
+ 1 more changes
In Release 17, the new MBS Frequency Selection Area (FSA) function was introduced to guide UE frequency selection for broadcast MBS sessions. The key addition is the MBS FSA ID, which is announced by RAN nodes in SIBs and communicated to the UE via service announcement, allowing the UE to compare IDs for frequency selection. Furthermore, the MB-SMF was given the capability to determine and provide these MBS FSA ID(s) to the AMF and NG-RAN during MBS session establishment if they are not provided by the Application Function.
In Release 18, the FSA function was enhanced to explicitly support RedCap UEs within MBS sessions, specifying that a single MBS FSA ID can be used for a session receivable by both RedCap and non-RedCap UEs. Furthermore, the release introduced the capability to provide radio frequency information to lower layers specifically for Broadcast and Groupcast mode V2X communications over the PC5 interface.
In Release 19, the FSA function was enhanced to explicitly support both NR RedCap and non-RedCap UEs within the same broadcast MBS session using the same MBS FSA ID(s), eliminating the need for separate sessions unless specifically required. Furthermore, the procedures were clarified so that the MB-SMF can now determine and provide MBS FSA ID(s) based on local configuration when they are not supplied by the Application Function. These updates standardized the role of the MB-SMF in the session establishment flow for broadcasting the MBS FSA ID(s) to the RAN via the AMF.
Explore further
Broader topics and technologies where FSA plays a role.
Defining Specifications
3GPP specifications that define or reference FSA, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.247 vj30 | 5G Multicast/Broadcast Service Architecture | Rel-19 |
| TS 24.578 vj00 | UE policies for A2X services in 5GS | Rel-19 |
| TS 24.587 vj30 | V2X Services Protocols for 5G System | Rel-19 |
| TS 24.588 vj00 | UE Policies for V2X Services in 5GS | Rel-19 |
| TS 26.517 vj10 | 5G MBS User Service Protocols and Formats | Rel-19 |
| TS 29.532 vj30 | MB-SMF Service Based Interface Protocol | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |