Description
Mobile Base Station Relay is a specialized network node defined in 3GPP that combines the functionalities of a relay node and a base station (gNB or ng-eNB in 5G) but is distinguished by its inherent mobility. Unlike fixed relays, an MBSR is integrated into a moving vehicle, such as a bus, train, aircraft, or unmanned aerial system (UAS/drone). Its primary role is to provide wireless access to UEs within its coverage area while itself connecting to the network via a wireless backhaul link to a donor base station (Donor gNB or DgNB). This creates a two-hop architecture: the wireless backhaul link (between DgNB and MBSR) and the access link (between MBSR and UEs).
Architecturally, the MBSR hosts a full gNB Distributed Unit (gNB-DU) and may host a gNB Centralized Unit (gNB-CU) as well, depending on the deployment. It connects to the 5G Core Network via the Donor gNB using the F1 interface over the wireless backhaul. The MBSR manages its own cell(s), broadcasting its own Physical Cell ID and system information. To the UEs it serves, it appears as a standard base station. A key technical challenge is managing the mobility of the MBSR itself, which involves handover procedures for the entire relay node and its associated UEs as it moves between donor cells.
The wireless backhaul can operate in-band (using the same spectrum as the access link) or out-of-band, and it utilizes advanced features like integrated access and backhaul (IAB) principles. The MBSR must handle dynamic topology changes, potential backhaul link degradation due to movement, and network slicing awareness to provide consistent service. Its operation is coordinated by the network, which manages resource allocation for both the backhaul and access links, mobility, and potentially the MBSR's trajectory in the case of controlled platforms like drones.
Purpose & Motivation
MBSR was created to address the need for flexible, on-demand, and mobile network coverage that cannot be cost-effectively provided by fixed infrastructure. It solves the problem of providing continuous high-quality service in moving vehicles (trains, buses, ships) and rapidly deploying coverage for temporary events, disaster recovery, or in remote areas.
It overcomes the limitations of traditional solutions like satellite backhaul (high latency, cost) or simple UE-to-network repeaters (which don't create a managed cell). Previous approaches often required complex multi-UE router setups or failed to provide a seamless, network-integrated experience with support for mobility, QoS, and network slicing. The MBSR, as a standardized 3GPP network node, allows the operator to fully manage the service as an extension of its own RAN.
The motivation stems from diverse use cases: providing enhanced connectivity on public transportation, enabling command-and-control for mobile drone swarms, ensuring coverage for first responders in dynamic emergency scenarios, and supporting connectivity for moving hotspots at large events. By standardizing the MBSR, 3GPP enables interoperable solutions for these advanced mobile relay scenarios within the 5G system framework.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (77 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Mobile Base Station Relay (MBSR) function was introduced to enhance location services for UEs connected to a moving relay node. Key enhancements include procedures for the LMF to obtain the location and velocity of the MBSR itself, often via the GMLC using a specific MBSR indication, and to use this data along with synchronized timing to improve the accuracy of the target UE's location estimate. Furthermore, specific privacy check exemptions were defined for when the MBSR's location is requested solely for locating another UE.
- Mobile Terminated SMS over NAS: 5GS Access Selection TS 23.501CR0201
- Storing of MPS indicator in non-volatile memory of mobile TS 24.501CR0123
- Resolution of editor's note on maximum length of the 5GS mobile identity IE TS 24.501CR0503
- Completion of mobile identity IE definition in messages TS 24.501CR0574
- Correction on 5GS mobile identity IE name TS 24.501CR0766
In Release 16, the MBSR (Mobile Base Station Relay) function introduced specific enhancements for location services to accurately estimate the position of a UE connected to a moving relay. Key new procedures included the LMF obtaining the location and velocity of the MBSR itself, often via the GMLC using a dedicated MBSR indication, and coordinating the timing of positioning measurements between the target UE and the MBSR. Furthermore, the release defined a privacy check exemption for the MBSR when its location is requested solely to determine the position of another target UE.
In Release 17, key enhancements for the Mobile Base Station Relay (MBSR) function focused on enabling accurate location services for UEs connected via a moving relay. Specifically, procedures were introduced for the LMF to obtain the location and velocity of the MBSR itself, using its cell ID and GPSI, and to use this data when estimating the target UE's position. Furthermore, the privacy check for the MBSR is skipped when its location is requested solely for determining the location of a target UE.
- Add the SMF shall provide the QoS flow description(s) for the PDU sessions used for relaying TS 24.501CR3595
- Triggering Service Request procedure due to lower layers request for ProSe layer-2 UE-to-network relay TS 24.501CR3683
- IPv6 prefix delegation via DHCPv6 for 5G ProSe layer-3 UE-to-network relay TS 24.501CR3775
- The timer for authentication and key agreement for 5G ProSe UE-to-network relay TS 24.501CR4318
- Secondary authentication via L3 relay TS 24.501CR4365
- Authentication and key agreement for 5G ProSe UE-to-network relay TS 24.501CR4219
+ 10 more changes
In Release 18, the MBSR (Mobile Base Station Relay) function introduced specific enhancements for Location Services (LCS). Key new capabilities include the support for the 5GC-MT-LR procedure when an MBSR is involved, requiring the LMF to obtain the MBSR's location, velocity, and cell ID to accurately estimate a target UE's position. Furthermore, the release defined mechanisms for MBSR authorization during registration and introduced a dedicated privacy check exemption for the MBSR when its location is requested solely to locate another UE.
- Support of Mobile Base Station Relay for Location Service TS 23.273CR0240
- MT-LR procedure for when a MBSR is involved in the location of a UE. TS 23.273CR0288
- Update of the Location Services involving Mobile Base Station Relay TS 23.273CR0339
- Introduction of Mobile Base Station Relay TS 23.501CR3813
- Introducing 5G ProSe ph2 function for KI#7 (Support of Emergency for UE-to-Network Relaying) TS 23.501CR3858
- Providing cell ID/TAC of MBSR for services TS 23.501CR3933
+ 38 more changes
In Release 19, the MBSR (Mobile Base Station Relay) function introduced new capabilities for location services, specifically enhancing procedures for accurate UE positioning when connected to a moving relay. Key additions include mechanisms for the LMF to obtain the location and velocity of the MBSR itself, to use this data in estimating the target UE's location, and to skip privacy checks for the MBSR when its positioning is solely for locating another UE. The release also defined support for the NL10 reference point between LMF and GMLC to handle location requests involving MBSR and MWAB (Mobile Wireless Access Backhaul).
- Adding support of Mobile Wireless Access Backhaul in 5GS TS 23.501CR5596
- General description of relaying media related information over N6 using an encapsulation protocol TS 23.501CR5711
- Enhancement of 5G ProSe capability for multi-hop relays TS 24.501CR6552
- Update 5GMM capability for 5G ProSe multi-hop relays TS 24.501CR6692
- Update 5GMM capability for IM relay TS 24.501CR6937
- Removal of the MoQ Relay Address in UPF Profile TS 23.501CR6181
+ 4 more changes
Explore further
Broader topics and technologies where MBSR plays a role.
Defining Specifications
3GPP specifications that define or reference MBSR, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.273 vj50 | 5G Location Services Stage 2 Architecture | Rel-19 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 28.875 vj00 | Study on IAB Node Management | Rel-19 |
| TS 29.515 vj50 | Ngmlc Service Based Interface Protocol | Rel-19 |
| TS 29.572 vj50 | Nlmf Service Based Interface Stage 3 | Rel-19 |