Sidelink Mobile Terminating Location Request

Description

SL-MT-LR (Sidelink Mobile Terminating Location Request) is a location service within 3GPP standards that permits an external entity, such as a location server or another UE, to request the geographical position of a target UE using sidelink communication. Unlike SL-MO-LR where the UE initiates the request, here the target UE receives the request terminated from an external source over the PC5 interface. This service is crucial for scenarios where a third party needs to ascertain the location of a device, such as in public safety operations, vehicle tracking, or network-assisted positioning. The procedures are detailed in specifications 23.273 for architectural aspects, 24.514 for protocol layers, and 38.355 for radio access network integration, ensuring a standardized approach across different network deployments.

The architecture for SL-MT-LR involves several key components: the requesting entity, which could be a UE or a network-based location server; the target UE, which is the device whose location is being sought; and the PC5 interface, which facilitates the direct sidelink communication. The requesting entity sends a location request message via the PC5 interface, containing parameters like target UE identification, required accuracy, and positioning method. Upon receipt, the target UE processes the request, which may involve activating its positioning modules, measuring signals such as SL-PRS from neighboring devices, or coordinating with other UEs for collaborative positioning. The target UE then computes or assists in computing its location, often using techniques like time-of-arrival or angle-of-arrival measurements, and returns the result to the requesting entity over the same sidelink channel.

SL-MT-LR operates through a series of signaling exchanges defined in the relevant 3GPP specs. Initially, the requesting entity formulates a location request message based on the application needs—for instance, in a V2X scenario, a roadside unit might request the location of nearby vehicles for traffic management. This message is transmitted over the SL-SCH (Sidelink Shared Channel) using the PC5 interface. The target UE, upon decoding the request, engages in measurement procedures, potentially involving reference signal exchanges with other UEs or infrastructure. If the target UE lacks sufficient capability, it may relay the request to a location server or use hybrid methods combining sidelink and cellular measurements. The computed location data is then encapsulated in a response message and sent back via PC5. This service is designed to be efficient, with mechanisms for privacy and security, such as authentication of requesting entities, to prevent unauthorized tracking. Its role in the network extends beyond mere positioning; it enables dynamic resource allocation and enhanced situational awareness in sidelink ecosystems, supporting everything from logistics to disaster recovery.

Purpose & Motivation

SL-MT-LR was developed to address the need for external entities to locate UEs in sidelink environments, where traditional network-based terminating location services are not feasible. In applications like V2X and ProSe, there is often a requirement for one device or system to determine the position of another—for example, emergency services locating a vehicle after a crash or a fleet manager tracking assets in remote areas. Previous methods relied on cellular network involvement, which could fail in coverage gaps or congested scenarios. SL-MT-LR enables direct, device-to-device location requests, improving reliability and reducing latency for critical operations.

Historically, Mobile Terminating Location Request (MT-LR) services in 3GPP were confined to core network interactions, requiring base station and core network elements to mediate requests. With the rise of sidelink communication in releases like Rel-16 for NR, this approach proved inadequate for direct communication use cases. SL-MT-LR emerged as a solution, extending terminating location capabilities to the PC5 interface. This was motivated by automotive and public safety sectors demanding robust positioning for collaborative awareness, such as in platooning or search-and-rescue missions, where devices must share location data without network overhead.

The creation of SL-MT-LR also supports regulatory compliance and commercial applications, enabling services like lawful interception or location-based advertising in sidelink contexts. By standardizing it in 3GPP, interoperability is ensured across diverse devices and networks, addressing the fragmentation seen in proprietary solutions. This evolution reflects a broader trend towards decentralized network functions, empowering devices to perform complex tasks independently while maintaining seamless integration with existing cellular infrastructure.