Sidelink Mobile Originating Location Request

Description

SL-MO-LR (Sidelink Mobile Originating Location Request) is a location service defined within the 3GPP framework that allows a UE to initiate a request to determine its own geographical position using sidelink communication channels. Sidelink refers to the direct device-to-device communication interface, standardized primarily for ProSe and V2X applications, operating independently of the cellular network's uplink and downlink. In this procedure, the UE acts as the originator, triggering the location request. The request is transmitted over the PC5 interface, the standardized sidelink communication interface, to a receiving entity, which could be another UE or a dedicated location server capable of processing sidelink-based positioning. The procedure involves signaling exchanges defined in specifications such as 23.273 for stage 2 architecture, 24.514 for protocol details, and 38.355 for radio access network aspects, ensuring interoperability across different implementations.

The architecture for SL-MO-LR integrates with the existing LTE and NR sidelink frameworks. Key components include the originating UE, which generates the location request message; the PC5 interface, which facilitates the direct communication; and the receiving entity, which processes the request. The receiving entity may utilize various positioning methods, such as observed time difference of arrival (OTDOA) or sidelink-specific measurements, to compute the location. The procedure supports both control plane and user plane location protocols, adapting to the available network conditions. In scenarios where network coverage is limited or absent, such as in remote areas or during network outages, SL-MO-LR enables critical location-based services by leveraging direct device-to-device links, enhancing reliability and reducing dependency on infrastructure.

SL-MO-LR works by the UE formulating a location request message containing necessary parameters like QoS requirements, positioning method preferences, and identification information. This message is sent via the PC5 interface using the sidelink shared channel (SL-SCH). Upon receipt, the receiving entity, if equipped with positioning capabilities, initiates measurement procedures. These may involve exchanging reference signals, such as SL-PRS (Sidelink Positioning Reference Signals), and performing timing or angle measurements. The computed location is then returned to the originating UE over the same PC5 interface. The service is designed to be low-latency and efficient, catering to real-time applications like emergency services in V2X or location-aware ProSe applications. Its integration into the broader 3GPP ecosystem ensures compatibility with network-based location services, providing a hybrid approach where sidelink and cellular positioning can complement each other based on availability and requirements.

Purpose & Motivation

SL-MO-LR was introduced to address the growing need for reliable location services in scenarios where traditional network-based positioning is impractical or unavailable. With the expansion of V2X and ProSe applications, there is a critical demand for devices to determine their location independently, especially in environments with poor cellular coverage, such as rural roads, tunnels, or disaster-stricken areas. Previous approaches relied heavily on network infrastructure, like base stations or GPS, which can be unreliable in these conditions. SL-MO-LR enables direct device-to-device location requests, enhancing resilience and ensuring continuous service for safety-critical applications like autonomous driving or emergency response.

Historically, location services in 3GPP networks were primarily network-centric, with procedures like Mobile Originating Location Request (MO-LR) depending on core network elements. The evolution towards sidelink communication in releases like Rel-14 for LTE and Rel-16 for NR highlighted the limitations of these methods in direct communication scenarios. SL-MO-LR fills this gap by extending location services to the sidelink domain, allowing UEs to leverage proximity-based positioning without network intervention. This is particularly motivated by the automotive industry's requirements for high-accuracy, low-latency positioning in V2X systems, where vehicles need to share location data for collision avoidance and traffic efficiency.

The creation of SL-MO-LR also supports regulatory and commercial needs, such as E911-like services in sidelink contexts, where users must be located during emergencies even without network access. By integrating into 3GPP standards, it ensures a standardized approach, reducing fragmentation and promoting interoperability across devices and networks. This addresses the limitations of ad-hoc solutions and paves the way for advanced applications in IoT and public safety, where dependable location awareness is paramount.