ProSe Discovery UE ID

Description

The ProSe Discovery UE ID (PDUID) is a temporary, layer-2 identifier used exclusively for ProSe Direct Discovery procedures in 3GPP networks, starting from Release 13. It is assigned by the ProSe Function in the network core to a UE that is authorized for ProSe services. The PDUID is used in the actual discovery messages broadcast over the PC5 interface (the direct device-to-device radio link) and monitored by other UEs. It serves to mask the permanent UE identity (like IMSI or SUPI) for privacy and to provide a handle for the discovery process. There are two primary models: Model A ("I am here") where a announcing UE broadcasts its PDUID, and Model B ("Who is there?/Are you there?") where a discoverer UE broadcasts a PDUID containing information about the UE it is looking for.

Architecturally, the assignment and management of the PDUID involve several network functions. The UE requests authorization for ProSe Discovery from the ProSe Function via the PC3 reference point. Upon successful authorization, the ProSe Function may assign one or more PDUIDs to the UE, along with associated parameters like a validity timer and discovery filters. The PDUID is then used by the UE's ProSe Protocol stack to formulate discovery messages. At the physical layer, these messages are sent using specific physical channels and resources allocated for sidelink communication (e.g., in LTE's SL-DCH or NR's SL-SCH). The receiving UE's ProSe application layer processes the received PDUID, often applying discovery filters provided by the ProSe Function to determine if the discovered UE is of interest.

The role of the PDUID in the network is to enable secure, private, and efficient device discovery for Proximity Services. It acts as a temporary alias, preventing long-term tracking of a UE based on its discovery broadcasts. The ProSe Function can map the PDUID back to the real UE identity for network-assisted discovery or for charging purposes. In public safety scenarios, where network coverage may be absent (Out-of-Coverage), UEs may use pre-configured or self-assigned PDUIDs. The identifier is thus a key enabler for a wide range of applications from social networking (finding friends nearby) to critical communications for first responders, forming the initial step before establishing a direct ProSe Communication session.

Purpose & Motivation

PDUID was created to address the specific need for a privacy-preserving and network-managed identifier for direct device discovery in LTE and 5G networks. Prior to ProSe standardization, device-to-device discovery relied on ad-hoc methods like Bluetooth or Wi-Fi Direct, which lacked integration with cellular network security, subscription management, and wide-area coordination. There was no standardized way for a cellular network to facilitate discovery between its subscribers while maintaining control and privacy. The PDUID solves this by providing a temporary, network-issued handle.

The historical context is the 3GPP's work on Proximity Services, initiated to support public safety requirements and commercial D2D applications. A core challenge was enabling discovery without constantly exposing permanent, trackable identifiers over the air. The PDUID, along with the ProSe Function, was designed to solve this privacy problem. It also addresses scalability; the network can allocate different PDUIDs for different discovery purposes (e.g., different apps on the same UE) and manage their lifecycle, preventing identifier exhaustion or conflict.

Furthermore, PDUID enables network-assisted discovery optimization. By controlling the assignment, the network can help UEs filter irrelevant discovery messages, saving battery life and radio resources. It is a foundational element that allows the cellular network to extend its services to direct device-to-device links, creating a hybrid communication paradigm. This was a significant evolution from traditional cellular architectures where all communication was necessarily routed through a base station, enabling new low-latency and off-grid use cases.