Proximity Services Direct Discovery

Description

ProSe Direct Discovery is a key component of the Proximity Services (ProSe) framework, introduced in Release 12. It defines a procedure by which a ProSe-enabled UE can discover other ProSe-enabled UEs in its vicinity using only the capabilities of the two UEs, leveraging E-UTRA or WLAN technology. This direct discovery process is fundamental for enabling D2D communication and proximity-based services, as it allows devices to become aware of each other's presence before establishing a communication session.

The procedure operates on the principle of direct radio signal exchange between UEs. A UE that wants to be discovered transmits discovery messages, while a UE that is looking for others listens for these messages. The discovery messages contain information such as the UE's identity, application identifiers, and other parameters. The discovery can be performed in two main models: Model A and Model B. In Model A, a UE announces its presence by broadcasting discovery messages, and other UEs monitor for these announcements. In Model B, a UE queries for specific UEs by sending solicitation messages, and the target UEs respond. Both models can operate with or without network assistance.

ProSe Direct Discovery can be network-controlled or autonomous. In network-controlled discovery, the network (specifically the ProSe Function) authorizes and provisions the UEs, allocates radio resources, and may assist in the discovery process by providing configuration parameters. In autonomous discovery, UEs use pre-configured parameters to perform discovery without network involvement, which is essential for out-of-coverage scenarios. The discovery process can be further categorized as open or restricted. Open discovery does not require explicit permission from the discovered UE, while restricted discovery requires the discovered UE to have authorized the discovering UE.

The radio interface used for direct discovery is the PC5 interface, which is based on E-UTRA sidelink. The discovery messages are transmitted using specific physical channels and protocols designed for sidelink communication. The UEs must be synchronized to ensure successful reception of discovery messages. Synchronization can be achieved via network signals (e.g., from eNBs) or through direct synchronization signals from other UEs when out of coverage.

Security and privacy are important considerations in ProSe Direct Discovery. The discovery messages may contain sensitive information, and mechanisms are defined to protect the privacy of UEs, such as using temporary identifiers and encryption. The ProSe Function manages the security aspects, including authorization and key distribution.

ProSe Direct Discovery enables a wide range of applications, including public safety (e.g., finding nearby first responders), social networking (e.g., finding friends nearby), and commercial services (e.g., targeted advertising). It is a foundational technology for D2D communication and has been enhanced in later releases to support 5G and new use cases.

Purpose & Motivation

ProSe Direct Discovery was created to enable UEs to discover each other directly, without relying on the core network for the discovery process. Traditional discovery mechanisms in cellular networks require the network to facilitate or mediate the discovery, which can be inefficient and may not work in out-of-coverage scenarios. ProSe Direct Discovery solves this by allowing UEs to discover each other using only their own radio capabilities, thus enabling proximity-based services in a more efficient and scalable manner.

The motivation for ProSe Direct Discovery came from the need for public safety communication, where devices must be able to discover each other even when network infrastructure is unavailable or congested. It also addressed commercial use cases, such as social networking and local advertising, where users want to find others nearby. The technology leverages the existing E-UTRA and WLAN capabilities of UEs, making it cost-effective and widely applicable.

By enabling direct discovery, ProSe Direct Discovery reduces the load on the core network, lowers latency, and enables new applications that require immediate awareness of nearby devices. It represents a shift towards more autonomous and device-centric operation in cellular networks, paving the way for advanced D2D and IoT services.