5G Proximity Services Direct Communication

Description

5G ProSe Direct Communication is a function defined in 3GPP TS 23.304 that supports communications between two or more UEs in proximity that are 5G ProSe-enabled. The communication uses user-plane transmission based on NR technology via a path that does not traverse any network node. In other words, the data flows directly over the radio interface between the participating UEs, using the PC5 reference point, rather than being routed through the gNB and core network. This is the 5G evolution of the Proximity Services (ProSe) concept originally introduced in LTE Rel-12/13.

Architecturally, 5G ProSe Direct Communication operates over the PC5 interface, which is the direct sidelink interface between UEs. The UEs must first be authorized for ProSe direct communication, which involves subscription checks in the UDM and policy provisioning from the PCF. The 5G ProSe layer is organized into several functional entities: the 5G ProSe Direct Discovery function, the 5G ProSe Direct Communication function, the 5G ProSe UE-to-Network Relay function, and the 5G ProSe UE-to-UE Relay function. Direct Communication specifically handles the establishment, maintenance, and release of sidelink data paths between UEs.

The operation of 5G ProSe Direct Communication involves several steps. First, UEs perform authorization and provisioning, obtaining ProSe policies that define whether they may communicate directly, under what conditions (e.g., whether they are served by the network or out of coverage), and with what QoS parameters. Second, the UEs discover each other, either through 5G ProSe Direct Discovery or through a pre-configured arrangement. Third, a direct link is established over PC5, with radio resource management handled either by the gNB (in-coverage, network-scheduled mode) or autonomously by the UEs (out-of-coverage, UE-autonomous mode). Fourth, user-plane data is exchanged directly between the UEs. The 5G ProSe layer supports both unicast and groupcast communication modes.

Key technical aspects include the use of NR sidelink (as specified in TS 38.300 and TS 38.331) for the radio interface, QoS management for sidelink bearers, and security procedures for direct communication (covered in TS 33.304). The 5G ProSe Direct Communication function also interacts with the 5G ProSe Direct Discovery function, since discovery is typically a prerequisite for communication. Management aspects are covered in TS 28.853 and TS 32.277, which define how ProSe services are managed and charged.

5G ProSe Direct Communication is important because it enables low-latency, high-reliability communication without infrastructure dependency. It is particularly valuable for public safety (e.g., police and firefighter communication when network coverage is unavailable), commercial applications (e.g., proximity-based social networking, local content sharing), and industrial IoT (e.g., machine-to-machine communication on a factory floor). By leveraging NR sidelink, it benefits from the higher data rates, lower latency, and improved reliability of 5G compared to LTE ProSe.

Purpose & Motivation

5G ProSe Direct Communication exists to provide reliable, low-latency device-to-device connectivity that does not depend on network infrastructure. Traditional cellular communication always routes user data through the base station and core network, which introduces latency, consumes network resources, and fails when coverage is unavailable. For many use cases — particularly public safety and mission-critical communications — this dependency is unacceptable. First responders need to communicate in basements, tunnels, or disaster areas where network infrastructure may be damaged or absent.

The concept originated with LTE ProSe in Rel-12/13, which introduced direct discovery and communication over the PC5 interface using LTE sidelink. However, LTE ProSe had limitations: lower data rates, limited QoS support, and a design optimized primarily for public safety. As 5G NR matured, it became possible to define a more capable ProSe framework leveraging NR sidelink, which offers wider bandwidths, flexible numerology, beamforming, and enhanced reliability.

5G ProSe Direct Communication addresses these limitations by providing a 5G-native direct communication capability. It supports higher throughput, better QoS differentiation, and integration with 5G core network functions such as the PCF and UDM for authorization and policy control. It also extends the scope beyond public safety to commercial and industrial applications, reflecting the broader 5G vision of connecting everything. The Rel-19 specification consolidates these capabilities into a coherent 5G ProSe architecture, and Rel-20 continues to refine and maintain the definition.