Proximity

Description

Proximity is a fundamental concept in 3GPP Proximity Services (ProSe), introduced in Release 12. It defines the condition under which a User Equipment (UE) is considered to be in proximity of another UE, enabling direct discovery and communication between devices without traversing the core network. The determination of proximity is based on specific criteria that can vary depending on whether the purpose is discovery or communication. These criteria are not strictly defined by 3GPP but are left to operator or application-specific implementations, allowing flexibility for different use cases such as public safety, social networking, and commercial services.

Architecturally, proximity determination can be performed either by the network or by the UEs themselves. In network-controlled discovery, the network (e.g., via the ProSe Function) evaluates proximity based on location information, radio measurements, or other parameters. In UE-autonomous discovery, the UE uses direct radio signals (e.g., discovery announcements) to detect nearby UEs. The proximity criteria may include distance thresholds, radio signal strength, or even application-layer context. The separation of criteria for discovery and communication means that a UE might be considered in proximity for discovery purposes but not for communication, or vice versa, depending on the required QoS, security, or resource availability.

The concept of proximity is central to the ProSe architecture, which includes entities such as the ProSe Function, ProSe Application Server, and the UEs. The ProSe Function is responsible for provisioning UEs with proximity criteria, authorizing discovery and communication, and potentially assisting in proximity determination. The criteria themselves are often expressed as a set of parameters that the UE or network can evaluate. For example, a proximity criterion could be a maximum distance between UEs, a minimum received signal strength, or a specific location area. The flexibility in defining these criteria allows operators to tailor ProSe services to their specific needs and regulatory environments.

In terms of operation, when a UE wants to discover another UE, it may send a discovery request to the network or broadcast a discovery message. The network or the UE then evaluates whether the proximity criteria are met. If proximity is determined, the UEs can proceed to establish a direct communication link, possibly using sidelink (PC5) interfaces. For communication, additional criteria such as link quality, security policies, and resource availability may be considered. The proximity determination can be periodic or event-triggered, depending on the service requirements.

Over the releases, the definition of proximity has remained largely stable, reflecting its foundational nature. However, its application has expanded with new features like wearable devices, vehicle-to-everything (V2X) communications, and public safety enhancements. The concept continues to evolve as part of the broader 5G ecosystem, where proximity-based services are expected to play a key role in ultra-reliable low-latency communications (URLLC) and massive machine-type communications (mMTC).

Purpose & Motivation

Proximity was introduced to enable direct device-to-device communication and discovery, which had not been possible in earlier cellular systems that relied solely on network-mediated connections. The primary motivation was to support public safety use cases where network infrastructure might be unavailable or congested, allowing emergency responders to communicate directly. Additionally, proximity-based services can offload traffic from the core network, reduce latency, and enable new commercial applications such as social networking, advertising, and local content sharing.

Before ProSe, all communication between UEs went through the core network, even if the devices were physically close. This was inefficient in terms of resource usage and latency. The concept of proximity allows the network to determine when UEs are close enough to communicate directly, thereby saving network resources and improving user experience. The separation of proximity criteria for discovery and communication provides flexibility to balance between the need to find nearby devices and the need to establish a reliable link.

The introduction of proximity in Release 12 was part of the broader ProSe work item, which aimed to standardize mechanisms for direct discovery and communication. It addressed the limitations of previous approaches that lacked standardized support for D2D, such as Bluetooth or Wi-Fi Direct, which operate in unlicensed bands and lack cellular-grade security and QoS. By integrating proximity into the cellular framework, 3GPP ensured that D2D services could be managed, secured, and monetized by operators.