Model A

Description

Model A is one of the two models for ProSe (Proximity Services) direct discovery defined in 3GPP. ProSe direct discovery allows UEs to discover other UEs in their proximity without going through the network, using direct radio signals. Model A involves a UE (the announcing UE) broadcasting a discovery message that says 'I am here', while other UEs (monitoring UEs) listen for these announcements. This model is typically used for restricted discovery, where the announcing UE only wants to be discovered by specific UEs that have been authorized.

In Model A, the announcing UE sends a discovery announcement message containing a ProSe Application Code or ProSe Restricted Code. This code is derived from the application layer and is used to identify the announcing UE's application or service. Monitoring UEs that are interested in that service will listen for the code and, upon receiving it, can decide to respond or take further action. The discovery process is controlled by the network through authorization and provisioning of the codes.

Model A is defined in 3GPP TS 23.303 (Proximity-based services) and related specifications. It supports both open and restricted discovery. In open discovery, any UE can monitor and discover the announcing UE. In restricted discovery, only UEs with the appropriate permissions can discover the announcing UE. Model A is particularly useful for public safety scenarios, where an incident commander might announce their presence to nearby responders, or for commercial applications like proximity-based advertising.

The architecture for Model A involves the ProSe Function in the core network, which handles authorization, provisioning of ProSe Application Codes, and charging. The UE must be authorized by the network to use ProSe direct discovery. The discovery message is sent on the PC5 interface, which is the direct radio interface between UEs. The PC5 interface uses specific physical channels and protocols for discovery, separate from the Uu interface used for network communication.

Model A differs from Model B, where a UE sends a 'who is there?' solicitation and another UE responds with 'I am here'. Model A is a push model, while Model B is a pull model. Both models are used in different scenarios, and the choice depends on the application requirements. Model A is simpler and more efficient for periodic announcements, while Model B is more suitable for on-demand discovery.

Purpose & Motivation

Model A was created to enable efficient proximity-based discovery for public safety and commercial applications. Before ProSe, UEs could only communicate via the network, which introduced latency and dependency on infrastructure. In public safety scenarios, such as disaster recovery, network infrastructure might be unavailable, so direct UE-to-UE discovery and communication are critical. Model A provides a simple, one-to-many announcement mechanism that allows a UE to advertise its presence and services to nearby UEs without network involvement.

The purpose of Model A is to support restricted discovery, where privacy and authorization are important. For example, a police officer may want to announce their presence only to other authorized officers, not to the general public. Model A allows the use of restricted ProSe codes that only authorized UEs can interpret. This solves the problem of unwanted discovery and ensures that only intended recipients can find the announcing UE.

Historically, Model A was introduced in 3GPP Release 12 as part of the initial ProSe specification. It was designed to meet the needs of public safety users, who required robust and resilient communication. Over time, Model A has been enhanced to support commercial use cases, such as social networking and proximity-based services. The evolution of Model A continues with 5G, where ProSe is integrated into the 5G system architecture, supporting new features like network slicing and enhanced QoS.