Model B (ProSe Direct Discovery Model)

Description

Model B is one of the two fundamental ProSe (Proximity Services) direct discovery models defined in 3GPP, the other being Model A. While Model A uses an announcing-monitoring approach where a UE broadcasts its presence, Model B employs a query-response mechanism. In Model B, a discovering UE sends a solicitation message asking 'who is there?' (a query for any UE that matches certain criteria) and/or 'are you there?' (a query for a specific UE). UEs that receive this solicitation and match the criteria respond with a response message. This model is particularly suited for restricted discovery scenarios where privacy and targeted communication are important.

Model B operates over the PC5 interface, which is the direct sidelink communication interface between UEs. The discovery process involves two roles: the Discoverer UE (which sends the query) and the Discoveree UE (which responds). The messages are transmitted using either the sidelink broadcast or unicast mechanisms, depending on the release and configuration. In restricted discovery, the Discoveree UE only responds if it has been authorized and configured to be discoverable by the Discoverer UE, often using a ProSe Discovery UE ID and associated discovery filters.

The architecture for Model B involves several network functions. The ProSe Function in the core network manages authorization, provisioning of discovery parameters, and charging. The UE stores discovery configuration including the ProSe Application ID, discovery filters, and security credentials. When a UE wants to discover another UE, it sends a Discovery Request message to the ProSe Function, which checks permissions and provides the necessary parameters. The UE then transmits the solicitation message over PC5. The receiving UE, if it matches the criteria, sends a response. This response may include information such as the ProSe Application ID or other identifiers.

Model B supports both open and restricted discovery. In open discovery, no explicit permission is needed for the Discoveree to be discovered, while restricted discovery requires that the Discoveree has granted permission to the Discoverer. The restricted discovery is often implemented using a ProSe Discovery UE ID and a Discovery Filter that contains the ProSe Application ID and other parameters. The security aspects are handled by the ProSe Function, which may use a ProSe Key Management Function to generate and distribute security keys for protecting the discovery messages.

In terms of protocol layers, Model B discovery messages are carried over the PC5 interface using the PC5 Discovery protocol. The messages are transmitted in specific resource pools configured by the network (for in-coverage UEs) or pre-configured (for out-of-coverage UEs). The physical layer uses SC-FDMA for sidelink transmission in LTE, and in 5G NR, it uses the NR sidelink design. The discovery messages include a discovery type, ProSe Application Code, and other fields. The response message may include the ProSe Application Code of the Discoveree.

Model B is particularly useful in public safety scenarios where a first responder needs to find other responders in the vicinity, or in commercial applications like social networking or proximity-based advertising. It allows for a more targeted discovery compared to Model A, as the query can be specific. The model has evolved over releases, with enhancements in security, efficiency, and support for new use cases such as vehicle-to-everything (V2X) communications. In later releases, Model B has been integrated with 5G ProSe features, including support for network slicing and QoS handling.

Purpose & Motivation

Model B was created to address the need for a more targeted and interactive proximity discovery mechanism in 3GPP networks. While Model A (announcing-monitoring) is efficient for broadcast-like discovery where a UE wants to be discovered by many others, it lacks the ability to initiate a discovery process based on specific criteria. Model B fills this gap by allowing a UE to actively query for other UEs, either generally ('who is there?') or specifically ('are you there?'). This is particularly important for restricted discovery scenarios where privacy and authorization are critical.

The motivation for Model B stems from public safety requirements, where first responders need to discover each other in a controlled manner. In such scenarios, it is essential that only authorized devices can discover and be discovered, and that the discovery can be initiated by the party seeking information. Model B provides the necessary query-response framework to support this. Additionally, commercial applications such as social networking, proximity-based services, and targeted advertising benefit from the ability to initiate discovery based on user actions or application logic.

Before Model B, proximity discovery in cellular networks was limited to network-based location services or Bluetooth/Wi-Fi based discovery, which lacked the integration with cellular infrastructure, security, and scalability. Model B, as part of the ProSe framework introduced in Release 12, provides a standardized, operator-controlled mechanism for direct discovery over the PC5 interface. It leverages the cellular network for authorization, security, and resource management, while enabling direct UE-to-UE communication. This offloads traffic from the core network and enables low-latency, high-reliability discovery for critical communications.