Land Mobile Radio user

Description

In 3GPP specifications, an LMR user is defined as a user of an LMR service who has a device with the capability to participate in legacy LMR services. The term is used in the context of interworking between Land Mobile Radio (LMR) systems and 3GPP mission critical services, such as Mission Critical Push To Talk (MCPTT). Like the term 'LMR system', the definition of 'LMR user' is stated to be for discussion purposes only and is out of scope of the present specification. This means that 3GPP does not standardize the characteristics of LMR users or their devices, but rather uses the term to describe the counterpart in interworking scenarios.

An LMR user typically operates a radio device that is designed for use on an LMR system, such as P25, TETRA, or DMR. These devices are often ruggedized and have features like push-to-talk buttons, emergency buttons, and long battery life. They communicate over narrowband channels and support group calls, private calls, and broadcast calls. LMR users are often public safety personnel (police, fire, EMS), but can also be in transportation, utilities, or other industries. The key aspect is that they are users of a legacy LMR system, not of a 3GPP network.

In the interworking architecture, an LMR user can communicate with a 3GPP mission critical user through an Interworking Function (IWF). The IWF maps the LMR user's identity to a 3GPP identity, translates protocols, and manages the media. From the perspective of the 3GPP network, the LMR user appears as a user of an external system. The 3GPP specifications define how the IWF handles group calls, private calls, and other services to ensure that LMR users can participate in group communications with 3GPP users. The LMR user's device does not need to support 3GPP protocols; the interworking is handled entirely by the IWF.

The term 'LMR user' has evolved in its definition. In Rel-14 and Rel-15, it was defined as 'A user of an LMR service, who has a device with the capability to participate in legacy LMR services.' This definition emphasized the device capability and the legacy nature of the service. In Rel-16, the definition was changed to 'a user of a device which allows participation in an LMR system.' This new definition is more concise and focuses on the device's ability to participate in an LMR system, without explicitly mentioning 'legacy' or 'LMR service'. This change aligns with the updated definition of 'LMR system' in Rel-16, which also removed the 'legacy' and 'out of scope' language.

The LMR user concept is important for mission critical interworking because it allows 3GPP to specify the behavior of the IWF without needing to define the LMR user's device or service in detail. It provides a clear boundary between the 3GPP system and the external LMR system. The term is used in specifications such as TS 23.283, TS 23.782, and TS 23.783, which cover mission critical interworking. As mission critical communications continue to evolve, the ability to interwork with LMR users will remain essential for a smooth transition and for coexistence of different technologies.

Purpose & Motivation

The LMR user term exists to facilitate interworking between legacy Land Mobile Radio systems and 3GPP mission critical services. When 3GPP began developing mission critical services like MCPTT, it recognized that many organizations would continue to use LMR systems for years to come. To enable communication between LMR users and 3GPP users, 3GPP needed a way to refer to the LMR side of the communication. The term 'LMR user' provides that reference, defining the user of an LMR device who can participate in interworked group calls.

The problem it solves is the interoperability gap between LMR and 3GPP systems. Without a standardized way to describe LMR users, 3GPP could not specify how the Interworking Function should handle identities, permissions, and media for these users. By defining the term, 3GPP allows vendors to implement interworking functions that can seamlessly connect LMR users to 3GPP mission critical users. This is particularly important for public safety agencies that may have a mix of LMR and 3GPP devices, or for mutual aid scenarios where different agencies use different technologies.

Historically, LMR users have relied on dedicated radio systems that are separate from commercial cellular networks. These systems are highly reliable but limited in data capabilities. As 3GPP mission critical services offer broadband capabilities, there is a desire to migrate to these services. However, migration will take time, and during the transition, LMR users must be able to communicate with 3GPP users. The LMR user term was introduced in Rel-14 as part of the first mission critical interworking specifications. Its definition was refined in Rel-16 to be more concise and to align with the updated LMR system definition. The term remains out of scope of 3GPP specifications, meaning that 3GPP does not define the LMR user's device or service, but only how to interwork with it.