Land Mobile Radio system

Description

In 3GPP specifications, an LMR system (Land Mobile Radio system) is defined as the collection of applications, services, and enabling capabilities required to provide LMR service for a mission critical organization. The term is used primarily in the context of interworking between legacy LMR systems and 3GPP mission critical services, such as Mission Critical Push To Talk (MCPTT). The definition is explicitly stated to be for discussion purposes only and is out of scope of the present specification, meaning that 3GPP does not standardize the internal details of LMR systems but rather focuses on the interworking functions.

LMR systems are traditional radio systems used by public safety, military, and professional organizations for group communication. They typically operate on dedicated frequencies and provide features such as push-to-talk, group calls, and broadcast calls. Examples of LMR technologies include P25 (Project 25), TETRA (Terrestrial Trunked Radio), and DMR (Digital Mobile Radio). These systems are characterized by high reliability, direct mode operation, and robust group communication capabilities. However, they are often proprietary and lack the broadband capabilities of modern cellular networks.

The 3GPP work on LMR interworking, starting from Release 14, aims to enable users of LMR systems to communicate with users of 3GPP mission critical services. This is achieved through interworking functions that translate between the protocols and procedures of the LMR system and those of the 3GPP system. The LMR system is treated as an external system, and the interworking function acts as a gateway. The specifications 23.283, 23.782, and 23.783 define the architecture and procedures for this interworking.

In the 3GPP architecture, the LMR system is connected to the 3GPP mission critical system via an interworking function (IWF). The IWF is responsible for mapping the LMR group call to the 3GPP group call, handling user identities, and ensuring that voice and data are correctly translated. The LMR system may also support the interworking of short data messages and location information. The goal is to allow a gradual migration from LMR to 3GPP mission critical services, where users of both systems can communicate seamlessly.

The definition of LMR system has evolved slightly over releases. In Rel-14 and Rel-15, it was defined as the collection of applications, services, and enabling capabilities required to provide LMR service for a mission critical organization. In Rel-16, the definition was changed to 'the collection of applications, services, and enabling capabilities providing a land mobile radio service offering group and private communications.' This change reflects a more precise description of the services offered by LMR systems, emphasizing group and private communications. The term remains out of scope of the specifications, indicating that 3GPP does not define the internal workings of LMR systems but only the interworking aspects.

Purpose & Motivation

The LMR system term exists to provide a clear reference point for interworking between legacy Land Mobile Radio systems and 3GPP mission critical services. As public safety organizations and other mission critical users began to migrate from narrowband LMR systems to broadband 3GPP networks (such as LTE and 5G), there arose a need for these two worlds to communicate. The LMR system term defines the external system that 3GPP networks must interwork with, without specifying its internal details. This allows 3GPP to standardize the interworking function while leaving the LMR system implementation to the respective LMR standards bodies.

The problem it solves is interoperability. Before this work, LMR users and 3GPP mission critical users could not communicate directly. An incident commander using an LMR radio could not talk to a responder using an MCPTT app on a smartphone. The LMR system concept, along with the interworking function, enables seamless communication between these groups. This is crucial during transition periods when organizations may have a mix of LMR and 3GPP devices, or when different agencies use different technologies.

Historically, LMR systems have been the backbone of mission critical communications for decades. They are highly reliable and offer features tailored to the needs of first responders, such as direct mode operation and group calls with fast call setup. However, they are limited in data capabilities and cannot support modern applications like video streaming or real-time location sharing. 3GPP mission critical services (MCPTT, MCVideo, MCData) were developed to provide these broadband capabilities. The LMR system term was introduced in Rel-14 as part of the first set of mission critical interworking specifications, acknowledging that LMR systems will continue to exist and need to be integrated with 3GPP systems. The evolution of the definition in Rel-16 reflects a refinement in understanding, but the core purpose remains the same: to facilitate interworking.