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An overview of the basic "local" PMR system

Private Mobile Radio (PMR) or as it is sometimes called Professional Mobile Radio is widely used for businesses as a very convenient way of communicating. The basic concept has been in use for many years and was firmly established prior to the introduction of the first cell phone systems, although systems including MPT1327 that provide trunking and TETRA enable far greater facilities.

The first PMR systems were initially set up to enable a set of mobile business users to maintain contact with a base. Organizations such as taxi firms, utility workers and the like all used these systems as they enabled them to maintain contact with their office. Additionally the emergency services used their own systems.

Initially the systems consisted of a base station with a number of mobile stations. Communication used a single frequency, with simplex push to talk transmissions. As pressure rose on the frequency allocations, often frequencies had to be shared. As the systems almost invariably used frequency modulation, squelch was employed so that the audio from the received was only switched on when a signal was present. Developments of this known as DTMF (dual tone multiple frequency) and CTCSS (continuous tone, coded squelch system) were used to enable only the required users to hear the call.

These systems were only able to communicate over relatively short distances. They used a single central base station to communicate with all the mobile stations. This considerably reduced their coverage area. To overcome this a system known as trunking was devised whereby several transmitters could be used and the signal was “trunked” to the correct station. Several systems are available for this but the one that has gained by far the widest use is specified as MPT 1327.

All the standards mentioned so far are analogue systems. The cellular telecommunications industry moved to digital technology to provide improved efficiency of spectrum usage along with a variety of new facilities. So too did the PMR industry with the induction of a system known as TETRA. . Originally the letters stood for Trans European Trunked Radio, but as the system is now being used beyond Europe the abbreviation now stands for TErrestrial Trunked RAdio. This system provided a far more flexible service along with all the other advantages of using a digital system.

 

 

 

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