Private Mobile Radio, PMR, Basics

- summary, overview or tutorial about the basics of Private, Professional, Public Access Mobile Radio, PMR or PAMR including the basic standards and specifications used for PMR mobile radio.

PMR, Private Mobile Radio or as it is sometimes called Professional Mobile Radio is widely used for businesses as a very convenient way of communicating. The basic concept of PMR mobile radio 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.

PMR mobile radio development

The first PMR mobile radio systems were initially set up to enable a set of mobile business users to maintain contact with a base using radio communications technology. Organisations 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 these radio communications systems consisted of a base station with a number of mobile radio stations. Communication used a single frequency, with "simplex" push to talk transmissions using a single channel for both transmitting and receiving. As pressure rose on the radio 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. Only when the appropriate DTF code of CTCSS frequencies were transmitted would the relevant receiver be activated

These radio communications systems were only able to communicate over relatively short distances. They used a single central base station to communicate with all the mobile radio 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.

PMR mobile radio basic concepts

In view of the fact that the antenna may be mounted on a high tower, coverage may extend up to distances of 50 kilometres, although ranges somewhat less than this are more usual, especially when antennas are not as high.

Licenses are allocated for operation on a particular channel or channels. The user can then have use of these channels to contact the mobile stations in his fleet. The base station may be run by the user himself or it may be run by an operating company who will hire out channels to individual users. In this way a single base station with a number of different channels can be run by one operator for a number of different users and this makes efficient use of the base station equipment. The base station site can also be located at a position that will give optimum radio coverage, and private lines can be provided to connect the users control office to the transmitter site.

As there is no incremental cost for the transmissions that are made, individual calls are not charged, but instead there is a rental for overall use of the system. For those users with their own licences they naturally have to pay for the licence and the cost of purchase and maintenance of the equipment.

Modulation used for PMR mobile radio

In general narrow band frequency modulation is the chosen form of modulation, although airport services use amplitude modulation. Typically a deviation of 2.5 kHz is used for FM and this enables a channel spacing of 12.5 kHz to be implemented. As the demands for PMR are high, it is necessary to make effective use of the channels available. This is achieved by re-using the frequencies in different areas. Base stations must be located sufficiently far apart so that interference is not experienced, and also selective calling techniques such as CTCSS and DTMF are used to ensure that as many mobiles as possible can use a given channel.

Selective calling

Many systems operate with the remote or mobile stations being able to hear all the calls being made. This may not always be satisfactory and a system of selective calling may be required. There are two ways of achieving this. One is to use a system known as Dual Tone Multiple Frequency (DTMF) signalling whereas the other uses Continuous Tone Coded Squelch System (CTCSS).

DTMF is s system that is widely used for telephone signalling and is almost universally used for touch tone dialling for landline telephones today. It uses set pairs of tones are used to carry the information. The eight frequencies used are 697, 770, 852, 941 Hz as what are termed the "low tones" and 1209, 1336, 1477, and 1633 Hz as what are termed the "high tones". One high and one low tone are used together and the various combinations are used to represent different numbers.

The relevant code consisting of one or more digits is sent and the station programmed to respond to the number, typically one or two digits responds by opening the squelch on the receiver to let the audio through. The disadvantage of this system is that if the receiver does not pick up the code at the instant the DTMF signalling takes place then it will not respond to any of the message. This can be a significant disadvantage because mobile stations often loose the signal for short periods as they are on the move.

The other widely used system is CTCSS. The system may also be referred to asd subaudible tones or PL tones (a Motorola trademark). As the name suggests it uses subaudible tones (below about 250 Hz) to carry the selection information. These are transmitted in addition to the normal voice channel, but as they appear below the normal audio range they are filtered out and they are not heard.

Only when the correct tone for the required station is transmitted will the squelch for that receiver be opened and the transmitted audio will be heard. The advantage of this system is that the subaudible tones are transmitted for the whole period of the transmission so if the signal fades at the beginning of the transmission is lost but later then increases in strength, the continuously transmitted tones will enable the squelch to open and the audio to be heard. Systems typically are able to provide up to 37 different tones, the lowest frequency of which is 67 Hz and the highest 250.3 Hz. This enables a variety of differnt mobiles to be called selectively.