Professional mobile radio (also known as private mobile radio (PMR) in the UK and land mobile radio (LMR) in North America) are field radio communications systems which use portable, mobile, base station, and dispatch console radios. Operation of PMR radio equipment is based on such standards as MPT-1327, TETRA and APCO 25 which are designed for dedicated use by specific organizations, or standards such as NXDN intended for general commercial use. Typical examples are the radio systems used by police forces and fire brigades. Key features of professional mobile radio systems can include:
- Point to multi-point communications (as opposed to cell phones which are point to point communications)
- Push-to-talk, release to listen a single button press opens communication on a radio frequency channel
- Large coverage areas
- Closed user groups
Use of VHF or UHF frequency bands
When Private or professional mobile radio (PMR) first started the systems simply consisted of a single base station with a number of mobiles that could communicate with this single base station. These systems are still in widespread use today with taxi firms and many others using them for communication. Now facilities such as DTMF and CTCSS provide additional calling selection. Because 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 their fleet. The base station may be run by the user themselves 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 that equipment.
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 several ways of achieving this, including Dual Tone Multiple Frequency (DTMF) signal and Continuous Tone Coded Squelch System (CTCSS).
Main article: DTMF
DTMF is a system that is widely used for telephone signalling and is almost universally used for touch tone dialing for landline telephones today. It uses set pairs of tones that are used to carry the information. The eight frequencies used are 697, 770, 852, 941 Hz which are termed the "low tones" and 1209, 1336, 1477, and 1633 Hz which are "high tones". One high and one low tone is used together and the various combinations are used to represent different numbers and symbols.
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 lose the signal for short periods as they are on the move.
Main article: CTCSS
Another widely used system is CTCSS, also referred to as 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 audio range passed by most mobile radios (roughly 300-3000Hz), they are filtered out and therefore 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 254.1 Hz. This enables a variety of different mobiles to be called selectively.
Modulation
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.
TETRA is a modern standard for digital Private Mobile Radio (PMR) and Public Access Mobile Radio (PAMR).
Work started on the development of the TETRA standards in 1990 and has relied on the support of the European Commission and the ETSI members. Experience gained in the development of the GSM cellular radio standard, as well as experience from the development and use of trunked radio systems has also been used to fashion the TETRA standard. In addition to this the process has gained from the co-operation of manufacturers, users, operators and industry experts. With this combined expertise the first standards were ready in 1995 to enable manufacturers to design their equipment to interoperate successfully.
TETRA allocates the channels to users on demand in both voice and data modes. Additionally national and multi-national networks are available and national and international roaming can be supported. For civil systems in Europe the frequency bands 410-430 MHz, 870-876 MHz / 915-921 MHz, 450-470 MHz, 385-390 MHz / 395-399.9 MHz, have been allocated for TETRA. Then for the emergency services in Europe the frequency bands 380-383 MHz and 390-393 MHz have been allocated. In addition to this, the whole or appropriate parts of the bands between 383-385 MHz and 393-395 MHz can be utilized.
Low speed packet data as well as circuit data modes are available, along with some form of encryption. The systems makes use of the available frequency allocations using Time Division Multiple Access (TDMA) technology with four user channels on one radio carrier with 25 kHz spacing between carriers.