Overview of Private Mobile radio (PMR)
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 DTMT and CTCSS provide additional calling selection.
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.
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.
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.
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