Professional mobile radio
They use channels in the VHF and UHF bands, giving them a limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain.
Repeaters installed on tall buildings, hills or mountain peaks are used to increase the range of systems.
Because the antenna may be mounted on a high tower, coverage may extend up to distances of fifty kilometres.
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.
The term PMR is often used by the public and magazine publishing to refer to the low power (500 milliwatt) PMR446 license exempt radio systems that consist of sixteen FM frequencies between 446.00625 and 446.19375 MHz for analog FM and thirty-two FDMA (digital) channels between 446.003125 and 446.196875 MHz.
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.
The simplest systems operate with all the radios on a frequency channel being able to hear all the calls being made.
A DTMF selective signaling PMRS system uses a code sequence of discrete audible tones, representing numbers, transmitted at the beginning of each voice message to address the transmission to a specific station or group of stations.
The DTMF (dual tone multifrequency) code is used, which is also universally used for touch-tone dialing in the worldwide public telephone network.
A disadvantage of this system is that since the DTMF code is sent only once at the beginning of a message, if the receiver does not pick up the code due to temporary noise or bad signal conditions the receiver will not turn on and it will miss the entire message.
This can be a significant disadvantage because mobile stations often lose the signal for short periods as they are on the move.
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.
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.
The first ruggedized high-speed smartphone based on the TETRA network was launched on 26 May 2011[1][2] A trunked version of the private mobile radio (PMR) concept that is defined under the standard MPT 1327 (MPT1327) is widely used and provides significant advantages over the simpler single station systems that are in use.
In view of the wider areas covered by these networks and the greater complexity, equipment has to be standardised so that suppliers can manufacture in higher volumes and thereby reduce costs to acceptable levels.
In order to be able to carry the audio information and also run the variety of organisational tasks that are needed the system requires different types of channel to be available.
The control channels use signalling at 1,200 bits per second with fast frequency shift keying (FFSK) subcarrier modulation.
For successful operation it is essential that the system knows where the mobiles are located so that calls can be routed through to them.
The MPT1327 standard describes several registration mechanisms, aimed at limiting the load on the control channel caused by mobiles roaming.
On a heavily loaded control channel, it is likely that two or more mobile radio units will try to transmit at the same time on the same random access timeslot.
Signalling on the forward control channel is nominally continuous with each slot comprising 64 bit code words.
This identifies the system to the mobile radio units and also provides synchronization for the following address codeword.
