Professional mobile radio

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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 and are sometimes based on such standards as MPT1327, TETRA and APCO 25 which are designed for dedicated use by specific organizations. 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

Introduction

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 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 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 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 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 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.

Overview of TETRA Private Mobile Radio (PMR)

TETRA is a modern standard for digital Private Mobile Radio (PMR) and Public Access Mobile Radio (PAMR). It offers many advantages including flexibility, security, ease of use and offers fast call set-up times. This makes it an ideal choice for many business communications requirements.

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 highly successful 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.

For users, the TETRA system offers fast call set-up times as well as direct mode operation between radios. Packet data as well as circuit data modes are available and the system provides excellent security along. The systems makes efficient use of the available frequency allocations using Time Division Multiple Access (TDMA) technology with 4 user channels on one radio carrier with 25 kHz spacing between carriers.

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. With the increased levels of international travel in business nowadays this enables a user to be always in contact with colleagues.

Although invisible to the user, there are a variety of frequency bands that are used by TETRA. 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 383-395 MHz and 393-395 MHz can be utilized should this be required.

TETRA is a growing standard for PMR and PAMR. It has shown itself to be very successful and is being implemented by civil and emergency services alike across Europe and beyond.

PMR Trunking using MPT1327

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. MPT1327 enables stations to communicate over wider areas as well as having additional facilities. In view of the very high cost of setting up trunked networks, they are normally run by large leasing companies or consortia that provide a service to a large number of users. 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. Most trunked radio systems follow the MPT1327 format.

To implement trunked PMR a network of stations is set up. These stations are linked generally using land lines, although optical fibres and point to point radio are also used. In this way the different base stations are able to communicate with each other.

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. These are the control channels of which there is one in each direction for each base station or Trunking System Controller (TSC).

A number of different control channels are used so that adjacent base stations do not interfere with one another, and the mobile stations scan the different channels to locate the strongest control channel signal. In addition to this there are the traffic channels. The specification supports up to 1024 different traffic channels to be used. In this way a base station can support a large number of different mobile stations that are communicating at the same time. However for small systems with only a few channels, the control channel may also act as a non-dedicated traffic channel.

The control channels use signalling at 1200 bits per second with fast Frequency Shift Keying (FFSK) subcarrier modulation. It is designed for use by two-frequency half duplex mobile radio units and a full duplex TSC.

For successful operation it is essential that the system knows where the mobiles are located so that calls can be routed through to them. This is achieved by base stations polling the mobile stations using the control channel.

To make an outgoing call the mobile transmits a request to the base station as requested in the control channel data stream from the base station. The mobile transmits its own code along with that of the destination of the call, either another mobile or a control office. The control software and circuitry within the base station and the central control processing area for the network sets up the network so that a channel is allocated for the audio (the traffic channel). It also sets up the switching in the network to route the call to the required destination.

To enable the mobile station to receive a call, it is paged via the incoming control channel data stream to indicate that there is an incoming call. Channels are allocated and switching set up to provide the correct routing for the call.

There is no method to "handover" the mobile from one base station to the next if it moves out of range of the base station through which a call is being made. In this way the system is not a form of cellular telephone. It is therefore necessary for the mobile station to remain within the service area of the base station through which any calls are being made.

The control channel signalling structure has to be defined so that all mobiles know what to expect and what data is being sent. Signalling on the forward control channel is nominally continuous with each slot comprising 64 bit code words. The first type is the Control Channel System Codeword (CSCC). This identifies the system to the mobile radio units and also provides synchronisation for the following address codeword. As mentioned the second type of word is the address codeword. It is the first codeword of any message and it defines the nature of the message. It is possible to send data over the control channel. When this occurs, botht he CSCC and the address codewords are displaced with the data appended to the address codeword. The mobile radio unit data structure is somewhat simpler. It consists fundamentally of synchronism bits followed by the address codeword.

There are a number of different types of control channel messages that can be sent by the base station to the mobiles:

Aloha messages -- Sent by the base station to invite and mobile stations to access the system.

Requests -- Sent by radio units to request a call to be set up.

"Ahoy" messages -- Sent by the base station to demand a response from a particular radio unit. This may be sent to request the radio unit to send his unique identifier to ensure it should be taking traffic through the base station.

Acknowledgements -- These are sent by both the base stations and the mobile radio units to acknowledge the data sent.

Go to channel messages -- These messages instruct a particular mobile radio unit to move to the allocated traffic channel.

Single address messages -- These are sent only by the mobile radio units. Short data messages -- These may be sent by either the base station or the mobile radio unit.

Miscellaneous messages -- Sent by the base station for control applications.

One of the problems encountered by mobile signalling systems is that of clashes when two or more mobile radio units try to transmit at the same time on the control channel. This factor is recognised by the system and is overcome by a random access protocol that is employed. This operates by the base station transmitting a synchronization message inviting the mobile radio units to send their random access message. The message from the base station contains a parameter that indicates the number of timeslots that are available for access. The mobile radio unit will randomly select a slot in which to transmit its request but if a message is already in progress then it will send its access message in the next available slot. If this is not successful then it will wait until the process is initiated again.

Although the data is transmitted as digital information, the audio or voice channels for the system are analogue, employing FM. However some work has been carried out to develop completely digital systems. The main systems are by Motorola, by Ericsson (EDACS) and Johnson (LTR). These systems have not gained such widespread acceptance. Prepared by PhD Ilir Imeri

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