Introduction

    If your business such that you need radio contact with several mobiles - a fleet
    of lorries and vans, say - there are other forms of radio telephony which are
    much cheaper than having all your vehicles fitted with cellular radio phones.
    After all, your truck drivers do not to be able to make unlimited nationwide
    or international telephone calls ; they only need to be able to maintain contact
    with their operating bases and their base dispatchers need to be able to control
    their movements.

    Until a few years ago the only possibility was a Private Mobile Radio (PMR)
    network of your own, with heavy capital and running cost and traffic handling
    constraints, especially if you needed coverage over a wide area of the country.

    The user of a  PMR  system  does not  have  access  to the  Public Switched
    Telephone Network (PSTN) and the geographical area covered is usually no
    larger than about  30  miles in diameter.  In a basic  PMR  system a dispatcher
    calls mobiles over a channel that is assigned for the duration of the conversation.

    PMR is considerably  cheaper  for a user  to  operate than cellular radio, both
    for connection to a  network  and  for  everyday  running  costs.  PMR  users
    generally require a despatch facility which allows mobiles to be simultaneously
    contacted via an open channel. This facility is not possible with cellular radio.

    PMR Concept

     A two-way PMR system may consist of just two transceivers, a base station
    and several  mobile transceivers, or several inter-connected base stations and
    a large number of mobiles.  Users of a PMR system are allocated a specific r.f.
    channel over which they can communicate. With the ever-increasing demand
    for mobile radio facilities, radio channels are becoming congested and users may
    have to share channels with other users.  Also, adjacent channel  interference
    is a potential problem. To overcome this congestion, a system known as trunked
    PMR , which allocates channels to users in a different way, is often employed.

   The simplest way of operating  a PMR system is shown in Fig 40 . The system
   operates in  the single-frequency simplex mode and uses a single aerial that is
   switched between the receiver  and the transmitter. A mobile is only able to
   communicate with the control.

    A different frequency may be used for each direction of transmission to provide
    double-frequency simplex operation of a radio channel. The basic idea is
    illustrated by Fig. 41. Although this method of operation would appear to
    be more expensive in its use of the frequency spectrum than single-frequency
    working, it allows systems operating on the same frequency to be more closely
    spaced  by a factor of about three. Channel carrier frequencies  are allocated
    on an area basis and so nine times as many channels in a given area can be
    accommodated if double-frequency working is used instead of single-frequency.
    Since two frequencies are required, the net gain is  about  4.5  times.

    The slightly more complex arrangement shown in Fig. 42  provides for
     two-way communication between the control centre and each mobile, as well
     as allowing for communication between two mobiles. Talkthrough may  be
     only possible under the control of the system operator, or it may be possible
     for selected mobiles to initiate talkthrough  without recourse to the control.
     For this system it is necessary to have either separate receive  and transmit
     aerials or a single aerial fed via a duplexer .

    The  allocation  of  actual  frequency  bands  to  be  used  is  determined  by
     the  telephone  administration and then the blocks of frequencies are selected
     within a band to give the  largest number of channels  together  with  the
     minimum  possible  level  of  intermodulation. A considerable number of
     different frequency bands, of various widths, have been allocated to land, sea
     and air mobile services in the VHF and UHFbands.

     Since frequencies in either the VHF band or the UHF band are employed, the
     service area provided by a base station transmitter is of limited size. This
     means  that a number of  interconnected base station may be required to cover
     a large area and Fig 43 shows a typical arrangement. Mobiles are not allocated a
     particular fixed carrier frequency; instead, a number of channels are provided
     and one of them is allocated to a mobile as and when required .  The operation
     of such a system is not as simple as it perhaps might seem, largely because
     the signal received by a mobile may be the resultant of the signals produced by
     more than one base station, or, alternatively, the mobile signal arriving at the
     control may have come via  two or more base station receivers .

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