f a typical coverage system. A progression from the remotely sited base is a 'Communal Base Using these techniques, a wide coverage system based on cellular Station' (DTI, 1988a). This operates in an automatic talkthrough coverage techniques may be achieved without the need for a rela- mode in full duplex on a shared user basis. Several groups of users, tively advanced central processing switch, nor the use of frequency each having inter-communication within their own fleet, employ the agile mobile or terminal units. Channel re-use may be achieved facilities of the remotely mounted equipment in a time shared mode. through the use of selective base transmitter operation dependent To ensure privacy between fleets, analogue or digital selective upon voting signal levels received from individual base station signalling is employed. This mode of operation is based upon the receivers. Trunked mobile radio 46/5 (46.1) Base A k=C-1 Base B Table 46.1 gives indications for typical 5, 10, and 20 channel mobile systems. In typical usage on a large system a dedicated Base C 'control' channel is permanently enabled, this carries signalling information, the protocol being dependent upon the system em- ployed, for reception by frequency scanning mobile units. In small usage systems the control channel may also be used as a traffic Base D channel when required. This improves channel utilisation at the expense of losing instantaneous call processing signalling during peak traffic periods. Table 46.1 Mobile trunked system performance Figure 46.4 Quasi-synchronous operation employs stable transmitters operating at a small frequency offset. Signal overlap produces received signal phase additions and No. of Grade Traffic (Erlang) No. of Mobiles cancellations Channe waiting Service time(s) Total Total channel channel Trunked mobile radio 46.3.1 Spectrum re-use 0.645 0.719 Over the years several improvements in spectrum conservation in PMR bands have been made in an effort to use available frequencies 0.846 to the maximum benefit possib