erent numbers of cells On moving away from the recognised base control transmitter, a low per cluster may be combined also in a typical wide area system RSSI indication or a high bit error rate in the mobile unit causes the where urban coverage is required together with rural coverage, each mobile to hunt for other acceptable control channels (Bye, 1989). cell thus being tailored to the required parameters. The roaming mobile unit thus re-registers as required onto different base transmitter areas when moving from area to area. The system 46.3.5 Base station coverage network switch updates its records to store the traffic area and channel the mobile is currently active on, hence routing incoming call indications to the mobile on the appropriate site and channel Although mobile units often effectively have an omnidirectional being used. aerial radiation pattern, efficiencies may be achieved in the overall number of base station sites required by the use of directional aerials. V.h.f systems are often used with omnidirectional or slightly 46.3.9 Communication tailored radiation patterns where local topography dictates to pro- vide wide area coverage with a minimum number of base station Once a call set up has been made, further signalling continues either sites, whilst those operating on u.h.f. (typically 450MHz and on the control channel until the called party answers, alternatively 900MHz systems) commonly use controlled aerial radiation pat- a traffic channel is allocated for further call processing dependent terns to give multiple cell clusters from a single base station site. upon the system being used. During communication, speech com- On elevated aerial sites in urban environments a degree of beam bined with supervisory signalling is employed. down-tilt is normally required on u.h.f. for effective coverage at Where a mobile is travelling from one cell coverage area to ground level. This practice may also be usefully employed in another, a 'handoff' action may occur where the base station signals achieving ma