rage area. Figure 46.3 of user transmission, and to the remote base station to limit the shows this concept, where receive audio is linked to the transmitter length of air time for a given user fleet in the time slot provided. for re-transmission under manual or automatic signalling control. Simplified PABX and PSTN telephone interconnect is also feasible 46.2.6 Quasi-synchronous operation with this system, with the portable unit controlling dialling and call clear down through analogue or digital signalling. An area or nationwide system using quasi-synchronous operation may be employed using cellular coverage on a single given channel, 46.2.4 Remote base station often combined with base station receivers having voting circuitry fitted to continuously detect and transmit received signal strength To achieve a controlled radio coverage range in the v.h.f./u.h.f. indications to the central control switch. Each transmitter is oper- spectrum used, a defined aerial site is normally required which may ated at a small but accurately controlled frequency offset from that be significantly at variance with the physical location of the base of its neighbour, to combat the effects of phase cancellation in station operator's console. In the case of an exclusive or shared received signals at mobile unit locations (Philips, 1980). This fre- channel allocation, a leased landline may be used to control a quency offset is typically 5Hz to 6Hz in a v.h.f. system operating remotely sited base station transmitter/receiver, alternatively a point with 12.5kHz channel separation, and 3Hz on u.h.f. systems. In to point u.h.f. or s.h.f. radio link may be used where geographical areas where two base station transmitter signals overlap, a beat note and propagation limitations allow. of this frequency will be present if both signals are within the capture effect range of the mobile receiver. The probability of a 46.2.5 Communal base stations complete null occurring diminishes where three or more signals overlap. Figure 46.4 gives an example o