0.939 14.09 mobile radio allocation has been already filled to capacity with no 0.885 17.70 further channels being available for new users, and the U.K. System 4 v.h.f. radiophone system some time ago was terminated due to 0.911 18.22 overcrowding. Other frequency bands such as u.h.f. as used for 0.953 19.06 private mobile radio communication also suffer from congestion to varying degrees, and in some countries further u.h.f. allocations such as 900MHz are used to supplement existing 450MHz alloca- tions for PMR use. 46.3.2 Trunked systems 46.3.3 Trunked area coverage Trunking may usefully be combined with area coverage techniques A trunking system typically used for mobile communication em- for use with frequency agile radio units under synthesiser control. ploys similar basic principles to landline telephony trunking, where Here the required area coverage is sectored, with adjacent sectors several radio channels are used to allow more efficient use of the being allocated different frequencies rather than the previously available spectrum by sharing a 'pool' of channels with other users. described quasi-synchronous coverage method operating on a This is the basis for most current analogue radio trunking systems single frequency. From carefully tailored planning of individual cell in use. The application of Erlang C may be used (Erlang, 1918) to coverage patterns, frequency re-use may be employed due to cap- provide approximate performance figures and hence grade of ser- ture effect performance of mobile receivers together with sufficient vice (DTI, 1986), as in Equation 46.1, where Pd is probability of geographical separation of cells operating on the same frequency. delay, C are the number of trunked channels, and A is the traffic load Due to the tailored coverage required, either v.h.f. for wide area in Erlangs. coverage, or u.h.f. for more closely defined coverage, is utilised. 46/6 PMR and Trunked Radio Systems 46.3.4 Cell clusters Semi-duplex or full duplex operation is used, the base transmitter control ch