nnel allocation investigated as an alternative to nickel cadmium, the charge/dis- from off-air received signalling. charge voltage gradient of these being similar to that of nickel cadmium thus allowing retrospective fitment. Nickel hydrogen of- fers approximately twice the capacity/volume ratio and four times 46.1.1 Spectrum usage the capacity/weight ratio of nickel cadmium. Lithium cells are commonly used in the limited number of cases where rechargeable The majority of PMR services employ sub-bands within the fre- capability is not required, these cells also having a significantly quency range 30MHz to 960MHz, these being periodically agreed higher capacity ratio compared with nickel cadmium cells. internationally in the World Administration Radio Conference. It must be noted however that the use of these bands are not common throughout the world, as the radio regulatory bodies of individual PMR systems countries often administer the use of the v.h.f. and u.h.f. spectrum to their own needs, based upon a 'non-interference' operation with other countries. A typical case is that of the Band III range of 46.2.1 Simplex 174MHz to 225MHz, which is used in the U.K. for trunked PMR but in the neighbouring country of France for Television Broadcast The simplest form of personal mobile communication is the use of services. a single 'open' frequency for both transmission and reception, this normally being termed a 'simplex' channel communication method. Here all parties to the communication use a common frequency, with all users within range having communication ability, as shown 46.1.2 Equipment in Figure 46.1. Spectrum conservation techniques in mobile radio equipment used Frequency A under digital agile frequency control combined with digital signall- ing have thus made standard the use of LSI control circuitry, with Frequency A Simplex resultant sophisticated two way communications equipment becom- Frequency B Half duplex ing relatively smaller each year. 46.1.2.1 Frequency generation and signal processing Single IC 