e standard with which the network complied. Since frequency allocations and other In rural areas, cells may be 20km to 30km in radius. In practice, basic parameters (such as channel spacing) have often been set at as cell sizes decrease, propagation effects, particularly in city areas, national level and not co-ordinated between countries, these local cause an increase in co-channel interference, even if the repeat factors resulted in different standards being adopted by different pattern is maintained. Also, as cell sizes decrease, it becomes countries. increasingly difficult to find suitable base station sites, which need In addition, in cases where the development of a standard has to be accurately positioned in order to keep to a regular pattern. started later, the opportunity has been taken to introduce new fea- The cost of providing and maintaining a large number of individ- tures made possible by advances in technology, further increasing ual base stations is also a factor, such that in addition to cell splitting, the diversity between standards. sectorisation of cells is commonly used in urban areas. From this wide range of differing system standards, four have In a regular cellular layout, co-channel interference will be re- become largely dominant and have been adopted in many countries, ceived from six surrounding cells which all use the same channel albeit still with some variations. These four are AMPS, TACS, NMT 47/8 Cellular radio systems Table 47.2 Comparison of system parameters NMT 450 NMT 900 C-450 Frequency band 800MHz 900MHz 450MHz 900MHz 450MHz Channel spacing 30kHz 25kHz 25kHz 12.5kHz 20kHz Speech modulation Deviation 12kHz 9.5kHz 5.0kHz 5.0kHz 4.0kHz Signalling Direct FSK Direct FSK Audio FFSK Audio FFSK Direct FSK Signalling bit rate 10kbit/s 8kbit/s 1200bit/s 1200bit/s 5280bit/s 'Overlay' signalling (both NMT 450 and NMT 900) and C450, and their basic system that operating licences were issued and true commercial exploita- parameters are shown in Table 47.2. Each of these is described in