igned for fixed PSTN use, it has proved to perform suffi- sake, but are aimed at improving the quality, capacity, and availa- ciently well over cellular paths, particularly to static mobiles, for the bility, and reducing the cost of mobile communications. In addition user to receive good service. to these step changes in 'generations' of system there are technical Conclusion 47/17 advances which are applicable to current systems. These include PCN is dealt with in detail in Chapter 48. The standard in Europe, techniques such as microcellular and intelligent networks. known as DCS1800 is based on the GSM standard but operating at 1800MHz. There is therefore unlikely to be a significant technical 47.7.1 Microcells difference between Cellular GSM and PCN, with microcellular techniques being equally applicable to either system. As the capacity of cellular systems has increased, cell sizes have In the USA, the use of CDMA, code division multiple access, is decreased, in some networks to as small as 0.5km radius, such that being trialled for PCS. CDMA works on the principle of transmit- controlling co-channel interface becomes a major problem. The use ting unique (orthogonal) codes to identify different users. Detection of microcells, that is, very small cells, is a way of increasing of signals is achieved by using correlating receivers such that other capacity still further. In a microcellular layout, base station antennas users appear as pseudonoise. CDMA thus allows a large number of are placed below the building height in urban areas, and low power users to share the same (wideband) radio channel. is used such that the propagation characteristics between base sta- There is considerable debate about the advantages and disadvant- tion and mobile are dominated by the street layout. Interference ages of CDMA, in particular how to control near/far user inter- from adjacent cells is blocked by buildings. ference; the extent to which this can be achieved is crucial to the Microcellular techniques allow significantly highe