requencies permit the use of small compact antennas requiring highly reliable communication. with high gains, and production of highly integrated low cost trans- ceivers. To produce systems with a high spectral efficiency it is essential to use extremely stable but costly oscillators. Thus cost The disadvantages of radio are as follows: considerations rather than spectral efficiency appear to be the im- portant influencing factors in the system design. Rainfall attenua- 1. Generally line-of-sight is required between terminals. tion becomes significant above about 10GHz and therefore the 2. Radio signals distort and/or attenuate due to the weather. range and non-availability of these systems are affected by this factor. Point-to-point radio systems are suitable for relatively large trans- mission capacity. However, for kilobit rate services it is difficult to 52.2.1 System operation provide spectrally efficient point-to-point systems at microwave frequencies for two reasons: it is difficult to make low loss filters The system comprises a central station (CS) and a number of with extremely narrow bandwidths, and power source instabilities subscriber outstations (OS) which are usually roof mounted and could result in a relatively significant transmission bandwidth re- have line of sight (LOS) paths to the CS. A block diagram of an quirement. These disadvantages are overcome by using point-to- urban multipoint system is shown in Figure 52.1. The CS would multipoint systems. There are two types of multipoint systems, one normally be located on a tall building with cable connections to a designed for urban use and the other for rural applications. nearby exchange or central office. Small (approx. 3m) earthstation Telephone Outstation exchange Central station Outstation Outstation Figure 52.1 Urban multipoint system operation 52/4 Point to multipoint urban and rural radio down-link time frame up-link time frame traffic time-slot management time-slot guard time traffic time-slots overheads subscriber's in up-link clock/b