lock diagram of a multipoint system installed, it acquires a timing reference signal from the CS, and 52.2.4 System details when commanded by it, the os transmits a burst in the up-link control time slot. The CS determines its range and instructs the OS A simplified block diagram of an urban multipoint system is shown to retard its transmissions suitably to correspond to that of an OS in Figure 52.3. The r.f. equipment at both the CS and OSs is located at a maximum distance from it. generally located outdoors with power and baseband (or intermedi- ate frequency, i.f.) signals carried between it and the indoor base- band equipment. Figure 52.4 shows a roof mounted os designed for 19GHz system (Hewitt, 1984; Bailey, 1987). The antenna is an offset parabola of 355mm diameter at the back of which is housed the r.f./i.f. unit and power supplies. A number of multipoint systems have been designed at frequen- cies between 10.5GHz and 26GHz employing binary or 4-level modulation schemes (Table 52.1). The system aggregate rates vary between 2Mbit/s and 16Mbit/s. These systems are usually used over relatively short hops (up to 7km to 10km), and therefore the domi- nant impairment mechanism is rainfall attenuation rather than at- mospheric multipath fading. Link budgets for such systems are illustrated in Table 52.2 with an example (Scott, 1984). The receiver sensitivity (-85dBm) is for a Table 52.1 Urban point-to-multipoint radio systems System 10.5GHz 19GHz 23GHz 26GHz Aggregate 2.048 8.192 0.832 16.384 bit rate (Mbits/s) Modulation 2 FSK 2 ASK FSK (CS to OS); (OS to Frame 0.125 (CS period to OS) Figure 52.4 A 19GHz roof mounted outstation 5 (OS to 52/6 Point to multipoint urban and rural radio carrier to noise (C/N) ratio of 11dB since, for binary FSK, the total tude shift keying (Shindo, 1981; Shindo, 1983a; Marchand, 1986). noise power is the sum of the receiver noise figure (8dB) and Transmitter power control have also been used to minimise received thermal noise of -104dBm in a noise bandwidth of 10MHz. power variatio