n. However, measurements have shown that ASK The system fade margin determines the outage due to rainfall. For requires about 7dB more C/N ratio than for differential frequency instance, in the U.K., for a maximum non-availability of 0.01% of shift keyed (DFSK) modulation. an average year, fade margins of 14.2dB and 17.0dB are respec- tively required at 17.7GHz and 19.7GHz (CCIR, 1990). 52.2.6 Channel plans Table 52.2 Link budget for a multipoint system (Scott, Channel plans are generally different for various systems designed 1984) over a frequency range of 10.5GHz to 26GHz (CCIR, 1990b). These are influenced to some extent by the modulation method selected, Direction of transmission source frequency variations and the general desirability of avoiding the use of RF filters with extremely narrow fractional bandwidths. OSs to CS CS to OSs Frequency (GHz) 52.2.7 Frequency re-use (a) Transmitted power (dBm) The CS antenna is usually a sector antenna with 90 degrees, 120 (b) Transmit antenna gain (dBi) degrees or 180 degrees coverage. The antenna patterns are generally (c) Receive antenna gain (dBi) shaped to ensure that adequate signal is available at OSs which are located at heights much lower (or higher) than the CS (Scott, 1984; (d) Fixed losses: feeder; filters; Murakami, 1983). In order to increase coverage, other systems at radome etc. (dB) different frequencies may be employed. (e) Receiver sensitivity (dBm) Figure 52.5 shows as an example a frequency re-use pattern using for 1 in 1000 bit error ratio 8 frequencies, 90 degrees sector antennas and 4 cell clusters (f) System gain in dB (a)+(b)+(c)+(d)-(e) (g) Path loss in dB for 10km 137.9 139.3 (h) System fade margin in dB (f)-(g) The path loss is the sum of the free space loss given by Equation 52.1 and the loss due to atmospheric water vapour absorption (CCIR, 1990a). Path loss = 20 log hop length (52.1) free space wavelength 52.2.5 Modulation/demodulation methods Spectral efficiency, ease of implementation and resistance to inter- ference are some of the co