sive, naturally, and the backlobe attenuation of a sector antenna is typically 15 dB. This is a substantial reduction in radio level but hardly elimination of radio signal. Thus, while it is easy to draw three or six sectors in Fig. 1.6 and picture each being served independently by its own antenna, there is sub- stantial backlobe interference. And the sector boundary has significant cross-interference as well because antennas do not have perfect directional filtering, as shown in Fig. 1.7. Wireless Telephone Figure 1.6 Antennas for 120-degree sectors. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts Radio Engineering Concepts Nearly Equal On Both Sectors Figure 1.7 The sector boundary. 1.9 The Link Budget We sometimes express the combination of path gains and losses in a link budget, as shown in Table 1.2. In this simple link budget, the power level at the receiver is - -79 dBm. The manufacturer's specifications or actual measurements might tell us that the re- ceiver noise level is - 105 dBm. The resulting S/N ratio (26 dB) is obtained by sub- tracting the noise 105 dBm) from the signal strength (-79 dBm). In this example, a required S/N ratio of 20 dB was used. Surplus S/N is the extra signal strength above the minimum requirement that the communications link has to spare. As the radio environment gets more complicated, we can add components to the link budget to reflect this. Of course, a link budget can be either an estimate of what we ex- pect or a display of actual, measured results. Sometimes the link-budget concept is an oversimplification, but other times it gives a clear picture and brings a radio environ- ment into perspective. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject t