Fade Margin

The reason for calculating a wireless link budget is to design and build a reliable wireless link. Microwave signals normally interact with many objects in their environment, as discussed throughout this chapter. Therefore, fading is a normal condition for all microwave links. To overcome the effects of this fading and to provide reliable service, every microwave link needs a certain amount of extra signal, over and above the minimum receiver threshold level. This extra signal is called the fade margin. Another term sometimes used for this extra signal is system-operating margin (SOM). Most wireless equipment manufacturers recommend a minimum fade margin of at least -1-10 dB to ensure reliable lmk performance. In general, the longer the link, the more fluctuation in signal levels and the greater the fade margin needs to be. Figure 2-25 shows a sample link budget calculation, including the calculated fade margin.

Figure 2-25. Link Budget and Fade Margin Calculations f-tflcelver Threshold

Sain or Losa —» t30 dßm -8 (Jörn *l2dEm -HD dBm +?4 <JBm -7 dßm -so dBm f-tflcelver Threshold

Sain or Losa —» t30 dßm -8 (Jörn *l2dEm -HD dBm +?4 <JBm -7 dßm -so dBm

Hessling—* +30 dBm +34 dBtn +3$dBm -74 dBm -5« dBm -57 rOJm +£3lJB Signal Levai FinJe Margta

Fadfi Margin = (Signal Arriving al RECfirmr) - {Raneivfir Thranlunld} = -£7dBrtt ■ (-80 dBrt) - -S7 dBm * so dSm Fade l/arg^n = +23 dB

Hessling—* +30 dBm +34 dBtn +3$dBm -74 dBm -5« dBm -57 rOJm +£3lJB Signal Levai FinJe Margta

Fadfi Margin = (Signal Arriving al RECfirmr) - {Raneivfir Thranlunld} = -£7dBrtt ■ (-80 dBrt) - -S7 dBm * so dSm Fade l/arg^n = +23 dB

By calculaticg thr fade ma rgin, you can ladedicr the relia bility of a wiselpss link. The 2-mile long link shown in Figure 2-25 has a fade margin of +23 dB. +23 dB is 13 dB more than the 10 dB fade margin needed to make this Mne peefo rm weliably; there-orPr yod can conclude that th is link is go , eg ho delive r excellent reliabi|i-yl

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