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Calculating STL Fade Margin
The transmitter power output is 10W, which corresponds to +40dBm. Because the net path loss for the system is 91.72dB, the latter is subtracted from the former yielding the received power level of -51.72dBm. The receiver sensitivity, given as 4µV, transforms to a sensitivity of -94.96dB. The resulting fade margin is 43.2dB.
Now that the fade margin is known, how is this number used to better understand the path? For starters, the greater the fade margin is, the more reliable a particular path will be. The fade margin can, however, be so large as to be indicative of excessive transmitter power; thus care must be exercised in that regard. From the fade margin, we can also predict the reliability of a path in terms of outage time.
The probability of an outage occurring on a particular path is given by the following equation:
In this equation "a" is assigned a value of 4 for very smooth terrain including over water, 1 for average terrain with some roughness, and 0.25 for mountainous, very rough, or very dry situations. The variable "b" is set to 0.5 for Gulf coast or other hot and humid areas, 0.25 for normal interior temperate areas, and 0.125 for mountainous or desert areas. The variables "f", "D", and "F" respectively are the path frequency in gighertz, the path length in kilometers, and the calculated fade margin in decibels. So our example becomes:
This is the probability of an outage occurring. The reliability of the path is just one minus this value. In this particular instance it would be 99.999992 percent, which works out to a predicted outage time of around 3 seconds per year. This is obviously a very reliable path. On the same path, a reduction in the fade margin to 30dB, results in a reliability of 99.9998 percent. This is about 55 seconds per year of outage and exceeds the old Ma Bell reliability standard of five nines. Finally a reduction in the fade margin to 20dB increases the outage time to a little over 9 minutes per year and the reliability is now down to 99.998 percent, or just shy of five nines.
The predicted outage time typically will not occur all at once, but rather tends to be spread out. Other environmental factors including precipitation, reflections, and the Fresnel zone incursions previously mentioned can eat away at the fade margin, so if you are laying out a path, don't settle for mediocrity in the numbers. Above all, keep track of your existing paths and where they run. Don't be the engineer that wakes up one morning to find a high rise building is now square in your path. That, by the way, is a true story.
Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.
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