WNI Global Challenger L Series, User Reference And Installation Manual

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Challenger L Series Digital Microwave User Manual
2-28 This manual is provided by WNI Global and its confidentiality is preserved. Without the pre-written consent from WNI Global,
anyone may not disclose, extract, cite or publish any part or all parts of this manual. Version No.: V2.0
2.5.3.2 Fade Margin Calculation
The fade margin is the difference between the actual received signal and the Challenger L Digital
Radio’s threshold for the modulation mode selected. The fade margin can be used to determine
availability and should be at least 10 dB.
2.5.3.3 Availability Calculation
Availability of the microwave path is a prediction of the percent of time that the link will operate
without producing an excessive BER due to multipath fading. Availability is affected by the
following:
•
Path length
•
Fade margin
•
Frequency
•
Terrain (smooth, average, mountainous, valleys)
•
Climate (dry, temperate, hot, humid)
Depending on the type of traffic carried over the link and the overall network design redundancy,
fade margin should be included to support the desired availability rate. Critical data and voice
may require a very high availability rate (99.999% or 5.3 minutes of predicted outage per year).
To improve availability, the fade margin can be increased by shortening the path length,
transmitting at a higher power level, or by using higher gain antennas.
Availability can be computed using the following formula, which is known as the Vigants Barnett
Method.
Availability = 100
×
(1 – P)
P = 2.5
×
10
-9
×
C
×
F
×
D
3
×
10
(-FM/10)
Where F is the frequency in MHz
D is the distance in miles
FM is the fade margin in dB
C is the climate/terrain factor as defined below:
Humid/Over Water: C = 4 (worst case channel)
Average Conditions: C = 1
Dry/Mountains: C = 0.25 (best case channel)
Example: Assume 21 dB fade margin, over 5 miles with average climate/terrain. The availability
comes out to be 99.9986. This corresponds to the link being unavailable for 7.6 minutes per year.