User's Manual

3. Pre-Installation Procedures 3-4

Where:

is the transmitter output power (in dBm)

TX ANT

is the gain of the transmit antenna (in dB)

RX ANT

is the gain of the receive antenna (in dB)

Path

is the Path loss, defined by: L

(dB) = 36.6 + 20log

(F*D)

Where:

F is the Frequency in MHz, D is the Distance of path in miles

This link budget is very important in determining any potential problems during

installation. The expected RSL and measured RSL should be close (+/- 5 to 10 dB)

3.2.2 Fade Margin Calculation

The fade margin is the difference between the actual received signal and the 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 for most cases but is ultimately

determined by required application reliability.

3.2.3 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.

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: C=4 for Humid/Over Water (worst case channel), C=1 for

Average Conditions, or C=0.25 for Dry/Mountains (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.