User Manual

User Reference and Installation Manual 3-11

MDS 5800 II

3.5.3.3 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

× 10

(-FM/10)

Where F is the frequency in MHz (5300 or 5800)

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, at 5.8 GHz. The

availability comes out to be 99.9986. This corresponds to the link being unavailable for 7.6

minutes per year.

3.5.3.4 Frequency Plan Determination

When configuring MDS 5800 II units in a point-to-point or consecutive point configuration, careful

engineering of the MDS 5800 II frequency plans and antenna locations should be performed in

order to minimize potential interference between nearby radios. Nearby radios should operate on

different frequencies, transmitting in the same band (high side or low side). When designing multi-

radio configurations, antenna size, antenna polarization, and antenna location are critical.