User's Guide

3 . I n sta lla t ion 9

3 .6 .3 .2 Fa de Margin Ca lculat ion

The fade m argin is t he difference between the act ual received signal and the Event- HD

digital radio’s t hreshold for the m odulat ion m ode selected. The fade margin can be used

to det erm ine availabilit y and should be at least 10 dB for m ost cases but is ultim at ely

det erm ined by required application reliability.

3 .6 .3 .3 Ava ilability Calculat ion

Availability of the m icrowave path is a predict ion of the percent of tim e that the link will

operat e without producing an excessive BER due t o m ultipath fading. Availability is

affect ed by the following:

• Path length

• Fade m argin

• Frequency

• Terrain (sm ooth, average, m ountainous, valleys)

• Clim ate ( dry, tem perat e, hot, hum id)

Depending on the t ype of t raffic carried over t he link and t he overall net work design

redundancy, fade m argin should be included t o support t he desired availabilit y rat e.

Critical data and voice m ay require a very high availability rate ( 99.999% or 5.3 m inut es

of predict ed outage per year) . To im prove availabilit y, the fade m argin can be increased

by shortening the pat h lengt h, t ransm itt ing at a higher power level, or by using higher

gain ant ennas.

Availability can be com put ed using t he follow ing form ula, which is known as t he Vigants

Barnett Method.

Availability = 100 × (1 – P)

P = 2.5 × 10

- 9

× C × F × D

× 10

( - FM/ 10)

Where F is t he frequency in MHz

D is t he dist ance in m iles

FM is t he fade m argin in dB

C is the clim ate/ t errain fact or as defined below:

Hum id/ Over Water: C = 4 (worst case channel)

Average Conditions: C = 1

Dry/ Mountains: C = 0.25 ( best case channel)

Exam ple: Assum e 21 dB fade m argin, over 5 m iles with average clim ate/ t errain. The

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

7.6 m inutes per year.