User Manual

VoiceXtender Modem Installer’s Guide

System Planning

2-4 Rev. A - 8/97

If a complete path analysis is necessary, Cylink Customer Support can help

with the calculation as long as exact coordinates of the location are known (U.

S. only).

Determining the Fade Margin

To achieve reliable communication, the radio link must have an average

received signal level high enough to protect the link against fluctuations in the

signal power due to multipath fading and other anomalous propagation

conditions. This safety factor is referred to as the fade margin.

The fade margin is a measure of how much additional signal attenuation the

system can endure without dropping below the required Bit Error Rate (BER)

level. A fade margin of 15 dB or more is sufficient in most situations.

Use the following formula to calculate the fade margin (see Figure 2-3):

Fade Margin = G

+ G

ANT

- L

where G

is the total system gain (measured in dB), G

ANT

is the total antenna

gain of both antennas (measured in dBi), L

is the total connector/cable loss of

all cables (measured in dB), and L

is the path loss (measured in dB). These

are described in more detail below.

The reliability of the link is related to the fade margin and is stated as

Availability (in percent) or annual outage time (in minutes).

Figure 2-3

Fade Margin Calculation