User's Manual

In-service commissioning | 170
Checking the fade margin
The fade margin is affected by many components in the system and is closely related to the received
signal strength. A link operating with a lower than expected fade margin is more likely to suffer from
degraded performance during fading conditions. A reduced fade margin can be due to operating the
link too close to the noise floor, or the presence of external interference. The fade margin of a link can
be asymmetrical (that is, different at each end).
Possible causes of low fade margin are as follows:
Problem Terminal
Low receive signal strength (see above table) Local and Remote
Interfering signals on the same, or very close to, the frequency of the
local terminal receiver.
Local
Intermodulation products that land on the same or very close to the
frequency of the local terminal receiver.
Local or Remote
Operating near the local receiver noise floor Local
To check the fade margin:
1. Confirm (and correct if necessary) the receive input level (see the previous test).
Note: If the receive input level is lower than expected, the fade margin may also be low.
2. Select Link > Performance > Summary and check the current BER of the link in its normal
condition is better than 10
-6
(If necessary, clear out any extraneous errors by clicking Reset
Counters).
3. Check the signal to noise (S/N) indication on the Link > Performance > Summary page. This
shows the quality of the signal as it is being processed in the modem. It should typically be better
than 30 dB. If it is less than 25 dB, it means that either the RSSI is very low or in-band interference
is degrading the S/N performance.
4. Temporarily reduce the remote site's transmit power using either an external attenuator or
SuperVisor (Remote > Terminal > Basic).
Note: Ideally, the transmit power of the remote site should be reduced by up to 20 dB, which will
require the use of an external 50 ohm coaxial attenuator capable of handling the transmit power
involved. In the absence of an attenuator, reduce the transmit power using SuperVisor.
5. Check and note the current BER of the link in its now faded condition (Again, if necessary, clear
out any extraneous errors (introduced by the power reduction step above) by clicking Reset
Counters).
6. Compare the unfaded and faded BER performance of the link (steps 2 and 4). Continue to reduce
the remote transmit power until either the BER drops to 10
-6
or the remote transmitter power has
been reduced by 20 dB.
Note: The fade margin of the link is expressed as a number (of dB) that the link can be faded
(transmitter power reduced) without reducing the BER below operating specifications (1 * 10
-6
BER). A 20 dB fade margin is adequate for most links.