Guardian Manual 001-5006-000 Rev 0
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P a g e
The signal strength (RSSI) at the receiver must exceed the receiver sensitivity by an amount known as the fade
margin to provide reliable operation under various conditions. Fade margin (expressed in dB) is the maximum
tolerable reduction in received signal strength, which still provides an acceptable signal quality. This compensates
for reduced signal strength due to multi-path, slight antenna movement or changing atmospheric losses. CalAmp
recommends a 30 dB fade margin for most projects.
Table 2-3
shows the RSSI versus Reliability.
Table 2-3
– RSSI Reliability
RSSI
Reliability
-100 dBm
Approximately 50% reliability. Fading may cause frequent data loss.
-90 dBm
Approximately 90% reliability. Fading will cause occasional data loss.
-80 dBm
Approximately 99% reliability. Reasonable tolerance to most fading.
-70 dBm
Approximately 99.9% reliability with high tolerance to fading.
2.6
RADIO INTERFERENCE
Interference is possible in any radio system. However, since the Guardian is designed for use in a licensed system,
interference is less likely because geographic location and existing operating frequencies are normally taken into
account when allocating frequencies.
The risk of interference can be further reduced through prudent system design and configuration. Allow adequate
separation between frequencies and radio systems. Keep the following points in mind when setting up your radio
system.
a.
Systems installed in lightly populated areas are least likely to encounter interference, while those in urban
and suburban areas are more likely to be affected by other devices.
b.
Directional antennas should be used at the remote end of the link. They confine the transmission and
reception pattern to a comparatively narrow beam, which minimizes interference to and from stations
located outside the pattern.
c.
If interference is suspected from another system, it may be helpful to use antenna polarization opposite
to the interfering system’s antennas. An additional 20 dB (or more) of attenuation to interference can be
achieved by using opposite antenna polarization.
d.
Check with your CalAmp sales representative or CalAmp Technical Services for additional options. The
Technical Services group has qualified personnel to help resolve your RF issues.
