User Manual Part 2

Lynx.GX Installation and Management
Clearing less than 60% of the 1
st
Fresnel zone will result in excess signal loss due to diffraction, in addition to
the calculated free-space loss.
Excessive antenna height resulting in clearance of the 2
nd
and higher order Fresnel zones sets up the likelihood
of multi-path reflection outages. The higher the number of the “cleared” Fresnel zones, the more likely that a
system multi-path reflection outage will occur when atmospheric refractivity changes.
AVAILABILITY
Availability of the microwave path is a prediction of the percent of time that the link operates without producing
an excessive bit error rate (BER) due to atmospheric fading only. The calculated availability number does not
include outages caused by multipath reflections off of the terrain surface. With proper path clearance, and in the
absence of direct interference, availability is affected by the following:
▪ Path length
▪ Fade margin
▪ Frequency
▪ Terrain (smooth, average, mountainous)
▪ Climate (dry, temperate, humid)
Depending upon the type of information carried over the link and the overall network design redundancy, you
may want to design for a specific availability rate. For example, if the data or voice traffic carried by the radio is
critical, the link can be designed for a very high availability rate (such as 99.999% or 5.3 minutes of predicted
outage per year).
You can increase the fade margin to improve availability either by making the path shorter or by using higher
gain antennas in conjunction with lower loss transmission line (using a higher quality transmission line,
shortening the length, or both). Mounting the RFU near the antenna (thereby shortening the transmission line)
is one means to assist in increasing fade margin.
FADE MARGIN
The fade margin is the difference between the actual received signal and the radio's threshold. Using the
formula provided in the previous section, you can calculate the anticipated RSL. Compare this RSL to the
specified threshold of the radio, and calculate the fade margin as the difference between the two signal levels.
Proxim Corporation recommends that you design your link to your desired availability standard, as discussed in
“Calculating Availability” above. However, independent of the availability standard, the following guidelines are
recommended for minimum fade:
▪ Greater than or equal to 15 dB for all paths, whenever possible, and always for path lengths greater than
two miles (3.2 kilometers).
▪ No less than 10 dB for any path length (this is not recommended, but can provide adequate performance if
the path length is very short—such as less than two miles (3.2 kilometers) over non-reflective terrain and in
non-refractive atmospheric conditions).
Appendix A. Installation Planning 47