AvMesh
®
RF Communications Systems
Installation & Service Manual
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15
The radius of the Fresnel zone increases as the distance between the transmitter and
receiver increases. Because of this, the curvature of the Earth can potentially impeach the
Fresnel zone and therefore cause signal loss.
In order to optimize the signal strength at the receiver, it is best to minimize the number of
out of phase signals reaching the receiving antenna by ensuring that the strongest signals
(i.e. in the first Fresnel zone) do not come into contact with obstacles. As a general rule of
thumb, the 1st Fresnel zone (the area from the line of sight to the outer boundaries of the
1st Fresnel zone) must be at least 60% clear in order to maintain a satisfactory connection.
The AvMesh
TM
Communications System operates in the unlicensed 2.4GHz ISM band.
Because of their cost-effectiveness, easy set-up and easy maintenance, unlicensed bands
are a popular choice for many other systems as well. Consequently, this makes them
susceptible to interference from other users operating on the same ISM band in the same
area, giving rise to in-band interference. This has the effect of raising the noise floor which
has the potential to mask the desired signal.
In wireless mesh networks, in-band interference is usually caused by the many nodes
operating within the mesh. Because the transmitted signals are very similar to each other,
it is highly likely that they will interfere with each other and affect the receivers’ ability to
receive the intended signal. The AvMesh
TM
Communications System mitigates this issue by
allowing the nodes to wait a predetermined amount of time before they transmit. As the
signal propagates down the runway, each node delays repeating the message by 0.6-3.2
seconds so that the nodes are not repeating over the top of one another and interfering
with one another.
3.6.2 In-band interference
