User's Manual

MDS 05-2708A01, Rev. D MDS TransNET I&O Guide 7
These requirements can be quickly determined in most cases. A possible
exception is the last item—verifying that an unobstructed transmission path
exists. Radio signals travel primarily by line-of-sight, and obstructions
between the sending and receiving stations will affect system performance. If
you are not familiar with the effects of terrain and other obstructions on radio
transmission, the discussion below will provide helpful background.
Terrain and Signal Strength
While the 900 and 2400 MHz bands offer many advantages over VHF and
lower UHF frequencies for data transmission, they are more prone to signal
attenuation from obstructions such as terrain, foliage or buildings in the trans-
mission path.
A line-of-sight transmission path between the Master station and its associ-
ated Remote site(s) is highly desirable and provides the most reliable commu-
nications link. A line-of-sight path can often be achieved by mounting the
station antenna on a tower or other elevated structure that raises it to a level
sufficient to clear surrounding terrain and other obstructions.
The importance of a clear transmission path relates closely to the distance to
be covered by the system. If the system is to cover only a limited geographic
area, say up to 3 miles (4.8 km), then some obstructions in the communica-
tions path can usually be tolerated with minimal impact. For longer range
systems, any substantial obstruction in the communications path could
compromise the performance of the system, or block communications
entirely.
Much depends on the minimum signal strength that can be tolerated in a given
system. Fade margin for a TransNET system does not usually have to be as
conservative as a licensed system. The nature of TransNET’s
frequency-hopping algorithm reduces the impact of frequency-selective
fading.
Although the exact figure will differ from one system to another, a Received
Signal Strength Indication (RSSI) of –100 dBm will often provide acceptable
performance. A higher signal strength is generally desirable and may be
needed in special cases—for example, urban deployment.
Conducting a Site Survey
If you are in doubt about the suitability of the radio sites in your system, it is
best to evaluate them before a permanent installation is begun. This can be
done with an on-the-air test (preferred method), or indirectly, using
path-study software.
An on-the-air test is preferred because it allows you to see firsthand the
factors involved at an installation site and to directly observe the quality of
system operation. Even if a computer path study was conducted earlier, this
test should be done to verify the predicted results.
The test can be performed by first installing a radio and antenna at the
proposed Master station site and then visiting each Remote site with a trans-
ceiver and a hand-held antenna.