User's Manual

MDS 05-2708A01, Rev. A MDS TransNET I/O Guide 9
A line-of-sight transmission path between the master station and its associated
remote site
(
s
)
is highly desirable and provides the most reliable
communications 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) for the MDS TransNET 900™, then some
obstructions in the transmission path can usually be tolerated with minimal
impact. For longer range systems, any substantial obstruction in the
transmission path could compromise the performance of the system, or block
transmission entirely.
Much depends on the minimum signal strength that can be tolerated in a given
system. Although the exact figure will differ from one system to another, a
Received Signal Strength Indication (RSSI) of –90 dBm or stronger will
provide acceptable performance in many systems. While the equipment will
work at lower signal strengths, this provides a “fade margin” to account for
variations in signal strength which may occur from time-to-time.
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 transceiver and a
hand-held antenna. (An RTU simulator—MDS Part No. 03-2512A01—can be
connected to each radio in the network to simulate data during this test.)
With the hand-held antenna positioned near the proposed mounting spot, a
technician can check for synchronization with the master station (shown by a
lit
SYNC
lamp on the front panel) and measure the reported RSSI value. If
adequate signal strength cannot be obtained, it may be necessary to mount the
station antennas higher, use higher gain antennas, or select a different site. To
prepare the equipment for an on-the-air test, follow the general installation
procedures given in this guide and become familiar with the operating
instructions given in Section 6.0, beginning on Page 20.
If time is short, and a site survey is impractical, a computer path study is a good
alternative. Factors such as terrain, distance, transmitter power, receiver
sensitivity, and other conditions are taken into account to predict the
performance of a proposed system. Contact MDS for more information on path
study services.