Terrain And Signal Strength; Conducting A Site Survey - Microwave Data Systems MDS 9810 Installation And Operation Manual

Terrain and Signal Strength

While the 900 MHz band offers many advantages over VHF and lower
UHF frequencies for data transmission, they are also more prone to
signal attenuation from obstructions such as terrain, foliage or buildings
in the transmission path.
A line-of-sight transmission path between the master station and its
associated remote site s is highly desirable and provides the most reli-
( )
able 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 dis-
tance 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 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 trans-
mission 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. RSSI can be measured with a Hand-Held Terminal
connected to the remote radio's connector. (See Section
DIAG(NOSTICS)
6.0, beginning on Page 28.)

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 ear-
lier, 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 sim-
ulate data during this test.)
MDS 05-3301A01, Rev. C MDS 9810 Installation and Operation Guide 9