User's Manual

40 P330 Data Sheet / User Guide
DRAFT
There are several items to note. First, the received signal has serrated look. Every 5-10 meters the
signal strength diminishes. As the separation distance increases the spacing between minima
increases. At the same time the depth of the minima also increases. These serrations are destructive
cancellation due to the Fresnel effect.
Second, at some point the received signal strength will drop to the point that the receiver will not be
able to acquire. For these settings (RF Channel 5, 64 MHz PRF, preamble length of 64 and 6.8Mbps)
will link failed at -94dB. This level is indicated by the red line.
Finally, the -94dB value that defines the location of this red line will vary from unit to unit by a few
dB. A prudent system designer will incorporate additional margin to insure that all of the units will
meet the system operational requirement.
Fig. 6-1: Plot of received signal strength (First Path Power) vs separation distance (range in
meters)
Defining a maximum operating range is ambiguous. While it is clear that ranges were measured out to
100m, it is equally clear that no range measurements were possible between 60 and 75m. In fact, the
signal was marginal at 30 meters and again at 40 meters. Depending on the requirements of the
application, the correct specification for maximum operating range could be anything from 30-100
meters, perhaps less if an allowance for margin was added. The correct answer for most applications
is probably 65 meters, but users who need a higher quality of service would argue for 30 meters.
It is also possible to mitigate the Fresnel effects at the cost of increasing the complexity of the overall
system. For example, assume it is necessary to monitor the location of many units as they move
through a fixed area. Furthermore, assume that the location of the units is determined by measuring
their distance to a set of fixed references mounted on the walls of the area. Since the Fresnel effects
are largely due to geometry, their effects can be mitigated in several ways. First, the infrastructure of
fixed references could be doubled such that all units would have at least three references which were
not at ranges that corresponded to Fresnel minima. Second, the fixed infrastructure could be outfitted
with multiple antennas each a different height. This would change the geometry such that one
antenna would be affected by the a Fresnel while the second could be counted on to have an
acceptable signal.
It becomes more difficult to define a maximum operating range if the units in the link are also
allowed to vary their height. In this a case, the location of the Fresnel nulls will move quite
dramatically. Suppose you were designing a system for tracking the location of miners in an open
area. Suppose further that the location tag was mounted on the miner’s helmet. In this case, the
location of the Fresnel nulls would be considerably different between tall miners and short ones. The
Fresnel nulls would change if any miner sat down or climbed on a tall vehicle.
While this can get complicated, the goal of this section is not complication. The objective is to
illustrate this phenomenon in a way that a system engineer can understand the issue and work to
define an appropriate solution. One approach is to require that the operating distances to limited to a