- Does
the Fox demodulate paging traffic (FLEX and POCSAG)?
- Yes.
Check out all the supported frequencies and specs above.
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- Can
I export Fox data into my favorite mapping program?
- Yes.
Fox supports a variety of post-processing software.
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- How
do I get the data into my PC?
- Use
the
Fox Data Logger software
to download the data off the PCMCIA storage card included with your
Fox.
What
is the significance of Rayleigh Fading in coverage measurements
with 40 Lambda criteria?
There
are two types of fading that are relevant to performing coverage
measurements, fast fading, also know as Rayleigh fading, and terrain
based fading due to
obstructions and propagation loss. The goal of coverage measurement
is to measure the local signal strength in presence of terrain
based fading.
Rayleigh fading is due to close in reflections. It produces drops in signal
strength that are only a fraction of a wavelength in size (often on the scale
of inches). These fades always exist and are only of interest in the sense
that coverage measurements must be made in a manner that rejects Rayleigh fading,
so that accurate measurements of terrain based fading can be made.
William C. Y. Lee derived a well accepted criteria for removing Rayleigh fading
and retaining slower terrain based fading, known as 40 Lambda averaging [W.C.
Y. Lee and Y. S. Yeh, “On the Estimation of the Second-Order Statistics of
Log Normal Fading in Mobile Radio Environment”, IEEE Trans. Comm., vol. 22,
June 1974, pp. 869-873] (You can find an easier to read version of this criteria
in most mobile communications text books). In order to remove Rayleigh fading,
you should average data for a time period equal to the time it takes to traverse
40 wavelengths in your measurement vehicle and you should have no less than
36 to 50 points in that time. For the remainder of this text we will assume
40 points to make the calculations easier.
The biggest misconception with applying the 40-Lambda criteria to coverage
measurements is that all measurements should be made with an averaging time
exactly equal to 40-Lambda. While 40-Lambda is the minimum filtering that needs
to be done to reject Rayleigh, it is not the most appropriate averaging time
for coverage analysis. In almost all coverage analysis, the 40-Lambda averaged
samples must be averaged further. Due to the accuracy and reporting rate of
position measurement devices (GPS for instance) and the real practical problem
of too much data this extra averaging is valid and necessary. Clearly, the
averages of averages is just a longer average, and the resulting data no longer
has an integration time of forty wavelengths.
That is not to say that the 40-Lambda criteria does not apply to coverage analysis.
In fact it is very important. From the criteria, the minimum sampling rate
is about once per wavelength at the fastest vehicle speed. At 800MHz, for example,
a wavelength is about a foot. Using the 40-Lambda criteria, an initial sampling
rate of 88 samples per second is needed to make measurements with a maximum
drive speed of 60 mile per hour.
It is important to realize that Rayleigh fading is a spatial phenomenon not
a time one. The averaging is actually intended to be done over a distance of
forty wavelengths. Most people performing coverage analysis average samples
spatially after they are collected. That is to say, all the samples within
a grid on a map are averaged to produce one point. To insure rejection of Rayleigh
fading, the grid size must be larger than forty wavelengths. This in general
is not a problem. At 800MHz, forty wavelengths is about 40 feet. Generally
much larger grid sizes are used.
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A Practical Example
A BVS Fox receiver and Gator transmitter are going to be used to do pre-site
survey at 800MHz. The Gator transmitter is hoisted up on a crane and the drivers
are sent out to perform the drive study.
The Fox receiver will be set to single channel mode with 4096 ADC samples.
This produces 1 report a second that is the average of 4096 samples. Since
we want to have at least one sample per wavelength, as per 40-Lambda, the maximum
speed would be 1ft*4096/sec or 2792 miles per hour! Obviously, the Fox can
be used for coverage analysis at any speed.
Each report is the average of all data received during that second, so we still
have an initial sampling rate of 4096 samples per second. This setting is the
most convenient since the internal GPS receiver in the Fox reports position
once per second. At this point we have statistically correct data samples each
with a position stamp.
The data should be spatially averaged in post processing. Since forty wavelengths
is only forty feet at this frequency, almost any convenient grid size will
do. Remember that the accuracy of GPS, with selective addition off, is only
about 30 meters, so grid sizes larger than that would be appropriate. In expensive
post processing software the grid size can be selected. When processing data
yourself, it is often convenient to make the grid based on Latitude and Longitude
from the GPS receiver. In the New York area, one degree of latitude is 69 miles
and one degree of longitude is 50 miles. Therefore, averaging together data
with the first three decimal places of the latitude and longitude the same
would be an easy way of getting an appropriate grid size. The smallest side
of that grid would be 264 ft.
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