Wireless Valley Software Validation

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AWPR Project

Wireless Valley Software Validation

October 12, 2005


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Introduction

Before using the Wireless Valley software package to predict the in-building coverageprovided by various 802.11 antennas, the software must be validated in a number of

ways. First, the software should be used to model the propagation of an antennas signal

while in free space. This should be done to determine whether or not the softwarepackage gives an accurate representation of received signal strengths with respect to howfar the receiver is from the antenna. This type of test has been performed on two

different types of antennas within the Wireless Valley software package. First, an

isotropic antenna was tested and then a directional cardioid antenna was tested.

After the two types of antennas were tested in the software package, the 802.11

propagation throughout a building was tested. This process was started by first locating

two access points inside of the test building. Once these access points were located, thetransmission parameters for each of the access points were researched so that the access

points could be modeled in Wireless Valley. Next, the group took four signal

measurements from each access point at various locations throughout the building using a802.11 test device known as a Grasshopper. After this was completed, the access

points, along with all of their transmission parameters, were entered into the Wireless

Valley software package. Upon initial simulation of the signal propagation throughout

the building, the software returned a number of false values. Therefore, the software wasadjusted until the software predicted the signal strength at the measured locations within

5 dB of the actual measured value. Adjustments were made to all of the following

parameters within Wireless Valley: the propagation model, the loss of various

materials, and the loss caused by distance.

Procedure

Isotropic Antenna in Free Space

As a first step towards validating the software, a number of calculations were performed

regarding an isotropic antenna in free space. Here is the formula for an isotropic antennain free space:

tPr log10.4log(20Plog10r

+

=

For the simplicity of this example, we will assume that the transmission power of the

antenna is 1 mW which is equivalent to 0 dBm. This simplifies the equation to thefollowing:


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=

r.4log(20Plog10 r

Next, the formula should be used to find out the radius of an isotropic antenna at various

receive signal levels. By using the logarithmic rules the equation can be changed into the

equation seen below.

=

4*10 )20(Pr/r

The only unknown variable in this equation is the wavelength, or . Lambda can be

calculated by using the following formula:

mHzX

smX

f

C123102.0

10437.2

/1039

8

=

=

=

Note that the frequency used in this formula is 2.437, which is the frequency of channel 6

in 802.11b. Once is known, the equation can be simplified to:

=

4*10

123102.0)20(Pr/

mr

By using this simplified equation, various values of received power (Pr) can be entered tofind out the corresponding radius. For this system, it is ideal to know the distance of theradius between -50 dBm and -90 dBm. If these values are entered into the formula, the

results seen in Table 1can be obtained.

Table 1 - Calculated Isotropic Radius Values

Pr Value Resulting Isotropic Radius-50 dBm 3.0878 m

-60 dBm 9.796146 m

-70 dBm 30.978 m

-80 dBm 97.961459 m

-90 dBm 309.78133 m

Using the Wireless Valley software, the user needs to validate the software by

simulating an isotropic antenna in free space. To simulate a free space environment, theantenna should be placed a significant distance above the ground. This will help reduceany changes in the radiation pattern which may occur due to the reflection or absorption

caused by the ground plane. Free space should be simulated in Wireless Valley by

placing both the antenna and the receiver 50 feet above the ground and by placing nowalls within the building layout. After this has been completed, the antenna thepropagation from the antenna can be simulated and the contour patterns for various signal


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levels will appear. The distance tool can then be used to find out how much distance is

between the antenna and the received signal level contour pattern.

In Figure 1, one can see the simulated signal strength levels for -50 dBm (red) and -60

dBm (yellow). By using the distance tool in Wireless Valley, as seen in Figure 1, the

user can determine that the radius of the isotropic antenna is 2.8 m when at a receivedsignal strength of -50 dBm.

Figure 1 - Simulated Isotropic Radius at -50 dBm

Next, the user can use the distance tool to measure the isotropic radius at -60 dBm

(yellow). As seen in Figure 2, the isotropic radius is 9.63 meters at -60 dBm.


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This process can be continued for all received signal strength levels. Figure 3shows thatthe simulated isotropic radius at -70 dBm (green) is 30.67 meters.


Figure 4below show the that the Wireless Valley software predicts a 96.84 meterradius at -80 dBm (blue).


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The final simulated received signal level value, -90 dBm (purple), shows that the radius

of an isotropic antenna at that level should be 306.58 meters as seen in Figure 5.


The values for the calculated isotropic radius and the software predicted radius at all thereceived signal strength levels can be found in Table 2below.

Table 2 - Calculated and Software-Prediction Radii at Various Received Signal Levels

Pr Value Calculated Isotropic

Radius

Wireless Valley Predicted

Isotropic Radius

Percentage Difference

between calculated and

software-predicted values

-50 dBm 3.0878 m 2.80 m - 9.3 %

-60 dBm 9.796146 m 9.63 m - 1.7 %

-70 dBm 30.978 m 30.61 m - 1.19 %

-80 dBm 97.961459 m 96.84 m - 1.14 %

-90 dBm 309.78133 m 306.58 m - 1.03 %


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By looking at the table above, one can clearly see that the software accurately simulates

an antenna in free space.


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Cardioid Antenna in Free Space

Next, for further validation of the software, a directed antenna must be tested in thesoftware. For the purpose of this validation, a 10 dB cardioid antenna will be used. The

distance for the antennas propagation can be calculated by using the following formula:

rFSttr GLGPP ++=

For simplicity, the gain of the receiver (Gr) and the transmitted power (Pr) will be set to

0. The gain of the cardioid antenna (Gt) should be 10 dB. This will allow us to simplifythe formula into the following:

FSr LP = 10

The formula for free space loss (LFS) is:

6.96log20log20 ++= GHzmiFS fDL

Distance in miles is the value that we are looking for and the frequency in Ghz is equal to2.437 GHz. This allow the free space formula to be simplified into the following

formula:

337.104log20 += miFS DL

By substituting the formula for free space loss into the original formula, the following

formula can be formed:

)337.104log20(10 += mir DP

Since the distance in miles is the desired value, the formula can be rearranged to appear

similar to the following:

mir DP

=

+

20

337.94

By using this formula, various received power values can be entered in order to find therange of the cardioid antenna. After plugging in values between -50 and -90 dBm, Table

3could be formed.


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Table 3 - Calculated Cardioid Range Values

Pr Value Calculated Antenna Range-50 dBm 9.7678 m

-60 dBm 30.88865 m

-70 dBm 97.678 m

-80 dBm 308.8864 m-90 dBm 976.784

After the values for the Cardioid antenna have been calculated, the antenna can be

entered into the Wireless Valley software package. Using methods, similar to those

used when simulating the isotropic antenna, the distances for the Cardioid can be found.By looking at Figure 1, one can see that the cardioid range at -50 dBm (red) is 9.75

meters.

Figure 6 - Simulated Cardioid Range at -50 dBm

Next, at -60 dBm (yellow) the simulated distance is 30.48 meters as seen in Figure 7.


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Third, the user can determine the distance of the cardioid at -70 dBm (green). As seen in

Figure 8, the software predicts that the range at this received signal level is 97.3 meters.


Next, the user can use the software to predict that the range of the cardioid at -80 dBm(blue) is 305.33 meters. This can be seen in Figure 9.


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As the final step of the cardioid validation process, the user can determine that thesimulated range of the antenna at -90 dBm (purple) is 969.63 meters. This can be seen in

Figure 10.


After the range for -90 dBm has been simulated, the following table can be formed.

Pr Value Calculated Isotropic Radius Wireless Valley Predicted

Isotropic Radius

Difference between

calculated and

software-predicted

values

-50 dBm 9.7678 m 9.75 m - 1.822 %

-60 dBm 30.88865 m 30.48 m - 1.32 %-70 dBm 97.678 m 97.3 m - 0.387 %

-80 dBm 308.8864 m 305.33 m - 1.15 %

-90 dBm 976.784 969.63 m - 0.732 %Table 4 - Calculated and Software-Predicted Ranges at Various Received Signal Levels

Like the results seen in Table 2, the results in Table 4show that Wireless Valleyprovides a very accurate simulated propagation distance in free space.


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802.11 Propagation Test

In order to validate the 802.11 propagation throughout the building, the RSSI values

received from two access points were used. A calibration of the softwares buildingparameters to reflect the true properties of the Research Park building was completed also

using the RSSI values of the two existing access points. The access points were modeledin the Wireless Valley software and placed at their approximate location on the floor

plan.

Figure 11 - Modeled Base Stations and Their Coverage

The software was set through a series of dialog boxes to create a prediction of the RSSIvalues in respect to an access point at any particular location; about 20+ values per AP.

The values were recorded as the predicted RSSI values for each AP. The measured RSSI

values were recorded using the Grasshopper 2.4GHz WLAN Receiver. TheGrasshopper was taken to the same locations inside the building and the RSSI values

were measured. Comparison of the measured and recorded RSSI values showed that theRSSI values inside the building were close to each other but not close enough. Todecrease the difference between the values, the absorption rating of the inner walls, outer

walls, doors, and glass windows were changed using the Partition Library and the Edit

Partition Category (shown in the figure below). After changing and re-predicting theRSSI values a few times, the softwares building parameters were calibrated to reflect the

true properties of the Research Park building.


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Figure 12 - Edit Dialog Boxes for Partitions

It was found that the software does not predict reliable RSSI values outside of the

building. This is due to the software inability to model outdoors conditions. This is due

to the softwares assumption that anything outside of a buildings walls is theoreticallyfree space. Therefore, when the signal goes beyond the exterior walls of the building, the

only factors affecting the signal propagation are free space loss and the loss caused by the

ground plane. The tables and figures below display a subset of the predicted andmeasured values of two of the access points, Star Vision (SV) and AdvantGX (AGX).

Star Vision

RSSI (dBm)

Predicted Measured

-82.5 -85

-74.3 -73

-65.2 -62

-64.7 -69

Table 5 - Star Visions

Measured vs. PredictedRSSI values


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Figure 13 - Star Vision's Predicted RSSI Values

AGX

RSSI (dBm)

Predicted Measured

-80.4 -80

-62.9 -63

-49.6 -50

-43.2 -42

Table 6- AdvantGX's

Measured vs. Predicted

RSSI values

Figure 14 - AdvantGX's Predicted RSSI Values

Therefore, calibration of the Wireless Valley software was achieved by importing

existing access points into the software into the layout of the first floor of the Texas

A&M half of the building in Research Park. The software was then used to predict theRSSI values from the associated access points at different locations on the floor plan.

The Grasshopper 2.4GHz WLAN Receiverwas then used to measure the actual RSSIvalues from the associated access points. Software parameters were adjusted until the

measured and software predicted RSSI values had the smallest deviation between them.

Documents

Wireless Valley Software Validation