Interference Calculations with TAP™ Composite Study
Q: How do I compute an interference study in TAP?
A: The Composite Study function in TAP can be used to evaluate the combination of signals from different sources. This can include simulcast or best-server studies when all of the signal sources are a desired signal on your system. If one or more signals are undesired potential interfering sources, you can also use the Composite function to determine the effects of the undesired interfering signals.
Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.
For example, suppose you have a base station with a desired signal at Castle Rock. Using HDCoverage you can compute the coverage area as shown:
By default, the areas where the computed field strength meets or exceeds the required field strength specified for the study are shown in green. Areas in red indicate insufficient signal strength based on the study parameters. (The colors and field strength levels displayed can be changed as desired.)
Now suppose that there is a potential interfering station at Cheyenne Mountain (about 40 miles to the south). You can run a study to compute the field strength in the same area resulting from the Cheyenne Mountain interfering signal:
As would be expected, the signal levels from that more distant site are significantly lower than the desired signal from the Castle Rock site
To see the effect of the undesired Cheyenne Mountain signal on the desired Castle Rock coverage, select the Composite Calculation function from the TAP6 Area menu:
Click the “Create a new study” button to create a new TAP Task:
A new Task ID will be assigned. Enter a description for the Composite Study you want to run.
Be sure the AutoDraw box is checked if you want the study map to be displayed in HDMapper automatically.
If the “Studies to Include” section is collapsed, click the heading to expand the section. Then click the check-box by each of the coverage studies you want to include in the Composite interference study.
This example only includes two area coverage calculations – one for the desired signal, one for the undesired signal. You can use the Composite Study function to include as many studies as needed for your application.
Click the Composite Area header to expand that section. Define the latitude and longitude limits of the rectangular area you want to use for the Composite study. For this example, click the “Get Limits from studies” button to compute the coordinates.
In this example, the two studies for the individual signals were computed for the identical area. It is also possible to include coverage studies that are not exactly congruent but include some partial overlap of the coverage areas. In that case you might want to define just the area of overlap and enter the coordinates manually. Or you can still use the “Get Limits from studies” button to define the bounding area for all the studies involved. The “Use Boundary from Shapefile” checkbox enables you to select a polygon object from a shapefile (such as your county boundary) and define the composite area as that shape.
Click the Composite Operation header to expand that section. In the pulldown list, select “Best Server” option. This type of Composite study enables you to set two parameters: The minimum required field for the desired signal, and the Capture Margin:
The Minimum Field value is the computed Required Field for the desired signal. If the desired signal is below this value, no service is provided. You can enter a field strength value manually, or you can double-click the text box to open the Mobile Facility database to lookup the required field value:
When you double-click the Minimum Field box, the Mobile Facility interface is displayed. Navigate to the record for the mobile configuration you want to use. The Required Field value is displayed in the Receiver Antenna section:
Click the Continue button in the Mobile Facility interface to close the form. The Required Field value is displayed for the Composite study:
The Capture Margin is the value required for the mobile receiver to discriminate between the desired and undesired signals. This value should be determined based on your equipment configuration, manufacturer’s specifications, channel separation from undesired signals, etc. The value used in this example is only for the purpose of this illustration. You must determine the appropriate value for your application.
The Capture Margin determines if the desired signal at each location is high enough above the highest undesired signal at that location for the mobile units to discriminate between the two.
Mobile service at each location requires that:
The desired signal meets or exceeds the Minimum Field signal level.
The desired signal meets or exceeds the Capture Margin above the highest undesired signal at that location.
Click the Save button to save the Composite Study parameters.
Click the Run button to start the study.
When the study is completed, if the AutoDraw box was checked in the study setup the results will be displayed in HDMapper:
The legend shows the four conditions resulting from this sample study:
Spec Not Met: in this example, the dark red areas show the locations where the Capture Margin was not achieved. Although the desired signal was adequately high, the undesired signal was high enough to cause interference and make the desired signal unusable.
Below 35.84 dBu: all of the signals (desired and undesired) were below the Minimum Required Field at the location, so no service is possible.
Castle Rock Interference Test – Castle Rock: these are areas where the desired signal meets both the Minimum Field requirement and the Capture Margin specification and the desired service is provided.
Castle Rock Interference Test – Cheyenne Mtn: these are areas where the undesired signal meets both the Minimum Field and Capture Margin requirements. In these areas the undesired signal may actually be usable, while the desired signal is not (depending on actual channel separation, etc.).
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