Set Up Link | Define Interferers | Set Up CommSystem | Interference Analysis | Interference Constraint
This brief tutorial is designed to introduce the CommSystem object, its setup and use in the analysis of RF interference phenomena.
Note: For this exercise you will need a valid license of STK Professional Edition and STK/Communications.
In this exercise you will:
Start STK and select Create a New Scenario. Set the Analysis Period from 1 Jul 2004 00:00:00.00 UTCG to 1 Jul 2004 02:00:00.00 UTCG. Click OK to dismiss the New Scenario Wizard.
Note: From this point forward, the instruction to "click OK" to capture a change made on a property page or dialog is implied. In general, if you simply close a page or dialog (e.g., by clicking Cancel or Close without having clicked Apply), all changes will be lost.
Add a Facility to the scenario and set its Latitude, Longitude, and Altitude to zero.
Attach a Receiver to the facility and name it GroundRcvr. Open its Definition page and select Complex Receiver Model as the Type. Set the LNA Gain to 30 dB. Click the Antenna tab and the Antenna's Orientation tab, and set the Azimuth of the antenna to 90 deg and the Elevation to 30 deg. Then click the Antenna Model Specs tab and decrease the Diameter of the antenna to 0.25 m.
Create a Satellite. Use the Orbit Wizard to define a Geosynchronous orbit, with Subsatellite Point set to 30 deg and other values left at their defaults. Name the satellite GeoSat.
Attach a Transmitter to the satellite and name it GeoXmtr. Open its Definition page, select Medium Transmitter Model as the Model Type. On the Model Specs tab, set the Frequency to 1.5 GHz and the Gain to 30 dB.
Use the Orbit Wizard to create a Satellite with a Circular orbit and an Altitude of 3500 km (leave other parameters at their default values). Name it LeoSat.
Select the GeoXmtr transmitter, and then select Copy from the Edit menu. Then select the LeoSat satellite and select Paste from the Edit menu. Rename the new transmitter LeoXmtr.
Tip: This is a quick way to create a new object having identical properties to an object already existing in the scenario.
Right-click the LeoSat satellite in the Object Browser. Select Satellite -> Walker.... Construct a constellation with 6 planes having 6 satellites per plane. Check Create Constellation and use the name LeoGroup. Click Create Walker before closing.
Open the GlobalAttributes page of the scenario’s 2D Graphics properties and clear the Show Ground Tracks / Routes option. This unclutters the 2D Graphics (Map) window display.
To set up a CommSystem object, you must first organize the relevant communications assets into three groups:
This grouping is accomplished with the help of the STK Constellation object.
Add a Constellation to the scenario and name it GeoXmtr. Open the Definition page of the constellation’s Basic properties, and add the GeoXmtr transmitter to the Assigned Objects area.
Create another constellation, name it GroundRcvr, open its Definition page, and add the GroundRcvr receiver to its Assigned Objects list.
Create yet another constellation, name it Interferers, and open its Definition page. Use the Selection filter to select all available transmitters.
Deselect the GeoXmtr transmitter, leaving only the LeoXmtr transmitters that are attached to satellites in the LEO group. Click the right arrow to move these to the Assigned Objects list.
Tip: To deselect an item in a list of previously selected items, hold down the Ctrl key and click the item to be deselected.
Now add a CommSystem object to the scenario. Open the Transmit page of the CommSystem’s Basic properties, and move the GeoXmtr constellation to the Selected Constellations list.
Use the same procedure to select the GroundRcvr constellation on the Receive page and the Interferers constellation on the IF Sources page. Open the Interval page of the CommSystem's Basic properties and set the step size to 60 secs.
Tip: It is a good idea at this stage to save the scenario.
Select the CommSystem in the Object Browser, and select Compute Data from the CommSystem menu.
Note: Interference computation may take several seconds.
Animate the scenario and observe the interference graphics display in the 2D and 3D Graphics windows.
Select the CommSystem object again, and right-click the CommSystem and select Report & Graph Manager. Create a Link Information Detailed report, with GroundRcvr as the Assigned Object.
In the report, compare values for link performance indicators with and without the effect of interference, for example: BER (without interference) and BER+I (with interference). Keep this report open.
Interference constraints for a receiver or transmitter allow you to take interference effects into account in setting minimum performance criteria for a communications link. Open the Interference page of the GeoXmtr transmitter’s Constraints properties and set the Min value for C/No+Io to -3 dB*MHz.
In other words, we are only interested in time intervals in which the communications link has a C/No value of at least -3 dB*MHz, taking the effects of interference into consideration.
Now return to the Link Information report you created in the last section, and select Refresh from the Report menu. Scroll to the C/(No+Io) column and note that no value less than -3 appears there. Scroll back to the Time column, and you will find gaps in the time entries, reflecting the fact that times when C/(No+Io) was less than -3 have been excluded.