Part 11: Introduction to Communications

STK Pro, STK Premium (Air), STK Premium (Space), or STK Enterprise
You can obtain the necessary licenses for this tutorial by contacting AGI Support at support@agi.com or 1-800-924-7244.

The results of the tutorial may vary depending on the user settings and data enabled (online operations, terrain server, dynamic Earth data, etc.). It is acceptable to have different results.

Capabilities covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Communications

Problem statement

Engineers and operators require a fast and easy way to set up, analyze, and optimize communication systems prior to employing them in the field. They want to simulate transmitters and receivers for a link budget analysis.

Solution

Use the STK® Communications capability and the host of analysis tools it provides to simulate how transmitters and receivers work in the field.

What you will learn

Upon completion of this tutorial, you will understand:

  • How to open and properly save a starter scenario visual data file (VDF).
  • How to use Transmitter and Receiver objects in the STK application.
  • How to generate a Link Budget report for your analysis.

Video guidance

Watch the following video. Then follow the steps below, which incorporate the systems and missions you work on (sample inputs provided).

Using the starter scenario (*.vdf file)

To speed things up and enable you to focus on this lesson's main goal, you will use a partially created scenario. The partially created scenario is saved as a visual data file (VDF) in your STK install.

Retrieving the starter scenario

  1. Launch STK ().
  2. Click Open a Scenario () in the Welcome to STK dialog box.
  3. Go to <STK install folder>\Data\Resources\stktraining\VDFs\.
  4. Select STK_Communications.vdf.
  5. Click Open.

Visual data files versus Scenario files

You must make sure that you save your work in STK as a scenario file (.sc) and not a visual data file (.vdf) by selecting Save As from the STK File menu. A VDF is a compressed version of an STK scenario, which makes them great for sending your work in STK to others. However, you want to use a scenario file while working with STK on your machine.

If you open a VDF file, STK keeps it as a VDF and does not automatically convert it to a scenario file. This means, STK does not change the file type of your scenario when you launch your scenario. You need to convert the VDF to a Scenario file using Save As.

Saving a VDF file as a Scenario file

Use Save As from the STK File menu to convert the VDF file that you opened into a scenario file.

  1. Open the File menu.
  2. Select Save As.
  3. Select the STK User folder on the left side of the Save As window.
  4. Click New Folder.
  5. Rename New Folder to match the title of the scenario.
  6. Open the folder you just created.
  7. Enter the name of the folder into File name field. This is the Scenario object's name.
  8. Open the Save as type drop-down menu.
  9. Select Scenario Files (*.sc).
  10. Click Save.

Analyzing a communications system

In this scenario, you will analyze communications between a geosynchronous satellite transmitter and a communications ground site receiver.

Selecting the relevant scenario objects

The starter scenario you loaded contains most of the objects required for your analysis. You will only use a portion of the available objects in the Object Browser in this tutorial, not all of them. There are extra objects because you can use this same scenario to complete other focused, feature-specific lessons about STK Communications.

  1. Select the checkbox for the following objects in the Object Browser:
    • GEO_Sat_West ()
    • Communication_Site ()
  2. Click Save ().

Save () often during this lesson!

Understanding the Communications capability

The STK Communications capability simulates the performance of communications systems in the context of their missions. With Communications, you can model all the physical components of a system including the radio frequency (RF) environment, assess the impacts of a wide variety of conditions and interference, conduct comprehensive link budget analyses, create reports of the results, and visualize them with 2D and 3D graphics.

Modeling a Simple Transmitter

Simulate the geosynchronous satellite's transmitter using a Transmitter object employing a Simple Transmitter model. The Simple Transmitter model is convenient when you do not have all the information necessary to model the transmitter in detail (such as during the system engineering process). The Simple Transmitter model uses an isotropic, omnidirectional antenna, which is an ideal spherical pattern antenna with constant gain.

Using a Simple Transmitter model, you can set up the RF carrier frequency, the EIRP, and data rate of the transmitter.

  • EIRP is the effective isotropic radiated power at the output of the transmit antenna. EIRP is expressed as the product of the power of the transmitting antenna and its gain.
  • Data rate is a compound dimension with data bits and time as simple dimensions.

Inserting a Transmitter object

Insert a Transmitter () object and attach it to GEO_Sat_West ().

  1. Select Transmitter () in the Insert STK Objects Tool.
  2. Select Insert Default () as the method.
  3. Click Insert...
  4. Select GEO_Sat_West () in the Select Object dialog.
  5. Click OK.
  6. Right-click on Transmitter1 () in the Object Browser.
  7. Select Rename.
  8. Rename Transmitter1 () to Downlink_Tx.

Configuring the Simple Transmitter model

Set up the RF carrier frequency, the EIRP, and data rate of the transmitter.

  1. Right-click on Downlink_Tx () in the Object Browser.
  2. Select Properties ().
  3. Select the Basic - Definition page when the Properties Browser opens.
  4. Look at the Transmitter Model field. Simple Transmitter Model is the default model.
  5. Select the Model Specs tab.
  6. Enter the following options:
    7. Option Value
    Frequency 5 GHz
    EIRP 5 kW
    Data Rate 1 Mb/sec
  7. Click Apply to accept your changes and to keep the Properties Browser open.

Adding a modulator to the transmitter

Communications allows you to select from multiple modulators, including user-defined modulators. Bi-phase shift keying (BPSK) is the default modulator in the STK application.

Change that to quadrature phase shift keying (QPSK). QPSK converts digital bits into pairs; this decreases the data bit rate to half, which allows space for other users on the same channel.

  1. Select the Modulator tab.
  2. Open the Name dropdown list.
  3. Select QPSK.
  4. Click OK to accept your changes and to close the Properties Browser.

Modeling a Simple Receiver

Simulate the ground communications site's receiver using a Receiver object employing a Simple Receiver model. The Simple Receiver model is convenient when you do not have all the information necessary to model the receiver in detail (such as during the system engineering process).

The Simple Receiver model uses an isotropic antenna which you cannot change. Set G/T (gain divided by the system noise temperature in kelvins) which expresses the performance of an entire receiver system.

Inserting a Receiver object

Insert a Receiver () object and attach it to Communication_Site ().

  1. Insert a Receiver () object using the Insert Default () method.
  2. Select Communication_Site () in the Select Object dialog.
  3. Click OK.
  4. Rename Receiver1 () to Downlink_Rx.

Configuring the Simple Receiver model

Set the G/T to express the performance of the entire receiver system.

  1. Open Downlink_Rx's () Properties ().
  2. Select the Basic - Definition page.
  3. Look at the Receiver Model field. Simple Receiver Model is the default model.
  4. Select the Model Specs tab.
  5. Enter 6 dB/K in the G/T field.

    6. Notice that Auto Track is turned on. The Frequency Auto Track option allows a receiver to track and lock onto the transmitter's carrier frequency with which it is currently linking, including any Doppler shift.

Adding a demodulator to the receiver

Communications enables you to select from a number of demodulators, including user-defined demodulators. Recall that your transmitter is using QPSK.

  1. Select the Demodulator tab.

    2. Notice that Auto-select Demodulator is turned on. If selected (which it is by default), the receiver automatically selects a demodulator that matches the modulation of the incoming signal. If this option is not selected, you must specify the type of demodulator. If the incoming signal's modulation does not match the modulation type of the selected demodulator, the STK application will set the Bit Error Rate (BER) to 0.5.

  2. Click OK to accept your changes and to close the Properties Browser.

Creating a Simple Link Budget report

A link budget created in the Access tool is known as a Simple Link Budget. The Link Budget report is a specialized Access report for basic link budget analysis and is available using the Link Budget button in the Reports frame of the Access tool.

  1. Right-click on Downlink_Rx () in the Object Browser
  2. Select Access… () in the shortcut menu.
  3. Expand () GEO_Sat_West () in the Associated Objects list once the Access Tool opens.
  4. Select Downlink_Tx ().
  5. Click .
  6. Click Link Budget... in the Reports frame.
  7. Take some time to look at the Simple Link Budget report.

Changes in data in columns such BER (Bit Error Rate) are likely caused by the satellite's inclination change which increases the range between the ground site and the satellite.

Saving your work

Clean up your workspace and save your scenario.

  1. Close any open reports, the Access Tool, and properties.
  2. Save () your work.

Summary

This was a quick introduction to Communications. You designed a preliminary system using Simple Transmitter and Receiver models and their inherited isotropic antennas. The system contained a very basic, one-way communications link between a geosynchronous satellite and a communications ground site. Based on your analysis, you determined that a viable communication link could be established between the satellite and the ground site.