Communications Part 1: Introduction to STK Communications

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

This lesson requires STK 12.5 or newer to complete it in its entirety.

Capabilities covered

This lesson covers the following STK Capabilities:

  • STK Pro
  • Communications

Problem statement

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

Solution

If you want to implement a communications system, you need an easy way to analyze and optimize your system. STK and the STK Communications capability provide a host of analysis tools to simulate how transmitters and receivers work in the field. To focus on learning Communications, you will start with a prepared scenario. This starter scenario contains most of the objects required for your analysis.

If you are new to STK, review the following Level 1 - Beginner tutorials first: Part 1: Build Scenarios and Part 2: Objects and Properties.

What you will learn

This tutorial is the first part of a five-part series that will introduce you to Communications. Part 1 will cover the following topics:

  • How to open and properly save a starter scenario (VDF file).

  • How to use Transmitter and Receiver objects in STK.

  • 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.
  8. Open the Save as type drop-down menu.
  9. Select Scenario Files (*.sc).
  10. Click Save.

Selecting relevant objects

In this scenario, you will analyze communications between a geosynchronous satellite transmitter and a communications ground site receiver. You will only use a portion of the objects in the Object Browser, not all of them. There are extra objects because you can use this same scenario to complete other lessons about STK Communications.

  1. Select the check box for the following objects in the Object Browser:
    • GEO_Sat_West ()
    • Communication_Site ()
  2. Click Save (). Save often during this lesson!

STK Communications capability

STK's 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 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

Using a Transmitter object, you will simulate a simple transmitter model on the geosynchronous satellite. 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 box.
  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

You will 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. Set the following:
    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 STK. You will 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: shortcut menu.
  3. Select QPSK.
  4. Click OK to accept your changes and to close the Properties Browser.

Modeling a simple receiver

You will use a simple receiver model on the ground communications site. 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. You will 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 box.
  3. Click OK.
  4. Rename Receiver1 () to Downlink_Rx.

Configuring a simple receiver model

You will 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 when the Properties Browser opens,.
  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 (default), the receiver automatically selects a demodulator that matches the modulation of the incoming signal. If 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, STK 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

Creating a link budget in the Access tool is referred to 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.

Summary

This was a quick introduction to STK Communications. You designed a preliminary system using the 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 have determined that a viable communication link can be established between the satellite and the ground site.

Save your work

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

What's next?

You are now ready to move onto the next tutorial Communications Part 2: Introduction to RF Environment, System Noise Temperature, and Terrain.