Using Communication Constraints to Design Communication Links

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.

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
  • Coverage
  • Analysis Workbench

Problem statement

Engineers and Operators require a quick way to model communications devices, add one or more constraints to them, and see how adjustments affect link performance.

Solution

Using STK's Communications capability, you can constrain access between a transmitter and a receiver to satisfy a variety of RF criteria. You will set up a link between a ground‐based receiver and a transmitter on a communications satellite and then impose certain communication constraints to observe the effects.

Video guidance

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

Create a scenario

Start by creating a scenario.

  1. Create a scenario () and name it Comm_Constraints.
  2. Change Start to 1 Jul 2016 16:00:00.000 UTCG and Stop to + 24 hrs.

Save Often!

Specifying the Radio Frequency (RF) Environment

Before defining parameters that are specific to the transmitter and receiver, it will be useful to select environmental models applicable to any communications link in the scenario.

  1. Open the scenario's () properties ().
  2. Select the Basic – Terrain page.
  3. Clear the Use terrain server for analysis check box.
  4. Go to the RF - Environment page.
  5. Select the Rain, Cloud & Fog tab.
  6. Select the Use check box in the Rain Model field.
  7. Click the Rain Loss Model ellipsis () and select the newest ITU (International Telecommunications Union) model installed on your machine.
  8. Select the Atmospheric Absorption tab.
  9. Select the Use check box.
  10. Click the Atmospheric Absorption Model ellipsis () and select the newest ITU model installed on your machine.
  11. Click OK.

Inserting a ground station

Use a Facility object, from the Standard Object Database, that is located in the central portion of the United States.

  1. Using the Insert STK Objects tool, insert a Facility () object using the From Standard Object Database method.
  2. Enter Offutt in the Name field of the Search Standard Object Data dialog box.
  3. Click Search.
  4. Select Offutt AFB SATCOM Terminal in the Search Results field; choose the Local Database result.
  5. Click Insert.
  6. Close the Search Standard Object Data dialog box.

Inserting a Satellite object

Add a new Satellite object to the scenario using the Orbit Wizard.

  1. Using the Insert STK Objects tool, insert a Satellite () object using the Orbit Wizard () method.
  2. Change the Satellite Name to "CommSat."
  3. Change Altitude to 1500 km under Definition.
  4. Leave the remaining default settings.
  5. Click OK.

Creating communication equipment

In this scenario, you will have a Transmitter object attached to the Satellite object that can downlink information to the Facility object. The Facility object will have a Receiver object that receives the downlink transmission. You can target both the Receiver and Transmitter objects. In STK, you will use a Sensor object as the targeting device.

Creating a pointing device for the satellite

  1. Using the Insert STK Objects tool (), insert a Sensor () object using the Insert Default method.
  2. When the Select Object dialog box appears, select CommSat and then click OK.
  3. In the Object Browser, rename the Sensor object to "TgtOffutt."
  4. Open TgtOffutt's () properties ().
  5. On the Basic - Definition page, in the Simple Conic field, change Cone Half Angle to two (2) deg. This is for situational awareness only.
  6. Go to the Basic - Pointing page.
  7. Set Pointing Type to Targeted.
  8. Move () Offutt_AFB_SATCOM_Terminal from the Available Targets list to the Assigned Targets list.
  9. Click OK.

Adding a downlink transmitter

Use a Complex Transmitter as the downlink transmitter.

  1. Using the Insert STK Objects tool (), insert a Transmitter () object using the Insert Default method.
  2. When the Select Object dialog box appears, select TgtOffut and then click OK.
  3. In the Object Browser, rename the Transmitter object to "DLXmtr."
  4. Open DLXmtr's () properties ().
  5. On the Basic - Definition page, make the following changes:
    6. Option Value
    Type Complex Transmitter Model
    Frequency 4.5 GHz
    Power 5 dBW
  6. Select the Antenna tab.
  7. Change the Design Frequency to 4.5 GHz.
  8. Click OK.

Adding a downlink receiver

Use a Medium Receiver Model as the downlink receiver.

  1. Using the Insert STK Objects tool (), insert a Receiver () object using the Insert Default method.
  2. When the Select Object dialog box appears, select Offutt_AFB_SATCOM_Terminal and then click OK.
  3. In the Object Browser, rename the Receiver object to "DLRcvr."
  4. Open properties ().
  5. On the Basic - Definition page, make the following changes:
    6. Option Value
    Type: Medium Receiver Model
    Gain: 20 dB
    Rain Model - Outage Percent 0.010
  6. Click Apply.

The Outage Percentage expresses, as a percentage, how much time you will allow to be sacrificed to rain outage during the year or, conversely, how much time the link must be maintained despite rain. In this case, you are specifying that the link must be maintained 99.99 percent of the year, rain or no rain, and other parameters, such as power or frequency, may need to be adjusted to meet this requirement.

Setting the System Noise Temperature

  1. Select the System Noise Temperature tab on DLRcvr's () Basic - Definition page.
  2. Select the Compute option.
  3. In the LNA field, set Noise Figure to 1.2 dB.
  4. In the Antenna Noise field, select Compute.
  5. Select the check boxes for Sun, Atmosphere, and Rain.
  6. Click Apply.

Modeling atmospheric refraction

Atmospheric refraction is the bending of an RF signal as it travels through the atmosphere, caused by the changing refractive index at different altitudes.

  1. Go to the Basic - Refraction page.
  2. Select the Use Refraction in Access Computations check box.
  3. Change Refraction Model to the available ITU model.
  4. Click OK.

This model computes the refracted elevation on the basis of inputs consisting of the nonrefracted elevation angle and the mean sea-level altitude of the receiver, using empirical criteria contained in ITU Recommendation. This completes the setup of the receiver.

Calculating Access

STK can now calculate the link budget between DLXmtr and DLRcvr. First determine if the receiver can talk to the transmitter.

  1. In the Object Browser, right-click DLRcvr () and select Access ().
  2. When the Access Tool opens, in the Associated Objects list, expand ()CommSat (), then TgtOffutt (), and select DLXmtr ().
  3. Click Compute.
  4. Bring the 2D Graphics window to the front.

Access highlights in the 2D visualization window will show portions of the satellite's ground track where there is access between the transmitter and receiver.

Clean Compute

In the remainder of this exercise, when you impose communication constraints that limit the periods of access on the basis of given RF criteria, it will be interesting to observe corresponding changes in the access graphics.

Generating a Link Budget report

  1. Return to the Access Tool () and click Report & Graph Manager.
  2. When the Report & Graph Manager opens, look in the Styles field and expand () the Installed Styles directory.
  3. Select the Link Budget - Detailed report.
  4. Click Generate.

Applying constraints

With the report open, you can change communication constraints and refresh the report to see the effects your constraints create on the link performance.

Applying a received isotropic power constraint

In the Link Budget - Detailed report, take a look at the values for received isotropic power (Rcvd. Iso. Power (dBW) or RIP), which is the product of the effective isotropic radiated power (EIRP) of the transmitter and the propagation losses (atmospheric, rain, and free space losses) without any contribution by the receiver. These values fall in the approximate range of ~ –149 to -133 dBW, according to the Link Budget Report.

Suppose it is desirable to exclude any link with an RIP value of less than –140 dBW.

  1. Open DLRcvr's () properties ().
  2. Go to the Constraints - Comm page.
  3. In the Rcvd Isotropic Power field, select the Min check box and set the value to -140 dBW.
  4. Click Apply. In the 2D Graphics window, you will see that the constraint you imposed has a significant impact on access times.

    5. RIP Constraint

  5. Return to the Link Budget - Detailed report.
  6. Click Refresh (). No time entries for RIP values less than –140 dBW will appear in the report.

Disabling the RIP Constraint

Remove the RIP constraint.

  1. Return to DLRcvr's () properties ().
  2. Go to the Constraints - Comm page.
  3. In the Rcvd Isotropic Power field, clear the Min check box.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click Refresh ().

Applying a Doppler Shift constraint

The Doppler Shift constraint relates to the ability of the receiver to adjust to increases in the frequency of the incoming signal as the satellite approaches its closest point to the receiver and to adjust to decreases in that frequency as the satellite moves away from that point. Recall that you specified a transmitter frequency of 4.5 GHz. If you look at the figures for received frequency (Rcvd. Frequency (GHz)) in the Link Budget Report, you will see that the receiver must accommodate a shift of about +/‐81 kHz in frequency to avoid losing any of the incoming signal.

Suppose the receiver is limited to a 50 kHz adjustment in either direction.

  1. Return to DLRcvr's () properties () and go to the Constraints - Comm page.
  2. In the Doppler Shift field, select both Min and Max check boxes.
  3. Set Min to -50 kHz.
  4. Set Max to 50 kHz.
  5. Click Apply.
  6. Return to the Link Budget - Detailed report and click Refresh ().

    7. You will notice that time entries for Received Frequencies less than 4.499950 GHz and greater than 4.500050 GHz are excluded. You can also see this in the Freq. Doppler Shift (kHz) column.

  7. Bring to 2D Graphics window to the front.

Doppler Constraint

Disabling the Min and Max Doppler Shift constraints

The 2D Graphics window clearly reflects the impact of this constraint. In fact, if you alternatively disable and enable the Min and Max options, you will see that the Doppler shift limitations are reflected alternatively in the approaching and departing ground tracks, just as you would expect.

  1. Return to DLRcvr's () properties () and go to the Constraints - Comm page.
  2. In the Doppler Shift field, clear both the Min and Max check boxes.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click Refresh ().

Applying a Flux Density constraint

Flux Density, usually measured in dB (watts/m^2), is the expression of the transmitter's radiated power, reduced by any applicable atmospheric losses, divided by the surface area of a sphere with a radius equal to the distance between the transmitter and receiver:

Flux Density

In the Link Budget - Detailed report, entries for Flux Density should range between approximately -114 and –99 dBW/m^2. Now suppose you wish to exclude from consideration links with a Flux Density less than –110 dBW/m^2.

  1. Return to DLRcvr's () properties () and go to the Constraints - Comm page.
  2. In the Flux Density field, select the Min check box.
  3. Set Min to -110 dBW/m^2.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click Refresh (). You will notice that time entries for Flux Density less than -110 dBW/m^2 are excluded.
  6. Bring to 2D Graphics window to the front.

2D Flux Density

The 2D Graphics view didn't change very much due to the loss of just a few time entries.

Disabling the Flux Density Constraint

  1. Return to DLRcvr's () properties () and go to the Constraints - Comm page.
  2. In the Flux Density field, clear the Min check box.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click Refresh ().

Applying carrier-to-noise ratio constraints

One of the most commonly used criteria for assessing link performance is carrier-to-noise ratio (C/N (dB) or CNR). This can be expressed independently of bandwidth as:

CNR Expression

For the communications link in this exercise, C/No and C/N fall in the approximate ranges 75 to 95 dB/Hz and ‐1 to 20 dB, respectively. You can constrain link performance with respect to either of these criteria. Now suppose you wish to exclude from consideration links with a carrier-to-noise ratio of less than 10 dB.

  1. Return to DLRcvr's () properties () and go to the Constraints - Comm page.
  2. In the C/N field, select the Min check box.
  3. Set Min to ten (10) dB.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click Refresh (). You will notice that time entries for C/N less than ten (10) dB are excluded.
  6. Bring to 2D Graphics window to the front.

C/N Constraint

Resetting the Receiver's properties

You can improve C/No and C/N via receiver as well as transmitter adjustments, since receiver gain appears in the numerator of both equations. Leave the C/N constraint in place, and from the receiver’s Basic – Definition page, try each of the following adjustments one at a time, resetting each parameter to its original value before proceeding to the next.

  1. Return to DLRcvr's () properties () and go to the Basic - Definition page.
  2. In the Model Specs tab, increase Gain to 25 dB.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click Refresh ().
  5. Note the changes in the C/No and C/N fields.
  6. Return to DLRcvr's () properties () and go to the Basic - Definition page.
  7. In the Model Specs tab, decrease the Gain to 20 dB.
  8. Click Apply.
  9. Return to the Link Budget - Detailed report and click Refresh ().

Your settings are back to the original settings. Continue with the remaining parameters one at a time, resetting each parameter to its original value before proceeding to the next like you did in steps one (1) through nine (9) above.

  • On the System Noise Temperature tab, reduce LNA Noise Figure to one (1) dB.
  • On the Model Specs tab, add (Right Hand or Left Hand) Circular Polarization. You must also set the polarization on the transmitter to the same type.
  • On the Additional Gains and Losses tab, add a Pre‐Receive gain of one (1) dB.
  • On the Model Specs tab, increase the Rain Model Outage Percent to 0.030.

At this point, Rain Model Outage should be back at 0.010 and the only Comm Constraint that is enabled is C/N. Everything else should be reset.

Disabling the constraints

  1. Return to DLRcvr's () properties () and go to the Constraints- Comm page.
  2. Clear all constraint check boxes.
  3. Click OK.
  4. Return to the Link Budget - Detailed report and click Refresh ().

Changing the Modulation Type

An interesting transmitter adjustment to try out is a change in the Modulation Type.

  1. Open DLXmtr's () properties () and go to the Basic - Definition page.
  2. Select the Modulator tab.
  3. Set Name to MSK.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click Refresh ().

    6. From here on, you can keep going back to the 2D Graphics window to see the constraint effects on access times.

Setting the Modulation back to BPSK

In digital modulation, minimum-shift keying (MSK) is a type of continuous-phase frequency-shift keying. The default modulation in STK is binary phase-shift keying (BPSK).

  1. Return to DLXmtr's () properties () and go to the Basic - Definition page.
  2. Select the Modulator tab and change Name back to BPSK.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click Refresh ().

Applying constraints to the digital systems

For digital communications systems, the receiver bit-energy-to-noise (Eb/No) ratio is defined as:

Digital System Definition

where T = the bit period. As reflected in the Link Budget report, the communications link in this exercise exhibits Eb/No values ranging from approximately 2.6 to 23 dB.

Applying Eb/No constraints

  1. Return to DLXmtr's () properties () and go to the Basic - Definition page.
  2. In the Model Specs tab, change Data Rate to 12 Mb/sec.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click the Refresh button. Note that Eb/No (dB) values increase.

As with CNR, link you can improve performance by tweaking receiver or transmitter parameters. This increases the bit period T, which leads directly to an improvement in Eb/No.

  1. Return to DLXmtr's () properties ().
  2. Go to the Constraints - Comm page.
  3. In the Eb/No field, select the Min check box.
  4. Change Min to 15 dB.
  5. Click Apply.
  6. Return to the Link Budget - Detailed report and click Refresh ().

Disabling the Constraint

  1. Return to DLXmtr's () properties () and browse to the Constraints - Comm page.
  2. In the Eb/No field, clear the Min check box.
  3. Click Apply.
  4. Go to the Basic - Definition page.
  5. Change Data Rate back to 16 Mb/sec.
  6. Click Apply.
  7. Return to the Link Budget - Detailed report and click Refresh ().

Applying Bit Error Rate constraints

A direct measure of link performance for a digital system is Bit Error Rate (BER), which expresses the probability that a bit will be received in error. According to the Link Budget report, BER values in this exercise range from approximately 1 x 10‐30 to 2.8x 10‐2. A typical desired BER is 1 x 10‐6 or lower.  

  1. Return to DLXmtr's () properties () and go to the Constraints - Comm page.
  2. In the Bit Error Rate field, select the Max check box.
  3. Change Max to 1e-006.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click Refresh ().

BER is a function of Eb/No, and you can improve it through adjustments in the receiver or transmitter, including reductions in Data Rate.

Disabling the constraint

  1. Return to DLXmtr's () properties () and go to the Constraints - Comm page.
  2. In the Bit Error Rate field, clear the Max check box.
  3. Click OK.
  4. Return to the Link Budget - Detailed report and click Refresh ().

Applying refracted elevation and range constraints

You can set constraints in terms of the refracted elevation and range of the transmitter with respect to the receiver. As a reminder, you are currently using the ITU refraction model in your analysis.

Applying the Elevation Angle constraint

  1. Open DLRcvr's () properties ().
  2. Go to the Constraints - Basic page.
  3. In the Elevation Angle field, select the Min check box.
  4. Set Min to five (5) deg.
  5. Click Apply.
  6. Return to the Link Budget - Detailed report and click click Refresh ().

This excludes links with satellites deemed to be too close to the horizon, which can be unreliable due to the relatively long path through the atmosphere to be traversed by the signal. You'll notice in the Link Budget - Detailed report, most if not all of your BER fall within acceptable range, which is at or below 1 x 10‐6.

Applying a refraction angle constraint

The calculation of the refracted elevation or range depends on the selected refraction model. As you may recall, you selected an ITU model satisfying empirical criteria.

  1. Return to DLRcvr's () properties () and go to the Basic - Refraction page.
  2. Change Refraction Model to Effective Radius Method. This computes the apparent elevation due to refraction.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click click Refresh ().

You will see improved Eb/No and BER values. Using this model, the 2D Graphics window should reflect a marginally larger portion of the satellite's orbit satisfying the elevation constraint than under the more empirically grounded ITU model. When you select the Use Refraction in Access Computations check box, STK computes the object visibility, range, elevation angle, and link angle of the antenna boresight with refraction taken into account.

Applying an everyday use of C/N constraints

It is a common practice for a receiver vendor to stipulate a minimum required C/N value that must be satisfied in order for the equipment to perform to specification. Imposing a C/N constraint on accesses between a receiver and a transmitter is an easy way to model this requirement in the design of communications links. For example, if the manufacturer specifies that its equipment requires a C/N value of at least five (5) dB to function properly, you can simply enter a Min value of five (5) for the C/N constraint, which will reduce the number and/or length of calculated periods of access between the receiver and transmitter.

For greater confidence in the quality and reliability of a link, it is a good idea to add a fade margin to other requirements that you must meet. A natural way to do this is to increase the Min value of the C/N constraint to include that margin. Thus, to model a fade margin of three (3) dB for a receiver requiring a minimum C/N value of five (5) dB, just set the Min value for C/N to eight (8) dB. You can then enjoy a higher degree of confidence in the access periods.

  1. Prior to moving on to the next section, place all properties back to their original settings.
  2. Close the report, Report & Graph Manager, Access Tool, and any object properties.
  3. Open the Analysis menu and select Remove All Accesses.

Using communication constraints in Coverage

You can use the Coverage Definition object to analyze a portion of a link budget (for example, C/N) over a wide area. Communication constraints will affect the coverage.

Inserting an Area Target

The Area Target object models a region on the surface of the central body.

  1. Insert an Area Target () object using the Select Countries and US States () method.
  2. Clear the US States check boxin the List Selections field.
  3. Select United_States_of_America in the list.
  4. Click Insert.
  5. Click Close.

Inserting a Coverage Definition object

  1. Insert a Coverage Definition () object using the Insert Default () method.
  2. Open the Coverage Definition object’s () properties ().
  3. Select the Basic – Grid page.

    Coverage analyses are based on the accessibility of assets (objects that provide coverage) and geographical areas. For analysis purposes, you can refine the geographical areas of interest further using regions and points.

  4. In the Grid Area of Interest field, change Type to Custom Regions.
  5. Select Area Targets in the drop-down menu below Custom Regions.
  6. Move () United_States_of_America to the Selected Regions list.

    7. The statistical data computed during a coverage analysis is based on a set of locations, or points, which span the specified grid area of interest.

  7. In the Grid Definition field, set Point Granularity to two (2) degrees.

Specifying an object class

Once you have defined the grid area, you can specify an object class or a specific object for the points within the grid. You can use the object to associate three types of information with the grid points: access constraints, basic object properties, and the shape of the ellipsoidal obstruction surface used by the Line Of Sight constraint.

  1. Click Grid Constraints Options....
  2. Set the Reference Constraint Class to Receiver.
  3. Select Offutt_AFB_SATCOM_Terminal/DLRcvr.
  4. Click OK to accept the changes to the Grid Constraints Option.

Assets properties enable you to specify the STK objects used to provide coverage.

  1. Go to the Basic - Assets page.
  2. Select DLXmtr.
  3. Click Assign.

STK automatically recomputes accesses every time you update an object on which the coverage definition depends, such as an asset. At times, you may want to recompute accesses manually.

  1. Go to the Basic - Advanced page.
  2. Clear the Automatically Recompute Accesses check box.
  3. Click OK to accept the changes to the Coverage Definition object.

Computing coverage accesses

  1. Right-click the CoverageDefinition () object in the Object Browser.
  2. Extend the CoverageDefinition menu item.
  3. Select Compute Accesses.

Inserting a Figure of Merit

The Coverage Figure of Merit object enables you to analyze coverage in various directions over time, using several attitude-dependent figures of merit.

  1. Insert a Figure Of Merit () object using the Insert Default method.
  2. Attach the Figure of Merit object to the Coverage Definition object.
  3. Name the Figure Of Merit object "AccessContraints".
  4. Open AccessConstraints' () properties ().

Measuring access constraints

Access constraints measure the value of various constraint parameters used to define visibility within STK.

  1. On the Basic - Definition page, change the Definition Type to Access Constraint.
  2. Change Constraints to C/N.
  3. Select the Enable check box in the Satisfaction section.
  4. Click OK.

Default Compute is set to Average. In this case, average is good when creating static contours for a finite time period.

Generating a Grid Stats report

The Grid Stats report summarizes the minimum, maximum, and average static value for the Figure Of Merit over the entire grid.

  1. Right-click AccessConstraints () in the Object Browser.
  2. Select Report & Graph Manager.
  3. Select the Grid Stats report in the Styles list and click Generate.

Be patient. Depending on your computer, this could take a few minutes.

Once STK generates the report, note the Maximum value for the Grid Stats. You will use the value to create contours on your 2D and 3D Graphics windows.

Creating map contours

You can specify how levels of coverage quality display in both the 2D and 3D Graphics windows.

  1. Open the AccessConstraints' () properties ().
  2. Go to the 2D Graphics - Static page.
  3. Make the following changes:
    4. Option Value
    % Translucency 20
    Show Contours Enabled
    Start Level 0
    Stop Level Round down the maximum integer from the Grid Stats report
    Step 1
  4. Click Add Levels.
  5. Change Start Color to red and End Color to blue.
  6. Select Natural Neighbor Sampling.
  7. Click Apply.
  8. Click Legend... . Bring the 2D Graphics window to the front to view the C/N contours on the map.

    9. C/N Contours

  9. When you are finished, close the legend.

Computing Grid Stats over time

The Grid Stats over Time report and graph summarize the minimum, maximum, and average of the Figure Of Merit's dynamic value over the entire grid as a function of time.

  1. Bring the Report & Graph Manager () to the front.
  2. Generate a Grid Stats Over Time report.
  3. Note the optimum time for C/N.
  4. Save the Grid Stats Over Time report as a Quick Report.
  5. Close the report and the Report & Graph Manager ().

SAVE YOUR WORK!