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, access between a transmitter and a receiver can be constrained 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, then impose various communication constraints and 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!

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 Use terrain server for analysis.
  4. select the RF - Environment page.
  5. Select the Rain & Cloud & Fog tab.
  6. Select Use In the Rain Model field.
  7. Select the newest ITU (International Telecommunications Union) model installed on your machine.
  8. Select the Atmospheric Absorption tab.
  9. Select Use.
  10. Select the newest ITU model installed on your machine.
  11. Click OK.

Insert 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: option.
  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 window.

Insert a Satellite

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 Satellite to "CommSat."
  3. Change Altitude: to 1500 km in the Definition field.
  4. Leave the remaining default settings.
  5. Click OK.

Communication Equipment

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

Satellite Pointing Device

  1. Using the Insert STK Objects tool, insert a Sensor () object using the Insert Default method.
  2. When the Select Object window appears, select CommSat and then click OK.
  3. In the Object Browser, rename the Sensor object "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. Browse to the Basic - Pointing page.
  7. Set the Pointing Type to Targeted.
  8. Move () Offutt_AFB_SATCOM_Terminal from the Available Targets list to the Assigned Targets list using the right arrow.
  9. Click OK.

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 window appears, select TgtOffut and then click OK.
  3. In the Object Browser, rename the Transmitter object "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.

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 window appears, select Offutt_AFB_SATCOM_Terminal and then click OK.
  3. In the Object Browser, rename the Receiver object "DLRcvr."
  4. Open DLRcvr's () 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 can 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.

Set the System Noise Temperature

  1. Select the System Noise Temperature tab.
  2. Select the Compute option.
  3. In the LNA field, set the Noise Figure to 1.2 dB.
  4. In the Antenna Noise field, enable Compute.
  5. Enable the Sun, Atmosphere, and Rain options.
  6. Click Apply.

Model 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. Select the Basic - Refraction page.
  2. Enable Use Refraction in Access Computations.
  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 non‐refracted 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.

Calculate Access

A Link Budget can now be calculated between DLXmtr and DLRcvr. Let's determine if the receiver can talk to the transmitter.

  1. In the Object Browser, right click on 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 communication constraints are imposed 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.

Generate a Link Budget Report

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

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.

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 (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. Browse to the Constraints - Comm page.
  3. In the Rcvd Isotropic Power field, enable Min: and set the value to -140 dBW.
  4. Click Apply. In the 2D visualization 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 the Refresh button.

Disable the RIP Constraint

No time entries for RIP values less than –140 dBW will appear in the report.

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

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 you specified a transmitter frequency of 4.5 GHz. If you now look at the figures for Received Frequency (Rcvd. Frequency (GHz)) in the Link Budget Report (refresh it if necessary, and make certain that all constraints are removed), 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 browse to the Constraints - Comm page.
  2. In the Doppler Shift field, enable both Min: and Max:.
  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 the Refresh button.

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

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

Doppler Constraint

Disable the Min and Max Doppler Shift Constraints

The 2D visualization window will clearly reflect 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 browse to the Constraints - Comm page.
  2. In the Doppler Shift field, disable both Min: and Max:.
  3. Click Apply.
  4. Return to the Link Budget - Detailed report and click the Refresh button.

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 whose radius equals 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 browse to the Constraints - Comm page.
  2. In the Flux Density field, enable Min:.
  3. Set Min: to -110 dBW/m^2.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click the Refresh button. 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.

Disable the Constraint

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

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 browse to the Constraints - Comm page.
  2. In the C/N field, enable Min:.
  3. Set Min: to ten (10) dB.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click the Refresh button. 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

Reset the Receiver's Properties

C/No and C/N can be improved via receiver as well as transmitter adjustments, since receiver gain appears in the numerator 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 browse 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 the Refresh button.
  5. Note the changes in the C/No and C/N field.
  6. Return to DLRcvr's () properties () and browse 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 the Refresh button.

Your settings are back to the original settings. Continue with the remaining steps 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).

  • 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.

Disable the Constraints

  1. Return to DLRcvr's () properties () and browse to the Constraints- Comm page.
  2. Remove all constraints.
  3. Click OK.
  4. Return to the Link Budget - Detailed report and click the Refresh button.

Modulation Adjustment

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

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

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

Set 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 browse 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 the Refresh button.

Constraints on Digital Systems

For digital communications systems, the receiver bit energy to noise 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 two (2.6) to 23 dB.

Energy Per Bit to Noise Power Spectral Density Ratio or Eb/No Constraints

  1. Return to DLXmtr's () properties () and browse 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 performance can be improved 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. Browse to the Constraints - Comm page.
  3. In the Eb/No field, enable Min:.
  4. Change Min: to 15 dB.
  5. Click Apply.
  6. Return to the Link Budget - Detailed report and click the Refresh button.

Disable the Constraint

  1. Return to DLXmtr's () properties () and browse to the Constraints - Comm page.
  2. In the Eb/No field, disable Min:.
  3. Click Apply.
  4. Browse 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 the Refresh button.

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 browse to the Constraints - Comm page.
  2. In the Bit Error Rate field, enable Max:.
  3. Change Max: to 1e-006.
  4. Click Apply.
  5. Return to the Link Budget - Detailed report and click the Refresh button.

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

Disable the Constraint

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

Refracted Elevation and Range Constraints

Constraints can be set 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.

Elevation Angle Constraint

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

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 (at or below 1 x 10‐6).

Refraction Angle

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 browser 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 the Refresh button.

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 “Use Refraction in Access Computations” is enabled, object visibility, range, elevation angle, and link angle of the antenna boresight are computed with refraction taken into account.

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 its 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 must be met. 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 (i.e. C/N) over a wide area. Communication constraints will affect the coverage.

Insert 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. Disable the US States option in the List Selections field.
  3. Select United_States_of_America in the list.
  4. Click Insert.
  5. Click Close.

Insert 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 analyses purposes, the geographical areas of interest are further refined using regions and points.

  1. In the Grid Area of Interest field, change the Type: to Custom Regions.
  2. Click the Select Regions button.
  3. Move () United_States_of_America to the Selected Regions list.
  4. Click OK to accept the changes to the Select Regions window.

    5. 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.

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

Specify 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. The object can be used 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 the Grid Constraints Options... button.
  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 allow you to specify the STK objects used to provide coverage.

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

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

  1. Select the Basic - Advanced page.
  2. Disable Automatically Recompute Accesses
  3. Click OK to accept the changes to the Coverage Definition object.

Compute Coverage Accesses

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

Insert 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.

Grid Stats

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

  1. Right-click on AccessConstraints () in the Object Browser.
  2. Select the 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 the report has been generated, note the Maximum value for the Grid Stats. You will use the value to create contours on your 2D and 3D Graphics windows.

Create Map Contours

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

  1. Open the AccessConstraints's () properties ().
  2. Select 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. Enable Natural Neighbor Sampling.
  7. Click Apply.
  8. Click the Legend... button.
  9. Bring the 2D Graphics window to the front to view the C/N contours on the map.

    10. C/N Contours

  10. When you are finished, close the legend.

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. Take 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!