Receiver Models

Complex Receiver Model

The Complex Receiver model enables you to select among a variety of analytical and realistic antenna models and to define the characteristics of the selected antenna type. On the receiver's Basic Definition properties page, you can set the following parameters for this model:

• Model Specs
• Antenna
• System Noise Temperature
• Demodulator
• Filter
• Additional Gains and Losses
• Interference

To specify the total system noise, you must specify gains and losses on the Model Specs tab as well as the noise temperature on the System Noise Temperature tab.

Model Specs

Use this tab to specify the following parameters:

Parameter	Description
Frequency	Enter a value or select the Auto Track check box.
Antenna to LNA Line Loss	Specify the loss of the transmission line between the antenna and the Low Noise Amplifier, in dB.
LNA Gain	Specify the gain of the Low Noise Amplifier, in dB.
LNA to Receiver Line Loss	Specify the loss of the transmission line between the LNA and the Receiver, in dB.
Rain Model	To use a rain model in your scenario, you must select a global Rain Model on the Scenario's RF Environment properties page. Then select the Use check box to enable the Rain Model for this receiver. Provide an Outage Percent to set the maximum percent outage for the rain model. To disable the Rain Model used in the calculation for this receiver, clear the Use check box. For rain model descriptions, see Rain Model. If you do not select the Rain Model at the scenario level, the rain model parameters are unavailable.
Link Margin	Select the Enablecheck box. Then choose a Link Margin type (BER, RIP, C/N, etc.) and specify a Threshold value. For descriptions of link types, see Link Margin. Clear the Enablecheck box to disable this parameter.

Antenna

You can select to embed an antenna model from the Component Browser or you can link to an antenna object. Antenna objects are listed in the Object Browser.

To embed an antenna model, select Embed as the Reference Type. On the Model Specs subtab, click the Antenna Models ellipsis button to select an antenna model. You can define polarization and orientation parameters for an embedded antenna using the Polarization and Orientation subtabs. For parameter definitions, see Antenna Orientation Methods and Polarization.

To link to an antenna object, select Linked as the reference type and select the antenna from the drop-down list. You can define polarization parameters for a linked antenna. For parameter definitions, see Polarization. You cannot modify the antenna's model specification and orientation parameters while in the receiver's basic properties. To modify these parameters, go to the antenna's basic properties.

If a transmitter or receiver is using a linked antenna, the geometric and vector constraints are not available for that transmitter or receiver because STK pulls the geometric and vector constraints from the antenna.

For information on antenna types and parameters, and how to link to an antenna on a sensor, see STK Antenna Models.

Polarization for Complex UAN Format and Complex ANSYS *.ffd Format antenna models is pulled from the external file that you added to the Antenna's Model Specs tab. Select Use on the Antenna's Polarization tab to enable the polarization specified in the file. You can ignore the remaining options on the polarization tab as they do not apply to these antenna models.

System Noise Temperature

You can set noise temperature by selecting Constant and entering a value directly. Alternatively, you can have STK calculate the system temperature for you. To do so, select Compute and specify the parameters that STK will use in calculating noise temperature. For parameter descriptions, see System Noise Temperature.

Demodulator

STK Communications enables you to select from a number of demodulators, including user-defined demodulators. Each demodulator has a defined modulation. The modulation determines two characteristics:

• One is the fraction of transmitter power contained within the receiver's bandwidth, computed in the Bandwidth Overlap Factor.
• The other is the translation between the signal-to-noise ratio (Eb/No) and the resulting bit error rate (BER). The BER curves in STK represent theoretical performance curves. When modeling real demodulators, you may want to use an external modulation type with a slightly degraded BER curve. Typical systems run within 1-2 dB of the theoretical values at a given bit error rate. STK assumes perfect bit synchronization when demodulating the data to obtain a BER.

Auto-select Demodulator. If selescted (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 BER to 0.5.

Name. This is the name of the demodulator that the receiver will use to demodulate the incoming signals. The demodulator's modulation determines the translation between the signal-to-noise ratio (Eb/No) and the resulting bit error rate (BER). If the modulation of the demodulator matches the modulation of the incoming signal, the demodulator will compute a BER. If it does not match, the demodulator will report a BER of 0.5. When you select a file-based modulation (Demodulators or Script Plugin Demodulators), you must specify a filename.

The BER curves used in STK represent theoretical performance curves. When modeling real demodulators, you may want to use an External demodulator with a slightly degraded BER curve. Typical systems run within 1-2 dB of the theoretical values at a given bit error rate. STK assumes perfect bit synchronization when demodulating the data to obtain a BER.

External demodulators

The external demodulator file enables you to specify a custom BER curve. Common uses of an external demodulator are to:

• incorporate Eb/No to BER curves that are unique to your particular modulation and encoding
• include nontheoretical performance degradations

The external file is comprised of special key words and associated user values. For more information, see External Demodulator File.

Script plugin demodulators

Script Plugin demodulators are user-defined scripts that enable you to define the demodulator, including its behavior. Script languages can be VB Script or MATLAB. Unlike the External file demodulators, which are static in nature, these can be time dynamic.

The plugin script is not automatically reloaded after you make changes to it. To reload the script, click Reload.

For a description of the script's input and output parameters, see Demodulator Arguments.

Filter

Receiver Bandwidth. Enter a value or select the Auto Scale check box.

Filter Model. To specify a filter model, first select the Use check box. Then click the Filter Models ellipsis to select a filter model. For more information, see Filter Models.

Additional gains and losses

Pre-Receive Gains/Losses. To define a gain or loss, go to the Additional Gains and Losses tab and click Add. Enter a brief description of the gain or loss in the Identifier field and provide its value in the Gain field. Remember to make it negative if you are entering a loss. Click Remove to delete an entry from the table. To modify an existing entry, simply edit the fields in the grid. The value in the Pre-Receive field will reflect the net value of all gains and losses recorded in the table.

Pre-Demodulation Gains/Losses. Add, modify, and delete entries in the same manner as for Pre-Receive gains and losses, described above.

For more information on modeling gains and losses that affect performance but are not defined using built-in analytical models, see Pre-Receive & Pre-Demod Gains & Losses.

Interference sources

You can add interference sources to an RF receiver and assess their impact on the performance of the receiver.

On the receiver's Basic Definition properties page, go to the Interference tab. You can turn on interference and then add or remove emitters from the assigned list. Only qualified assets will appear in the Available Emitters list, including the following:

• Transmitter objects configured with one of the RF transmitter models (no laser models)
• Radar objects configured with either the Monostatic or Bistatic Transmitter Radar systems

Also, you can set constraints for an interfering emitter and include the effects of the interference in Coverage and Attitude Coverage analyses.

Enabling and disabling interference effects

Select or clear the Use check box to enable or disable the computation of interference effects. Use this check box to turn off interference analysis temporarily without having to dismantle your interference setup; i.e., it is not necessary to remove assigned emitters, turn off constraints, etc. Disabling interference computation will save you processing time if you want to make and test other changes in the scenario without having to recompute interference each time. This can be especially helpful in a scenario that includes many interfering emitters.

Assigning interference sources

To add an emitter from the Available Emitters list to the Assigned Emitters list, double-click it or select it and click the right arrow button. To remove it from the Assigned Emitters list, double-click it or select it and click the left arrow button.

If you want to add or remove whole classes of objects, proceed as follows:

1. In the Selection filter area, select object check box(es) to make objects available by class.
2. To assign all the objects of that class (those classes) as assigned emitters, click the right arrow button. Make sure not to highight an individual emitter before clicking the arrow button.
3. You cannot remove all the objects of that class (those classes) from assigned emitters, click the left arrow button. Make sure not to highight an individual emitter before clicking the arrow button.

You can also use multiselect techniques to manage large lists of Available and Assigned Emitters.

Assigned Emitters can occupy the receiver's parent object in the STK object browser, but interference computations will only occur if the receiver and interferer are physically separated by an offset distance.

Include active communication systems

At the bottom of the Interference tab, select the Include Active CommSystems Interference Emitters check box to account for interference from these sources.

Effects of interference on Coverage and Attitude Coverage

You can select an RF receiver object as the associated class in defining the basic grid for Coverage Definition or Attitude Coverage. If you then select Access Constraint as the Figure of Merit Definition, the available constraints for the receiver, including those that take interference into account, appear in the Constraints list.