Receiver Models

Laser Receiver Model

The Laser Receiver model enables you to model free space laser communications in the near infrared, visual, and ultraviolet bands.

Model Specs

Frequency. Enter the desired value or select the Auto Track option.

Optical Detector. The following parameters define the optical detector:

Use APD Detector Model. Computes the signal-to-noise ratio for an Avalanche photodetector.

Gain. The gain of the detector in dB.

Efficiency. A measure of the light-to-current conversion efficiency, ranging from 0 to 100 percent.

Dark Current (Amps). The current output of the detector under conditions of no light.

Noise Factor. The detector's noise factor value expressed in dB.

Noise Temperature. The detector's noise value in degrees K.

Load Impedance. The detector's load impedance in ohms.

Link Margin. The Link Margin type (BER, RIP, C/N, etc.) and threshold value. Valid if Enable is selected. For descriptions of link types, see Link Margin.

The above parameters are used to compute signal-to-noise ratio and Eb/N0 for the laser communications link.

The receiver gain value, if zero or negative, is computed from the equation:

Grcvr = (4A/2)

where A = optics aperture area, = laser wavelength = c/f, and = optics efficiency.

The receiver gain value is set to this value for the non-tracking receiver types. For the auto-tracking receiver type, the gain value is checked at each time step. If it is set to zero or negative, the gain is computed by using the above equation and the Doppler shifted frequency received by the receiver at that time instant.

Lambert, Stephen G., and William L. Casey, Laser Communications in Space, Artech House, Inc. (1995)

Gagliardi, Robert M., and Sherman Karp, Optical Communications, Krieger Publishing Co. (1988)

Gagliardi, Robert M., Satellite Communications, Chapter 10, New York: Van Nostrand Reinhard (1991).

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 and click the browse button to select an antenna from the Component Browser. You can define polarization and orientation parameters for an embedded antenna. 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.

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

  • A linked antenna is the focus of the communications link, which means that all geometry and vector computations are carried out on the linked antenna instead of on the receiver to which that antenna is linked.
  • The reference type is only available for transmitters, receivers, and radar objects that are not a child of a Sensor object. If the one of these objects is a child to a Sensor, the only option is to use the embedded antenna model. For more information, see Linking to an Antenna that Resides on a Sensor.

Propagation Loss Models

To apply the Beer-Bouguer-Lambert Law Propagation Loss Model, select Enable. For a description of this model and its parameters, see Beer-Bouguer-Lambert Law Propagation Loss Model.

Demodulator

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

  1. The fraction of transmitter power contained within the receiver's bandwidth (computed in the Bandwidth Overlap Factor).
  2. The translation between the signal-to-noise ratio (Eb/No) and the resulting bit error rate (BER). The BER curves used 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. Perfect bit synchronization is assumed when demodulating the data to obtain a BER.

Auto-select Demodulator. If checked (default), the receiver automatically selects a demodulator that matches the modulation of the incoming desired signal. If not checked, you must specify the type of demodulator. If the incoming signal’s modulation does not match the modulation type of the selected demodulator, the BER will be set to 0.5.

Name. The name of the demodulator to be used by the receiver 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 BER will be reported as 0.5. When a file-based modulation is selected (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. Perfect bit synchronization is assumed when demodulating the data to obtain a BER.

External Demodulators

The external demodulator file allows you to specify a custom BER curve. Common uses of an external demodulator is to incorporate Eb/No to BER curves which are unique to the user's particular modulation and/or encoding, or to include non-theoretical 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 allow you to define the demodulator including its behavior. Script languages can be VB Script, Perl, or MATLAB. Unlike the External file demodulators which are static in nature, these can be time-dynamic, if so desired.

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 the desired value or select the Auto Scale option.

Filter Model. To specify a filter model, select Use and browse to a filter model. For more information, see Filter Models.

Additional Gains and Losses

Pre-Receive Gains/Losses. To define a Gain or Loss, click Add. Enter a brief description of the Gain or Loss in the Identifier field and its value in the Gain field. Remember to make it negative if a Loss is being entered. Once the values are entered, you can 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.

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