Laser Receiver Model
The Laser Receiver model enables you to model free space laser communications in the near-infrared, visual, and ultraviolet bands. On the Laser Receiver properties page, under Basic - Definition, the following specification tabs appear:
Model Specs
Use this tab to specify the following parameters:
Frequency. Enter the desired value or select the Auto Track option.
Optical Detector. The following parameters define the optical detector:
| Parameter | Description |
|---|---|
| Type |
Use the drop-down menu to select one of the following optical detector types:
|
| Gain | Enter the gain of the detector, in dB. |
| Efficiency | This is a measure of the light-to-current conversion efficiency, ranging from 0 to 100 percent. |
| Dark Current | Enter the current output of the detector under conditions of no light. |
| Noise Factor | Enter the detector's noise factor value, in dB. |
| Noise Temperature | Enter the detector's noise value, in degrees K. |
| Load Impedance | Enter 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.
STK uses these parameters to compute signal-to-noise ratio and Eb/N0 for a laser communications link.
STK computes the receiver gain value, if zero or negative, from the equation:
Grcvr = (4pA/λ2)u
where A = optics aperture area, λ = laser wavelength = c/f, and u = optics efficiency.
The receiver gain value is set to this value for the nontracking receiver types. For the autotracking 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 instant in time .
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. 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.
- 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 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.
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.
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 .
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 the Use check box and click the ellipsis to 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, go to the Additional Gains and Losses tab and click . 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 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.