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 that are unique to a particular modulation and/or encoding, or to include non-theoretical performance degradations. The external file is comprised of special keywords and associated user values.
Below is a table of allowable keywords and their descriptions.
External Demodulator Data File Keywords
| Keyword | Description | Valid Values | Default Value |
|---|---|---|---|
| Description | Optional, one-line description of the contents of the file. | N/A | N/A |
| ModulationName | A name given to the type of modulation that this demodulator can successfully demodulate. If the modulation of the incoming signal does not match this value, a BER of 0.5 is reported. |
> 0.0 |
2.0 |
| InterpolationScale | Optional keyword which specifies the interpolation data scale. The demodulator data file format requires the Eb/No values to be in dB and BER values on a linear scale. The Eb/No is converted to a linear scale internally. Due to the nature of the data samples, the linear scale may not lend itself to a suitable interpolation process. You can specify the scales for the x and y axes to be linear or on a log scale. STK interpolates the table as necessary to determine the appropriate bit error rate for a particular bit energy level. If the bit energy is smaller than the first value in the table, the bit error rate for the first value is used. If the bit energy is larger than the last value in the table, a default bit error rate of 1.0e-30 is used to indicate no errors. |
Linear |
|
| OrderOfInterpolation |
Optional keyword that specifies the interpolation order for computing BER at the computed Eb/No value. If the Eb/No value falls between BER data points, the data is interpolated to compute BER. The default interpolation process is curve fitting a first order polynomial. Yet, you can choose any value from 1 to 5. Order 1 does a piecewise linear approximation and interpolate values on a straight line between points. Order 1 is the most stable. Higher order curve fitting can cause oscillation (ringing) in the computed data, especially when the original data points have sudden changes in values. |
1-5 |
1 |
| BERPoints | Indicates the start of the BER data points. The data points must follow BERPoints and must consist of (Eb/No, BER) pairs separated by white space. |
N/A |
N/A |
InterpolationScale Keyword Valid Values
| Scale | Description |
|---|---|
| Linear | Scale data for both axes is converted to linear. |
| SemiLogX | X-axis ( Eb/No) data is retained on a log scale, while BER remains on a linear scale. |
| SemiLogY | Y-axis (BER) data is converted to a log scale, while Eb/No is changed to a linear scale. |
| LogLog | Both axes are on a log scale. |
Sample Demodulator Data File Formats
Format 1
The following is a minimum sample file format that specifies a custom BER curve.
Description MIL-STD-188-165B OQPSK, rate 7/8 (punctured 1/2). ModulationName MIL-STD-188-165B-OQPSK877 BERPoints 2.0 5.0E-01 2.2 5.0E-01 2.4 4.9E-01 2.6 3.4E-01 2.8 2.4E-01 3.0 1.6E-01 3.2 1.1E-01 3.4 7.1E-02 3.6 4.6E-02 3.8 2.9E-02 4.0 1.8E-02 4.2 1.1E-02 4.4 6.9E-03 4.6 4.1E-03 4.8 2.4E-03 5.0 1.4E-03 5.2 7.8E-04 5.4 4.3E-04 5.6 2.4E-04 5.8 1.3E-04 6.0 6.8E-05 6.2 3.5E-05 6.4 1.8E-05 6.6 9.2E-06 6.8 4.6E-06 7.0 2.2E-06 7.2 1.1E-06 7.4 5.1E-07 7.6 2.3E-07 7.8 1.1E-07 8.0 4.7E-08 8.2 2.0E-08 8.4 8.4E-09 8.6 3.4E-09 8.8 1.3E-09 9.0 5.0E-10 9.2 1.8E-10 9.4 6.3E-11 9.6 2.1E-11 9.8 6.5E-12 10.0 2.0E-12 10.2 5.5E-13 10.4 1.5E-13 10.6 3.7E-14
Format 2
The following is a sample file format that specifies a custom BER curve and type of interpolation.
Description My personal description goes here. ModulationName MyModulation OrderOfInterpolation 1 InterpolationScale Linear BERPoints 2.0 1.0e-2 3.0 1.0e-3 4.0 1.0e-4 5.0 1.0e-5 6.0 1.0e-6 7.0 1.0e-7 8.0 1.0e-8 9.0 1.0e-9 10.0 1.0e-10 11.0 1.0e-11 12.0 1.0e-12 13.0 1.0e-13 14.0 1.0e-14 15.0 1.0e-15