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Scatter Plot Examples
This section presents scatter plots that illustrate how the blocks in the RF Impairments library distort a signal modulated by 16-ary quadrature amplitude modulation (QAM). The usual 16-ary QAM constellation without distortion is shown in the following figure:
The scatter plots illustrate the effects of the following four blocks:
As the scatter plots show, the first two blocks distort both the magnitude and angle of points in the constellation, while the last two alter just the angle.
You can create these scatter plots with models similar to the following, which produces the scatter plot for the Memoryless Nonlinearity block:
The model uses the Rectangular QAM Modulator Baseband block, from AM in the Digital Baseband Modulation sublibrary of the Modulation library. You can control the power of the block's output signal by the Normalization method parameter.
Memoryless Nonlinearity Block
The Memoryless Nonlinearity block applies a nonlinear distortion to the input signal. This distortion models the AM-to-AM and AM-to-PM conversions in nonlinear amplifiers. The block provides five methods, which you specify by the Method parameter, for modeling the nonlinear characteristics of amplifiers:
In the model shown in the preceding figure, the Method parameter is set to Ghorbani model. The following figure shows the scatter plot the model generates.
For another example of a scatter plot produced using this block, see the reference page for the Memoryless Nonlinearity block.
I/Q Imbalance Block
You can generate the next scatter plot by replacing the Memoryless Nonlinearity block in the 16-ary QAM Model with the I/Q Imbalance block. Set the block's I/Q amplitude imbalance (db) parameter to 10 and the I/Q phase imbalance (deg) parameter to 30.
For more examples of scatter plots produced using this block, see the reference page for the I/Q Imbalance block.
Phase/Frequency Offset Block
You can generate the next scatter plot by replacing the Memoryless Nonlinearity block in the 16-ary QAM Model with the Phase/Frequency Offset block. Set the block's Frequency offset (Hz) parameter to 0 and the Phase offset (deg) parameter to 70.
The Frequency offset (Hz) parameter adds a constant to the phase of the signal. The scatter plot corresponds to the standard constellation rotated by a fixed angle of 70 degrees.
The Frequency offset (Hz) parameter determines the rate of change of the signal's phase. In this example, Frequency offset (Hz) is set to 0, so the scatter plot always falls on the grid shown in the preceding figure. If you set Frequency offset (Hz) to a positive number, the points on the scatter plot fall on a rotating grid, corresponding to the standard constellation, which revolves at a constant rate in the counterclockwise direction. For an example, see the reference page for the Phase/Frequency Offset block.
Phase Noise Block
You can generate the next scatter plot by replacing the Memoryless Nonlinearity block in the 16-ary QAM Model with the Phase Noise block. Set the Phase noise level (dBc/Hz) parameter to -60 and the Frequency offset (Hz) parameter to 100.
The phase noise adds a random error to the signal's phase, so that the points in the scatter plot are spread in a radial pattern around the constellation points.
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