Add Saleh Model of Power Amplifier to 16-QAM Signal in Simulink

This example applies the Saleh method of memoryless nonlinearity to a 16-QAM modulated signal. To show the Saleh model of a power amplifier, the example applies exaggerated levels that are not typical for modern radios.

In this example, the cm_mnl_saleh_16qam model 16-QAM modulates a signal containing a complete set of constellation points and passes them to Memoryless Nonlinearity blocks configured to apply the Saleh method with the default AM/AM and AM/PM distortion settings, AM/AM distortion alone, and AM/PM distortion alone. The model includes Constellation Diagram blocks after each Memoryless Nonlinearity block so you can analyze the impact of each impairment on the constellation.

In the model, you can click the Plot Transfer Curve Callback Button (Simulink) block to run the testPA.m helper function. The testPA helper function plots this amplifier transfer curve for the Saleh method in its default configuration to show the nonlinear input-to-output signal. After amplification, plotted constellation points get displaced according to the input-to-output characteristics of the amplifier model. The voltage for each constellation point determines the direction and magnitude of distortion for each point in the constellation.

This set of constellation diagrams shows the constellation distortion for the amplifier configured to distort the amplitude and phase individually and together.

• The AMAM diagram shows the signal with amplitude-to-amplitude distortion but no amplitude-to-phase distortion. AM/AM distortion displaces constellation points radially away from or toward the origin. For the Saleh method and specified operating characteristics, the inner corner constellation points move away from the origin and the outer corner constellation points move toward the origin. The noncorner constellation points move a negligible amount.

• The AMPM diagram shows the signal with amplitude-to-phase distortion but no amplitude-to-amplitude distortion. AM/PM distortion causes rotation of constellation points. Constellation points farther from the origin are more displaced than points closer to the origin.

• The AMAM and AMPM diagram shows the signal with amplitude-to-amplitude distortion and amplitude-to-phase distortion. In this diagram, the constellation points move away from or toward the origin and rotate counter-clockwise.

This scatter plot of the constellation points shows points displaced by AM/AM distortion only. The scatterplot function used here plots constellation point locations labeled to highlight the relative distance each point has moved from the ideal constellation point location. To analyze the magnitude and direction each constellation point moved, you would need to replot the input and output characteristic curve for the current operating characteristics.

To explore the model try adjusting settings for the Memoryless Nonlinearity blocks to:

• Apply different impairment model methods.

• Apply different levels of impairments.

• Update the testPA.m helper function to plot and analyze input and output characteristics at nondefault Memoryless Nonlinearity block settings.