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Add RF Impairments to DQPSK Signal

This example applies RF impairments to a signal modulated by the differential quadrature phase shift keying (DQPSK) method. To show the RF impairments, the example applies exaggerated levels that are not typical levels for modern radios.

In this example, the slex_rcvrimpairments_dqpsk model DQPSK-modulates a random signal and applies various RF impairments to the signal. The model uses impairment blocks from the RF Impairments library. The InitFun callback function initializes simulation variables. For more information, see Model Callbacks (Simulink).

After the impairment blocks, the signal forks into two paths. One path applies DC blocking, automatic gain control (AGC), and I/Q imbalance compensation to the signal before demodulation. The signal on the correction path is adjusted by the DC Blocker, AGC, and I/Q Imbalance Compensator blocks. Because the signal is DQPSK modulated, no carrier synchronization is required. The second path goes directly to demodulation. After demodulation, an error rate calculation is performed on both signals. The model includes Constellation Diagram blocks after modulation, before correction, and after correction so that you can analyze the constellation.

When the model runs, constellation diagrams plot the signal at these stages in the simulation:

  • The After Modulation constellation diagram shows the reference DQPSK-modulated signal constellation.

  • The Before Correction constellation diagram shows the attenuated and distorted signal constellation.

  • The After Correction constellation diagram shows the signal has been amplified and improved after the correction blocks.

The error rate for the demodulated signal without AGC is primarily caused by free space path loss and I/Q imbalance. The QPSK modulation minimizes the effects of the other impairments.

Error rate for corrected signal:   0.000
Error rate for uncorrected signal: 0.042

To explore the model try:

  • Adjusting RF impairment settings, rerun the model, and notice the changes to the constellation diagrams and error rates.

  • Modifying the model to add an equalizer stage before the demodulation. Equalization has inherent ability to reduce some of the distortion caused by impairments. For more information, see Equalization.

See Also

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