This model applies RF impairments to a signal modulated by differential quadrature phase shift keying (DQPSK). To demonstrate and visualize the RF impairments, levels applied in this model are exaggerated and not representative of typical levels for modern radios.
A random signal is DQPSK modulated and various RF impairments are applied to the signal. The model uses impairment blocks from the RF Impairments library. 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. Since 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. To analyze the constellation, the model includes Constellation Diagram blocks after modulation, before correction, and after correction.
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.
Open the example model
After Modulation diagram, shows the clean reference DQPSK modulated constellation.
The transmitted signal is distorted by various RF impairments. The
Before Correction diagram shows the attenuated and distorted constellation.
The signal on the correction path is adjusted by the DC Blocker, AGC Block, and I/Q Imbalance Compensator blocks. The
After Correction diagram shows the constellation has been amplified and improved after the correction blocks.
Error rate for corrected signal: 0.000 Error rate for uncorrected signal: 0.042
To run the model yourself, open the example using the button provided or by entering
open slex_rcvrimpairments_dqpsk at the MATLAB® command prompt. Adjust the RF impairments, rerun the model, and notice the changes to the constellation diagrams and error rates. Consider 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.