Advanced Communication Engg. Lab (EC-804) - Important Questions

Explain the generation of Differential Phase Shift Keying (DPSK) signal. Draw a detailed block diagram showing the differential encoder, PSK modulator (I/Q paths if used), and necessary filtering. For the bit sequence 1101001, sketch the expected phase transitions and corresponding time-domain waveform at the modulator output. Describe the steps required to implement this transmitter in the laboratory using standard lab equipment (signal generators, modulators, filters, oscilloscope).

Derive the bit error rate (BER) expression for coherent detection of DPSK in an additive white Gaussian noise (AWGN) channel. Express the final result in terms of $E_b/N_0$ and compare the resulting BER with that of coherent BPSK. Explain the reason for any performance difference.

Describe noncoherent detection of DPSK. Derive the BER for noncoherent DPSK detection in AWGN and compare its performance with coherent DPSK. Sketch qualitatively the BER versus $E_b/N_0$ curves for coherent DPSK, noncoherent DPSK, and coherent BPSK on the same plot and explain the relative positions.

Design a laboratory experiment to implement DPSK generation and detection using a differential encoder and an I/Q modulator/demodulator. Provide the block diagram, list of required instruments and components, step-by-step test procedure, points to measure (waveforms, phase transitions, SNR), and sample expected oscilloscope and spectrum analyzer observations.

Draw and explain the constellation representation appropriate for DPSK (showing phase transitions rather than absolute symbol locations). Explain how differential encoding maps input bits to phase changes (for example, mapping a '0' to no phase change and a '1' to a phase change of $\pi$). For a sequence of three bits, illustrate the resulting phase transitions explicitly.

Analyze the effect of phase noise and carrier frequency offset on the detection of DPSK signals. Quantify how a constant phase offset of $\theta_0$ affects the differential decision metric, and discuss why DPSK offers robustness to slowly varying phase compared to coherent PSK. Propose laboratory mitigation techniques for phase noise and frequency offset.

For a practical DPSK receiver in the laboratory, describe algorithms and circuitry used for carrier recovery and symbol synchronization. Discuss trade-offs between differential detection (no explicit carrier recovery) and coherent demodulation with carrier recovery in terms of complexity, performance, and sensitivity to frequency/phase errors. Outline a simple lab procedure to validate the chosen synchronization approach.