Tutorial on QPSK and Demonstrating It with pSim Plus

Introduction

Quadrature Phase Shift Keying (QPSK) is a popular modulation scheme used in coherent fiber optic communication systems. QPSK encodes two bits per symbol by modulating the phase of the carrier signal to one of four possible values (0, π/2, π, 3π/2 radians).

pSim Plus Simulator

Latitude Design Systems' pSim Plus photonic circuit simulator supports modeling and simulating QPSK coherent links end-to-end. This tutorial walks through setting up and running a basic QPSK link simulation in pSim Plus.

The key components are:

1. Transmitter QPSK Precoder and I/Q Modulator

2. Laser Source

3. Optical Fiber with Nonlinearity and Polarization Mode Dispersion

4. Coherent Receiver with Hybrid 90° and Photodiodes

5. DSP Algorithms like Clock Recovery and Carrier Recovery

6. BER Calculation

The example coherent link model shown above includes the Tx_DQPSK_Array component which handles QPSK precoding, an ideal laser source LaserCW, the fiber transmission link components like FiberNLS and FiberNLS_PMD, the coherent receiver components like Hybrid90deg and Photodiode, and finally the DSP stages like ClockRecovery4D and the error analyzer BER_4D.

Setting up the simulation is straightforward using pSim Plus' graphical interface to instantiate and connect the components. Key parameters to adjust include:

• Laser linewidth and power

• Modulation rate and number of symbols

• Fiber type, length, nonlinearity, PMD coefficients

• Local oscillator linewidth

• DSP algorithms and loop bandwidth

Once set up, the simulation can be run in either frequency or time domain modes. Frequency domain is faster for analyzing steady-state behavior, while time domain captures transient effects. Results are available through visualizers like constellation diagrams and eye diagrams, as well as calculating parameters like BER and EVM.

Here is the constellation after transmission showing some phase noise and nonlinear distortion:

Conclusion

pSim Plus makes it easy to vary parameters like fiber length, modulation rate, DSP settings and more to optimize the link performance. Its combination of photonic device modeling with advanced DSP algorithms enables comprehensive simulation of modern coherent communication systems like QPSK all in one powerful tool.