MATLAB Implementation of Complete QPSK System: From Random Sequence to Symbol Level with M-Sequence Spread Spectrum and Modulation/Demodulation

This article presents a detailed implementation of a complete QPSK system using MATLAB, covering the entire process from random sequence generation to symbol level mapping, m-sequence direct sequence spread spectrum, QPSK modulation/demodulation, and RF modulation/demodulation. The implementation begins with generating random binary sequences using MATLAB's rand() or randi() functions, which are then mapped to symbol levels through constellation mapping algorithms. The program employs m-sequences for direct sequence spread spectrum, utilizing shift register implementations for both spreading and despreading operations. QPSK modulation is achieved using I/Q modulation techniques with cosine and sine carriers, while demodulation implements coherent detection with phase recovery algorithms. RF modulation and demodulation stages simulate realistic transmission conditions including carrier frequency shifting and channel effects. Each processing stage includes detailed implementation considerations, such as using MATLAB's communication toolbox functions for pulse shaping and matched filtering, and custom algorithms for timing recovery and symbol synchronization. The program architecture allows easy extension to other modulation schemes and communication systems, providing a versatile framework for implementing various digital signal processing functionalities in communication systems.