MATLAB Implementation of Complete QPSK Demodulation

This document provides a detailed discussion of the complete QPSK demodulation process. The full demodulation workflow includes both carrier synchronization and bit synchronization, where bit synchronization utilizes the early-late gate algorithm while carrier synchronization employs decision-directed feedback technology. We will systematically introduce the principles and operational methods for each step to facilitate better understanding and application of QPSK demodulation techniques. The implementation involves generating matched filters using raised-cosine pulses, implementing the early-late gate algorithm for symbol timing recovery by comparing early and late sampling instances, and applying decision-directed carrier phase recovery through phase error detection based on demodulated symbols. Key MATLAB functions include comm.QPSKDemodulator for basic demodulation, comm.SymbolSynchronizer for timing recovery, and custom phase-locked loop implementations using phasediff and loop filter functions for carrier recovery.