Implementation of a QPSK Modulation System for Bandpass Transmission

This model implements a QPSK modulation system for transmission over a bandpass channel with center frequency fc = 100 Hz and bandwidth B = 30 Hz, where additive white Gaussian noise (AWGN) is introduced at the receiver. The system utilizes square-root raised cosine (SRRC) filters with an excess bandwidth parameter α = 0.18, which can be implemented using MATLAB's rcosdesign function with appropriate roll-off factor and filter length specifications. The receiver architecture incorporates a non-data-aided (NDA) timing recovery loop followed by a decision-directed (DD) phase recovery loop, typically implemented using interpolation filters and phase-locked loop (PLL) algorithms. The implementation details include comprehensive modeling of both modulator and demodulator components, covering required circuit elements, parameter configurations, and signal processing blocks. Performance analysis examines how system behavior is affected by varying noise conditions, filter characteristics, timing recovery parameters, and phase recovery loop settings. The model also explores potential enhancements for improved performance, such as implementing higher-order modulation schemes like 16-QAM, incorporating advanced error correction techniques including LDPC or turbo codes, and optimizing channel coding methods using Reed-Solomon or convolutional coding algorithms. These improvements facilitate better understanding and optimization of QPSK system performance under various channel conditions. Key implementation aspects include: - Symbol mapping using constellation points at ±1 ± j - SRRC filter design with specified roll-off factor - Timing recovery using Gardner algorithm implementation - Phase recovery through Costas loop implementation - BER calculation through Monte Carlo simulation