A Blind Despreading and Demodulation Method for QPSK-DSSS Signal With Unknown Carrier Offset Based on Matrix Subspace Analysis

Direct sequence spread spectrum (DSSS) signals are widely used in various military and civilian communication systems. It adopts the pseudorandom (PN) sequence to modulate baseband signal before transmission, leading to some unique and useful properties such as lower working signal to noise ratio (SNR), strong anti-jamming capability, and robustness to the effect of multi-path fading. In past few years, blind despreading for DSSS signals has been a hot topic in cognitive radio field. Previous works are mostly concentrated on BPSK-DSSS signals, and have developed theories and methods to estimate the PN sequence effectively. But unfortunately, most of them are powerless to process QPSK-DSSS signals. This work proposes a method based on matrix subspace analysis to recover the PN sequences and transmitted symbols for QPSK-DSSS signals with unknown carrier offset under non-cooperative reception occasion. To recover spreading sequences from subspace of self-covariance matrix, we derive the theoretical expressions of eigenvector by studying the complex hermit matrix decomposition and then investigate the influence of carrier residual. Finally, a structure is designed to complete blind despreading and demodulation from the received waveform to bit stream. Simulation verifies pretty good bit-error-rate (BER) performance of proposed algorithm.