DSSS Chip-Wise Faster-than-Nyquist Signaling with DPSK for Robust Carrier Synchronization

Direct sequence spread spectrum (DSSS) signals possess strong and undesirable cyclostationary features when used in covert communications. This paper addresses their mitigation via chip-wise faster-than-Nyquist (FTN) signaling and time-varying pseudo-random SNR-optimal spreading sequences. We append a differential phase-shift keying (DPSK) encoder to the DSSS/FTN system to use a non-coherent differential receiver that facilitates carrier synchronization in challenging scenarios involving phase, frequency, and frequency-rate offsets. The proposed DSSS/FTN system proves robust with a bit-error-probability performance close to its traditional DSSS/Nyquist counterpart. Our results pave the way for designing low probability of detection/intercept waveforms without incurring a noticeable extra complexity at the receiver side.