Spread-Spectrum Continuous-Phase-Modulated Signals for Satellite Navigation

Spread-spectrum continuous-phase-modulated (SS-CPM) signals are investigated with application to satellite navigation. In particular, focus is devoted to the special subclasses of SS-CPM with semi-integer modulation index h greater than one. This constitutes a novelty with respect to traditional navigation signal design on one hand (which is based on direct-sequence spread-spectrum (DS-SS) signals with rectangular pulse) and with traditional communication SS-CPM with h < 1 on the other. Main output is that setting the modulation index h > 1 to semi-integer values allows the SS-CPM signals to behave like having subcarriers, exactly in the same way as it happens for the binary offset carrier (BOC) modulation. The main advantage is that CPM ensures a constant envelope at the transmission side, thus being appealing when these features are required at the system level. After a description of signal properties, focus is devoted to ranging code synchronization, by exploitation of the theory on time-delay estimation (TDE). A few delay-lock loop schemes are also presented, based on an offset quadrature phase-shift keying (OQPSK) approximation of the signal at the receiver side, achieved by Laurent's decomposition at the receiver side. Proposed loops performance are analyzed in terms of Cramer-Rao bound (CRB), rms tracking error, and multipath robustness. An application of SS-CPM for a two-rate-service scheme is proposed.