Real-time implementation of all-digital optical time transfer for optical satellite links

Several architectures for autonomous optical synchronisation (time transfer) and ranging between satellites have been proposed, which generally require custom transceiver implementations with additional components. These architectures are not well suited or even feasible to the needs of communications transceivers for optical satellite links. In this paper, we analyze the feasibility of integrating time transfer functionality into an optical communications link primarily intended for data transfer. A fully digital system in which all signal processing occurs in the digital domain is presented. The transmitted optical waveform consists primarily of data, as well as a data field containing observables necessary for computing precise and stable two-way time transfer. At the receiver, the signal is detected and demodulated using conventional intradyne signal processing algorithms. The time transfer observables are then obtained at the frame detection by using the timing recovery subsystem to estimate the time of arrival. To validate the feasibility of the proposed time transfer function, a real-time, 5 GBit/s QPSK FPGA-based laboratory demonstrator was developed.