Time-Misalignment Estimation in Overlapped DVB-S2X Waveforms

Low signal strength, resulting from the distance between satellites and the Earth's surface, remains a primary challenge in integrating them into terrestrial network systems. Nevertheless, the current density of satellite constellations presents an opportunity to design cost-effective receivers capable of utilizing diversity combining techniques to overcome this issue. However, any combining technique relies heavily upon time synchronism between the received signals, which until now has presented a limitation for practical applications in satellite communication systems. This paper proposes a procedure for compensating large symbol time misalignment and estimating the remaining fractional one between two DVB-S2X waveforms (from different satellites) overlapped in time, frequency, and under noise-limited scenarios. We propose a fractional-time-misalignment estimator using the data-aided early late gate (ELG) principle. We evaluate the proposed estimator using the Walsh-Hadamard-based pilot sequences available in the DVB-S2X waveforms. At the same time, we present an estimation of large misalignments based on the correlation properties of the start-of-super-frame (SOSF) field (which uses longer Walsh-Hadamard sequences than the pilots). Simulation results show that the proposed synchronization methods provide an unbiased estimation even when the noise power levels match that of the received signals.