  Integrated satellite-terrestrial communications networks aim to exploit both
the satellite and the ground mobile communications, thus providing genuine
ubiquitous coverage. For 5G integrated low earth orbit (LEO) satellite
communication systems, the timing advance (TA) is required to be estimated in
the initial random access procedure in order to facilitate the uplink frame
alignment among different users. However, due to the inherent characteristics
of LEO satellite communication systems, e.g., wide beam coverage and long
propagation delays, the existing 5G terrestrial uplink TA scheme is not
applicable in the satellite networks. In this paper, we investigate
location-based TA estimation for 5G integrated LEO satellite communication
systems. We obtain the time difference of arrival (TDOA) and frequency
difference of arrival (FDOA) measurements in the downlink timing and frequency
synchronization phase, which are made from the satellite at different time
instants. We propose to take these measurements for either UE geolocation or
ephemeris estimation, thus calculating the TA value. The estimation is then
formulated as a quadratic optimization problem whose globally optimal solution
can be obtained by a quadratic penalty algorithm. To reduce the computational
complexity, we further propose an alternative approximation method based on
iteratively performing a linearization procedure on the quadratic equality
constraints. Numerical results show that the proposed methods can approach the
constrained Cramer-Rao lower bound (CRLB) of the TA estimation and thus assure
uplink frame alignment for different users.
