Two-Dimensional Joint Acquisition of Doppler Factor and Delay for MC-DS-CDMA in LEO Satellite System

Low Earth Orbit (LEO) satellite systems have received considerable attentions for its advantages and potential applications in global communication. Yet LEO satellite-ground communication is challenged by the huge Doppler shift. This article is devoted to studying how to deal with this challenge in Multi-carrier Direct Sequence Code Division Multiple Access (MC-DS-CDMA) systems in the presence of multi-path and partial band interferences for LEO satellite-ground link in urban areas. To this end, we design a two-dimensional joint acquisition algorithm. Specifically, we acquire the delay and Doppler factor by solving the Maximum Likelihood (ML) problem with decision variable falling into two-dimensional uncertain region of the estimated parameters. We then use grid-based searching method to access the ML estimates of the delay and Doppler factor at the cost of acceptable complexity. We also derive the detection probability and Mean Square Error (MSE) in both Additive White Gaussian Noise (AWGN) and Rayleigh fading channels. It is shown that the performance of MC-DS-CDMA with the proposed algorithm significantly outperforms its single-carrier counterpart in Rayleigh fading channel, and is proved to be more tolerant to partial band interferences for both AWGN and Rayleigh fading scenarios.