OTFS-Based Super Resolution Channel Estimation in Multi-Satellite Coordinated Transmission

Cohesive clustered satellite (CCS) system can utilize multi-satellite coordinated transmission (MSCT) to enhance the sum rate and provide direct satellite-to-device connectivity, which is regarded as a key component for low Earth orbit (LEO) satellite-integrated Internet. Considering the high-mobility LEO satellites and fractional Doppler interference (FDI) due to the non-integer Doppler tap, we utilize orthogonal time frequency space (OTFS) modulation to mitigate the complex delay-Doppler effects on a linear time-varying (LTV) channel. Then, we analyze the impact of FDI and the block circulant matrix with circulant block (BCCB) structure on the OTFS channel matrix, and derive the approximate super resolution (SR) relationships between the integer and fractional Doppler channel matrices. Furthermore, to improve communication efficiency and reliability of MSCT, we propose an OTFS-based super resolution-fractional Doppler channel estimation (SR-FCE) scheme, and introduce an enhanced low-correlation-zone periodic sequence (ELPS) superimposed on the OTFS frame to lower the peak-to-average power ratio. In the coarse estimate stage of SR-FCE scheme, a low-resolution channel matrix is obtained via the threshold method, followed by the fractional Doppler network (FracNet) to extract FDI parameters for high-resolution channel matrix reconstruction. Simulation results validate the feasibility of the SR-FCE scheme in MSCT, and outperforms the state-of-the-art schemes in terms of normalized mean squared error and bit error rate.