Interference Suppression for Spaceborne NOMA: A Delay-Doppler Domain Shaping Method

The large channel dynamics of low-earth-orbit satellite introduces random time-frequency offsets on the multiple uplink signals. The resulting multi-user interference (MUI) reflects a user-specific circular shifting property in the delay-Doppler (D-D) domain. This paper proposes a D-D domain shaping method which exploits this property to ease the MUI suppression for spaceborne non-orthogonal multiple access (NOMA) with a low-complexity successive interference cancellation (SIC) receiver. Specifically, a shaping filter with non-uniform power density is optimized and identically employed by multiple users to enlarge power gaps and facilitate receiving in a statistical manner. A novel mixed-level SIC structure is also designed to effectively alleviate the resulting inconstant power ordering at D-D domain. Theoretical capacity analysis reveals that the proposed scheme promises to suppress the MUI especially in symmetric channels. Simulation results provide a rate gain over 20% compared with the conventional NOMA under typical satellite communication settings.