Secrecy Rate Analysis of Satellite Communications With Frequency Domain NOMA

Due to the inherent broadcasting nature and broad coverage of satellite, satellite communications are well known to be vulnerable to security threats. Since the distance difference from satellite to terrestrial terminals is negligible, the channels of different users are similar, posing a challenge of achieving secure satellite downlink transmission in the presence of eavesdroppers (Eves). In this paper, we consider satellite communications in areas without terrestrial networks converge, and investigate the physical layer security in the satellite downlink. To achieve a positive secrecy rate, a frequency domain non-orthogonal multiple access (FD-NOMA) scheme and an according multiuser cooperative scheme are proposed. Particularly, by adopting the FD-NOMA, the spectrum efficiency can be improved at the cost of raising inter-user interference (IUI), and the inherent IUI is elegantly leveraged to suppress the signal-to-interference-plus-noise ratio (SINR) of Eves while the intended SINR of legitimate users can be enhanced by the cooperative scheme. The secrecy rate of satellite communications with FD-NOMA is analyzed, and a tight lower bound is derived, which is validated via numerical results. In addition, the secrecy rate is found to be affected by the level of spectral overlapping, and there exists an optimal spectral overlapping factor (SOF) which can maximize the secrecy rate.