Channel Modeling and Evaluation on Terahertz Orbital Angular Momentum (OAM)-Based Intersatellite Communications

Providing continuous bandwidth of several tens of gigahertz, terahertz (THz) intersatellite links (ISLs) hold great promise for high-speed space communications. To address the lack of spatial degree of freedom (DoF) to further enhance the capacity of THz ISLs, orbital angular momentum (OAM) is introduced, which realizes orthogonal signal transmission in the mode domain. However, the satellite perturbation caused by the oblateness of the Earth, called the $J_{2}$ effect, leads to beam deviation, which induces undesired OAM mode transition and results in interchannel interference (ICI). In this article, a deterministic channel model for OAM-based THz ISLs considering $J_{2}$ perturbation is proposed. Specifically, an analytical solution of the relative motion equation (RME) of satellite pairs is derived to precisely characterize the beam deviation induced by $J_{2}$ perturbation. Then, the OAM mode decomposition is derived for deviated THz OAM beams, based on which the intended signals and interference signals are modeled. By incorporating the $J_{2}$ perturbation characterization into the OAM mode decomposition, the interference power and capacity of OAM-based THz ISL can be determined. For OAM-based THz ISLs, simulation results indicate that: 1) the capacity first increases and then decreases with growing deviation; 2) with proper deviation (~1.5 km), the implementation of OAM can provide double or higher peak capacity compared to THz ISLs without OAM; 3) with significant deviation (>3 km), a larger receiver aperture decreases the capacity; and 4) the proper radius of the receiver aperture is 0.5 m and satellite pairs should calibrate after 180 min at 600 km operating in dual-OAM mode multiplexing to restrict deviation and ensure high data rates.