Pointing-Error-Induced Fading in an Open-Loop THz Uplink with Hardware Impairments

TL;DR

The paper tackles open-loop pointing for ground-to-THz NTN uplinks, where narrow beams and rapid satellite motion make precise alignment critical. It introduces a mechanical motion model of a ground-based alt-azimuth mount that includes latency, sampling, and trapezoidal acceleration constraints, and integrates resulting pointing errors into a link-budget via a pointing-loss model. By evaluating multiple satellite passes and control strategies, it shows that an optimized velocity profile (Profile C) significantly reduces pointing deviation and fading, especially for narrow beams, while a naive maximum-velocity strategy degrades performance. The work provides practical design insights and performance guidelines for robust high-frequency uplinks under realistic hardware constraints, including how to balance beam directivity with pointing tolerance and latency effects. Future work includes fine-tuning time steps, enabling adaptive velocity control, incorporating atmospheric effects, and examining end-to-end impact on NTN systems.

Abstract

We analyze the open-loop mechanical tracking performance of a sub-Terahertz (sub-THz) and Terahertz (THz) uplink communication system. These high-frequency bands enable multi-gigabit links through large bandwidths and narrow beams, but require precise pointing to overcome spreading loss. A tracking system can be used to orient horn antennas toward mobile targets. We develop a mathematical model that captures the mechanical dynamics of a real tracking system, which includes motion latency and acceleration and velocity limits, to quantify pointing errors during satellite passes and integrate these effects into the link budget. We evaluate the trade-offs between beam directionality and pointing tolerance across different Low Earth Orbit (LEO) satellite trajectories and control strategies. The results link the hardware limitations to the communications performance, providing design guidelines for high-frequency Non-Terrestrial Network (NTN) uplink under practical mechanical constraints.

Figures (6)

• Figure 1: Phases of movement at each time step. Velocity and position refer to a generic angular coordinate.
• Figure 2: of the pointing error for the three considered trajectories with maximum altitude $\ang{47}$, $\ang{70}$, and $\ang{83}$, and the three considered velocity profiles A, B, and C.
• Figure 3: Lagging behavior of velocity profile B for the $\ang{83}$ trajectory.
• Figure 4: Oscillatory behavior of the mount with velocity profile A and C for $\ang{47}$ trajectory: the WAIT, LATENCY, and MOVE phases are clearly distinguishable for each movement.
• Figure 5: of the pointing loss $L_p$ for the three considered trajectories with maximum altitude $\ang{47}$, $\ang{70}$, and $\ang{83}$, for the three considered velocity profiles A, B, and C (color-coded), and for the three different values of gain over Half Power Beamwidth (HPBW) $G_t / \text{HPBW}$ (coded in the line patterns).