Terahertz Band for High-Altitude Platform Station Communications: Performance Analysis, Potential, and Challenges

The Terahertz (THz) band (0.1–10 THz) attracts attention due to its massive bandwidth, potentially enabling ultra-high data rate with ultra-low latency. High-altitude platform stations (HAPSs) are envisioned as nodes of a non-terrestrial network (NTN) that complements terrestrial networks, particularly for connecting the unconnected and addressing temporary needs, such as disaster relief. THz HAPS communication is a promising solution for NTN, since HAPS is quasi-stationary in nature, located in the stratosphere, thereby avoiding beam misalignment and molecular absorption loss issues in the THz band. In this paper, we provide an overview of THz band HAPS communication, considering HAPS-to-ground, HAPS-to-HAPS, HAPS-to-satellite, and HAPS-to-uncrewed aerial vehicle links. We present simulation results of outage probability, ergodic capacity, and spectral efficiency under beam misalignment fading and turbulence fading in comparison to millimeter wave and sub-6 GHz bands. In addition, we summarize existing THz HAPS studies to reveal ongoing efforts. Moreover, we discuss open issues and research directions for THz band HAPS-related links. Overall, this paper draws a clear picture of THz HAPS communication by reporting its potential and challenges.