GHz-to-THz broadband communications for 6G non-terrestrial networks

Recently, Terahertz (THz) band communications at various atmospheric altitudes have been studied due to larger bandwidth availability and reduced water vapor concentrations at higher atmospheric altitudes as compared to sea level. In this paper, as special cases of 6G aerial communication networks, we consider: (1) Low Altitude Platform-to-High Altitude Platform (LAP-to-HAP), (2) HAP-to-HAP, and (3) HAP-to-Satellite (HAP-to-SAT) GHz-to-THz broadband communications over (1-1000) GHz by analyzing total path loss and total usable bandwidth. For obtaining realistic absorption loss at practical altitudes, we employ the International Telecommunications Union's (ITU) model using the standard weather profile. We consider four practical carrier frequencies offering low absorption loss values i.e., f1 = 0.140 THz (D band: 110-170 GHz), f2 = 0.300 THz (275-325 GHz), f3 = 0.750 THz, and f4 = 0.875 THz for analyzing total path loss. Numerical results show that due to improved atmospheric conditions particularly above 23 km altitudes, increasing the Rx-HAP altitude in HAP-to-HAP communications promises a lower total path loss of up to 7.7 %, even at the cost of an increase in the Tx-Rx-HAP distance from 1 km to 34 km, promising Tbps rates for 6G non-terrestrial communications. Additionally, total usable bandwidth analysis demonstrates that with total antenna gains of 80 dBi, bandwidth in the order of 100s of GHz is usable for the LAP-to-HAP scenario, the entire considered broadband is usable for the HAP-to-HAP scenario between 16 km to 50 km, and the HAP-to-SAT scenario between a HAP at 19 km and SAT at 100 km, truly showcasing the potential of employing GHz-to-THz broadband communications cognitively over (1-1000) GHz for various practical 6G non-terrestrial networks.