Terahertz-band communications at various atmospheric altitudes

Official URL: https://risc01.sabanciuniv.edu/record=b2579272_(Table of contents)

Terahertz (THz) band (0.1-10 THz) communications shows a huge potential for the prospective beyond 5G and 6G wireless systems, as the band offers large bandwidth and data rates as compared to the existing sub 6 GHz bands, which are almost saturated. Traditionally, the THz band has been employed for the sea-level-short range communication, due to the large absorption loss the THz waves experience by the water vapor molecules present in the atmosphere, which decreases across higher atmospheric altitudes, where the communication over the THz band can be highly leveraged among various practical aerial vehicles. In this dissertation, first, we investigate path loss over the THz band (0.75-10 THz) at various atmospheric altitudes, ranges and directions by realistically calculating the THz absorption loss. Four practical altitudes are considered, corresponding to Drone-to-Drone (Dr2Dr), Jet plane-to-Jet plane (J2J), Unmanned Aerial Vehicle-to-Unmanned Aerial Vehicle (U2U), and near-space Space-to-Space (S2S) communications. Following comparison and validation with two real-world experimental results from the literature measured at the sea-level, Line by Line Radiative Transfer Model (LBLRTM) is used to obtain realistic THz transmittance values for each altitude case and setting. Numerical results show that as the altitude increases, the water vapor concentration decreases, enabling the communication over the THz band to be more feasible as compared to the sea-level communication. Moreover, the total usable bandwidth results over the THz band exhibit that the upper bounds of 8.218 THz, 9.142 THz and 9.250 THz are usable up to the transmission distance of 2 km for J2J, U2U and S2S communication cases, respectively. Next, the THz band is further explored for the identical four aerial communications at the practical altitudes, variable directions and distances, under realistic weather and channel fading conditions, due to beam misalignment and also multi path fading. A channel model for aerial communications at THz band is proposed to calculate the common flat bands (CFB) for frequency-selective path gain and the colored noise spectrums, both of which are highly affected by the atmospheric conditions. An extensive capacity analysis is presented, considering equal power (EP) and water-filling (WF) allocation, showing that when there is no fading, capacity for aerial links is several orders of magnitude larger than the sea-level capacity. For both the proposed CFB and the Standard (STD) approaches, the sea-level capacity is enhanced by an order of magnitude for the drones, which is doubled for the jet plane scenario, which is further tripled for UAVs, which is again increased by another order of magnitude for the satellite communications. When ergodic capacity is computed for the fading scenarios, it is shown that the impact of fading vanishes at higher altitudes. Sea-level ergodic capacity is increased by an order of magnitude for drone-to-drone communications, providing several Tbps at 10 m, while 10s of Tbps is achievable among jet planes and UAVs, and several 100s of Tbps is possible for satellites/cubesats at 1 km under fading, suggesting that THz band is a promising alternative for aerial communications. Then, we consider various realistic mobility scenarios of THz-enabled Dr2Dr links over the THz band (0.75-4.4 THz), also incorporating real drone mobility traces. Additionally, we consider real THz antennas over the THz band (0.75-4.4 THz) under various drone mobility scenarios for evaluating the true potential of utilizing the THz band among drone communications. For maximizing the capacity, we propose a channel selection scheme, MaxActive, which intelligently selects THz narrowband channels promising the maximum capacity. Then, we propose a joint process of channel selection, beamwidth adjustment and power control for the capacity maximization. Based on this joint process, we compare the MaxActive scheme with the CFB scheme as well as the STD narrowband scheme in terms of capacity and spectral efficiency using both WF and EP allocations. It is inferred that the beamwidth misalignment highly affects the THz band Dr2Dr communication. Moreover, the link performance of the MaxActive scheme even with EP approaches the MaxActive and STD schemes with WF, while clearly outperforming CFB and STD with EP.