Modeling and analysis of terahertz non-terrestrial networks

Abstract

With the recent advances in space and aerial technologies, integrating satellites, aerial vehicles, and ground stations within the non-terrestrial network (NTN) architecture has emerged as a promising solution for the global coverage requirement of the sixth-generation (6G) communication systems. To provide extreme data rates globally with NTNs, the utilization of high frequencies can be considered, among which the millimeter wave (mmWave) and terahertz (THz) bands arise as prominent candidates since they demonstrate certain advantages over their counterparts. Though mmWave/THz transmission is sensitive to atmospheric conditions and weather-dependent effects, the multi-layer structure of the NTN architecture and high-gain directional antennas/arrays enable reliable communication in the mmWave and THz bands. Hence, the mmWave/THz NTNs have gathered considerable interest from the academic community. Within the scope of this thesis, the channel characteristics, feasibility, and statistical performance of THz NTNs are investigated and analyzed. Throughout the thesis, a statistical model for the impact of antenna misalignment is proposed, and performance analyses for various communication scenarios are presented by taking the hardware imperfections into consideration. First, the influence of antenna misalignment on the mmWave and THz communications is investigated. Since highly directional antennas/arrays with pencil-sharp beams are utilized in mmWave/THz transmission, the perfect alignment between transmitter and receiver is critically important. When the antennas/arrays are misaligned due to non-stable positioning, moving platforms, or jitters, the received power significantly reduces, referred to as pointing errors. To characterize this phenomenon, a simple analytical model for the pointing error in mmWave/THz links is proposed. Unlike the existing models, the proposed model incorporates the antenna element radiation pattern and the array design. To accomplish this, the main lobes of the antenna element radiation pattern and the array factor are modeled as Gaussian beams. Then, the main lobe of the array pattern is formulated by accounting for the maximum gain and 3 dB beamwidth of the antenna element and number of array elements, element spacing, and 3 dB beamwidth of the array factor, which is validated via electromagnetic simulations in CST Microwave Studio. By using the main lobe of the array pattern, the statistics of the pointing error are derived, and it is shown that the pointing error follows a special case of the negative log-Gamma distribution with the shape parameter of 2 and the scale parameter depending on the antenna/array designs and jitter variance. To examine the impact of the antenna and array design parameters on the system performance through pointing errors, outage probability of a horizontal high-altitude platform station (HAPS)-to-HAPS communication scenario is analyzed. The results have revealed that antenna/array designs are as influential as the jitter variance, and they cannot be neglected. Secondly, link budget analysis for mmWave/THz NTNs is presented to investigate their feasibility by considering all possible uplink/downlink inter-layer and intra-layer communication scenarios. With this objective, large- and small-scale channel effects are taken into account to obtain the received power in various channel conditions. Afterwards, the received signal-to-noise ratio (SNR) is evaluated by considering the frequency-dependent noise characteristics. Then, the maximum achievable data rate is found for a wide range of system and channel parameters to quantify the performance in inter-layer and intra-layer links. The results have illustrated that the extreme losses in ground-satellite links can be reduced to a compensable level owing to the multi-layer structure of the NTN architecture, enabling multi-gigabit links. More precisely, the total propagation distance in ground-satellite links can be divided into two distinct segments by utilizing aerial nodes. The first segment spans the distance between satellites and aerial nodes in the upper atmospheric layers where the atmosphere remains sufficiently thin to meet negligible absorption. In contrast, the second layer lies within the lower atmospheric layer. Here, even though the distance is shorter, the denser concentration of absorber particles combined with weather-dependent effects lead to more frequent absorption. Thirdly, a dual-hop inter-satellite THz communication system's outage and symbol error rate (SER) performance are examined. In the system of interest, it is assumed that the direct line-of-sight (LOS) is not available between the source and destination low earth orbit (LEO) satellites, and the communication is established with the aid of another LEO satellite at a higher altitude that employs the variable-gain amplify-and-forward (AF) relaying. To assess the performance of the system, the statistics of the end-to-end SNR are derived in the presence of free-space loss, absorption loss, ionospheric loss, fading, and pointing errors. Moreover, the asymptotic outage analysis is performed to gain insights about the system performance. It is shown that reliable inter-satellite communication links can be established with feasible transmit power levels. Fourthly, a dual-hop multi-HAPS THz NTN is considered under hardware impairment noise. In this setup, the HAPS system that provides the maximum SNR utilize the variable-gain AF relaying to aid the transmission from the satellite to the ground station. To obtain the outage probability, asymptotic outage probability, and ergodic capacity expressions, the statistics of the end-to-end SNR are obtained. The impacts of hardware impairment levels, zenith angles, and atmospheric conditions on the system performance are illustrated by using these statistics. The findings reveal that hardware impairment noise results in a power loss in the outage performance and reduces the overall system capacity. This adverse effect can be mitigated by increasing the number of HAPS systems. Furthermore, the results have shown that the system performance is determined by either fading or pointing error characteristics in high SNR region. Finally, the SER and ergodic capacity performance of a dual-hop THz NTN are investigated under in-phase (I)/quadrature (Q) imbalance. In the considered system model, multiple HAPS systems act as relays to assist transmission by utilizing the variable-gain AF relaying. At the ground station, the received signals from all HAPS systems are combined by using the maximal ratio combining technique. To quantify the performance, the statistics of the received SNR are derived in the presence of free-space loss, absorption loss, fading, pointing errors, and receiver I/Q imbalance. The impacts of various satellite and HAPS deployments and amplitude/phase mismatch levels are illustrated. It is demonstrated that error floors and lower capacity limits are observed due to I/Q imbalance. High SNR analysis have shown that the system performance depends on either fading or pointing error parameters in the case of perfect hardware. In addition, the results have revealed that the SER performance remains almost the same regardless of the atmospheric conditions for low zenith angles in HAPS-to-ground station link. The main contributions of this thesis to the current literature can be summarized as follows: First, to characterize the impact of antenna and array designs on the pointing error characteristics in mmWave/THz transmission, a simple analytical model is proposed. By using this model, the variation in the system performance in the presence of different types of antenna elements and/or different array configurations can be investigated, enabling more realistic performance analysis and system design. Secondly, the feasibility assessment of mmWave/THz NTNs is performed by considering various channel conditions. It is demonstrated that reliable mmWave/THz communication links can be achieved in NTNs owing to the multi-layer structure of the NTN architecture. Afterwards, the performance analysis for a dual-hop inter-satellite THz communication system is presented. The results have revealed that satisfactory performance can be achieved with feasible transmit power levels. As the fourth contribution, the performance of a dual-hop multi-HAPS NTN is analyzed in the presence of hardware impairment noise. The performance limits due to the noise caused by non-ideal equipment are illustrated. Finally, the influence of I/Q imbalance led by the amplitude/phase mismatches in the local oscillators on a dual-hop multi-HAPS NTN is examined. From the results, it is inferred that receiver I/Q imbalance leads to error floors and lower capacity limits, deteriorating the system performance.