Transmission strategies for UAV-enabled next-generation wireless networks : energy- and spectrum-efficiency perspectives

국문 초록 (Abstract)

미래 무선 네트워크의 많은 시나리오 들에 대한 수요는 무인 항공기 기반 통신 시스템에 의하여 충족되고 있다. 만일 지상의 기지국이 가동이 불가능한 상황이 되거나 특정 지역에 트래픽이 몰리는 상황이 발생할 경우, 무인 항공기 기반 기지국을 그 자리에 배치시키는 방식이 유망한 기술로써 연구되고 있다. 따라서 이러한 상황에서 무인 항공기들을 전력 소비, 주파수 자원 관리, 3차원 배치, 안테나 파라미터 결정 등 여러 요인들을 고려하여 효율적으로 배치시키는 전략에 대한 연구가 매우 중요해지고 있다. 본 학위논문에서는 이러한 무인 항공기 기반 이동통신 시스템의 프론트홀과 백홀 링크에서 사용 가능한 에너지 및 가용 주파수 측면에서 효율적인 새로운 송신 기법을 제시하였고, 이를 테라헤르츠 대역의 무선 광통신 시스템에서도 적용시킬수 있는지에 대한 가능성을 검증하였다. 첫째, 다수의 지향성 안테나를 장착한 무인 항공기 기지국들의 배치 상황에서, 지상 장비들을 에너지 효율적으로 서비스하는 클러스터링 알고리즘을 제안한다. 이는 지향성 안테나의 타원 모양의 빔 패턴에 착안하여 무인 항공기의 3차원 배치를 결정 함으로써 셀 간섭 없이 에너지 효율적으로 지상 장비들을 서비스한다. 둘째, 앞서 시나리오를 확장하여 수동 및 능동 재구성 가능 표면을 항공 플랫폼에 장착하여 상공에 띄움으로써 무인 항공기 기지국들에게 높은 확률의 가시 백홀 링크를 제공하는 알고리즘을 제안한다. 제안한 알고리즘의 흐름에 따라 다수의 무인 항공기들에 대한 백홀 구조를 저복잡도로 최적화 함으로써 지상 백홀 소스의 송신 전력을 최소화할 수 있는 결과를 얻는다. 마지막으로 이러한 무인 항공기 기반의 통신 시스템의 초고주파 테라헤르츠 대역으로의 확장 가능성을 보인다. 구체적으로, 20 km 거리의 상공-지상 간 무선 광통신 링크에 대한 실현 가능성을 20 km 링크의 대기 환경을 모두 반영한 광통신 채널 에뮬레이터와 FPGA 기반 송수신 장비를 통합한 프로토타입을 사용하여 검증한다. 본 학위논문의 연구 결과를 토대로 모든 항공 플랫폼 기반 통신 네트워크를 커버할 수 있는 새로운 송신 체계를 설립할 수 있으며, 이러한 통신 체계가 기존의 송신 시스템에 비하여 성능 향상이 있음을 보일 수 있었다.

다국어 초록 (Multilingual Abstract)

The demand for future wireless networks is being satisfied for various scenarios through non-terrestrial network (NTN) with unmanned aerial vehicles (UAVs). When there are issues concerning the wireless network regarding ground base stations (BSs) malfunctioning or a massive increase in network traffic and coverage holes, deployment of the UAVs with mounted base stations (UAV-BSs) is becoming a promising solution due to its flexibility in mobility, low deployment cost, and rich line-of-sight (LoS) links. Therefore, it is worth investigating the efficient deployment strategies of UAVs concerning several factors, e.g., power control, spectrum usage, geometric parameters, and antenna specifications. In this study, we attempt to examine techniques involving novel transmission strategies that enhance energy- and spectrum-efficiency for both fronthaul and backhaul links of UAV-enabled environments, and provide prospective directions for the application of terahertz (THz) spectrum range by showing the feasibility of the long-distance free-space optical (FSO) communication links. In the first part, we consider a multiple-UAV-BSs scenario, where UAV-BSs are equipped with directional antennas. The orientation and the 3D deployment of the UAV-BSs and the beamwidths of the antennas are adjusted by the proposed ellipse clustering algorithm to provide energy-efficient elliptical coverage to ground users without inter-cell interference. The second part expands on the previous scenario by offering rich-LoS backhaul links to UAV-BSs through aerial reconfigurable intelligent surfaces (RIS). A novel framework is proposed to optimize the structure of the backhaul link, with the aim of minimizing the transmit power of the backhaul source, while maintaining low complexity. In the final part, the potential of UAV-enabled communication systems is explored in terms of the THz spectrum. The feasibility of long-distance FSO links between high-altitude UAVs and ground users is investigated. Video signal transmissions are successfully carried out through an FPGA-based FSO link prototype with a channel emulator that models the atmospheric channel reliably. In conclusion, we propose a novel framework that covers air-to-everything (A2X) wireless networks, which are designed to achieve better performance compared to conventional terrestrial-only transmission scenarios.

목차 (Table of Contents)

• Chapter1. Introduction................................................... 1
• 1.1 Introduction........................................................ 1
• 1.2 Overview of Contributions........................................... 5
• 1.3 Notations........................................................... 7
• Chapter 2. A2G Communications: Novel Interference-Free Deployment of Multiple UAV-BSs: An Energy-Efficiency Perspective........................ 8
• 2.1 System Model and Problem Formulation .............................. 10
• 2.2 Energy-Efficient Ellipse Clustering.................................... 14
• 2.2.1 Hierarchical Clustering Based on Silhouette Index . . . . . . . . . . . . . . 16
• 2.2.2 Ellipse Clustering........................................... 16
• 2.2.3 Complexity of the Ellipse Clustering Algorithm ................ 18
• 2.3 Determining UAV-BS 3D Location.................................... 20
• 2.4 Simulation Results.................................................. 24
• 2.5 Conclusion......................................................... 27
• Chapter 3. A2A Communications: An Energy-Efficient Aerial Backhaul System with Reconfigurable Intelligent Surface.............................. 28
• 3.1 System Model...................................................... 32
• 3.1.1 Aerial Backhaul Model ...................................... 32
• 3.1.2 Aerial-RIS Beamforming Model .............................. 37
• 3.1.3 Problem Formulation ........................................ 40
• 3.2 Proposed Aerial-RIS Setup Algorithm................................. 41
• 3.2.1 Minimizing the Numerator: Determining q..................... 41
• 3.2.2 Maximizing the Denominator: Determining {N ̄ }, ρ ̄, and Θ . . . . . . 44
• 3.2.3 Determining {Pm}m∈M ...................................... 49
• 3.2.4 Complexity Analysis......................................... 53
• 3.3 Implementation of Active RIS........................................ 54
• 3.4 Simulation Results.................................................. 58
• 3.4.1 Simulation Setup............................................ 58
• 3.4.2 Reliability of Aerial-RIS Placement and Phase Alignment . . . . . . . 60
• 3.4.3 Transmit Power Comparisons of Passive-RIS-Equipped Scenario under Several Effects ........................................ 65
• 3.4.4 Transmit Power Comparisons of Active-RIS-Equipped Scenario under Several Effects ........................................ 70
• 3.5 Conclusion......................................................... 73
• Chapter 4. Opportunities to THz Spectrum: Free-Space Optical Communications for 6G Long-Range Wireless Networks: Feasibility Study and Prototype Validation............................................................. 79
• 4.1 Long-Range FSO Communication System for 6G Wireless Network: Challenges and Solution.................................................. 83
• 4.1.1 Motivation for the Proposed Real-Time FSO Link Demonstration 83
• 4.1.2 Contribution of the Proposed Real-Time FSO Link Demonstration 86
• 4.2 System Architecture of FSO Link Prototype ........................... 87
• 4.2.1 Path-loss Model of the FSO Communication Link .............. 87
• 4.2.2 Hardware and Data Setup..................................... 91
• 4.2.3 Proposed Techniques for Link-Quality Enhancement . . . . . . . . . . . . 92
• 4.2.4 Video Signal Transmission under Various Channel Conditions . . . 96
• 4.3 Conclusion......................................................... 97
• Chapter5. Conclusion and Future Work................................... 99
• 5.1 Summary........................................................... 99
• 5.2 Future Work........................................................100
• References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
• Abstract in Korean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

참고문헌 (Reference)

1. Roadmap to free space optics, A. Trichili, vol. 37, no. 11, pp. A184–A201, , 2020

2. Weiszfeld’s method: Old and new results, S. Sabach, A. Beck, vol. 164, no. 1, pp. 1–40, , 2015

3. Beyond 100G single sideband PAM-4 transmission with silicon dual-drive MZM, X. Ruan, vol. 31, no. 7, pp. 509–512, , 2019

4. Outdoor FSO communications under fog: Attenuation modeling and performance evaluation, M. A. Esmail, vol. 8, no. 4, pp. 1–22, , 2016

5. Silhouettes: a graphical aid to the interpretation and validation of cluster analysis, P. J. Rousseeuw, vol. 20, pp. 53–65, , 1987

6. Wideband QAM-over-SMF/turbulent FSO downlinks in a PON architecture for ubiquitous connectivity, D.-N. Nguyen, vol. 475, p. 126281, , 2020

7. Ward’s hierarchical agglomerative clustering method: Which algorithms implement Ward’s criterion?, F. Murtagh, vol. 31, no. 3, pp. 274–295, , 2013

8. Performance evaluation of a novel converged architecture for digital-video transmission over optical wireless channels,, N. Cvijetic, vol. 25, no. 11, pp. 3366–3373, , 2007