Common Path-based Full-duplex Mobile Free Space Optical Communication System

Abstract

As the unmanned aerial vehicle (UAV) market expands and diversifies, there has been a significant increase in mobile data consumption. With the rise of 5G mobile communication using millimeter-wave bands, the existing radio frequency (RF) bandwidth is becoming increasingly scarce. Furthermore, with the use of UAV for transmitting high-quality images, photos, and other personal or sensitive data, there is a heightened need for security. Simultaneously, there is an increasing demand for large-capacity and long-distance data transfers, either between inter-UAVs or from ground control system (GCS)-to-UAV. As a solution to this situation, compared to traditional RF communication, the free space optical communication (FSOC) technology, which boasts ultra-wide bandwidth and high security, is emerging as a potential solution and is consequently being studied widely. Moreover, as a next-generation communication method, the FSOC technology offers advantages over traditional RF communication. These include long-distance propagation due to the directness of lasers, reduced weight and size owing to smaller antenna size, low power consumption, immunity to electromagnetic interference, and the ability to utilize unlicensed bands. Despite these advantages, the FSOC technology requires a line-of-sight (LOS) between both FSOC terminals to establish a link. For this reason, a pointing, acquisition, and tracking (PAT) system is essential. Especially, for mobile FSOC systems designed to be mounted on UAV, there are more delicate considerations required than for typical FSOC systems. In this thesis, the objective was to develop a mobile FSOC system technology capable of Gbps full-duplex large-capacity transmission between GCS and UAV. Following this approach, a “common-path based Gbps full-duplex mobile FSOC system” was meticulously developed. This sophisticated system facilitates simultaneous PAT processes and data communication, eliminating the need for additional beacon beam sub-system. Based on these technologies, design guidelines for mobile FSOC systems intended for mounting on UAV have been applied. Consequently, a “common-path based mobile FSO terminal” with beamforming capabilities and a “PAT system for mobile FSOC terminal” designed for QPD-based optimal tracking of target FSO terminal have been developed. Firstly, to enhance the connection between GCS and UAV, a Gbps full-duplex mobile FSOC terminal based on a common optical path with adaptive beamforming capabilities was proposed. Practical guidelines and valuable information for the design and fabrication of the terminal were also provided. The impact on geometric loss and volume that varies depending on the aperture diameter of the mobile FSOC terminal was analyzed. From this, guidelines were provided for the appropriate aperture diameter for the design of the mobile FSOC terminal. Additionally, by estimating the changes in beam intensity and rotatable angle due to the divergence angle variations from beamforming, valuable information was provided to enhance optical tracking efficiency between mobile FSOC terminals. Secondly, a common-path based mobile FSOC system with a QPD-based PAT system was reported. In the QPD-based PAT system, both simulation and experimental methods were used to analyze the relationship between the incident beam size on the QPD and the size of the QPD. From this analysis, it was determined that the incident beam size on the QPD has a significant impact on tracking efficiency. Therefore, to ensure a robust FSOC link between the GCS and UAV under dynamic environments, a PAT system that takes into account the incident beam size on the QPD needs to be considered. Based on this, it was experimentally demonstrated that when the incident beam size on the QPD nearly matches the size of the QPD, the tracking performance improves by up to 4.25 times. This enhanced tracking performance was demonstrated to assist in quickly establishing a reliable mobile FSOC link. Lastly, by presenting and analyzing a comparison with various studies related to mobile FSOC systems for flight platforms, it was confirmed that the common-path based mobile FSOC system possesses various advantages such as the use of fewer optical components, the capability to operate the PAT system without additional beacon beam sub-systems, and structural miniaturization. This was verified through simulations and both indoor/outdoor experiments, demonstrating 1.25 Gbps full-duplex communication over a length of data link starting from a minimum of 50 m and extending slightly beyond 100 m.