无线光通信具有速率高、带宽大、兼容性好等优点，能够为多个应用场景提供高速接入。然而，已有工作大多集中于提升单用户系统性能，无法满足日益增长的多用户通信网络需求。本文主要研究无线光通信多址技术，包括以正交频分多址接入（Orthogonal frequency division multiple access，OFDMA）为代表的正交多址接入和非正交多址接入（Non-orthogonal multiple access，NOMA）技术。基于这两种多址方式，本文设计面向室内和水下等场景的多用户无线光通信系统。首先，论文介绍了OFDMA和NOMA两种多址技术的基本原理和特性。从理论角度分析了OFDMA的实现原理和基本的系统模型，并讨论了其内涉及的关键技术。同时，重点介绍了NOMA技术编码方式和系统架构，详细分析了NOMA解码关键技术。其次，论文为自由空间典型的下行和设备到设备（Device to device，D2D）链路分别设计了基于微型发光二极管（μLED）和垂直腔面发射激光器（Vertical cavity surface emitting laser，VCSEL）等光源的多用户无线光通信系统。基于高性能的μLED，下行链路带宽达1 GHz，2米的传输距离下用户传输总速率达2 Gbps。所设计的D2D链路，能够同时服务具有不同信道状态的两个用户，使系统的总速率达到2.7 Gbps。再次，论文设计了能提供大覆盖角度的自适应白光结构，同时满足室内照明和高速通信需求。在算法方面，新设计的NOMA结构可有效提高频谱效率，相比传统NOMA速率提高超过30%。在架构方面，所提出的基于颜色转换材料和光敏聚合物薄膜的白光控制器，可提供光源发射半角超过30°的大照度区域。该结构的自适应性使其能同时满足中心和边缘用户的通信需求，保证系统整体性能的稳定性。最后，论文评估了水下信道中OFDMA和NOMA两种多址方式的性能，设计了采用波分复用和偏振复用支持8用户的水下无线光通信系统。在仿真中，不论有/无湍流作用，NOMA都能实现比OFDMA更高的通信速率。在试验中，4通道8用户的水下NOMA无线光通信总速率达到18.75 Gbps。同时，实验验证了在气泡湍流下，所采用的NOMA结构具有对抗信道衰减、实现可靠通信的能力。
Optical wireless communication (OWC) has considerable advantages including high speed, high bandwidth and good compatibility, which makes it a promising approach to providing high-speed access for many practical scenarios. However, most of the previous works focused on improving the performance of a single user, making it hard to meet the needs of boosting multi-user communication networks. This thesis mainly investigates multiple access technology in OWC, including typical orthogonal multiple access represented by orthogonal frequency division multiple access (OFDMA), and non-orthogonal multiple access (NOMA). Based on those two method, the thesis designs different high-performance multi-user OWC systems for indoor and underwater applicaitons. Firstly, the basic principles and characteristics of OFDMA and NOMA are introduced. The implementation principle of OFDMA and its basic system model are analyzed theoretically, and corresponding key technologies involved are discussed. Meanwhile, the encoding method and system architecture of NOMA are introduced, and its key NOMA decoding technology is analyzed in detail.Secondly, multi-user OWC systems based on micro LED (μLED) and vertical cavity surface emitting laser (VCSEL) are respectively demonstrated for typical downlink and device to device (D2D) communication in free-space environment. Using a home-made high-performance μLED, the downlink bandwidth is up to 1 GHz. Within the transmission distance of 2 m, the sum rate reaches over 2 Gbps. The designed D2D link can simultaneously support two users with different channel condiitons, and the sum rate of 2.7 Gbps is achieved for the overall system.Thirdly, an adaptive white-lighting scheme is proposed to provide a large coverage angle, which meets dual requirements of illumination and high-speed communication. For the software algorithm, a novel NOMA method is designed to improve the spectral efficiency. It increases the sum rate by more than 30% when compared to the traditional NOMA algorithms. For the hardware structure, a white light controller composed of a perovskite color-converting material and a photopolymer film can provide a large illuminating area with an emitting semi-angle of more than 30°. The proposed adaptive scheme can meet the communication needs of both center and edge users, ensuring the stable performance of the whole system.Finally, the performance of OFDMA and NOMA in the underwater environment is evaluated numerically, and an 8-user underwater experimental system is demonstrated using wavelength division multiplexing and polarization multiplexing scheme. In simulation systems, with or without the effects of turbulence, NOMA always outperforms OFDMA in the regard of capacity. Experimentally, a 4-channel underwater NOMA system is demonstrated to achieve a sum rate of 18.75 Gbps for 8 users. Meanwhile, it is verified that the NOMA modality is capable of resisting channel attenuation caused by bubble turbulence, and ensures a considerate communication performance.
