Development of High Throughput Intelligent Indoor Optical Wireless Communication Systems

Abstract

The proliferation of mobile devices in indoor environments has led to insatiable demand for high-speed, high-bandwidth data connectivity. Optical wireless communication (OWC) is seen as a promising technology capable of meeting this increasing demand for rapid and dependable wireless connectivity across various length scales, from access to distribution to long-haul communication. Within the realm of OWC, data transmission is accomplished through intensity modulated LED or laser-based transmitters, directing signals toward user devices equipped with high-speed photo-receiver front-ends. One prominent application of OWC operates within the visible wavelength range, referred to as Visible Light Communication (VLC) or Li-Fi (Light fidelity). VLC’s primary objective is to harness the existing infrastructure of solid-state lighting, particularly white LED or blue laser technology, which can be converted using remote phosphors. This dual-purpose lighting infrastructure has the potential to simultaneously deliver illumination and communication services, and it provides THz of unlicensed bandwidth (BW), with a high degree of spatial reuse, and high security. According to the literature, almost 80% of mobile data traffic is indoors; this underscores the pressing need to make the OWC system compatible and eye-safe for indoor environments with beam steering with intelligence as well as pathloss optimization by designing the optical transmitter. The aim of this work is to design an eye-safe optical system for the diffused light source transmitter to increase the overall throughput of the link by making it intelligent and optimizable for mobile users. Thesis starts with the introduction part, setting the stage by addressing the pressing need to handle the increasing demand for data traffic and proposing potential solutions. It provides an overview of the existing work in the field and identifies key challenges. The chapter also outlines parameters used for characterizing both the illumination source and communication performance. For illumination source, parameters such as correlated color temperature (CCT), Color rendering index (CRI), CIE XY color illuminant, and the percentage of the blue component in the white spectrum are discussed. Communication characterization involves parameters such as BER, SNR, and data rate. This chapter introduces the thesis’s central themes, challenges, and solutions proposed in subsequent chapters to address the evolving demands of data traffic and communication. Next chapter, focuses on pathloss optimization for variable link lengths using raytracing. It identifies suitable lens pairs for transmitters and receivers and optimizes their positions to reduce pathloss. Pathloss optimization allows for practical links(25 to 300 cm) with lux levels ≤50 lux. The chapter implements a blue laser-down-converted white light VLC system for 25 cm and 300 cm links, achieving a narrow beam spread and low lux levels (approximately 45 and 16 lux). Communication experiments at 1.5 Gbps result in a BER below FEC thresholds, enabling gigabit-class communication with minimal illuminance. This low-illuminance VLC holds promise for coexistence with existing indoor lighting infrastructure. The subsequent chapter talks about the simple, cost-effective transmitter lens decentering- based mechanical beam steering technique for mobile users. The chapter also discusses lens aberrations like coma and field curvature resulting from lens decentering. Optical ray tracing optimizes collection efficiency and assesses off-axis aberrations. Experimental results show a maximum steering angle of 7.1◦, providing 30 cm of coverage per cm of lens de-center for a 300 cm link with intensity below the MPE level. Communication experiments achieve BER below FEC limit for 75 cm and 60 cm receiver coverage at 1.25 Gbps and 1.5 Gbps data rates. Although the setup has limitations in coma, astigmatism, and field curvature, these can be addressed with a more sophisticated lens combination or de-centered liquid lenses for non-mechanical steering. In the next chapter, a hybrid Laser-LED transmitter module is presented for indoor optical wireless communication, featuring closed-loop, non-mechanical beam steering. It uses a laser diode (NIR) for communication and a white LED ring for illumination. This hybrid transmitter decouples data communication and illumination performance, customizing each of emitter performance to specific needs. Dual-axis non-mechanical beam steering is achieved using off-centred liquid lenses, offering horizontal angles from -7.6◦, to 7.6◦, and vertical angles from -1.7◦, to 2.6◦, at a 1.5 m receiver distance. With M-QAM/OFDM and adaptive loading, a data throughput of 5.15 Gbps is achieved for the diffused laser beam with steering. the final piece of work of the thesis introduces the use of Optical Intelligent-Reconfigurable Surface (o-IRS) to steer multiple laser beams for multiple users, coupled with pre-compensated aberration correction. SLM serves as the reconfigurable surface, partitioned into segments dedicated to each of the two laser beams. o-IRS independently directs laser beams towards mobile users by applying blaze gratings or Fresnel lens phase profiles in combination with aberration-corrected phase profiles using Zernike polynomial functions, thereby enhancing overall data throughput. Intelligent machine vision camera is used to identify, differentiate, and track the receivers. The system demonstrates data communication for two channels over a 1.6 m link with a BER close to the FEC limit (3.8 × 10−3). It achieves multi-gigabit-per-second data rates of 4 Gbps and 1.5 Gbps for short-haul indoor optical communication with two channels. Hand-off experiments are conducted as mobile receivers move between distinct coverage areas of two transmitters, facilitated by programmable RF switches for data switching and machine vision-based position sensing. Finally conclusion of the thesis is discussed of this thesis, summarizing its major contributions to the field of VLC as well as outlining potential future directions that hold promise and could further advance the field of VLC