Implementation of angular diversity receiver for VLC-IoT sensor networks

Abstract

The demand for wireless spectrum is increasing, leading to radio frequency (RF) congestion. As wireless technologies advance and applications such as smart cars, e-health, smart grids, and the Internet of Things (IoT) proliferate, the problem intensifies. Visible light communication (VLC) systems are suggested as a solution, offering a large unlicensed spectrum ideal for radiosensitive environments such as hospitals and aircrafts. However, a significant challenge for VLC implementation is achieving wide communication coverage due to a limited field of view (FOV). This thesis aims to explore and develop methods to enhance communication coverage in VLC-based IoT (VLC-IoT) systems.

The thesis first analyses the performance of angular diversity receiver (ADR) configurations in indoor multi-cell VLC scenarios, focussing on square pyramid (SP), square frustum (SF), hexagonal pyramid (HP), and hexagonal frustum (HF) shapes. Through numerical and simulation analysis, performance is assessed using spatial multiplexing (SMP) and receive diversity techniques, including selection combining (SC), equal gain combining (EGC), and maximum ratio combining (MRC). The results from SMP applications indicate that the HF ADR provides the widest coverage for multi-stream transmission due to its largest number of photodiodes (PDs). However, for receive diversity, the HP and HF configurations with six sides PDs do not outperform the SP and SF types with four PDs, one on each side in terms of bit error rates (BERs) performance when using SC.

Subsequently, this thesis analyses the performance of ADR systems in single-cell VLC scenario and proposes a novel ADR system in IoT sensor networks. The numerical analysis applying the combining techniques indicates that any combination of ADR types and diversity techniques can enhance performance. Therefore, a SF ADR is implemented with the EGC due to the low complexity. The proposed ADR system enhanced received signal-to-noise ratio (SNR), improved BERs, and provided more than double the coverage of the link. Furthermore, the effectiveness of the system is demonstrated with real-time sensor transmission and a duplex VLC communication link, showcasing its advantages in practical IoT applications.

Following that, the integrated VLC and RF (VLC-RF) system using the ADR for secure industrial IoT applications is presented. The prototype with the custom-built ADR and analogue front end (AFE) is connected to the backbone network such as a wireless cloud server. As the fundamental VLC performance, SNR, secrecy capacity (SeCap) and BER coverage is measured. The result indicates that the prototype has better physical layer security (PLS) performance than the single PD receiver, and the coverage of the link is drastically expanded compared to the initial design. Furthermore, to evaluate the feasibility of the IoT application, real-time sensor network applications are demonstrated using the proposed integrated VLC-RF ADR system. These experiments, including real-time sensor transmission, image transmission, and integration of the robot arm system, indicate the benefits in a practical scenario by showing low latency.

Finally, the wavelength division multiplexing (WDM) based VLC prototype using the colour PDs is implemented. This is an additional experimental study to show the method for ADR based multiplexing system. The BERs performance of three different wavelength channels (red, green, blue) is shown in comparison with the single PD receiver. The result shows that WDM can be possible using the colour PD and the ADR WDM system enhances SNR and communication range.