Study of MIMO techniques for optical wireless communications

Abstract

With its huge spectral resource, optical wireless communication (OWC) has emerged as a promising complementary technology to the radio frequency (RF) communication systems. OWC provides data communications for a variety of user applications and it can be deployed using simple, low-cost, low-power and energy-efficient component. In order to enhance capacity, reliability and/or coverage of OWC, multiple-input-multiple-output (MIMO) systems are employed to exploit additional degrees of freedom, such as the location and angular orientation of optical sources and detectors. However, the implementation of MIMO systems is faced with challenges such as the strong correlation and multipath propagation in indoor OWC channels, system synchronisation, as well as inter-channel interference (ICI) due to multiple parallel data transmission. This dissertation investigates MIMO OWC systems which utilises transmission techniques with reduced complexity. A detailed study and performance evaluation of the techniques in terms of capacity, spectral efficiency and error rates is conducted through theoretical analysis, simulation and experiments. The system performance is investigated under different constraints imposed by impairments such as interference, synchronization and channel correlation. Optical spatial modulation (OSM) is studied as a low complexity technique using multiple light sources to enhance system capacity. A generalised framework for implementing OSM with energy efficient pulse position modulation scheme is devised. This framework supports other variants of OSM, and it can be adapted to satisfy varying system requirement such as spectral and energy efficiencies. The performance of the OWC system is investigated in indoor line-of-sight (LOS) propagation. The error performance of the system is analysed theoretically and matched by simulation results. Also, the system performance is evaluated with experiments to demonstrate feasibility. Furthermore, the performance of OSM MIMO techniques in the realistic indoor scenario is considered by taking into account the multiple reflections of the transmitted signal from room surfaces. This is motivated by the recent drive towards high-speed Gigabits per second (Gbps) data communication, where the inter-symbol interference (ISI) caused by the multipath propagation may pose a major bottleneck. A model of the multipath-induced ISI is presented to account for signal spreading and then applied to formulate the error performance analysis. The impact of multipath-induced power penalty and delay spread on system performance is demonstrated using their spatial distributions across the coverage area. Additionally, the impact of timing synchronization problems on the error performance of different variants of the OSM MIMO techniques is investigated. While most works related to SM have assumed a perfect synchronization among the multiple transmitter and receiver elements, such assumption pose a challenge in practical deployment. Hence, the need to examine the impact of synchronisation error that can result from clock jitters and variations in propagation delay. Synchronisation error analyses of OSM schemes are presented, and the tolerance of each scheme to timing synchronization errors is demonstrated. To further enhance system capacity, this thesis also explores spatial multiplexing MIMO technique with orthogonal frequency division multiplexing (OFDM). The central objective is to propose and apply techniques to address the correlation of the indoor optical wireless channel and the frequency selectivity due to the limited bandwidth of LEDs. To address these two effects, a joint coding of paired information symbols was applied in a technique termed pairwise coding (PWC). This technique is based on rotated symbol constellation and it offers significant performance improvement. The error performance of the proposed system is evaluated through simulation and experimental demonstration. PWC proved to be effective over varying degrees of bandwidth limitation and under different channel conditions.