Study of MIMO techniques for optical wireless communications

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

Design guidelines for spatial modulation

A new class of low-complexity, yet energyefficient Multiple-Input Multiple-Output (MIMO) transmission techniques, namely the family of Spatial Modulation (SM) aided MIMOs (SM-MIMO) has emerged. These systems are capable of exploiting the spatial dimensions (i.e. the antenna indices) as an additional dimension invoked for transmitting information, apart from the traditional Amplitude and Phase Modulation (APM). SM is capable of efficiently operating in diverse MIMO configurations in the context of future communication systems. It constitutes a promising transmission candidate for large-scale MIMO design and for the indoor optical wireless communication whilst relying on a single-Radio Frequency (RF) chain. Moreover, SM may also be viewed as an entirely new hybrid modulation scheme, which is still in its infancy. This paper aims for providing a general survey of the SM design framework as well as of its intrinsic limits. In particular, we focus our attention on the associated transceiver design, on spatial constellation optimization, on link adaptation techniques, on distributed/ cooperative protocol design issues, and on their meritorious variants

Iterative receiver for hybrid asymmetrically clipped optical OFDM

This paper proposes an iterative receiver to enhance the performance of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) in optical wireless communication systems. In HACO-OFDM scheme, asymmetrically clipped optical OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) signals are transmitted simultaneously, which is more spectrally efficient compared with ACO-OFDM and PAM-DMT. However, the existing HACO-OFDM receiver directly recovers the signals in the frequency domain, which could not eliminate the interference thoroughly between ACO-OFDM and PAM-DMT signals and limits its performance. In our proposed receiver, the ACO-OFDM and PAM-DMT signals are detected in the frequency domain and regenerated in the time domain. After that, they are subtracted from the received signals iteratively. Thus, ACO-OFDM and PAM-DMT signals can be distinguished. By taking advantage of the signal symmetry properties of ACO-OFDM and PAM-DMT in the time domain, pairwise clipping is utilized to further reduce the effect of noise and estimation error, resulting in improved performance. In addition, unequal power allocation is proposed to guarantee that ACO-OFDM and PAM-DMT signals have similar performance in HACO-OFDM systems. Simulation results show that the proposed method provides significant signal-to-noise ratio gain over the conventional receiver for both equal and unequal power allocations at the cost of slightly increased complexity

Visible Light Communication (VLC)

Visible light communication (VLC) using light-emitting diodes (LEDs) or laser diodes (LDs) has been envisioned as one of the key enabling technologies for 6G and Internet of Things (IoT) systems, owing to its appealing advantages, including abundant and unregulated spectrum resources, no electromagnetic interference (EMI) radiation and high security. However, despite its many advantages, VLC faces several technical challenges, such as the limited bandwidth and severe nonlinearity of opto-electronic devices, link blockage and user mobility. Therefore, significant efforts are needed from the global VLC community to develop VLC technology further. This Special Issue, “Visible Light Communication (VLC)”, provides an opportunity for global researchers to share their new ideas and cutting-edge techniques to address the above-mentioned challenges. The 16 papers published in this Special Issue represent the fascinating progress of VLC in various contexts, including general indoor and underwater scenarios, and the emerging application of machine learning/artificial intelligence (ML/AI) techniques in VLC