Wireless optical backhauling for optical attocell networks

The backhaul of tens and hundreds of light fidelity (LiFi)-enabled luminaires constitutes a major challenge. The problem of backhauling for optical attocell networks has been approached by a number of wired solutions such as in-building power line communication (PLC), Ethernet and optical fiber. In this work, an alternative solution is proposed based on wireless optical communication in visible light (VL) and infrared (IR) bands. The proposed solution is thoroughly elaborated using a system level methodology. For a multi-user optical attocell network based on direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and decode-and-forward (DF) relaying, detailed modeling and analysis of signal-to-interference-plus- noise (SINR) and end-to-end sum rate are presented, taking into account the effects of inter-backhaul and backhaul-to-access interferences. Inspired by concepts developed for radio frequency (RF) cellular networks, full-reuse visible light (FR-VL) and in-band visible light (IB-VL) bandwidth allocation policies are proposed to realize backhauling in the VL band. The transmission power is opportunistically minimized to enhance the backhaul power efficiency. For a two-tier FR-VL network, there is a technological challenge due to the limited capacity of the bottleneck backhaul link. The IR band is employed to add an extra degree of freedom for the backhaul capacity. For the IR backhaul system, a power-bandwidth tradeoff formulation is presented and closed form analytical expressions are derived for the corresponding power control coefficients. The sum rate performance of the network is studied using extensive Monte Carlo simulations. In addition, the effect of imperfect alignment in backhaul links is studied by using Monte Carlo simulation techniques. The emission semi-angle of backhaul LEDs is identified as a determining factor for the network performance. With the assumption that the access and backhaul systems share the same propagation medium, a large semi-angle of backhaul LEDs results in a substantial degradation in performance especially under FR-VL backhauling. However, it is shown both theoretically and by simulations that by choosing a sufficiently small semi-angle value, the adverse effect of the backhaul interference is entirely eliminated. By employing a narrow light beam in the back-haul system, the application of wireless optical backhauling is extended to multi-tier optical attocell networks. As a result of multi-hop backhauling with a tree topology, new challenges arise concerning optimal scheduling of finite bandwidth and power resources of the bottleneck backhaul link, i.e., optimal bandwidth sharing and opportunistic power minimization. To tackle the former challenge, optimal user-based and cell-based scheduling algorithms are developed. The latter challenge is addressed by introducing novel adaptive power control (APC) and fixed power control (FPC) schemes. The proposed bandwidth scheduling policies and power control schemes are supported by an analysis of their corresponding power control coefficients. Furthermore, another possible application of wireless optical backhauling for indoor networks is in downlink base station (BS) cooperation. More specifically, novel cooperative transmission schemes of non-orthogonal DF (NDF) and joint transmission with DF (JDF) in conjunction with fractional frequency reuse (FFR) partitioning are proposed for an optical attocell downlink. Their performance gains over baseline scenarios are assessed using Monte Carlo simulations

High speed energy efficient incoherent optical wireless communications

The growing demand for wireless communication capacity and the overutilisation of the conventional radio frequency (RF) spectrum have inspired research into using alternative spectrum regions for communication. Using optical wireless communications (OWC), for example, offers significant advantages over RF communication in terms of higher bandwidth, lower implementation costs and energy savings. In OWC systems, the information signal has to be real and non-negative. Therefore, modifications to the conventional communication algorithms are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing (OFDM) promise to deliver a more efficient use of the communication capacity through adaptive bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM (DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated discrete multitone (PAM-DMT) – have been introduced in the literature. The current work investigates the recently introduced scheme subcarrier-index modulation OFDM as a potential energy-efficient modulation technique with reduced peak-to-average power ratio (PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore, an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM, PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based modulation approach which is electrically and optically more efficient than any other OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD) communication systems. Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed communication in OWC. This work presents a generalised approach for analysing nonlinear distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form analytical solution for an arbitrary memoryless distortion of the information signal and has been proven to work for the majority of the known unipolar OFDM-based modulation techniques - DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM. The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm device is demonstrated. The capabilities of using such devices at practical transmission distances are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in comparison to DCO-OFDM

A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications

The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined

LiFi Transceiver Designs for 6G Wireless Networks

Due to the dramatic increase in high data rate services, and in order to meet the demands of the sixth-generation (6G) wireless networks, researchers from both academia and industry have been exploring advanced transmission techniques, new network archi- tectures and new frequency bands, such as the millimeter wave (mmWave), the infrared, and the visible light bands. Light-fdelity (LiFi) particularly is an emerging, novel, bidirectional, high-speed and fully networked optical wireless communication (OWC) technology that has been introduced as a promising solution for 6G networks, especially for indoor connectivity, owing to the large unexploited spectrum that translates to signifcantly high data rates. Although there has been a big leap in the maturity of the LiFi technology, there is still a considerable gap between the available LiFi technology and the required demands of 6G networks. Motivated by this, this dissertation aims to bridge between the current research literature of LiFi and the expected demands of 6G networks. Specifcally, the key goal of this dissertation is to fll some shortcomings in the LiFi technology, such as channel modeling, transceiver designs, channel state information (CSI) acquisition, localization, quality-of-service (QoS), and performance optimization. Our work is devoted to address and solve some of these limitations. Towards achieving this goal, this dissertation makes signifcant contributions to several areas of LiFi. First, it develops novel and measurements-based channel models for LiFi systems that are required for performance analysis and handover management. Second, it proposes a novel design for LiFi devices that is capable of alleviating the real behaviour of users and the impurities of indoor propagation environments. Third, it proposes intelligent, accurate and fast joint position and orientation techniques for LiFi devices, which improve the CSI estimation process and boost the indoor location-based and navigation-based services. Then, it proposes novel proactive optimization technique that can provide near-optimal and real-time service for indoor mobile LiFi users that are running some services with high data rates, such as extended reality, video conferencing, and real-time video monitoring. Finally, it proposes advanced multiple access techniques that are capable of cancelling the efects of interference in indoor multi-user settings. The studied problems are tackled using various tools from probability and statistic theory, system design and integration theory, optimization theory, and deep learning. The Results demonstrate the efectiveness of the proposed designs, solutions, and techniques. Nevertheless, the fndings in this dissertation highlight key guidelines for the efective design of LiFi while considering their unique propagation features

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

Spectrum and energy efficient digital modulation techniques for practical visible light communication systems

The growth in mobile data traffic is rapidly increasing in an unsustainable direction given the radio frequency (RF) spectrum limits. Visible light communication (VLC) offers a lucrative solution based on an alternative license-free frequency band that is safe to use and inexpensive to utilize. Improving the spectral and energy efficiency of intensity modulation and direct detection (IM/DD) systems is still an on-going challenge in VLC. The energy efficiency of inherently unipolar modulation techniques such as pulse-amplitude modulation discrete multitone modulation (PAM-DMT) and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) degrades at high spectral efficiency. Two novel superposition modulation techniques are proposed in this thesis based on PAM-DMT and ACO-OFDM. In addition, a practical solution based on the computationally efficient augmented spectral efficiency discrete multi-tone (ASE-DMT) is proposed. The system performance of the proposed superposition modulation techniques offers significant electrical and optical power savings with up to 8 dB in the electrical signal-to-noise ratio (SNR) when compared with DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM). The theoretical bit error ratio (BER) performance bounds for all of the proposed modulation techniques are in agreement with the Monte-Carlo simulation results. The proposed superposition modulation techniques are promising candidates for spectrum and energy efficient IM/DD systems. Two experimental studies are presented for a VLC system based on DCO-OFDM with adaptive bit and energy loading. Micrometer-sized Gallium Nitride light emitting diode (m-LED) and light amplification by stimulated emission of radiation diode (LD) are used in these studies due to their high modulation bandwidth. Record data rates are achieved with a BER below the forward error correction (FEC) threshold at 7.91 Gb/s using the violet m-LED and at 15 Gb/s using the blue LD. These results highlight the potential of VLC systems in practical high speed communication solutions. An additional experimental study is demonstrated for the proposed superposition modulation techniques based on ASE-DMT. The experimentally achieved results confirm the theoretical and simulation based performance predictions of ASE-DMT. A significant gain of up to 17.33 dB in SNR is demonstrated at a low direct current (DC) bias. Finally, the perception that VLC systems cannot work under the presence of sunlight is addressed in this thesis. A complete framework is presented to evaluate the performance of VLC systems in the presence of solar irradiance at any given location and time. The effect of sunlight is investigated in terms of the degradations in SNR, data rate and BER. A reliable high speed communication system is achieved under the sunlight effect. An optical bandpass blue filter is shown to compensate for half of the reduced data rate in the presence of sunlight. This thesis demonstrates data rates above 1 Gb/s for a practical VLC link under strong solar illuminance measured at 50350 lux in clear weather conditions

Optical Wireless Communications Using Intelligent Walls

This chapter is devoted to discussing the integration of intelligent reflecting surfaces (IRSs), or intelligent walls, in optical wireless communication (OWC) systems. IRS technology is a revolutionary concept that enables communication systems to harness the surrounding environment to control the propagation of light signals. Based on this, specific key performance indicators could be achieved by altering the electromagnetic response of the IRSs. In the following, we discuss the background theory and applications of IRSs and present a case study for an IRS-assisted indoor light-fidelity (LiFi) system. We then highlight some of the challenges related to this emerging concept and elaborate on future research directions

Enhanced energy and spectrum efficiency in visible light communications

In recent years, there has been a surge in data traffic, leading to the investigation of using optical frequencies in conjunction with radio frequency (RF) wireless communication systems. One such technology is visible light communication (VLC), which uses light-emitting diodes (LEDs) in the visible light spectrum to transmit data. VLC has gained popularity for short-range wireless connections due to its energy efficiency, low-cost, and wide availability of front-end devices. However, one of the main challenges in designing a VLC system is improving its energy and spectral efficiency. This thesis aims to investigate techniques and determine the most effective methods for enhancing the energy and spectral efficiency of VLC systems. The thesis examined methods for optimising the bias point of an LED to benefit from increasing bandwidth at higher driving current while minimising the resulting signal distortion. The approaches are based on allowing for some nonlinear distortion or reducing signal swing/signal-to-noise ratio (SNR) while benefiting from higher bandwidth at higher driving currents. A framework is presented to estimate the attainable capacity under both conditions. Simulation results showed that the optimal bias point does not lie in the middle of the dynamic range. This was verified through a PAM-based VLC experiment, which showed that the transmission rate can be increased by choosing the optimal bias current instead of the midpoint of the linear range. Subsequently, VLC with probabilistic shaping (PS) is studied to optimise the distribution of source symbols and improve system performance. In this study, the error performance of PS is analysed, and closed-form analytical expressions are provided. The results show that PS outperforms the conventional uniform distribution and significantly reduces the required SNR to achieve a certain error probability. To demonstrate the practical application of PS in VLC, it was implemented in conjunction with optical orthogonal frequency-division multiplexing (OFDM) modulation. This allowed for continuous and adaptive loading of information bits to the channel response, resulting in an efficient use of available modulation bandwidth and transmission rates close to the channel capacity limits. In the two experimental demonstrations, a single low-power LED and a wavelength-division multiplexing (WDM) system using three off-the-shelf LEDs were used to achieve bit rates of 1.13~Gbps and 10.81~Gbps, respectively, representing increases of 27.13\% and 25.7\% over the traditional bit-power loading technique. Finally, an alternative approach towards enhancing the energy of VLC systems is introduced using frequency shift chirp modulation (FSCM). The error performance of FSCM was analysed in different types of channels, and a proof-of-concept experiment was conducted to demonstrate its potential use in VLC systems. FSCM offers improved robustness in band-limited, frequency-selective channels compared to other modulation techniques. This makes it a promising choice for integrating into VLC systems, particularly in low-power and low-rate application scenarios

Quantum Machine Learning for 6G Communication Networks: State-of-the-Art and Vision for the Future

The upcoming 5th Generation (5G) of wireless networks is expected to lay a foundation of intelligent networks with the provision of some isolated Artificial Intelligence (AI) operations. However, fully-intelligent network orchestration and management for providing innovative services will only be realized in Beyond 5G (B5G) networks. To this end, we envisage that the 6th Generation (6G) of wireless networks will be driven by on-demand self-reconfiguration to ensure a many-fold increase in the network performanceandservicetypes.Theincreasinglystringentperformancerequirementsofemergingnetworks may finally trigger the deployment of some interesting new technologies such as large intelligent surfaces, electromagnetic-orbital angular momentum, visible light communications and cell-free communications – tonameafew.Ourvisionfor6Gis–amassivelyconnectedcomplexnetworkcapableofrapidlyresponding to the users’ service calls through real-time learning of the network state as described by the network-edge (e.g., base-station locations, cache contents, etc.), air interface (e.g., radio spectrum, propagation channel, etc.), and the user-side (e.g., battery-life, locations, etc.). The multi-state, multi-dimensional nature of the network state, requiring real-time knowledge, can be viewed as a quantum uncertainty problem. In this regard, the emerging paradigms of Machine Learning (ML), Quantum Computing (QC), and Quantum ML (QML) and their synergies with communication networks can be considered as core 6G enablers. Considering these potentials, starting with the 5G target services and enabling technologies, we provide a comprehensivereviewoftherelatedstate-of-the-artinthedomainsofML(includingdeeplearning),QCand QML, and identify their potential benefits, issues and use cases for their applications in the B5G networks. Subsequently,weproposeanovelQC-assistedandQML-basedframeworkfor6Gcommunicationnetworks whilearticulatingitschallengesandpotentialenablingtechnologiesatthenetwork-infrastructure,networkedge, air interface and user-end. Finally, some promising future research directions for the quantum- and QML-assisted B5G networks are identified and discussed