29 research outputs found
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
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
Efficient Analytical Calculation of Non-line-of-sight Channel Impulse Response in Visible Light Communications
This study provides an analytical method to calculate the non-line-of-sight (NLoS) channel impulse response (CIR) in visible light communication (VLC) systems based on intensity modulation and direct detection (IM/DD). In this method, the NLoS channel is decomposed into a number of components with different propagation categories. These propagation categories are defined according to the number of reflections and the reflective surfaces that the light undergoes. The CIR corresponding to each light propagation category is analysed and the overall NLoS CIR is approximated by the combination of the calculated CIR components in different propagation categories. The proposed method has the major advantage of offering accurate results with very low computational complexity. Typically, a NLoS CIR with a time resolution of 0.1 ns can be generated within a second in MATLAB. Furthermore, the analytical results derived herein could be used as an analytical tool for the VLC channel characterisation study in future research
Downlink system characterisation in LiFi Attocell networks
There is a trend to move the frequency band for wireless transmission to ever higher frequencies
in the radio frequency (RF) spectrum to fulfil the exponentially increasing demand in wireless
communication capacity. Research work has gone into improving the spectral efficiency of
wireless communication system to use the scarce and expensive resources in the most efficient
way. However, to make wireless communication future-proof, it is essential to explore ways
to transmit wirelessly outside the traditional RF spectrum. The visible light (VL) spectrum
bandwidth is 1000 times wider than the entire 300 GHz RF spectrum and is, therefore, a viable
alternative. Visible light communication (VLC) enables existing lighting infrastructures to provide
not only illumination but also wireless communication. In conjunction with the concept
of cell densification, a networked VLC system, light fidelity attocell (LAC) network, has been
proposed to offer wide coverage and high speed wireless data transmission. In this study, many
issues related to the downlink system in LAC networks have been investigated.
When analysing the downlink performance of LAC networks, a large number of random channel
samples are required for the empirical calculation of some system metrics, such as the
signal-to-interference-plus-noise ratio (SINR). However, using state-of-the-art approaches to
calculate the non-line-of-sight (NLoS) channel component leads to significant computational
complexity and prolonged computation time. An analytical method has been presented in this
thesis to efficiently calculate the NLoS channel impulse response (CIR) in VLC systems. The
results show that the proposed method offers significant reduction in computation time compared
to the state-of-the-art approaches.
A comprehensive performance evaluation of the downlink system of LAC networks is carried
out in this thesis. Based on the research results in the literature in the field of optical wireless
communication (OWC), a system level framework for the downlink system in LAC networks
is developed. By using this framework, the downlink performance subject to a large number
of parameters is evaluated. Additionally, the effect of varying network size, cell deployment
and key system parameters are investigated. The calculation of downlink SINR statistics, cell
data rate and outage probability are considered and analysed. The results show that the downlink
performance of LAC networks is promising in terms of achievable data rate per unit area
compared to other state-of-the-art RF small-cell networks.
It is found that co-channel interference (CCI) is a major source of signal impairment in the
downlink of LAC network. In order to mitigate the influence of CCI on signal distortion in
LAC networks, widely used interference mitigation techniques for RF cellular systems are borrowed
and extensively investigated. In this study, fractional frequency reuse (FFR) is adapted
to the downlink of LAC networks. The SINR statistics and the spectral efficiency in LAC
downlink system with FFR schemes are evaluated. Results show that the FFR technique can
greatly improve the performance of cell edge users and as well the overall spectral efficiency.
Further performance improvements can be achieved by incorporating angular diversity transmitters
(ADTs) with FFR and coordinated multi-point joint transmission (JT) techniques
Channel modelling for visible light communication systems
Visible Light Communications (VLCs) have been identified as a potential solution for mitigating the looming Radio Frequency (RF) spectrum crisis. Having the ability to provide illumination and communication at the same time, this technology has been considered as one of the most promising communication technologies for future wireless networks. VLCs are a viable candidate for short-range indoor applications with very high data rates. In terms of outdoor applications, Vehicular VLCs (VVLCs) play an important role in vehicular ad hoc networks and Intelligent Transportation Systems (ITS). Adopting visible light in vehicular networks offers a great potential to enhance road safety and traffic efficiency towards accident-free driving. For the sake of VLC system design and performance evaluation, it is indispensable to develop accurate, efficient, and flexible channel models, which can fully reflect the characteristics of VLC channels.
In this thesis, we first give a comprehensive and up-to-date literature review of the most important indoor Optical Wireless Communications (OWCs) measurement campaigns and channel models, primarily for Wireless Infrared Communications (WIRCs) and VLCs. Consequently, we can identify that an appropriate channel model for VLC systems is currently missing in the literature. This Ph.D. project is therefore devoted to the modelling of VLC channels for both indoor and outdoor communication systems.
Second, a new Two-Dimensional (2D) stationary Field of View (FoV) one-ring Regular-Shape Geometry Based Stochastic Model (RS-GBSM) for VLC Single-Input Single-Output (SISO) channels is proposed. The proposed model considers the Line-of-Sight (LoS) and Single-Bounce (SB) components. VLC channel characteristics are analysed based on different positions of the Photodetector (PD) and FoV constraint.
Third, we propose a new 2D stationary multiple-bounce RS-GBSM for VLC SISO channels. The proposed model employs a combined two-ring and confocal ellipse model. This model is sufficiently generic and adaptable to a variety of indoor scenarios since the received signal is constructed as the summation of the LoS, SB, Double-Bounce (DB), and Triple-Bounce (TB) rays with different powers.
Fourth, a new 2D mobile RS-GBSM for vehicular VLC SISO channels is proposed. The proposed model combines a two-ring model and a confocal ellipse model, and considers SB and DB components in addition to LoS component. Unlike conventional models, the proposed model considers the light that is reflected off moving vehicles around the Transmitter (Tx) and Receiver (Rx), as well as the light that is reflected off the stationary roadside environments. Vehicular VLC channel characteristics are analysed along different distance ranges between 0 and 70 m and different PD heights.
Fifth, we propose a novel Three-Dimensional (3D) mobile RS-GBSM for vehicular VLC Multiple-Input Single-Output (MISO) channels. The proposed model combines two-sphere and elliptic-cylinder models. Both the LoS component and SB components, which are reflected off moving vehicles and stationary roadside environments, are considered. The proposed 3D RS-GBSM has the ability to study the impact of the vehicular traffic density on the received power and jointly considers the azimuth and elevation angles by using the von Mises-Fisher (VMF) distribution.
In summary, this work proposes new realistic VLC channel models which are useful for the design, test, and performance evaluation of advanced indoor and outdoor VLC systems. Furthermore, it identifies important directions that can be considered in future research, and helps propose new applications that require the development of more realistic channel models before the actual implementation
Performance Analysis of Train Communication Systems
Trains are considered as a highly efficient transport mode which generate significant
challenges in terms of their communication systems. For improved safety, to cope with
the expected rapid increase in traffic, and to meet customer demands, an enhanced
and reliable communication system is required for high-speed trains (HSRs). Mobile
phone and laptop users would like to make use of the non-negligible time that they
spend commuting but current HSR communication systems have a foreseeable end
to their lifetime and a reliable, efficient, and fast communication replacement system
has become essential. Encouraged by the use of existing power line networks for
communication purposes, this research investigates the possibility of developing a train
communication system based on the use of overhead line equipment (OLE). The ABCD
transfer line model is developed to represent the transfer function of the OLE channel
and is evaluated using computer simulations. The simulations of the OLE system used
are based on orthogonal frequency division multiplexing as the chosen modulation
scheme.
Within the train, for the provision of broadband services, developing a reliable
communication system which is a combination of power line communication and
optical wireless communication services using visible light communication (VLC)
is considered. Mathematical methods were developed for these networks to assess
the overall capacities and outage probabilities of the hybrid systems. Derivation of
such analytical expressions offered opportunities to investigate the impact of several
system parameters on the performance of the system. To assess the possibility of
improving the performance of the proposed integrated systems, their performance in
the presence of different relaying protocols has been comprehensively analyzed in
terms of capacity and outage probability. This thesis studied the outage probability and
energy per bit consumption performance of different relaying protocols over the VLC
channel. Accurate analytical expressions for the overall outage probability and energyper-bit consumption of the proposed system configurations, including the single-hop
and multi-hop approaches were derived.
It was found that the transfer function of the OLE channel can be represented by the
two-port network model. It was also revealed that transmission over OLE is negatively
affected by the speed of the train, frequency, and length of the OLE link. In train, relay-based communication systems can provide reliable connectivity to the end-user.
However, choosing an optimal system configuration can enhance system performance.
It was also shown that increasing relay numbers on the network contributes to the total
power consumption of the system
Visible Light Communications for Indoor Applications
The field of visible light communications (VLC) has undergone a rapid development in
recent years. The increased utilization of light emitting diodes (LEDs) has opened new
possibilities for especially indoor services such as broadband internet connection and po-
sitioning. Thus, a research within VLC is the main focus of the thesis and is divided into
two main parts. At rst, the multiband carrier-less amplitude and phase (m-CAP) mod-
ulation, introducing a newly adopted format for spectrally e cient VLC links, is under
investigation using both theoretical and experimental approaches. The recommendations
for m-CAP transmitter site design are proposed. Next, the channel modeling of indoor
VLC is investigated with emphasis on the dynamically changing environments caused by
moving people and non-line of sight (NLOS) propagation and new statistical models are
derived.Katedra elektromagnetického pol
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
Broadband optical wireless communications for the teleoperation of mining equipment
The current level of technological advancement of our civilization serving more than seven
billion human population requires new sources of biotic and abiotic natural resources. The
depletion and scarcity of high-grade mineral deposits in dry land are forcing the Natural Re-
sources industry to look for alternate sources in underwater environments and outer space,
requiring the creation of reliable broadband omnidirectional wireless communication systems
that allows the teleoperation of exploration and production equipment. Within these ob-
jectives, Optical Wireless Communications (OWC) are starting to be used as an alternative
or complement to standard radio systems, due to important advantages that optical wave-
lengths have to transmit data: potential for Terabit/s bit rates, broadband operation in
underwater environments, energy e ciency and better protection against interference and
eavesdropping. This research focus in two crucial design aspects required to implement
broadband OWC systems for the teleoperation of mining equipment: high bandwidth wide
beam photon emission and low noise omnidirectional Free-Space Optical (FSO) receivers.
Novel OWC omnidirectional receivers using guided wavelength-shifting photon concentra-
tion are experimented in over 100 meters range vehicle teleoperation.Master of Science (MSc) in Natural Resources Engineerin