26 research outputs found
Design and Investigation of RGB-type LED Visible Light Communication System
This paper examines the feasibility of a Red Green Blue (RGB)-type Light Emitting Diode (LED) Visible Light Communication (VLC) system based on wavelength division multiplexing (WDM). Each color in the RGB-LED is individually modulated to increase the data rate by three times as compared to the single channel modulation approach used in conventional VLC system. Color filters are employed to detect separately the RGB signals at the receiver side. The proposed system utilized a reflector to improve the performance and the system is lens-free. In this work, an approach of approximated WDM testing is adopted due to the incapability of multiplexing by the microcontroller at higher data rate. The proposed system is demonstrated to transmit and receive data at a maximum distance of 1.4m, with total data transmission speed of 345.6 kbps using standard WDM, while a total speed of 1.5 Mbps up to maximum distance of 1.2m and 3 Mbps up to maximum distance of 0.7m is achieved by the approach of approximated WDM testing
DESIGN AND ANALYSIS OF VISIBLE LIGHT COMMUNICATION AND POSITIONING SYSTEMS
Ph.DDOCTOR OF PHILOSOPH
Visible Light Positioning using Received Signal Strength for Industrial Environments
There is a forecast for exceptional digital data traffic growth due to the digitisation
of industrial applications using the internet of things. As a result, a great need for
high bandwidth and faster transmission data rates for future wireless networks
has emerged. One of the considered communication technologies that can assist in satisfying this demand is visible light communications (VLC). VLC is an
emerging technology that uses the visible light spectrum by mainly utilising lightemitting diodes (LEDs) for simultaneous indoor lighting and high bandwidth wireless communication. Some of the applications of VLC are to provide high data
rate internet in homes, offices, campuses, hospitals, and several other areas.
One of these promising areas of application is for industrial wireless communications. The research project will provide a review of VLC applications intended
for industrial applications with an emphasis on visible light positioning (VLP). In
this research work, a three-dimensional (3D) positioning algorithm for calculating
the location of a photodiode (PD) is presented. It solely works on measured powers from different LED sources and does not require any prior knowledge of the
receiver’s height unlike other works in the literature. The performance of the proposed VLP algorithm in terms of positioning error is evaluated using two different
trilateration algorithms, the Cayley–Menger determinant (CMD) and the Linear
Least Squares (LLS) trilateration algorithms. The evaluation considers different
scenarios, with and without receiver tilt, and with multipath reflections. Simulation results show that the CMD algorithm is more accurate and outperforms
the LLS trilateration positioning algorithm. Furthermore, the proposed method
has been experimentally assessed under two different LED configurations, with
different degrees of receiver tilt, and in the presence of a fully stocked storage
rack to examine the effect of multipath reflections on the performance of VLP
systems. It was observed from simulations and experimental investigations that
the widely used square LED-configuration results in position ambiguities for 3D
systems while a non-lattice layout, such as a star-shaped configuration, is much
more accurate. An experimental accuracy with a 3D median error of 10.5 cm
was achieved using the CMD algorithm in a 4 m × 4 m × 4.1 m area with a
horizontal receiver. Adding receiver tilt of 5◦ and 10◦
increases the median error
by an average of 29% and 110%, respectively. The effect of reflections from the
i
storage rack has also been thoroughly examined using the two mentioned trilateration algorithms and showed to increase the 3D median positioning error by
an average of 69% in the experimental testbed for the areas close to the storage
rack. These results highlight the degrading effect of multipath reflections on VLP
systems and the necessity to consider it when evaluating these systems. As
the primary consideration for positioning systems in industrial environments is
for mobile robots, the encouraging results in this thesis can be further improved
though the use of a sensor fusion method
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
`THz Torch' technology: secure thermal infrared wireless communications using engineered blackbody radiation
The thermal (emitted) infrared frequency bands, from 20 to 40 THz and 60 to 100 THz, are best known for applications in thermography. This underused and unregulated part of the spectral range offers opportunities for the development of secure communications. The `THz Torch' concept, operating between the THz and mid-infrared ranges, was recently introduced. This technology fundamentally exploits engineered blackbody radiation, by partitioning thermally-generated spectral power into pre-defined frequency channels; the energy in each channel is then independently pulsed modulated to create a robust form of short-range secure communications in the far/mid-infrared. In the thesis, the development of `THz Torch' wireless communications systems will first be introduced. State-of-the-art THz technologies, infrared sources and detectors, as well as near-infrared and visible light communications technologies, will be reviewed in Chapter 2. Basic single-channel architecture of the `THz Torch' technology will be presented in Chapter 3. Fundamental limits for the first single-channel proof-of-concept demonstrator will be discussed, and possible engineering solutions will be proposed and verified experimentally. With such improvements, to date, octave bandwidth (25 to 50 THz) single-channel wireless links have been demonstrated with >2 kbit/s data rate and >10 cm transmission distance. To further increase the overall end-to-end data rate and/or the level of security, multiplexing schemes for `THz Torch' technologies are proposed in Chapter 4. Both frequency division multiplexing (FDM) and frequency-hopping spread-spectrum (FHSS) working demonstrators, operating between 10 and 100 THz spectral range, will be implemented. With such 4-channel multiplexing schemes, measured bit error rates (BERs) of <10−6 have been achieved over a transmission distance of 2.5 cm. Moreover, the integrity of such 4-channel multiplexing system is evaluated by introducing four jamming, interception and channel crosstalk experiments. Chapter 5 gives a detailed power link budget analysis for the 4-channel multiplexing system. The design, simulation and measurement of scalable THz metal mesh filters, which have potential applications for multi-channel `THz Torch' technology, will be presented in Chapter 6. The conclusions and further work are summarised in the last chapter. It is expected that this thermodynamics-based approach represents a new paradigm in the sense that 19th century physics can be exploited with 20th century multiplexing concepts for low cost 21st century ubiquitous security and defence applications in the thermal infrared range.Open Acces
Organic Materials for Photonics: Properties and Applications
Photonics will play a key-role for the future development of ICT and healthcare and organic semiconductors are promising candidates to fulfil the capacity of photonics and deliver on its promises. This “photonics revolution” relies on novel and more performing materials, tailored for the specific requirements of real-world applications, and on reliable and cheap technologies, which can attract investments to address the transition from academia to industry. In this dissertation, I will report my findings on conjugated polymers suitable for photonic applications and demonstrate their use into low-cost photonic structures, as proof of concept. The first part is dedicated to the study of an aggregation-induced emission polymer, whose fluorescence is enhanced in the aggregated solid-state thanks to the restrictions of intramolecular rotations in contrast to typical planar conjugated polymers. I will show its exceptional fundamental photophysical properties which enable the reduction of non-radiative pathways and makes it attractive for its use in organic light-emitting diodes. In the second part, I will present the application of conjugated polymers into flexible all-polymer microcavities fabricated through a low-cost process based on spin coating. The incorporation of functional defects in periodic dielectric structures with optical feedback will enable the change in the photonic density of states. I will report the investigation on photonic resonators embedding an aggregation-induced polymer emitting in the visible and a novel near-infrared oligomer, assessing high quality factors and tuning of their radiative rates to achieve low threshold optically pumped lasers. In the last part, I will show the infiltration of conjugated engineered materials into porous silicon microcavities to enable a novel class of photonically-enhanced chips for communications and sensing. A cheap electrochemical technique has been employed to fabricate one-dimensional resonators, which I characterized fully to demonstrate the variation of the photonic density of states and an efficient approach to novel hybrid photonic devices
The influence of dipolar doping on charge injection and transport in small molecular organic semiconductors
The present work investigates the effect of dipolar doping on charge injection and charge carrier dynamics in organic semiconducting thin films. In this context, the term dipolar doping refers to the dilution or doping of a non-polar matrix molecule with a polar guest. For this purpose, the hole-conductors N,N ’-Di(1-naphthyl)-N,N ’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) and 4,4-N,N ’-Dicarbazole-1,1’-biphenyl (CBP) will serve as the host molecules. Dopants include Tris-(8-hydroxyquinoline)aluminum (Alq3) and OXD-7 (1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene. The main focus, however, is on the system NPB:Alq3, which has also been studied extensively in the past [1–3]. In general, doping of (organic) semiconductors is a well known concept to tune conductivity [4] or optimize emitting properties of OLEDs [5]. The specific effect of doping with polar species, however, was not thoroughly investigated so far, although many guest molecules are indeed polar [6]. Because organic molecules are extended objects, their orientation with respect to the substrate surface [7], other molecules in the film [8] or e.g. the direction of light output from OLEDs [9] plays a crucial role in device performance. The key figure of polar molecules in this regard is their permanent dipole moment, arising from the non-uniform charge distribution on the molecule. If this dipole moment does not orient perfectly isotropic, it will lead to the build-up of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this being a known fact [1, 7, 10], the implications of such high potentials on charge transport and injection within and into an organic layer stack have only been studied recently [3, 7, 11]. Dipolar doping now allows to introduce and tune the GSP in a former unpolar organic material [2]. The concentration dependence of the magnitude of the GSP in dipolar doped systems is the first major part of this work. Additionally, dipolar doping can be utilized to create hole conducting films that exhibit a GSP, which allow to study the impact of film polarization also in the hole conducting layer (HTL) of an OLED. In neat film, a GSP was previously seen mostly for electron conductors [6, 12]. Therefor, the prototypical, hole conducting mixture NPB:Alq3 is investigated at different doping concentrations and with varied substrate material with respect to hole injection and charge transport. The mixtures are studied in single-layer, monopolar devices with only the HTL present, as well as bilayer OLEDs with Alq 3 -doped NPB as HTL and neat Alq3 as electron transport layer, respectively. The latter are treated as metal insulator semiconductor (MIS) structures following and applying our recently developed method of charge extraction by linearly increasing voltage (CELIV) on polar OLEDs [13,14].
Furthermore, ultraviolet photoelectron spectroscopy allows to compare the electrical observations with the energy alignment between contact and doped NPB. For all device types, an optimum in device performance and carrier injection for moderate doping concentrations of about 5% is found. By comparing all different methods with a focus on charge injection barriers, a complex relationship of carrier transport, substrate workfunction, modified injection and the effect of polarization is found. This effectively manipulates charge carrier injection across the metal-organic interface and charge transport in the device.Die vorliegende Arbeit befasst sich mit den Auswirkungen von polarer Dotierung auf Ladungsträgertransport und -injektion in organischen Halbleitern. „Polare Dotierung“ bezieht sich hierbei auf das Verdünnen oder Dotieren einer unpolaren Matrix aus organischen Molekülen mittels polaren Gast-Molekülen. Für diesen Zweck dienen die organischen Lochleiter N,N ’-Di(1-Naphthyl)-N,N ’-Diphenyl-(1,1’-Biphenyl)-4,4’-Diamin (NPB) und 4,4-N,N ’-Dicarbazole-1,1’-Biphenyl (CBP) als Matrix. Dotiert werden sie mit Tris-(8-Hydroxyquinolin) Aluminium (Alq3) und 1,3-bis[2-(4-tert- butylphenyl)-1,3,4-oxadiazo-5-yl]Benzen (OXD-7). Der Fokus dieser Arbeit liegt allerdings auf dem System NPB:Alq3, welches auch vorher bereits gründlich untersucht wurde [1–3]. Im Allgemeinen ist die Dotierung von (organischen) Halbleitern eine etablierte, zentrale Methode zur Optimierung der Leitfähigkeit [4] oder der Emittereigenschaften von organischen Leuchtdioden [5]. Die Folgen einer Dotierung mit polaren Molekülen im Speziellen wurde allerdings bisher noch nicht systematisch untersucht, obwohl viele häufig verwendete Dotanden durchaus polare Moleküle sind [6]. Organische Moleküle sind ausgedehnte Objekte mit komplexen Formen, deren Orientierung zur Substratoberfläche [7], zu anderen Molekülen in der Schicht [8] oder auch zum Emissionsvektor einer OLED [9] einen großen Einfluss auf die Effizienz des Bauteils hat. Die wichtigste Eigenschaft von polaren Molekülen ist in diesem Zusammenhang ihr permanentes Dipolmoment, das sich auf eine ungleichmäßige Verteilung der Elektronendichte im Molekül zurückführen lässt. Falls sich das Dipolmoment nicht vollständig isotrop orientiert, hat es ein makroskopisches Oberflächenpotential (engl. giant surface potential, GSP) bzw. eine dielektrische Polarisation der gesamten Dünnschicht zur Folge. Die Existenz des GSP ist bereits seit einiger Zeit bekannt [1, 7, 10]. Seine Auswirkungen auf den Ladungstransport bzw. -injektion in Bezug auf organische Halbleiter werden erst seit kurzem häufiger untersucht [3,7,11]. Das Konzept der polaren Dotierung erlaubt nun, ein GSP in ursprünglich unpolare organische Matrizen einzubauen [2]. Mittels Dotierung lassen sich auch polare Lochleiter herstellen, die im weiteren Verlauf der Arbeit eine Untersuchung der Auswirkungen des GSP auf die Lochleiterschicht in einer OLED erlauben. In undotierten Schichten wurde ein GSP bisher hauptsächlich in Elektronenleitern beobachtet [6, 12]. Exemplarisch wird dazu das Mischsystem NPB:Alq3 in verschiedenen Konzentrationen und auf verschiedenen Substraten in Bezug auf Lochinjektion und Lochtransport näher untersucht. Es werden sowohl monopolare Bauteile, die ausschließlich Lochtransport aufweisen, als auch vollständige OLEDs verwendet. Bei OLEDs kommt dabei eine im Laufe dieser Arbeit mit entwickelte neue Methode zum Einsatz, die diese OLEDs wie sog. Metall-Isolator-Halbleiter-Dioden (engl. metalinsulator-semiconductor, MIS) behandelt und Rückschlüsse auf die Energiebarriere für Ladungsinjektion erlaubt [13, 14]. Des Weiteren stehen Ergebnisse aus der ultravioletten Photoelektronenspektroskopie zur Verfügung, die einen Vergleich mit der Ausrichtung der Energieniveaus von NPB am Kontakt ermöglichen. Mittels des Vergleichs verschiedener Messmethoden im Bezug auf die Injektionseigenschaften kann der Effekt des GSP auf verschiedene Parameter im Bauteil nachgewiesen werden, deren Zusammenspiel in sämtlichen Bauteiltypen zu einem Optimum bei moderaten Dotierkonzentrationen von ca. 5% führt