1,721,097 research outputs found
Researching Widening Participation (WP) in Engineering - PTAS Project Data
No full textThis data set is a record of the survey carried out as part of the PTAS project titled researching widening participation in Engineering
Impact of VLC on Light Emission Quality of White LEDs
No full textData set for the published paper:
W. O. Popoola, "Impact of VLC on Light Emission Quality of White LEDs," in Journal of Lightwave Technology, vol. 34, no. 10, pp. 2526-2532, May15, 15 2016. doi: 10.1109/JLT.2016.254211
Underwater optical wireless communication with subcarrier intensity modulation: an experimental demonstration
No full textThis paper presents an experimental study of an underwater optical wireless communications system (UOWC). The potential of UOWC and how it can operate alongside acoustic based communications in a hybrid system is outlined. The channel characteristics, in terms of the inherent optical properties of water, are described and the impact on data transmission assessed. We present data transmission with a blue laser diode using both on-off keying (OOK) and subcarrier intensity modulation (SIM). These modulation schemes are evaluated in both still and turbulent water, demonstrating the inherent resilience to turbulence of SIM as well as the impact of turbulence on signal transmission. We show a maximum data rate in the range of Gbps, showing the potential for high speed underwater communications links using UOWC
Underwater optical wireless communications in turbulent conditions: from simulation to experimentation
Full text linkUnderwater optical wireless communication (UOWC) is a technology that aims to apply high speed optical wireless communication (OWC) techniques to the underwater channel. UOWC has the potential to provide high speed links over relatively short distances as part of a hybrid underwater network, along with radio frequency (RF) and underwater acoustic communications (UAC) technologies. However, there are some difficulties involved in developing a reliable UOWC link, namely, the complexity of the channel. The main focus throughout this thesis is to develop a greater understanding of the effects of the UOWC channel, especially underwater turbulence. This understanding is developed from basic theory through to simulation and experimental studies in order to gain a holistic understanding of turbulence in the UOWC channel.
This thesis first presents a method of modelling optical underwater turbulence through simulation that allows it to be examined in conjunction with absorption and scattering. In a stationary channel, this turbulence induced scattering is shown to cause and increase both spatial and temporal spreading at the receiver plane. It is also demonstrated using the technique presented that the relative impact of turbulence on a received signal is lower in a highly scattering channel, showing an in-built resilience of these channels. Received intensity distributions are presented confirming that fluctuations in received power from this method follow the commonly used Log-Normal fading model. The impact of turbulence - as measured using this new modelling framework - on link performance, in terms of maximum achievable data rate and bit error rate is equally investigated.
Following that, experimental studies comparing both the relative impact of turbulence induced scattering on coherent and non-coherent light propagating through water and the relative impact of turbulence in different water conditions are presented. It is shown that the scintillation index increases with increasing temperature inhomogeneity in the underwater channel. These results indicate that a light beam from a non-coherent source has a greater resilience to temperature inhomogeneity induced turbulence effect in an underwater channel. These results will help researchers in simulating realistic channel conditions when modelling a light emitting diode (LED) based intensity modulation with direct detection (IM/DD) UOWC link.
Finally, a comparison of different modulation schemes in still and turbulent water conditions is presented. Using an underwater channel emulator, it is shown that pulse position modulation (PPM) and subcarrier intensity modulation (SIM) have an inherent resilience to turbulence induced fading with SIM achieving higher data rates under all conditions. The signal processing technique termed pair-wise coding (PWC) is applied to SIM in underwater optical wireless communications for the first time. The performance of PWC is compared with the, state-of-the-art, bit and power loading optimisation algorithm. Using PWC, a maximum data rate of 5.2 Gbps is achieved in still water conditions
Enhanced carrierless amplitude and phase modulation for optical communication systems
Full text linkThis thesis develops and investigates enhanced techniques for carrierless amplitude and phase
modulation (CAP) in optical communication systems. The CAP scheme is studied as the
physical layer modulation technique due to its implementation simplicity and versatility, that
enables its implementation as a single carrier (CAP) or multi-carrier technique (m-CAP).
The effect of timing jitter on the error performance of CAP is first investigated. The
investigation indicates that synchronization is a critical requirement for CAP receiver and as
a result, a novel low-complexity synchronization algorithm is developed with experimental
demonstration for CAP-based visible light communication (VLC) systems. To further reduce
the overall link complexity, a fractionally-spaced equalizer (FSE) is considered to mitigate the
effects of inter-symbol interference (ISI) and timing jitter. The FSE implementation, which
eliminates the need for a separate synchronization block, is shown through simulation and
VLC experimental demonstration to outperform symbol-spaced equalizers (SSE) that are
reported in literature for CAP-based VLC systems.
Furthermore, in this thesis, spectrally-efficient index modulation techniques are developed for
CAP. The proposed techniques can be divided into two broad groups, namely spatial index CAP
(S-CAP) and subband index CAP (SI-CAP). The proposed spatial index techniques leverage
the fact that in VLC, multiple optical sources are often required. The spatial CAP (S-CAP)
transmits CAP signal through one of Nt available LEDs. It is developed to reduce equalization
requirement and improve the spectral efficiency of the conventional CAP. In addition to the bits
transmitted through the CAP symbol, the S-CAP encodes additional bits on the indexing/spatial
location of the LEDs. The generalised S-CAP (GS-CAP) is further developed to relax the
S-CAP limitation of using a single LED per symbol duration. In addition to the S-CAP scheme,
multiple-input multiple-output (MIMO) techniques of repetitive-coded CAP (RC-CAP) and
spatial multiplexing CAP (SMux-CAP) are investigated for CAP. Low-complexity detectors
are also developed for the MIMO schemes. A key challenge of the MIMO schemes is that they
suffer power penalty when channel gains are similar, which occur when the optical sources are
closely located. The use of multiple receivers and power factor imbalance (PFI) techniques
are proposed to mitigate this power penalty. The techniques result in significant improvement
in the power efficiency of the MIMO schemes and ensure that the spectral efficiency gain is
obtained with little power penalty.
Finally, subband index CAP (SI-CAP) is developed to improve the spectral efficiency of
m-CAP and reduce its peak-to-average power ratio (PAPR). The SI-CAP encodes additional
information bits on the selection of ‘active’ subbands of m-CAP and only modulate data
symbols on these ‘active’ subbands. The error performance of the proposed SI-CAP is
evaluated analytically and verified with computer-based simulations. The SI-CAP technique is
also experimented for both VLC and step-index plastic optical fibre (SI-POF) communication
links. The experimental results show that for a fixed power efficiency, SI-CAP achieves higher
data rate compared tom-CAP. For example, at a representative bit error rate (BER) of 10-5, the
SI-CAP achieves a data rate and power efficiency gain of 26:5 Mb/s and 2:5 dB, respectively
when compared to m-CAP. In addition, an enhanced SI-CAP (eSI-CAP) is developed to
address the complexity that arises in SI-CAP at higher modulation order. The results of the
experimental demonstrations in VLC and 10 m SI-POF link shows that when compared with
m-CAP, eSI-CAP consistently yields a data rate improvement of between 7% and 13% for
varying values of the SNR
Study of MIMO techniques for optical wireless communications
Full text linkWith 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
Non-linear equalisation techniques for high-speed step-index plastic optical fibre communication
Full text linkStep-index plastic optical fibres (SI-POF) have become a promising candidate as the media for
short-range in-home and automotive networks due to their low cost and their ease of installation.
However, they have the smallest bandwidth compared to the other optical fibres. Therefore,
high-speed communication over SI-POF results in inter-symbol interference (ISI) that linearly
distorts the signal. Moreover, there are non-linearities from the optical front-end that further
degrade the SI-POF performance.
A straightforward solution is to use non-linear equalisers (NLE) with the SI-POF system as
they compensate for the non-linear distortions while mitigating the channel ISI. Three NLEs
– transversal decision feedback equaliser (DFE), Volterra equaliser/DFE, and the multi-layer
perceptron-based equaliser/DFE (MLPDFE) – have been introduced in the literature. High-order modulation formats – like pulse amplitude modulation (PAM), carrier-less amplitude and
phase modulation (CAP), and discrete multi-tone (DMT) – can be used in combination with the
NLE to overcome the bandwidth limitation further. Thus, the thesis deals with the performance
of these NLEs for PAM, CAP, and DMT transmission in order to achieve high data rates (from
several hundreds of megabits-per-second (Mbps) to gigabits-per-second (Gbps)) in SI-POF.
The contributions of this research work are in threefold: firstly, a simulation model is used to
evaluate and compare the performance of the NLEs for PAM and CAP schemes. The study
shows that for a highly non-linear SI-POF with higher PAM (or CAP) modulation order, the
MLPDFE offers higher data rates than the Volterra DFE followed by the transversal DFE. This
simulation study is further verified with various experiments. For instance, the MLPDFE offers
an error-free bit rate of about 6.2 Gbps over a 30 m SI-POF while the transversal DFE offers
about 5 Gbps at similar SI-POF length. A computational complexity comparison of each NLE
shows that the transversal DFE requires the least computing requirement, and the VOLT2DFE
has higher computational order than the MLPDFE.
Secondly, the work investigates a recently introduced frequency domain NLE (FD-NLE) for
DMT transmission over SI-POF. It explores the performance of the FD-NLE for DMT with
clipping distortion in a highly non-linear SI-POF system. The FD-NLE is shown in this case as
the better choice than the conventional frequency domain equaliser. With insight from the FD-NLE for DMT, both Volterra and the MLP equalisers are translated to the frequency domain for
PAM and CAP transmission over SI-POF. A computational complexity analysis shows that implementing the NLEs (with PAM and CAP) in the frequency domain reduces their complexity
by at least 60% if there are more than 16 feedforward taps for the equaliser.
Finally, extensive experiments are carried out to evaluate and compare the bit error rate (BER)
performances and the computational complexity of the modulation schemes with their respective NLEs. The comparisons show that for a short-length SI-POF of up to 30 m, representing
benign channel conditions, bit-loaded DMT with FD-NLE offers the best performance requiring the least complexity and the least transmitted electrical power. However, at longer lengths,
PAM with MLPDFE gives the best performance. CAP with the MLPDFE demands the highest
computational complexity and the transmitted electrical power
Measurement of Droplets Effect on Free Space Optical Link and Mathematical Modelling of Multi-phase Flow
Full text linkHigh-speed quantum key distribution in the presence of detector dead time
No full textQuantum technology, particularly quantum key distribution (QKD), has garnered increasing research interest in recent years. Despite significant advancements, the key generation rate of current QKD systems remains constrained by practical limitations, including imperfections in quantum sources, the performance of quantum detectors, and the characteristics of the quantum channel. This thesis primarily investigates the bandwidth limitations of quantum detectors caused by the dead time effect under high-speed conditions, while also addressing imperfections in quantum sources and channels through the study of the decoy-state method and the dispersive characteristics of underwater channels.
The quantum detector's dead time effect, which happens after each photon detection, limiting the maximum quantum bit transmission rate and consequently the key generation rate of QKD systems. Qubit transmission at the sub-dead-time regime (i.e., faster than the reciprocal of dead time) can introduce different security loopholes. In this thesis, we propose the use of a detector array instead of a single detector to measure the quantum state of individual photons in discrete variable QKD systems, showing that it can significantly alleviate the limitations induced by detectors' dead time. A dead-time compensated BB84 scheme is introduced, allowing qubit transmission at the sub-dead-time regime. Consequently, novel analytical expressions can be derived for the sifted bit rate (SBR) and secret key rate (SKR) of the proposed QKD system, showing excellent match with the Monte Carlo simulation results. A remarkable gain is observed for the BB84 system employing detector arrays, showing a potential M-fold improvement of SBR at high qubit transmission rates, where M is the size of the array.
In practice, weak coherent sources are commonly used as substitutes for perfect single-photon sources. However, these sources can sometimes emit more than one photon per pulse, enabling Eve to perform photon number splitting (PNS) attacks. The decoy-state method has been proposed to overcome this issue. Moreover, the key generation rate of QKD systems is also constrained by the practical limitations of detectors. In this thesis, we propose using four linked single-photon detectors (SPDs) in a high-speed decoy-state QKD system, considering the dead time effect of the SPDs. We introduce a Markov chain model to describe the impact of dead time on the operation of the proposed decoy-state BB84 system. This allows us to derive accurate analytical expressions for the sifted bit rate (SBR) and secret key rate (SKR) of the proposed QKD system, which show excellent agreement with Monte Carlo simulation results. We compare our more accurate model with the coarse model used in prior works, demonstrating that our model better captures system performance under different channel and noise conditions. Finally, we optimize the intensity of the signal source, determining the optimal signal intensity under various conditions, and compare the SKR with and without the optimal intensity.
In practical scenarios, underwater channels, such as those in coastal and harbor waters, exhibit dispersive characteristics. At higher qubit transmission rates, this dispersion causes photons from previous pulses to arrive within the same time window as current pulses, leading to inter-symbol interference (ISI). To evaluate this issue, this thesis subdivides the clock period into smaller chips to analyze the impact of ISI on the performance of QKD systems. A chip-scale Markov chain model is built to characterize the influence of detector dead time on qubit transmission in the sub-dead-time regime. Analytical expressions for SBR, QBER, and SKR are derived, and the effects of ISI are observed under various channel conditions
Vehicular visible light communications
Full text linkTo enable high-speed wireless communications, higher frequencies in the radio
frequency (RF) spectrum are required to satisfy the increased demand for
communication capacity. Even though researchers and industry have improved
the cellular network architecture to increase the data rate, as well as decrease
the latency and achieve a better quality of services, most of the RF spectrum
has been allocated for specific use. Therefore, it is expected to find an
alternative to expand the spectrum. The emergence of visible light
communication (VLC) provides a promising alternative to RF-based
communications. Light-emitting diodes (LEDs) have been adopted in many
illumination applications, due to the significant improvement in solid-state
lighting technology. The investigation of VLC began in close-distance indoor
scenarios and, over time, was adapted to wider applications.
When it comes to transportation, in Intelligent Transportation System (ITS),
VLC is also a powerful candidate. The current LED headlights can be connected
with a modulator module and therefore, are possible to transmit signals as a
transmitter. The structure of the vehicle will remain unchanged and the reuse
of headlights, to some extent, saves energy. Therefore, it is possible to integrate
VLC into vehicles to allow vehicular VLC (V-VLC) in vehicle-to-vehicle (V2V)
and vehicle-to-infrastructure (V2I) schemes. However, the signal-to-noise ratio
(SNR) performance of VLC in outdoor scenarios is limited for many reasons,
such as adverse weather conditions, artificial light and solar irradiance.
The
transmitter itself, the headlight, has a narrow irradiation angle and is able to
show a higher directionality, which also limits the coverage and the data rate.
In terms of the analytical channel models in vehicular communication (VC),
most of the existing models are static and the same as the indoor scenarios.
However, a moving vehicle changes its orientation and distances of transceivers,
which leads to a time-varying communication channel. This time-related
channel shows uncertainty, diversity and dependency. However, most of the
current research lacks the analysis of this time-related channel. An analytical
method has been presented in this thesis to calculate the time-related channel
DC gain in dynamic VC systems. This method relates the road roughness to
the change in the orientation of the transceivers by using the mechanical
movement models and therefore, is able to give a more realistic result, with
regards to a real road measurement. Moreover, other vehicle-related variables
also influence the SNR on the rough road, which is novel in the work.
In addition to the dynamic channel models, the low sensitivity of the receiver is
another challenge for outdoor environments. A single-photon avalanche diode
(SPAD) was applied to receive the signals, which enabled a longer
communication distance. However, the non-linearity of the SPAD and its
unique mechanism limited the performance and therefore, the SNR expression
for a specific modulation method should be derived differently and expressed
explicitly. In this thesis, the mechanism of the SPAD was analyzed and the
SNR expression in single-input single-output (SISO) and orthogonal frequency
division multiplexing (OFDM)-based vehicular communications was derived, in
terms of the SPAD parameters and positions of transceivers.
From the SISO perspective, the results showed many limitations and therefore,
multi-input multi-output (MIMO) scenarios were introduced to subsequently
improve the coverage. Due to the existing non-linearity issue and the
uniqueness of the mechanism in the SPAD array, the channel gain cannot be
obtained directly. In this part of the work, the SNR in MIMO scenarios was
expressed. In terms of the number of transceivers, some typical scenarios were
considered. In addition, a general 2 × 2 spatial multiplexing (SM)-MIMO was
also studied. Further performance improvements can be achieved by employing
a longer sampling duration and reducing the spatial correlation
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