33 research outputs found
Advanced optical fibre communication via nonlinear Fourier transform
Optical fibre communication using the Nonlinear Fourier transform (NFT) is one of the
potential solutions to tackle the so-called capacity crunch problem in long-haul optical fibre
networks. The NFT transforms the nonlinear propagation of temporal signal, governed by
the nonlinear Schr¨odinger equation (NLSE), into simple linear evolutions of continuous and
discrete spectra in the so-called nonlinear spectral domain. These spectra and the corresponding
nonlinear spectral domain, defined by the NFT, are the generalized counterparts of the linear
spectrum and frequency domain defined by the ordinary Fourier transform. Using the NFT,
the optical fibre channel is effectively linearised, and the basic idea is to utilize degrees of
freedom in the nonlinear spectral domain for data transmission. However, many aspects of this
concept require rigorous investigation due to complexity and infancy of the approach. In this
thesis, the aim is to provide a comprehensive investigation of data transmission over mainly
the continues spectrum (CS) and partly over of the discrete spectrum (DS) of nonlinear optical
fibres. First, an optical fibre communication system is defined, in which solely the CS carries
the information. A noise model in the nonlinear spectral domain is derived for such a system by
asymptotic analysis as well as extensive simulations for different scenarios of practical interest.
It is demonstrated that the noise added to the signal in CS is severely signal-dependent such
that the effective signalling space is limited. The variance normalizing transform (VNT) is used
to mathematically verify the limits of signalling spaces and also estimate the channel capacity.
The numerical results predict a remarkable capacity for signalling only on the CS (e.g., 6
bits/symbol for a 2000-km link), yet it is demonstrated that the capacity saturates at high power.
Next, the broadening effect of chromatic dispersion is analysed, and it is confirmed that some
system parameters, such as symbol rate in the nonlinear spectral domain, can be optimized so
that the required temporal guard interval between the subsequently transmitted data packets
is minimized, and thus the effective data rate is significantly enhanced. Furthermore, three
modified signalling techniques are proposed and analysed based on the particular statistics
of the noise added to the CS. All proposed methods display improved performance in terms of
error rate and reach distance. For instance, using one of the proposed techniques and optimized
parameters, a 7100-km distance can be reached by signalling on the CS at a rate of 9.6 Gbps.
Furthermore, the impact of polarization mode dispersion (PMD) is examined for the first time,
as an inevitable impairment in long-haul optical fibre links. By semi-analytical and numerical
investigation, it is demonstrated that the PMD affects the CS by causing signal-dependent
phase shift and noise-like errors. It is also verified that the noise is still the dominant cause
of performance degradation, yet the effect of PMD should not be neglected in the analysis of
NFT-based systems. Finally, the capacity of soliton communication with amplitude modulation
(part of the degrees of freedom of DS) is also estimated using VNT. For the first time,
the practical constraints, such as the restricted signalling space due to limited bandwidth,
are included in this capacity analysis. Furthermore, the achievable data rates are estimated
by considering an appropriately defined guard time between soliton pulses. Moreover, the
possibility of transmitting data on DS accompanied by an independent CS signalling is also
validated, which confirms the potentials of the NFT approach for combating the capacity
crunch
Spatial and Wavelength Division Joint Multiplexing System Design for Visible Light Communications
The low-pass characteristics of front-end elements including light-emitting
diodes (LEDs) and photodiodes (PDs) limit the transmission data rate of visible
light communication (VLC) and Light Fidelity (LiFi) systems. Using multiplexing
transmission techniques, such as spatial multiplexing (SMX) and wavelength
division multiplexing (WDM), is a solution to overcome bandwidth limitation.
However, spatial correlation in optical wireless channels and optical filter
bandpass shifts typically limit the achievable multiplexing gain in SMX and WDM
systems, respectively. In this paper, we consider a multiple-input multiple
output (MIMO) joint multiplexing VLC system that exploits available
degrees-offreedom (DoFs) across space, wavelength and frequency dimensions
simultaneously. Instead of providing a new precoder/post-detector design, we
investigate the considered joint multiplexing system from a system
configuration perspective by tuning system parameters in both spatial and
wavelength domains, such as LED positions and optical filter passband. We
propose a novel spatial clustering with wavelength division (SCWD) strategy
which enhances the MIMO channel condition. We propose to use a state-of-the-art
black-box optimization tool: Bayesian adaptive direct search (BADS) to
determine the desired system parameters, which can significantly improve the
achievable rate. The extensive numerical results demonstrate the superiority of
the proposed method over conventional SMX and WDM VLC systems
Impact of Device Orientation on Error Performance of LiFi Systems
Most studies on optical wireless communications (OWCs) have neglected the
effect of random orientation in their performance analysis due to the lack of a
proper model for the random orientation. Our recent empirical-based research
illustrates that the random orientation follows a Laplace distribution for a
static user equipment (UE). In this paper, we analyze the device orientation
and assess its importance on system performance. The reliability of an OWC
channel highly depends on the availability and alignment of line-of-sight (LOS)
links. In this study, the effect of receiver orientation including both polar
and azimuth angles on the LOS channel gain are analyzed. The probability of
establishing a LOS link is investigated and the probability density function
(PDF) of signal-to-noise ratio (SNR) for a randomly-oriented device is derived.
By means of the PDF of SNR, the bit-error ratio (BER) of DC-biased optical
orthogonal frequency division multiplexing (DCO-OFDM) in additive white
Gaussian noise (AWGN) channels is evaluated. A closed-form approximation for
the BER of UE with random orientation is presented which shows a good match
with Monte-Carlo simulation results. Furthermore, the impact of the UE's random
motion on the BER performance has been assessed. Finally, the effect of random
orientation on the average signal-to-interference-plus-noise ratio (SINR) in a
multiple access points (APs) scenario is investigated.Comment: 10 pages, 11 figures, journa
OFDM-Based Optical Spatial Modulation
Spatial modulation (SM) has proven to be a promising
multiple-input-multiple-output (MIMO) technique which provides high energy
efficiency and reduces system complexity. In SM, only one transmitter is active
at any given time while the rest of them remain silent. The index of the active
transmitter carries information. This spatial information is in addition to the
data carried by the constellation symbols in the signal domain. Therefore, SM
increases the transmission rate of the communication system compared to
single-input-single-output and space-time block coding (STBC)-MIMO. For signal
domain data encoding, orthogonal frequency division multiplexing (OFDM) has
been widely adopted. The key benefits in multi-carrier intensity-modulation and
direct-detection (IM/DD) systems are: i) the capability to achieve high
spectral efficiency and ii) the ability to effectively mitigate direct-current
(DC) wander effects and the impact of ambient light. However, current off-the
shelf light emitting diodes (LEDs) which are used as transmit entities are
primarily bandwidth limited. Thus, there are benefits of combining SM and OFDM
to enhance transmission speeds while maintaining low complexity. In this paper,
the two most common OFDM-based SM types, namely frequency domain SM (FD-SM) and
time domain SM (TD-SM), are investigated for optical wireless communications
(OWC). Moreover, proof-ofconcept experimental results are presented to showcase
practical feasibility of both techniques. The obtained results are also
compared with Monte Carlo simulations. The results show that TDSM with an
optimal maximum-a-posteriori-probability (MAP) detector significantly
outperforms FD-SM. It can be inferred from the results that TD SM is a strong
candidate among OFDM-based optical SM systems for future optical IM/DD wireless
communication systems
Invoking Deep Learning for Joint Estimation of Indoor LiFi User Position and Orientation
Light-fidelity (LiFi) is a fully-networked bidirectional optical wireless
communication (OWC) that is considered a promising solution for high-speed
indoor connectivity. Unlike in conventional radio frequency wireless systems,
the OWC channel is not isotropic, meaning that the device orientation affects
the channel gain significantly. However, due to the lack of proper channel
models for LiFi systems, many studies have assumed that the receiver is
vertically upward and randomly located within the coverage area, which is not a
realistic assumption from a practical point of view. In this paper, novel
realistic and measurement-based channel models for indoor LiFi systems are
proposed. Precisely, the statistics of the channel gain are derived for the
case of randomly oriented stationary and mobile LiFi receivers. For stationary
users, two channel models are proposed, namely, the modified truncated Laplace
(MTL) model and the modified Beta (MB) model. For LiFi users, two channel
models are proposed, namely, the sum of modified truncated Gaussian (SMTG)
model and the sum of modified Beta (SMB) model. Based on the derived models,
the impact of random orientation and spatial distribution of LiFi users is
investigated, where we show that the aforementioned factors can strongly affect
the channel gain and system performance
The movement-rotation (MR) correlation function and coherence distance of VLC channels
Adaptive transmission based on instantaneous channel state information is an important methodology to improve data rates of mobile users, which requires the periodic update of channel variations. Different from radio frequency (RF) channels, whose variations are governed by Doppler and multi-path effects, visible light communication (VLC) channel variations are mainly related to receiver movements and rotations. In this article, a movement-rotation (MR) correlation function is proposed to measure VLC channel variations with the changes in receiver location and orientation. The correlation function of VLC channel gain in the time domain can then be approximated by the MR correlation function, which is an important criterion for the design of data transmission frames. It is verified that the approximation by MR correlation function can approach the actual simulation and experiment results of VLC channel gain correlation function in the time domain. In addition, experiment and simulation results are presented to investigate variation characteristics of VLC channels in different scenarios. It is shown that a receiver movement of several decimeters or a change of 10-20 degrees in the inclined angle of the receiver is required in a typical scenario in order to observe a distinguishable change of VLC channel gain
Hybrid multiplexing in OFDM-based VLC systems
In conventional visible light communication (VLC) systems with multiple light-emitting diodes (LEDs) and multiple photodiodes (PDs), high data rate transmission with limited modulation bandwidth can be achieved via spatial multiplexing (SMP) or wavelength division multiplexing (WDM). However, the number of multiplexing channels is limited by the strong spatial correlation in SMP and by the inter-colour crosstalk in WDM. In this paper, we propose a multiple-input multiple-output (MIMO) hybrid multiplexing (HMP) VLC system which avoids the disadvantages of SMP/ WDM and explores the degrees-of-freedom (DoFs) in space and wavelength domains jointly. With appropriate system configuration, a MIMO channel matrix with a better channel condition in HMP can be obtained. Eventually, it is able to increase the number of multiplexing channels and support higher data rate transmission
Physical Layer Security for Visible Light Communication Systems:A Survey
Due to the dramatic increase in high data rate services and in order to meet
the demands of the fifth-generation (5G) networks, researchers from both
academia and industry are exploring advanced transmission techniques, new
network architectures and new frequency spectrum such as the visible light
spectra. Visible light communication (VLC) particularly is an emerging
technology that has been introduced as a promising solution for 5G and beyond.
Although VLC systems are more immune against interference and less susceptible
to security vulnerabilities since light does not penetrate through walls,
security issues arise naturally in VLC channels due to their open and
broadcasting nature, compared to fiber-optic systems. In addition, since VLC is
considered to be an enabling technology for 5G, and security is one of the 5G
fundamental requirements, security issues should be carefully addressed and
resolved in the VLC context. On the other hand, due to the success of physical
layer security (PLS) in improving the security of radio-frequency (RF) wireless
networks, extending such PLS techniques to VLC systems has been of great
interest. Only two survey papers on security in VLC have been published in the
literature. However, a comparative and unified survey on PLS for VLC from
information theoretic and signal processing point of views is still missing.
This paper covers almost all aspects of PLS for VLC, including different
channel models, input distributions, network configurations,
precoding/signaling strategies, and secrecy capacity and information rates.
Furthermore, we propose a number of timely and open research directions for
PLS-VLC systems, including the application of measurement-based indoor and
outdoor channel models, incorporating user mobility and device orientation into
the channel model, and combining VLC and RF systems to realize the potential of
such technologies