17 research outputs found
Analysis of OFDM-based intensity modulation techniques for optical wireless communications
Optical wireless communication (OWC) is a promising alternative to radio frequency (RF) communication
with a significantly larger and unregulated spectrum. Impairments in the physical
layer, such as the non-linear transfer characteristic of the transmitter, the dispersive optical wireless
channel and the additive white Gaussian noise (AWGN) at the receiver, reduce the capacity
of the OWC system. Single-carrier multi-level pulse position modulation (M-PPM) and multilevel
pulse amplitude modulation (M-PAM) suffer from inter-symbol interference (ISI) in the
dispersive channel which reduces their capacity even after channel equalization. Multi-carrier
modulation such as optical orthogonal frequency division multiplexing (O-OFDM) with multilevel
quadrature amplitude modulation (M-QAM) is known to maximize the channel capacity
through bit and power loading. There are two general signal structures: bipolar Gaussian signal
with a direct current (DC) bias, i.e. DC-biased O-OFDM (DCO-OFDM), or unipolar half-
Gaussian signal, employing only the odd subcarriers, i.e. asymmetrically clipped O-OFDM
(ACO-OFDM). In this thesis, the signal distortion from the transmitter nonlinearity is minimized
through pre-distortion, optimum signal scaling and DC-biasing.
The optical front-ends impose minimum, average and maximum optical power constraints, as
well as an average electrical power constraint, on the information-carrying signals. In this thesis,
the optical signals are conditioned within these constraints through optimum signal scaling
and DC-biasing. The presented analysis of the optical-to-electrical (O/E) conversion enables
the derivation of the electrical signal-to-noise ratio (SNR) at the receiver, including or excluding
the additional DC bias power, which is translated into bit-error rate (BER) performance.
In addition, a generalized piecewise polynomial model for the non-linear transfer characteristic
of the transmitter is proposed. The non-linear distortion in O-OFDM is translated by means of
the Bussgang theorem and the central limit theorem (CLT) into attenuation of the data-carrying
subcarriers at the receiver plus zero-mean complex-valued Gaussian noise. The attenuation
factor and the variance of the non-linear distortion noise are derived in closed form, and they
are accounted towards the received electrical SNR. Through pre-distortion with the inverse of
the proposed piecewise polynomial function, the linear dynamic range of the transmitter is
maximized, reducing the non-linear distortion to double-sided signal clipping.
Finally, the OWC schemes are compared in terms of spectral efficiency and electrical SNR
requirement as the signal bandwidth exceeds the coherence bandwidth of the optical wireless
channel for a practical 10 dB linear dynamic range. Through optimum signal scaling and DCbiasing,
DCO-OFDM is found to achieve the highest spectral efficiency for a target SNR, neglecting
the additional DC bias power. When the DC bias power is counted towards the signal
power, DCO-OFDM outperforms PAM with linear equalization, approaching the performance
of the more computationally intensive PAM with non-linear equalization. In addition, the average
optical power in O-OFDM is varied over dynamic ranges of 10 dB, 20 dB and 30 dB.
When the additional DC bias power is neglected, DCO-OFDM is shown to achieve the Shannon
capacity, while ACO-OFDM exhibits a 3 dB gap which grows with higher SNR targets.
When the DC bias power is included, DCO-OFDM outperforms ACO-OFDM for the majority
of average optical power levels with the increase of the SNR target or the dynamic range
A Novel PAPR Reduction in Filter Bank Multi-Carrier (FBMC) with Offset Quadrature Amplitude Modulation (OQAM) Based VLC Systems
The peak to average power ratio (PAPR) is one of the major problem with multicarrier-based systems. Due to its improved spectral efficiency and decreased PAPR, Filter Bank Multicarrier (FBMC) has recently become an effective alternative to the orthogonal multiplexing division (OFDM). For filter bank multicarrier communication/offset quadrature amplitude modulation-Visible light communication (FBMC/OQAM-VLC) systems is proposed a PAPR reduction technique. The suggested approach overlaps the proposed FBMC/OQAM-based VLC data signal with the existing signals. Non-redundant signals and data signals do not overlap in the frequency domain because data signals are scattered on odd subcarriers whereas built signals use even subcarriers. To reduce the effects of large-amplitude signal reduction, the suggested technique converts negative signals into positive signals rather than clipping them off as in conventional FBMC-based VLC systems. The PAPR reduction and bit error rate (BER) are realized using a scaling factor in the transformed signals. Complementary cumulative distribution function(CCDF) and BER are used to calculate the performance of the proposed approach. The presented study found that FBMC/OQAM-VLC systems to achieve a good trade-off between PAPR reduction and BER
PAPR reduction in multicarrier modulation techniques based visible light communication systems
Visible light communication (VLC) is an optical wireless communication (OWC) technology that has the potential to provide high data rate transmission for indoor applications.
VLC is a promising alternative technology with a large and unlicensed spectrum to complement the congested radio frequency (RF) based communication in order to meet the exponential growth and popularity of smart devices, data intensive services and applications.
The use of low-cost commercially available front-end devices further highlights the attraction of VLC system. However, nonlinear dynamic range of front-end devices and optical channel impairments limit full exploitation of VLC available modulation bandwidth. To fully benefit from the inherent resources and mitigate these limitations, multicarrier modulation (MCM) techniques are adopted. However, these techniques are affected by high peak-to-average power ratio (PAPR) which imposes constraints on the limited dynamic range of the front-end devices and the average radiated optical power. The main focus throughout this thesis is to reduce the high PAPR of MCM modulation techniques-based VLC system by implementing pilot-assisted (PA) technique. Additionally, performance of PAPR reduced modulation techniques is investigated through analytical, simulation, and experimentally.
This thesis first presents background of VLC system principles including the front-end devices, VLC channel, system impairments and challenges, and employed solutions. The principles, limitations, and performance of MCM modulation variants that are implemented in this work are presented. Moreover, principles of PAPR challenge in MCM based VLC, PAPR evaluation, impact on the transmitted signal as well as the existing PAPR reduction techniques are discussed. Looking at the gap, a PA is implemented as PAPR reduction technique which is presented in this work including its implementation and performance.
Following that, multiple experimental studies on PAPR reduction of PA technique are presented. Two experimental demonstrations on the efficacy of PA PAPR reduction for PAM-DMT and DCO-OFDM based VLC using a single blue LED are presented. These studies are comparing the bit-error-rate (BER) performance of the proposed systems with conventional counterparts over a range of sampling rate. This shows that, the proposed systems perform better than conventional systems without PAPR reduction. The results are validated through simulation. Other two experimental studies on the previous systems with parameters optimisation and available modulation bandwidth utilisation are presented, which show that the proposed systems outperform the conventional systems in terms of BER. This is followed by investigating the PA PAPR reduction effect on the achievable data rate of a wavelength division multiplexing (WDM) based VLC system using three different LEDs for PAPR reduced DCO-OFDM and PAM-DMT systems. The proposed systems have achieved more than 8% data rate higher than that of conventional systems without BER performance degradation.
Finally, analytical investigation of clipping noise that leads to distortion in a VLC system due to front-end devices limitations is presented. To mitigate the clipping noise, PAPR of the system is reduced by the PA technique. The analytical BER performance of the system with PAPR reduction is verified through simulation and then compared to that of the conventional system without PAPR reduction at similar clipping levels. The PA proposed system shows better BER performance at all clipping levels
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
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 Tb/s indoor optical wireless access system using VCSEL arrays
This paper presents a proof-of-concept for Tb/s infrared (IR) indoor optical wireless networks. We introduce a novel double tier access point architecture based on array of arrays of vertical cavity surface emitting lasers (VCSELs) to deliver beyond Tb/s aggregate capacity. For a given indoor environment, the optimal access point architecture is designed. The downlink performance is analysed throughout the coverage area and the spatial distribution of signal-to-interference-plus-noise ratio (SINR) and data rate are obtained. Numerical results demonstrate that with a single access point in a 25 m2 indoor area, data rates of at least 10 Gb/s per beam are achieved almost everywhere and the aggregate data rate can exceed 2 Tb/s
Study, analysis and application of Optical OFDM, Single Carrier (SC) and MIMO in Intensity Modulation Direct Detection (IM/DD)
With the rapid growth of wireless data demands and saturation of radio frequency (RF) capacity,
visible light communication (VLC) has become a promising candidate to complement
conventional RF communication, especially for indoor short range applications. However the
performance of the system depends on the propagation and type of system used. An optical
Orthogonal Frequency Division Multiplexing (O-OFDM) together with multiple input multiple
output (MIMO) in different scenario and modulation techniques are studied in the thesis.
A novel optical wireless communication (OWC) multi-cell system with narrow field of view
(FOV) is studied. In this system the intensity modulated beam from four light sources are
used for communication. The system allows beams to be concentrated in specific areas of
the room to serve multiple mobile devices with low interference and hence increase system
capacity. The performance of asymmetrically clipped optical orthogonal frequency division
multiplexing (ACO-OFDM), direct current biased optical OFDM (DCO-OFDM) and single
carrier (SC) modulation are then compared in this system considering single user and multiusers
scenarios. The performance of the multi-cell is compared with single cell with wide FOV.
It is shown that the capacity for multi-cell system increases with the number of users to 4 times
the single user capacity. Also the findings show that multi-cell system with narrow beams can
outperform a single wide beam system in terms of coverage area and hence average throughput
of about 2.7 times the single wide beam system capacity.
One of the impairments in line of sight (LOS) OWC systems is coverage which degrades the
performance. A mobile receiver with angular diversity detectors in MIMO channels is studied.
The objective is to improve the rank of the channel matrix and hence system throughput. Repetition
coding (RC), spatial multiplexing (SMP) and spatial modulation (SM) concepts are used
to evaluate throughput across multiple locations in a small room scenario. A novel adaptive
spatial modulation (ASM) which is capable of combating channel rank deficiency is devised.
Since the receiver is mobile, the channel gains are low in some locations of the room due to
the lack of LOS paths between transmitters and receivers. To combat the situation adaptive
modulation and per antenna rate control (PARC) is employed to maximise spectral efficiency.
The throughputs for fixed transmitters and receivers are compared with the oriented/inclined
detectors for different cases. Angular diversity detectors offer a better throughput improvement
than the state of the art vertical detectors, for example in ASM angular diversity receiver gives
throughput of about 1.6 times that of vertical detectors. Also in SMP the angular detectors
offer throughput about 1.4 times that of vertical detectors. SMP gives the best performance
compared to RC, SM and ASM, for example SMP gives throughput about 2.5 times that of RC
in both vertical detectors and angular diversity receivers. Again SMP gives throughput about 6
times that of SM in both vertical detectors and angular diversity receivers. Also SMP provides
throughput about 2 times that of ASM in both vertical detectors and angular diversity receivers.
ASM exhibit improvement in throughput about average factor of 3.5 times SM performance in
both vertical detectors and angular diversity detectors.
As the performance of the system may be jeopardized by obstructions, specular and diffuse
reflection models for indoor OWC systems using a mobile receiver with angular diversity detectors
in MIMO channels are considered. The target is to improve the MIMO throughput
compared to vertically oriented detectors by exploiting reflections from different reflecting surfaces
in the room. The throughput across multiple locations in the small room by using RC,
SMP and SM approaches is again evaluated. The results for LOS only channels against LOS
with specular or diffuse reflection conditions, for both vertical and angular oriented receivers
are then compared. The results show that exploiting specular and diffuse reflections provide
significant improvements in link performance. For example the reflection coefficient (α) of
0.9 and the antenna separation of 0.6 m, RC diffuse model shows throughput improvement of
about 1.8 times that of LOS for both vertical detectors and angular diversity receivers. SM
diffuse model shows throughput improvement of about 3 times that of LOS for both vertical
detectors and angular diversity receivers. ASM diffuse model shows throughput improvement
of about 2 times that of LOS for both vertical detectors and angular diversity receivers. SMP
diffuse model shows throughput improvement of about 1.5 times that of LOS for both vertical
detectors and angular diversity receiver
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