27 research outputs found
Interference mitigation techniques for optical attocell networks
The amount of wireless data traffic has been increasing exponentially. This results in the shortage
of radio frequency (RF) spectrum. In order to alleviate the looming spectrum crisis, visible
light communication (VLC) has emerged as a supplement to RF techniques. VLC uses light
emitting diodes (LEDs) for transmission and employs photodiodes (PDs) for detection. With
the advancement of the LED technology, LEDs can now fulfil two functions at the same time:
illumination and high-speed wireless communication. In a typical indoor scenario, each single
light fixture can act as an access point (AP), and multiple light fixtures in a room can form a
cellular wireless network. We refer to this type of networks as âoptical attocell networkâ. This
thesis focuses on interference mitigation in optical attocell networks.
Firstly, the downlink inter-cell interference (ICI) model in optical attocell networks is investigated.
The conventional ray-tracing channel model for non-line-of-sight (NLOS) path is studied.
Although this model is accurate, it leads to time-consuming computer simulations. In order
to reduce the computational complexity, a simplified channel model is proposed to accurately
characterise NLOS ICI in optical attocell networks. Using the simplified model, the received
signal-to-interference-plus-noise ratio (SINR) distribution in optical attocell networks can be
derived in closed-form. This signifies that no Monte Carlo simulation is required to evaluate
the user performance in optical attocell networks.
Then, with the knowledge of simplified channel model, interference mitigation techniques using
angle diversity receivers (ADRs) are investigated in optical attocell networks. An ADR
typically consists of multiple PDs with different orientations. By using proper signal combining
schemes, ICI in optical attocell networks can be significantly mitigated. Also, a novel
double-source cell configuration is proposed. This configuration can further mitigate ICI in
optical attocell networks in conjunction with ADRs. Moreover, an analytical framework is
proposed to evaluate the user performance in optical attocell networks with ADRs.
Finally, optical space division multiple access (SDMA) using angle diversity transmitters is
proposed and investigated in optical attocell networks. Optical SDMA can exploit the available
bandwidth resource in spatial dimension and mitigate ICI in optical attocell networks.
Compared with optical time division multiple access (TDMA), optical SDMA can significantly
improve the throughput of optical attocell networks. This improvement scales with the number
of LED elements on each angle diversity transmitter. In addition, the upper bound and
the lower bound of optical SDMA performance are derived analytically. These bounds can
precisely evaluate the performance of optical SDMA systems. Furthermore, optical SDMA
is shown to be robust against user position errors, and this makes optical SDMA suitable for
practical implementations
Light-Fidelity as Next Generation Network Technology: A Bibliometric Survey and Analysis
This paper delivers a systematic review and a bibliometric survey analysis of Light-Fidelity (Li-Fi) indoor implementation in Next Generation Network (NGN). The main objective of this study is to design a communication network based on NGN-Li-Fi for the indoor implementation which aims to increase user Quality of Service (QoS). The main merits and contributions of this study are the thorough and detailed analysis of the review, both in literature surveys and bibliometric analysis, as well as the discussion of the implementation model challenges of Li-Fi in both indoor and outdoor environments. The issue articulated in an indoor communication network is the possibility of intermittent connectivity due to barriers caused by line-of-sight (LOS) between the LED transmitter and receiver, handover due to channel overlap, and other network reliability issues. To realize the full potential and significant benefits of the Next Generation Network, challenges in indoor communication such as load-balancing and anticipating network congestion (traffic congestion) must be addressed. The main benefit of this study is the in-depth investigation of surveys in both selected critical literatures and bibliometric approach. This study seeks to comprehend the implications of Next Generation networks for indoor communication networks, particularly for visible light communication channels
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Design and Analysis of Free-space Optical Communications Systems for Next Generation Short-range Wireless Networks
This dissertation focuses on indoor free-space optical communications systems for use in short range wireless networks. We propose that current radio frequency wireless links be augmented or replaced with optical frequency links due to overcrowding in the radio frequency spectrum. Optical frequencies contain hundreds of terahertz of unregulated bandwidth and offer a physical layer of protection due to the inherently line-of-sight nature of near infrared light. We first present a hybrid optical and radio frequency link based on inexpensive LEDs in which a downlink is established optically, while the uplink is routed through preexisting radio frequency channels. Second, an all optical dual channel laser-based communication system is implemented consisting of a medium bandwidth wide-angle optical femtocell and a high-speed line-of-sight optical attocell. This approach balances the diverse needs of end users by providing links optimized for both mobility and bandwidth. Lastly, we demonstrate a high-power vertical cavity surface emitting laser (VCSEL) array transmitter for use in optical femtocells. A complimentary design for a distributed current laser driver is also presented
Interference mitigation in LiFi networks
Due to the increasing demand for wireless data, the radio frequency (RF) spectrum has
become a very limited resource. Alternative approaches are under investigation to support
the future growth in data traffic and next-generation high-speed wireless communication
systems. Techniques such as massive multiple-input multiple-output (MIMO), millimeter
wave (mmWave) communications and light-fidelity (LiFi) are being explored. Among
these technologies, LiFi is a novel bi-directional, high-speed and fully networked wireless
communication technology. However, inter-cell interference (ICI) can significantly restrict the
system performance of LiFi attocell networks. This thesis focuses on interference mitigation
in LiFi attocell networks.
The angle diversity receiver (ADR) is one solution to address the issue of ICI as well as
frequency reuse in LiFi attocell networks. With the property of high concentration gain and
narrow field of view (FOV), the ADR is very beneficial for interference mitigation. However,
the optimum structure of the ADR has not been investigated. This motivates us to propose the
optimum structures for the ADRs in order to fully exploit the performance gain. The impact
of random device orientation and diffuse link signal propagation are taken into consideration.
The performance comparison between the select best combining (SBC) and maximum ratio
combining (MRC) is carried out under different noise levels. In addition, the double source
(DS) system, where each LiFi access point (AP) consists of two sources transmitting the same
information signals but with opposite polarity, is proven to outperform the single source (SS)
system under certain conditions.
Then, to overcome issues around ICI, random device orientation and link blockage, hybrid
LiFi/WiFi networks (HLWNs) are considered. In this thesis, dynamic load balancing (LB)
considering handover in HLWNs is studied. The orientation-based random waypoint (ORWP)
mobility model is considered to provide a more realistic framework to evaluate the performance
of HLWNs. Based on the low-pass filtering effect of the LiFi channel, we firstly propose
an orthogonal frequency division multiple access (OFDMA)-based resource allocation (RA)
method in LiFi systems. Also, an enhanced evolutionary game theory (EGT)-based LB scheme
with handover in HLWNs is proposed.
Finally, due to the characteristic of high directivity and narrow beams, a vertical-cavity
surface-emitting laser (VCSEL) array transmission system has been proposed to mitigate
ICI. In order to support mobile users, two beam activation methods are proposed. The
beam activation based on the corner-cube retroreflector (CCR) can achieve low power
consumption and almost-zero delay, allowing real-time beam activation for high-speed users.
The mechanism based on the omnidirectional transmitter (ODTx) is suitable for low-speed
users and very robust to random orientation
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
Analysis of random orientation and user mobility in LiFi networks
Mobile data traffic is anticipated to surpass 49 exabyte per month by 2021. Smartphones, as
the main factor of generating this huge data traffic (86%), are expected to require average speed
connection of 20 Mbps by 2021. Light-fidelity (LiFi) is a novel bidirectional, high-speed and
fully networked optical wireless communication and it is a promising solution to undertake
this huge data traffic. However, to support seamless connectivity in LiFi networks, real-time
knowledge of channel state information (CSI) from each user is required at the LiFi access point
(AP). The CSI availability enables us to achieve optimal resource allocation and throughput
maximization but it requires feedback transmitted through the uplink channel. Furthermore,
the important aspects of the indoor LiFi channel such as the random orientation of user device,
user mobility and link blockage need to be carefully analysed and effective solutions should be
developed.
In contrast to radio frequency (RF) channels, the LiFi channel is relatively less random. This feature
of LiFi channel enables a potential reduction in the amount of feedback required to achieve
high throughputs in a dynamic LiFi network. Based on this feature, two techniques for reducing
the amount of feedback in LiFi cellular networks are proposed: 1) limited-content feedback
scheme based on reducing the content of feedback information and 2) limited-frequency feedback
scheme based on the update interval. It is shown that these limited-feedback schemes can
provide almost the same downlink performance as full feedback scheme. Furthermore, an optimum
update interval which provides maximum bidirectional user equipment (UE) throughput,
has been derived.
Device orientation and its statistics is an important determinant factor that can affect the users
throughput remarkably in LiFi networks. However, device orientation has been ignored in many
previous performance studies of LiFi networks due to the lack of a proper statistical model. In
this thesis, a novel model for the orientation of user device are proposed based on experimental
measurements. The statistics of the device orientation for both sitting and walking activities are
presented. Moreover, the statistics of the line-of-sight (LOS) channel gain are calculated. The
influence of random device orientation on the received signal-to-noise-ratio (SNR) and bit-error
ratio (BER) performance of LiFi systems has been also evaluated.
To support the seamless connectivity of future LiFi-enabled devices in the presence of random
device orientation, mobility and blockage, efficient handover between APs are required. In this
thesis, an orientation-based random waypoint (ORWP) mobility model is proposed to analyze
the performance of mobile users considering the effect of random device orientation. Based on
this model, an analysis of handover due to random orientation and user mobility is presented.
Finally, in order to improve seamless connectivity, a multi-directional receiver (MDR) configuration
is proposed. The MDR configuration shows a robust performance in the presence of user
mobility, random device orientation and blockage
Resource Allocation for Cooperative Transmission in Optical Wireless Cellular Networks With Illumination Requirements
This work has been partially funded by the Spanish MECD FPU fellowship program granted to the author Borja GenovĂ©s GuzmĂĄn, the Catalan Government under Grant 2017-SGR-1479, and the Spanish Government under the national project âTERESA-ADAâ with ID no. TEC2017-90093-C3-2-R and TEC2017-90093-C3-1-R (MINECO/AEI/FEDER, UE)
Design and performance analysis of optical attocell networks
The exponentially increasing demand for high-speed wireless communications will no longer
be satisfied by the traditional radio frequency (RF) in the near future due to its limited spectrum
and overutilization. To resolve this imminent issue, industrial and research communities have
been looking into alternative technologies for communication. Among them, visible light communication
(VLC) has attracted much attention because it utilizes the unlicensed, free and safe
spectrum, whose bandwidth is thousand times larger than the entire RF spectrum. Moreover,
VLC can be integrated into existing lighting systems to offer a dual-purpose, cost-effective and
energy-efficient solution for next-generation small-cell networks (SCNs), giving birth to the
concept of optical attocell networks.
Most relevant works in the literature rely on system simulations to quantify the performance
of attocell networks, which suffer from high computational complexity and provide limited
insights about the network. Mathematical tools, on the other hand, are more tractable and
scalable and are shown to closely approximate practical systems. The presented work utilizes
stochastic geometry for downlink evaluation of optical attocell networks, where the co-channel
interference (CCI) surpasses noise and becomes the limiting factor of the link throughput. By
studying the moment generating function (MGF) of the aggregate interference, a theoretical
framework for modeling the distribution of signal-to-interference-plus-noise ratio (SINR) is
presented, which allows important performance metrics such as the coverage probability and
link throughput to be derived. Depending on the source of interference, CCI can be classified
into two categories: inter-cell interference (ICI) and intra-cell interference. In this work,
both types of interference are characterized, based on which effective interference mitigation
techniques such as the coordinated multipoint (CoMP), power-domain multiplexing and successive
interference cancellation (SIC) are devised. The proposed mathematical framework is
applicable to attocell networks with and without such interference mitigation techniques.
Compared to RF networks, optical attocell networks are inherently more secure in the physical
layer because visible light does not penetrate through opaque walls. This work analytically
quantifies the physical-layer security of attocell networks from an information-theoretic point of
view. Secrecy enhancement techniques such as AP cooperation and eavesdropper-free protected
zones are also discussed. It is shown that compared to AP cooperation, implementing secrecy
protected zones is more effective and it can contribute significantly to the network security