60 research outputs found
Optical MIMO communication systems under illumination constraints
Technology for wireless information access has enabled innovation of 'smart' portable consumer devices. These have been widely adopted and have become an integral part of our daily lives. They need ubiquitous connectivity to the internet to provide value added services, maximize their functionality and create a smarter world to live in. Cisco's visual networking index currently predicts wireless data consumption to increase by 61% per year. This will put additional stress on the already stressed wireless access network infrastructure creating a phenomenon called 'spectrum crunch'.
At the same time, the solid state devices industry has made remarkable advances in energy efficient light-emitting-diodes (LED). The lighting industry is rapidly adopting LEDs to provide illumination in indoor spaces. Lighting fixtures are positioned to support human activities and thus are well located to act as wireless access points. The visible spectrum (380 nm - 780 nm) is yet unregulated and untapped for wireless access. This provides unique opportunity to upgrade existing lighting infrastructure and create a dense grid of small cells by using this additional 'optical' wireless bandwidth. Under the above model, lighting fixtures will service dual missions of illumination and access points for optical wireless communication (OWC).
This dissertation investigates multiple-input multiple-output (MIMO) optical wireless broadcast system under unique constraints imposed by the optical channel and illumination requirements. Sample indexed spatial orthogonal frequency division multiplexing (SIS-OFDM) and metameric modulation (MM) are proposed to achieve higher spectral efficiency by exploiting dimensions of space and color respectively in addition to time and frequency. SIS-OFDM can provide significant additional spectral efficiency of up to (Nsc/2 - 1) x k bits/sym where Nsc is total number of subcarriers and k is number of bits per underlying spatial modulation symbol. MM always generates the true requested illumination color and has the potential to provide better color rendering by incorporating multiple LEDs. A normalization framework is then developed to analyze performance of optical MIMO imaging systems. Performance improvements of up to 45 dB for optical systems have been achieved by decorrelating spatially separate links by incorporating an imaging receiver. The dissertation also studies the impact of visual perception on performance of color shift keying as specified in IEEE 802.15.7 standard. It shows that non-linearity for a practical system can have a performance penalty of up to 15 dB when compared to the simplified linear system abstraction as proposed in the standard. Luminous-signal-to-noise ratio, a novel metric is introduced to compare performance of optical modulation techniques operating at same illumination intensity. The dissertation then introduces singular value decomposition based OWC system architecture to incorporate illumination constraints independent of communication constraints in a MIMO system. It then studies design paradigm for a multi-colored wavelength division multiplexed indoor OWC system
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
Multi-Hop Wireless Optical Backhauling for LiFi Attocell Networks: Bandwidth Scheduling and Power Control
The backhaul of hundreds of light fidelity (LiFi) base stations (BSs)
constitutes a major challenge. Indoor wireless optical backhauling is a novel
approach whereby the interconnections between adjacent LiFi BSs are provided by
way of directed line-of-sight (LOS) wireless infrared (IR) links. Building on
the aforesaid approach, this paper presents the top-down design of a multi-hop
wireless backhaul configuration for multi-tier optical attocell networks by
proposing the novel idea of super cells. Such cells incorporate multiple
clusters of attocells that are connected to the core network via a single
gateway based on multi-hop decode-and-forward (DF) relaying. Consequently, new
challenges arise for managing the bandwidth and power resources of the
bottleneck backhaul. By putting forward user-based bandwidth scheduling (UBS)
and cell-based bandwidth scheduling (CBS) policies, the system-level modeling
and analysis of the end-to-end multi-user sum rate is elaborated. In addition,
optimal bandwidth scheduling under both UBS and CBS policies are formulated as
constrained convex optimization problems, which are solved by using the
projected subgradient method. Furthermore, the transmission power of the
backhaul system is opportunistically reduced by way of an innovative fixed
power control (FPC) strategy. The notion of backhaul bottleneck occurrence
(BBO) is introduced. An accurate approximate expression of the probability of
BBO is derived, and then verified using Monte Carlo simulations. Several
insights are provided into the offered gains of the proposed schemes through
extensive computer simulations, by studying different aspects of the
performance of super cells including the average sum rate, the BBO probability
and the backhaul power efficiency (PE).Comment: 36 pages, 21 figures, 1 tabl
Komunikace viditelným světlem (VLC) pro LED veřejné osvětlení a další venkovní aplikace
This diploma thesis focuses on visible light communications, mainly in outdoor appliances. Whole text is divided into two blocks. First segment serves as a research, focusing on experiments carried by different university teams all over the world, which are published as IEEE conference papers. Another half discuss our own VLC prototyping, its limitations and possible upgrade paths.Tato práce se zabývá problematikou komunikace viditelným spektrem světla. Celý text je rozvržen do dvou velkých bloků, přičemž první slouží jako rešerše prací jiných týmů ze všech koutů světa, publikovaných na IEEE konferencích. Druhá část pojednává o samotném měření a prototypu komunikačního řetězce. Zmíněna je konstrukce, limitace i možnosti budoucích vylepšení.440 - Katedra telekomunikační technikyvýborn
Energy and spectral efficiency of multi-tier LiFi networks
In this paper, multi-tier LiFi networks are studied in terms of energy efficiency (EE) and spectral efficiency (SE), which are crucial metrics for LiFi system design. We derived a closed-form expression of the user association probability for different tiers using stochastic geometry based Poisson Voronoi Tessellation (PVT) LiFi network. The performance metrics of the network, EE and SE, are analyzed in terms of different parameters such as transmit power and Lambertian index. Performance evaluations and numerical results show that multi-tier LiFi networks have an optimum transmit power in which EE is maximized. Besides, increasing the transmit power does not increase SE after passing a threshold point. The resulting trade-off between EE and SE is presented
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