234 research outputs found
Performance evaluation of FSO communication systems over weak atmospheric turbulence channel for eastern coast of South Africa.
Masters Degree. University of KwaZulu-Natal, Durban.Free space optical (FSO) communication, otherwise known as optical wireless communication (OWC), is an established line-of-sight telecommunication technique which utilises an optical signal carrier to propagate modulated signals in the form of a light wave (visible or infrared) over the atmospheric medium. It has numerous advantages, including ease of deployment, large bandwidth, cost effective, full duplex high data rate throughput, protocol independence, highly secured data rate transmission, unregulated frequency spectrum, limited electromagnetic interference, and minimum amount of power consumption.
With all the inherent advantages in FSO systems, the technology is impaired by atmospheric turbulence. Atmospheric turbulence occurs due to the persistent random changes of the refractive index as a result of variations in atmospheric temperature and pressure. This results in fluctuations in the irradiance of the laser (simply referred to as scintillation), which may lead to attenuation of optical signals in the FSO communication system. Thus, atmospheric attenuation and turbulent conditions have negative effects on the performance and ease of deployment of FSO communication systems. In this dissertation, we examine the performance of FSO systems over weak atmospheric turbulence channel for the eastern coast of South Africa.
We evaluate the feasibility of the FSO link and how to improve the reliability by estimating the link margin, probability of attenuation exceedance, power scintillation index, overall power loss due to attenuation and turbulence, link budget estimate for different link lengths and wavelengths. The FSO system availability estimated for the eastern coast of South Africa is above 99% for link distances ranging from 1 km-4 km at 850 nm, 950 nm and 1550 nm. It is also observed that the FSO link availability increases with corresponding increase in wavelengths.
Adopting the Kim model to estimate the atmospheric attenuation at 850 nm wavelength, the attenuation due to scattering contributes 9.47% to the absolute atmospheric losses while the atmospheric turbulence loss contributes 90.53% to the overall power loss at a link range of 4 km. Using the Ferdinandov model for a link range of 4 km at 950 nm wavelength, the attenuation due to scattering contributes 8.81% to the total power loss while the atmospheric turbulence loss contributes 91.19% to the overall power loss.
It is observed that the attainable link distance increases with increase in atmospheric visibility status. The FSO system availability reduces with increase in the propagation link distance. Furthermore, it is found that the fading loss from scintillation effects strongly depends on the power scintillation index. An increase in the power scintillation index, causes an increase in the fading loss. Thus, the power scintillation index also increases per unit increase in transmission link length and refractive index.
The compensation margin for such atmospheric fading loss increases with decrease in accessible FSO system bound probability. Therefore, for a highly reliable FSO system link, extra margin must be incorporated to compensate for fading loss caused by scintillation
Integrated Electronics for Wireless Imaging Microsystems with CMUT Arrays
Integration of transducer arrays with interface electronics in the form of single-chip CMUT-on-CMOS has emerged into the field of medical ultrasound imaging
and is transforming this field. It has already been used in several commercial products such as handheld full-body imagers and it is being implemented by commercial and academic groups for Intravascular Ultrasound and Intracardiac Echocardiography. However, large attenuation of ultrasonic waves transmitted through
the skull has prevented ultrasound imaging of the brain. This research is a prime
step toward implantable wireless microsystems that use ultrasound to image the
brain by bypassing the skull. These microsystems offer autonomous scanning
(beam steering and focusing) of the brain and transferring data out of the brain for
further processing and image reconstruction.
The objective of the presented research is to develop building blocks of an integrated electronics architecture for CMUT based wireless ultrasound imaging systems while providing a fundamental study on interfacing CMUT arrays with their
associated integrated electronics in terms of electrical power transfer and acoustic
reflection which would potentially lead to more efficient and high-performance
systems.
A fully wireless architecture for ultrasound imaging is demonstrated for the
first time. An on-chip programmable transmit (TX) beamformer enables phased
array focusing and steering of ultrasound waves in the transmit mode while its
on-chip bandpass noise shaping digitizer followed by an ultra-wideband (UWB)
uplink transmitter minimizes the effect of path loss on the transmitted image data
out of the brain. A single-chip application-specific integrated circuit (ASIC) is de-
signed to realize the wireless architecture and interface with array elements, each
of which includes a transceiver (TRX) front-end with a high-voltage (HV) pulser,
a high-voltage T/R switch, and a low-noise amplifier (LNA). Novel design techniques are implemented in the system to enhance the performance of its building
blocks.
Apart from imaging capability, the implantable wireless microsystems can include a pressure sensing readout to measure intracranial pressure. To do so, a
power-efficient readout for pressure sensing is presented. It uses pseudo-pseudo
differential readout topology to cut down the static power consumption of the sensor for further power savings in wireless microsystems.
In addition, the effect of matching and electrical termination on CMUT array
elements is explored leading to new interface structures to improve bandwidth
and sensitivity of CMUT arrays in different operation regions. Comprehensive
analysis, modeling, and simulation methodologies are presented for further investigation.Ph.D
Radio Channel Characterization for Future Wireless Networks and Applications
The new frontier of Above-6GHz bands is revolutionizing the field of
wireless telecommunications, requiring new radio channel models to support
the development of future Giga-bit-per-second systems. Recently, deterministic
ray-based models as Ray Tracing are catching on worldwide thanks to their
frequency-agility and reliable predictions. A modern 3D Ray Tracing developed
at University of Bologna has been indeed calibrated and used to investigate the
Above-6GHz radio channel properties. As starting point, an item-level electromagnetic
characterization of common items and materials has been achieved successfully
to obtain information about the complex permittivity, scattering diagrams and
even de-polarization effects, both utilizing Vector Spectrum Analyzer (at 7-15GHz)
and custom Channel Sounder (at 70GHz). Thus, a complete tuning of the Ray Tracing
has been completed for Above-6GHz frequencies. Then, 70GHz indoor doubledirectional
channel measurements have been performed in collaboration with TU
Ilmenau, in order to attain a multidimensional analysis of propagation mechanisms
in time and space, outlining the differences between Below- and Above-6GHz propagation.
Furthermore, multi-antenna systems, as Multiple-Input-Multiple-
Output (MIMO) and Beamforming have been taken into considerations, as strategic
technologies for Above-6GHz systems, focusing on their implementation, limits
and differences. Finally, complex system simulations of Space-Division-Multiple-
Access (SDMA) networks in indoor scenarios have been tested, to assess the capabilities
of Beamforming. In particular, efficient Beam Search and Tracking algorithms
have been proposed to assess the impact of interference on Multi-User Beamforming
at 70GHz and, also, novel Multi-Beam Beamforming schemes have been tested
at 60GHz to investigate diversity strategies to cope with NLOS link and Human
Blockage events. Moreover, the novel concept of Ray-Tracing-assisted Beamforming
has been outlined, showing that ray-based models represent today the promising
key tools to evaluate, design and enhance the future Above-6GHz multi-antenna
systems
1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface
A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance
Beam scanning by liquid-crystal biasing in a modified SIW structure
A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium
Adaptation techniques in optical wireless communications
The need for high-speed local area networks to meet the recent developments
in multimedia and video transmission applications has recently focused interest
on optical wireless communication. Optical wireless systems boast some
advantages over radio frequency (RF) systems, including a large unregulated
spectrum, freedom from fading, confidentiality and immunity against
interference from electrical devices. They can satisfy the dual need for mobility
and broadband networking. However, optical wireless links are not without
flaws. They are affected by background noise (artificial and natural light
sources) and suffer from multipath dispersion. The former can degrade the
signal-to-noise ratio, while the latter restricts the maximum transmission rate
available.
The aim of this thesis is to investigate a number of techniques to overcome
these drawbacks and design a robust high-speed indoor optical wireless system
with full mobility. Beam delay and power adaptation in a multi-spot diffusing
system is proposed in order to increase the received optical signal, reduce the
delay spread and enable the system to operate at higher data rates. The thesis
proposes employing angle diversity receivers and imaging diversity receivers as
in order to reduce background noise components. Moreover, the work
introduces and designs a high-speed fully adaptive optical wireless system that
employs beam delay, angle and power adaptation in a multi-spot diffusing
configuration and investigates the robustness of the link design in a realistic
indoor office. Furthermore, a new adaptive optical wireless system based on a
finite vocabulary of stored holograms is introduced. This method can effectively
optimise the spots’ locations and reduce the design complexity of an adaptive
optical wireless system. A fast adaptation approach based on a divide-andconquer
methodology is proposed and integrated with the system to reduce the
time required to identify the optimum hologram. The trade-off between complexity and performance enhancement of the adaptive finite holograms
methods compared with the original beam power and angle adaptation is
investigated
Satellite Communications
This study is motivated by the need to give the reader a broad view of the developments, key concepts, and technologies related to information society evolution, with a focus on the wireless communications and geoinformation technologies and their role in the environment. Giving perspective, it aims at assisting people active in the industry, the public sector, and Earth science fields as well, by providing a base for their continued work and thinking
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