Thesis (doctoral)--Universität Duisburg-Essen, 2003. / Includes bibliographical references (p. 115-123).

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2006. / Dr. Stevenson J. Kenney, Committee Member ; Dr. Paul G. Steffes, Committee Chair ; Dr. Gregory D. Durgin, Committee Member ; Dr. Aaron D. Lanterman, Committee Member ; Dr. Robert G. Roper, Committee Member.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 94-96). Also available in electronic version.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 150-159). Also available in electronic version.

Thesis (M.Eng.)(Electronic)--University of Pretoria, 2007. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

M.Ing. / Over the past few years, fibre Bragg gratings have emerged as very important components in the photonics environment. Their discovery has greatly revolutionised the design of many complex devices, introducing simplicity and cost effectiveness. Extensive research has been invested into identifying possible areas of application for fibre Bragg gratings. This has resulted in fibre Bragg gratings finding a comfortable niche in the fields of sensors and optical communication systems. This study focuses on the possible application of fibre Bragg gratings in wireless local area networks. Wireless local area networks are rapidly becoming a market of great potential for the investor. To sustain the impressive image of this market, research initiatives should strive to secure cost-effective solutions for the implementation of wireless local area networks. With reduced costs on wireless local area network products, the demand for these products is expected to escalate. Research conducted at the COBRA Institute, has produced a novel concept of optically distributing data signals across a network, and later transmitting them at radio frequencies between antenna sites. This concept, earmarked for wireless local area networks, uses a fast tuneable laser source, graded index polymer optical fibre and a periodic optical filter to generate the radio frequency microwave signals. The periodic filter in this network system serves to up-convert the frequency of the signal to radio-frequency levels. The filter is an important component of the network and it is situated at the antenna site. For wireless local area networks that require broad coverage, the number of periodic optical filters deployed for the system can be high. This can have a negative impact on the cost of the network. This research proposes a low-cost Fabry-Perot filter designed from fibre Bragg gratings, to replace the optical periodic filter discussed in the network mentioned above. The work presented in this study consolidates the design theories of Fabry-Perot filters and fibre Bragg gratings. The grating-based Fabry-Perot filter is modelled using coupled-mode theory, simulated using the transfer matrix method and fabricated via the strain controlled phase mask technique. The figures of merit that describe the spectral performance of the Fabry-Perot filter (i.e. free spectral range, minimum resolvable bandwidth, finesse and contrast factor) are compared to calculations associated with classical designs. The final part of this study presents experimental measurements of the generated radio frequency microwave signal. The purpose of these measurements is firstly to demonstrate the feasibility of the entire concept of generating radio frequency microwave signals using optical frequency multiplication. The second goal for the measurements is to benchmark the performance of the new grating-based Fabry-Perot filter against expected results approximated through calculations. Both goals were achieved with encouraging observations.

Please read the abstract in the section 00front of this document / Thesis (DPhil (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

Surpassing the fundamental limits that govern all electromagnetic structures, such as reciprocity and the delay-bandwidth-size limit, will have a transformative impact on all applications based on electromagnetic circuits and systems. For instance, violating principles of reciprocity enables non-reciprocal components such as isolators and circulators, which find application in full-duplex wireless radios, radar, biomedical imaging, and quantum computing systems. Overcoming the delay-bandwidth-size limit enables ultra-broadband yet extremely-compact devices whose size is not fundamentally related to the wavelength at the operating frequency. The focus of this dissertation is on using time-variance as a new toolbox to overcome these fundamental limits and re-imagine circuit and system design. Traditional non-reciprocal components are realized using ferrite materials that loose their reciprocity under the application of external magnetic bias. However, the sheer volume, cost and weight of these magnet based non-reciprocal components coupled with their inability to be fabricated in conventional semiconductor processes, have limited their application to bulky and large-scale systems. Other approaches such as active-biased and non-linearity based non-reciprocity are compatible with semiconductor processes, however, they suffer from other poor linearity and noise performance. In this dissertation, using passive transistor switch as the modulating element, we have proposed the concept of spatio-temporal conductivity modulation and have demonstrated a gamut of non-reciprocal devices ranging from gyrators to isolators and circulators. Through novel circuit topologies, for the first time, we have demonstrated on-chip circulators with multi-watt input power handling, operation at high millimeter-wave frequencies, and tailor made circulators for emerging technologies such as simultaneous-transmit-and-receive MRI and quantum computing. Delay-bandwidth-size trade-off is another fundamental electromagnetic limit, that constrains the delay imparted by a medium or a device within a fixed footprint to be inversely proportional to the signal bandwidth. It is this limit that governs the size of any microwave passive devices to be inversely proportional to its operating frequency. As a part of this dissertation, through intelligent clocking of switched capacitor networks we overcame the delay-bandwidth-size limit, thus resulting in infinitesimal, yet broadband microwave devices. Here we proposed a new paradigm in wave propagation where the properties such as the propagation delay and characteristic impedance does not depend on the constituent elements/materials of the medium, but rather heavily rely on the user-defined modulation scheme, thereby opening huge opportunities for realizing highly-reconfigurable passives. Leveraging these concepts, we demonstrated wide range of reciprocal an non-reciprocal devices including ultra-compact delay elements, highly-reconfigurable microwave passives, ultra-wideband circulators with infinitesimal form-factors and dispersion-free chip scale floquet topological insulators. Application of these devices have also been evaluated in real-world systems through our demonstrations of wideband, full-duplex receivers leveraging switched capacitors based true-time-delay interference cancelers and floquet topological insulator based antenna interfaces for full-duplex phased-arrays and ultra-wideband beamformers. Furthermore, to cater the growing RF and microwave needs of future, large-scale quantum computing systems, we demonstrated a low-cryogenic, wideband circulator based on time modulation of superconducting devices. This superconducting circulator is expected to operate alongside the superconducting qubits, inside a dilution refrigerator at 10mK-100mK, thus enabling a tightly integrated quantum system. We also presented the design and implementation of a cryogenic-CMOS clock driver chip that will generate the clocks required by the superconducting circulator. Finally, we also demonstrated the design and implementation of a low-noise, low power consumption, 6GHz - 8GHz cryogenic downconversion receiver at 4K for cryogenic qubit readout.

No description available.

In this dissertation, we study energy efficient transmission in wireless communication networks. The general problem of energy efficient transmission over wireless networks is formulated into optimization problems for the following distinct (but inter-related) settings: Problem Setting 1: The minimization of energy (power) consumption given a system throughput and other constraints, and Problem Setting 2: The maximization of system throughput given energy (power) constraints. Under Problem Setting 1, we focus on energy efficient transmission problems over wideband channels. The first result we obtain is as follows: We consider a two user multiple access channel. In this multiple access channel, previous research shows that cooperation with respect to block error rate is only possible if two transmitters share their sources completely. However, we find that a modified pulse position modulation with synchronization enables cooperation without complete sharing of their sources between two transmitters if we replace a block error rate requirement with a normalized error rate constraint. Normalized error rate, a quantity that resembles bit error rate, is developed in this work as an error metric that is of value in practical communication systems. We show full cooperation between two transmitters without sharing their sources by deriving that the minimum energy per bit required for reliable transmission reduces by quarter compared with the minimum energy per bit required for point-to-point channels. Next, we generalize this analysis to a cognitive communication framework with a wideband cognitive transmitter, which can causally sense signal levels over multiple frequency bands, and a cognitive receiver. We assume that multiple legitimate users already exist in the system and each one transmits in its own non-overlapping frequency band. In this setting, from order statistical analysis, we show that the wideband cognitive transmit-receive pair is able to communicate reliably with minimum energy as if the legitimate users were absent from the system, while causing negligible interference to bandlimited legitimate users. The wideband cognitive transmit-receive pair employs a strategy defined as opportunistic group orthogonal signaling to achieve the minimum energy per bit. Under Problem Setting 2, we investigate the impact of correlation and transmit and receive strategies on the throughput of multiple antenna broadcast channels in cellular networks. With perfect channel state information at the transmitter, it is well known that dirty paper coding (DPC) is the optimal multi-user broadcast transmission method. However, with partial channel state information at the transmitter, the picture changes significantly. Specifically, since multi-user transmission is unable to employ DPC perfectly, singleuser transmission strategies can have a better performance than multi-user transmission strategies when we have a small number of users and correlated antenna gains. We explore the trade-offs between the single-user and multiuser MIMO transmission strategies. Order statistical analysis provides us with both analytical expressions and insights about these trade-offs. We verify that the analytical framework that we develop is accurate by checking the values obtained against numerical results. From this analysis, we confirm that 'mode switching' between single-user and multi-user MIMO transmission schemes is necessary for maximizing throughput for emerging MIMO solutions. Finally, we suggest an adaptive mode switching algorithm between single-user and multi-user MIMO transmission strategies based on this analytical framework. / text