The potential of Calcium-Vanadium garnet loaded binary composites for use in the production of planar frequency agile microwave devices has been investigated. A WR90 rectangular waveguide system using the transmission/reflection technique has been used to compare effective medium theory predicted permittivities and permeabilities for unmagnetised and transversely magnetised composites with actual measured composite properties. Use of the the Bruggeman effective medium theory with manufacturer supplied garnet permittivity and values of garnet permeability calculated using simple empirical models were demonstrated to be as accurate as predictions made using the measured properties of the composites' individual constituents. Errors in predicted material properties for unmagnetised and transversely magnetised samples relative to measured data were less than 5% across almost half of the 8.2 -12.4GHz measurement band and within a worst-case error of 15% across the whole band. A series of end-coupled linear microstrip resonators using garnet-loaded composite substrates has been fabricated and tested. Tunabilities in resonance of up to 3.9% at 9.2GHz have been demonstrated for DC magnetic bias fields applied transverse to the microwave magnetic field component. An entirely new and previously unpublished broadband microstrip technique for the measurement of the effective permittivity and permeability of bulk gyromagnetic and gyromagnetic material loaded composite substrates subject to transverse DC magnetic bias fields is presented. This will have wide application in the design of frequency agile microwave integrated circuits.

Multimedia services provided through wireless networks, such as mobile television and video calls, have recently attracted great attention. These systems require higher data rates, better communication quality, and wider channel bandwidth compared with traditional wireless network services, such as voice calling and text messaging. In response to these demands, multiple-input multiple-output (MIMO) employing multiple antennas at both transmitter and receiver, has been investigated in recent years. Nevertheless, the major impediment in MIMO based wireless systems is the cost of the hardware due to the requirement of the complete radio frequency (RF) chain for each transmit and receive path. One technique named antenna subset selection has been proposed which can reduce the hardware complexity, for example, provide and smaller number of RF chains which are reconfigurable to serve multiple antennas, but retain good communication performance, such as increasing data rates and improving communication quality. On the other hand, network service providers have in recent years established wideband communication systems in order to provide more services and higher bandwidth to customers. However, this development lowers the communication link quality, since signals transmitted in wideband communication systems suffer frequency-selective fading. In order to reduce the fading, orthogonal frequency division multiplexing (OFDM) as a potential infrastructure in the fourth generation mobile communication networks is developed. Referring to the cost-performance ratio, an attractive future wireless system named antenna selection based MIMO-OFDM is considered to be widely utilized in civil wireless communications in near future. The working theory of antenna selection based MIMO-OFDM systems can be simply represented as that a data stream at each selected transmit/receiver antenna is sent/receive over a number of narrow band orthogonal subcarriers. This thesis addresses analysis of wireless channel and performance investigation of future wireless communication systems, such as MIMO and OFDM structures. Moreover, a novel significantly low computational complexity algorithm is introduced in this thesis, which is proposed for antenna selection MIMO-OFDM systems on the basis of multiple selection criteria. It is shown that the proposed selection algorithm clearly reduces the computational complexity load of the selection process and efficiently selects the optimum antenna subset of antenna selection MIMO-OFDM systems. The thesis concludes by outlining the advantage of the proposed antenna selection technique and points out its potential role in future wireless communications.

Metamaterials are artificially constructed "materials" which are formed from arrays of engineered elements. By designing individual elements as well as their interactions, the propagation of electromagnetic (EM) waves within the structure can be manipulated, so that new responses can be realised which may not be found in nature. The subject of this research concerns the study of miniaturised elements with strong EM responses so that the constructed metamaterial can better approximate an ordinary low-loss material. The research involves designing miniaturised magnetic resonators operating in the microwave frequency range. A novel resonator prototypes, so-called “helical resonators”, have been successfully designed and fabricated whose physical sizes can fall below 1% of the free space wavelength at resonance. This contributes to a size reduction of 90% compared with previously published work. In addition, an analytical model has been developed, so that the resonance parameters of a helical resonator have explicit expressions. In particular, a constant optimal metallic fill ratio is demonstrated to exist, which can achieve a minimum resonant frequency and a maximum miniaturisation for any given outmost dimension of the element. The accuracy of the model has been verified by both simulation and experiment. The frequency responses of fabricated helical elements were measured using a vector network analyser and a pair of small loop non- resonant dipole probes, and the parameters were extracted using the phase frequency fit method which proves to have the best accuracy and robustness. The properties of a regular square array of helical resonators are subsequently investigated, which can be regarded as a two-dimensional metamaterial. A relevant analytical model has been developed, which characterises the array as an equivalent sheet with surface current distributions, rather than an artificial medium with finite thickness. The relation between the macroscopic EM fields and the small scale properties of individual helical resonators are then established. In particular, the helical resonators are observed to be inherently chiral, thus the assembled interlocking array exhibits dichroism. The transmission coefficients for the circular EM waves with two different polarisation states have been derived, which have been verified by simulation and measurement results as well. In addition, it has been theoretically demonstrated that the resonator elements and their spacings can be engineered, so that the circular EM wave with one particular polarisation state can be totally attenuated around the element resonance, while the other state suffers negligible attenuation. A quadratic relation between the optimal array spacing and the elements’ quality factor has been demonstrated.

Free-space optics (FSO), or wireless optical communications, has received extensive research due to its promise of practically limitless bandwidths. However, FSO has challenges yet to be met for a cost effective realisation. This D.Phil thesis explores a solution using a ferro-electric liquid crystal spatial light modulator (FLC SLM) and binary phase holograms to significantly reduce the hardware complexity of an FSO system with auto-alignment and turbulence compensation. The theory of binary phase hologram is presented and extended to obtain a new algorithm that is suitable for a FLC SLM. The algorithm is able to be used in a demonstration system to broadcast data streams to multiple receivers, showing the capability of using FLC SLM to form any beam configuration. An FSO transmitter is then developed that uses retro-reflectors as markers for the receivers. The transmitter combines an imaging system with the FLC SLM as a reconfigurable beam steering system for acquiring the retro-reflector location. The FLC SLM is also used to reduce aberrations in the optics, resulting in a significant increase in the transmitted beam power density. The accuracy of the acquisition is measured to give a small steering error without the use of a closed loop controller. An optical turbulence simulator, using the principals of binary phase hologram, is constructed to simulate optical beam propagation in turbulent conditions. The simulator accurately produces aberrations that have the same statistics with the theoretical prediction. Analysis of the phase distortion due to turbulence is performed and a wavefront sensorless turbulence compensation method based on the FLC SLM gives significant reduction in calculated bit error rates. New scintillation index derivation for multiple optical beams is described and then used to demonstrate further decrease in bit error rates.

<p>The complex and distributed nature of wireless networks have traditionally made allocation of network resources between network stakeholders a challenging task. In the next generation of wireless networks, allocation mechanisms must be able to address these traditional challenges while also addressing new challenges. New challenges arise as networks adopt changing business relationships between existing stakeholders, introduce new stakeholders with diverse interests, integrate intelligent and autonomous systems, and contend with emerging security threats. To address these new challenges, wireless network engineers will require a fundamental understanding of systems consisting of strategic decision makers with competing interests. Our contribution to this understanding is threefold: First, we study a novel moral hazard that that can occur when payment mechanisms are used to incentivize cooperation between multi-hop network nodes. Second, we introduce a network sharing framework that enables 5G/beyond-5G mobile operators to split shared infrastructure costs subject to a regulatory constraint on the cost structure of the shared network. Lastly, we study reliable communication over an adversarial channel in which the adversary can compute side-information subject to a practical computational bound. For each of the above three topics, we provide both analytical and numerical studies from which we derive insights into the design of allocation mechanisms.</p>

Personal computing devices are becoming more and more popular. These devices are able to collaborate with each other using wireless communication technologies, and then support many applications. Some interesting examples of these are healthcare, context-aware computing, and sports training. In any such applications, security is of vital importance. Firstly, sensitive personal data is always collected in these applications, thus confidentiality is usually required. Secondly, authenticity and integrity of data or instructions are always critical; incorrect data or instructions are not only useless, but also harmful in some cases. This thesis analyses the security requirements of personal networks, and develops a number of multi-channel security protocols. With the help of out-of-band channels, especially no-spoofing and no-blocking out-of-band channels, these protocols can bootstrap security in personal networks. In particular, three kinds of security protocols have been studied: protocols that use human-controlled channels, protocols that use visible light communications, and protocols that use intra-body communications. Interesting trade-offs have been discovered among communication, computation and security, resulting from different channel implementations and protocols.

In order to improve the sensitivity of an optical receiver, the gain and the collection area of the photo-detectors within the receiver should be increased. Detectors with internal gain such as avalanche photodiodes (APD) are usually used to increase the sensitivity of the receiver. One problem with APDs is the sensitivity of their gain to their bias voltage, which makes them challenging to be fabricated in a standard CMOS process due to variations in their gain. However, when an APD is biased over its breakdown voltage, it is sensitive to a single photon, hence, referred to as a single photon avalanche diodes (SPAD). The SPADs are photon-counting detectors, which are less sensitive to their bias voltage, and can be integrated with rest of the electronic circuitry that form an optical receiver. An avalanche diode requires dedicated circuits to be operated in the SPAD mode. These circuits make the diode insensitive to an incident photon for a duration that is known as deadtime. Unfortunately, The collection area of the PD, APD, and SPADs are limited to their capacitance. Hence, a large photo-detector leads to a larger capacitance, which reduces the bandwidth of the receiver. In this thesis, a photon counting optical receiver based on an array of SPADs is proposed which increases the collection area with a low output capacitance. The avalanche diode and peripheral circuits which operate and readout-out the SPAD array are fabricated in the commercially available UMC 0.18 μm CMOS process. Initially, the avalanche diode is tested and characterised. A high performance circuit is then designed and tested which is able to achieve short deadtimes up to 4 ns. Once the photon counting operation of the SPAD is verified, a numerical model is developed to investigate the influence of several factors, including the deadtime, on the performance of the photon-counting detector in a communication link. Based on the simulation results, which show the advantages of an array over a single detector, a prototype detector array of 64 asynchronous SPADs is designed and tested. This array uses a high-speed readout mechanism which is inspired by the current steering digital-to-analogue converters. Bit error ratio tests (BERT) verify the photon counting capability of the proposed detector, and a bit error rate of 1E-3 has been achieved at data rate of 100 Mbps. In addition, the array of SPAD is compatible with a front-end of conventional optical receiver which uses a photodiode as a photo detector.

Ultra-wideband (UWB) radio techniques have long promised good contrast and high resolution for imaging human tissue and tumours; however, to date, this promise has not entirely been realised. In recent years, microwave imaging has been recognised as a promising non-ionising and non-invasive alternative screening technology, gaining its applicability to breast cancer by the significant contrast in the dielectric properties at microwave frequencies of normal and malignant tissues. This thesis deals with the development of two novel imaging methods based on UWB microwave signals. First, the mode-matching (MM) Bessel-functions-based algorithm, which enables the identification of the presence and location of significant scatterers inside cylindrically-shaped objects is introduced. Next, with the aim of investigating more general 3D problems, the Huygens principle (HP) based procedure is presented. Using HP to forward propagate the waves removes the need to apply matrix generation/inversion. Moreover, HP method provides better performance when compared to conventional time-domain approaches; specifically, the signal to clutter ratio reaches 8 dB, which matches the best figures that have been published. In addition to their simplicity, the two proposed methodologies permit the capture of a minimum dielectric contrast of 1:2, the extent to which different tissues, or differing conditions of tissues, can be discriminated in the final image. Moreover, UWB allows all the information in the frequency domain to be utilised, by combining information gathered from the individual frequencies to construct a consistent image with a resolution of approximately one quarter of the shortest wavelength in the dielectric medium. The power levels used and the specific absorption rates are well within safety limits, while the bandwidths satisfy the UWB definition of being at least 20% of the centre frequencies. It follows that the methodologies permit the detection and location of significant scatterers inside a volume. Validation of the techniques through both simulations and measurements have been performed and presented, illustrating the effectiveness of the methods.

In this thesis, research is described which leads to the proposal of a link-directionality-based dual channel MAC (Medium Access Control) protocol (DCP) for carrier sense wireless ad hoc networks. It attempts to double the capacities of such networks using an industrial standard (the single-channel IEEE 802.11 protocol) as a benchmark. Simulations show that the proposed scheme can increase the capacities to more than 1.7 times of the single-channel IEEE 802.11 protocol in large-scale random network topologies. The algorithm, however, requires extra radio spectrum resource which could be costly. In addition to DCP, a signal-to-interference ratio comparison algorithm (SCA) is proposed to further release the protocol constraints imposed by the virtual carrier-sensing mechanism. Interestingly, while the capacity of the pure DCP decreases when link lengths are short, the capacity of the pure SCA increases when link lengths are short. The two algorithms compensate for the downside of each other to bring about a more uniform capacity improvement. Simulations show that the integrated scheme can further increase the network throughputs to more than 2.13 times in random topologies. This thesis also clarifies inter-link interference in wireless ad-hoc networks by using link-directional interference graphs (l-graph). By colouring the l-graphs, independent data streams obtained by Multi-Carrier Code Division Multiple Access (MC-CDMA) are assigned for transmitting up-link and down-link traffic separately in order to eliminate the hidden-node and exposed-node problems in wireless local area networks (WLAN). Finally, a generic approach for capacity analysis is proposed to show that the concept of link-directionality can also be adopted with other network models, protocols and parameter settings. However, in certain scenarios where links are densely packed together, the advantage of using link-directionality could be diminished. Therefore, the proposed generic approach for capacity analysis allows one to determine whether channel allocations according to link-directionalities should be applied to a given network.

This thesis explores the use of single-photon avalanche diodes (SPADs) for visible light communications (VLC). The high sensitivity of SPADs can potentially enhance the performance of VLC receivers. However, a SPAD-based system has challenges that need to be addressed before it can be considered as a viable option for VLC. The first challenge is the susceptibility of SPAD-based receivers to variations in ambient light. The high sensitivity of SPADs is advantageous for signal detection, but also makes SPADs vulnerable to variations in ambient light. In this thesis, the performance of a SPAD-based receiver is investigated under changing lighting conditions. Analytical expressions to quantify performance are derived, and an experiment is conducted to gain further understanding of system performance. It is shown that a SPAD-based receiver is highly sensitive to illumination changes when on-off keying (OOK) is employed, and that pulse-position modulation (PPM) is a preferred modulation scheme as it is more robust. The second challenge is broadcasting to SPAD-based receivers with different capabilities. A traditional broadcasting scheme is time-sharing, whereby a transmitter sends data to receivers in an alternating manner. Broadcasting to SPAD-based receivers is challenging as receivers may have diverse capabilities. In this thesis, a new multiresolution modulation scheme is proposed, which can potentially improve system performance over the traditional timesharing approach. The performance of the proposed scheme is analyzed, and a proof-of-concept experiment is performed to demonstrate its viability.