The work presented in this thesis examines mode conversion characteristics of corrugated horns. The outcome of this investigation is the ability to produce optimum designs for corrugated horns. Analysis of propagation characteristics of corrugated conical horns is extended to the throat region where the complex representation of eigenvalues must be used to obtain an accurate field representation. A modal matching technique to the several is described using spherical modes and applied investigation of the scattering properties of types of discontinuities which may occur in corrugated horns. Computer modelling of a corrugated horn is described using the modal matching technique. The corrugated horn is divided up into a number of conical horn sections, each section propagating spherical modes. The propagating characteristics of each section are computed and modal matching is used to predict the scattering matrix for each junction between sections. The scattering matrices are then cascaded so that the complete horn can be modelled on the computer and an optimised design performed. Radiation properties of corrugated horns are studied by taking the influence of mode conversion in the horn into account. Comprehensive theoretical investigation into return loss performance, mode conversion characteristics and radiation properties of corrugated horns are described and substantiated by the experimental results . A phase cancellation method is presented, which can be used to design a profiled corrugated horn with low return loss and low higher order modes presented at the aperture.

The IEEE 802.15.4 standard commonly known as Zig8ee is a wireless sensor targeted at applications that require low data rate, low power and less expense. IEEE 802.15.4 standard is limited to a through-rate of 250kbps maximum providing supports for small packet file transitions and is design to provide highly efficient connectivity. Hence, IEEE 802.15.4 is not designed and cannot be used to transfer large amounts of data. Therefore, in this research MPEG-4 video transmission over ZigBee is the aim as its bandwidth is too low and the limitation could become a real problem which makes the video transmission over IEEE 802.15.4 networks difficult to achieve. Due to the low bandwidth of the ZigBee any large amount of data needs to be optimized at the targeted bitrate. Optimization techniques are widely used in engineering and computer science as well as being used in real environment applications to overcome complex issues and in particular; an artificial intelligence technique known as Particle Swann Optimization (PSO) has becoming very popular. PSO is a population-based stochastic optimization technique, inspired by the social behavior of flocks of birds, and colonies of ants and bees. Such intelligence is decentralized, self-organized and distributed throughout an environment and used by swarm to solve problems. In this research, the problem solving strategy decided on the use of PSO to optimize the transmission of MPEG-4 video over Zig8ee, which requires a much lower computation and accordingly it can be executed faster. A novel solution to transmit MPEG-4 over IEEE 802.15.4 has been developed and this research further utilizes a technique to regulate the quantization patterns and output an optimal frame rate by using an "Adaptive Scalar Quantization", which prevents excessive data loss of MPEG-4 video over IEEE 802.15.4 transmission. The computer simulation results confirm that adaptive scalar quantization video coding do improve the quality of picture and reduce data loss and prevents excessive data loss, and use of particle swarm optimization can improve QoS and empower video within the MPEG-4 compression technique to be transmitted over IEEE 802.15.4 standard compared to conventional MPEG video transmissions.

A steady signal exerts two complementary effects on a noisy acoustic environment: one is to add energy, the other is to create order. The ear has evolved mechanisms to detect both effects and encodes the fine temporal detail of a stimulus in sequences of auditory nerve discharges. Taking inspiration from these ideas, this thesis investigates the use of regular timing for sonar signal detection. Algorithms that operate on the temporal structure of a received signal are developed for the detection of merchant vessels. These ideas are explored by reappraising three areas traditionally associated with power-based detection. First of all, a time-frequency display based on timing instead of power is developed. Rather than inquiring of the display, "How much energy has been measured at this frequency? ", one would ask, "How structured is the signal at this frequency? Is this consistent with a target? " The auditory-motivated zero crossings with peak amplitudes (ZCPA) algorithm forms the starting-point for this study. Next, matters related to quantitative system performance analysis are addressed, such as how often a system will fail to detect a signal in particular conditions, or how much energy is required to guarantee a certain probability of detection. A suite of optimal temporal receivers is designed and is subsequently evaluated using the same kinds of synthetic signal used to assess power-based systems: Gaussian processes and sinusoids. The final area of work considers how discrete components on a sonar signal display, such as tonals and transients, can be identified and organised according to auditory scene analysis principles. Two algorithms are presented and evaluated using synthetic signals: one is designed to track a tonal through transient events, and the other attempts to identify groups of comodulated tonals against a noise background. A demonstration of each algorithm is provided for recorded sonar signals.

Cooperative networks have developed as a useful technique that can achieve the same advantage as multi-input and multi-output (MIMO) wireless systems such as spatial diversity, whilst resolving the difficulties of co-located multiple antennas at individual nodes and avoiding the effect of path-loss and shadowing. Spatial diversity in cooperative networks is known as cooperative diversity, and can enhance system reliability without sacrificing the scarce bandwidth resource or consuming more transmit power. It enables single-antenna terminals in a wireless relay network to share their antennas to form a virtual antenna array on the basis of their distributed locations. However, there remain technical challenges to maximize the benefit of cooperative communications, e.g. data rate, asynchronous transmission and outage. In this thesis, therefore, firstly, a modified distributed quasi-orthogonal space-time block coding (M-D-QO-STBC) scheme with increased code gain distance (CGD) for one-way and two-way amplify-and-forward wireless relay networks is proposed. This modified code is designed from set partitioning a larger codebook formed from two quasi-orthogonal space time block codes with different signal rotations then the subcodes are combined and pruned to arrive at the modified codebook with the desired rate in order to increase the CGD. Moreover, for higher rate codes the code distance is maximized by using a genetic algorithm to search for the optimum rotation matrix. This scheme has very good performance and significant coding gain over existing codes such as the open-loop and closed-loop QO-STBC schemes. In addition, the topic of outage probability analysis in the context of multi-relay selection from $N$ available relay nodes for one-way amplify-and-forward cooperative relay networks is considered together with the best relay selection, the $N^{th}$ relay selection and best four relay selection in two-way amplify-and-forward cooperative relay networks. The relay selection is performed either on the basis of a max-min strategy or one based on maximizing exact end-to-end signal-to-noise ratio. Furthermore, in this thesis, robust schemes for cooperative relays based on the M-D-QO-STBC scheme for both one-way and two-way asynchronous cooperative relay networks are considered to overcome the issue of a synchronism in wireless cooperative relay networks. In particular, an orthogonal frequency division multiplexing (OFDM) data structure is employed with cyclic prefix (CP) insertion at the source in the one-way cooperative relay network and at the two terminal nodes in the two-way cooperative network to combat the effects of time asynchronism. As such, this technique can effectively cope with the effects of timing errors. Finally, outage probability performance of a proposed amplify-and-forward cooperative cognitive relay network is evaluated and the cognitive relays are assumed to exploit an overlay approach. A closed form expression for the outage probability for multi-relay selection cooperation over Rayleigh frequency flat fading channels is derived for perfect and imperfect spectrum acquisitions. Furthermore, the M-QO-STBC scheme is also proposed for use in wireless cognitive relay networks. MATLAB and Maple software based simulations are employed throughout the thesis to support the analytical results and assess the performance of new algorithms and methods.

The present dissertation deals with electrically small loop antennas, namely antennas which are small compared to the wavelength. The. single turn loop antenna is an effective `small' transmitting antenna. Small being defined as having a maximum physical dimension, which is less than the wavelength of operation. Electrically small antennas are those whose overall length is less than one-sixth of a wavelength. Most applications of loop antennas are in the HF (3-30 MHz) VHF (30-300 MHz) and UHF (300-3000 MHz) bands. The fundamental limitations, namely the antenna Q, of small antennas derived in the past by several authors, are reviewed and unified, yielding simple formulae for both the antenna Q and bandwidth of an antenna as a function of its size. This theoretical work enables us to understand deeply the physical phenomena occurring in small antennas, such as the importance of the stored reactive energy or the increase of losses when an antenna is miniaturised. An important point regarding electrically small antennas is that their performance is closely related to their electrical size. The product of the bandwidth and the gain is a function of the size of the antenna, so that the gain can only be increased at the expense of the bandwidth, and vice versa. Furthermore, an electrically small antenna is highly dependent on the environment in which the antenna operates, which must be taken into account. The environment comprises both the device on which the antenna is mounted and the surroundings This thesis describes a technique to determine the radiation efficiency of an electrically small antenna, primarily by measuring the radiation Q of the antenna at the input terminals at a number of frequencies. This is called the Q-bandwidth method. From this all antenna mode radiation resistances, the antenna loss resistances, the antenna efficiency, and input impedance can all be derived from the measurements. The results show that traditional formulas for antenna Q and radiation resistance do not predict what is measured in the laboratory or in the field. The measurements show that the fundamental formula for electrically small antennas has been breached.Furthermore thermal heat balance and field strength measurements are used to confirm the Q-bandwidth method. Computer simulation in general agrees with current theory but not with the measurements as the current theory does not predict the extra radiation modes described in this thesis. This thesis addresses some of the discrepancies.

Delay tolerant networking has received considerable attention from the research commu nity in the recent years. Advances in wireless and mobile technologies have enabled new application scenarios where intermittent disconnections are common and not exceptional. Examples include communication in sparse mobile ad hoc networks for emergency support, infestation-based systems for connectivity in remote areas and data collection in sensor networks for wildlife monitoring. At the same time, most of the existing research work in mobile networking is based on the assumption that a path exists between the sender and the receiver. Therefore, new communication paradigms and techniques have to be designed to make communication possible in these environments. In this thesis we present the design, implementation and evaluation of the Context-aware Adaptive Routing (CAR) protocol for delay-tolerant unicast communication in intermit tently connected mobile ad hoc networks. The protocol is based on the idea of exploiting nodes as carriers of messages among network partitions to achieve their delivery. The choice of the best carrier is made using Kalman filter based prediction techniques and utility theory. We argue that movement of the nodes and potential future colocation with the recipient of the messages can be used to make intelligent forwarding decisions. We then discuss the design of a realistic mobility model based on social network theory in order to test the routing protocol. An evaluation of the proposed model using real traces and its mathematical formalisation are also presented. The performance of the Context-aware Adaptive Routing protocol in terms of delivery ratio, delay and overhead are evaluated using simulations based on the proposed mobility model and purely random ones. Finally, we discuss the implementation of CAR over an opportunistic networking frame work (Haggle), outlining possible applications of the general principles at the basis of the proposed approach.

Most modern radar systems use a monostatic configuration and exploit wide bandwidth to achieve high down range resolution used for target detection and classification. However increasing pressure on occupancy of the radio spectrum and the requirement for ever more accurate target classification poses serious challenges to future radar designs. Techniques based on narrow band radar are thus being investigated as a mean of reducing spectral occupancy. This approach can be coupled with the use of a multiple radar (multistatic) geometry to provide a potentially powerful technique for improving target detection and classification even beyond that of conventional systems. A multistatic topography allows the application of tomographic techniques for target imaging. Tomography is a process by which a two-dimensional cross-sectional image of an object is obtained via illumination from a variety of differing angles in a variety of differing planes. In radar tomography observations from multiple radar locations enable a three dimensional projection in Fourier space. In this way a three dimensional image of an object can be constructed using techniques such as Backprojection. The use of a narrow band waveform in multistatic radar tomography trades resolution achieved by bandwidth for resolution achieved by spatially diverse angular imaging. This thesis reports a detailed investigation into a range of narrow band, multistatic geometries and techniques to obtain high resolution imaging of moving targets. Images processed using Synthetic Aperture Radar (SAR) and Inverse SAR (ISAR) configurations, modified to emulate multistatic narrow band configurations, have been investigated for both real and simulated data. The effect on the spatial resolution due to masking, ambiguity and coherency of targets consisting of both sparse and dense scatterers was analysed under a range of conditions. A cross range resolution of A/4 was achieved using simulated data. This analysis was also extended to the case of real data of typical ground targets. In this situation the data is inevitably significantly affected by noise but a resolution of A/2 was achieved. This study concludes with a comparison of modeled narrow band system performance with theoretical predictions leading to a preliminary assessment of the capability of the narrow band radar tomographic imaging technique for potential applications.

This thesis reports the advancement of two forms of Fabry-Pérot fibre optic hydrophone for the characterisation of high intensity and high pressure ultrasound fields. The first form of fibre optic hydrophone was realised using a multi-layered hard dielectric Fabry-Pérot interferometer deposited at the tip of an optical fibre. The acoustic transduction mechanism is based on the detection of acoustically induced changes in the optical thickness of the spacer layer. Several hydrophone designs were realised and an extensive investigation into the acoustic performance of the hydrophones was conducted. Each hydrophone design is anticipated to be capable of characterising acoustic fields of high pressure, with working ranges of ~90MPa. In addition, the hydrophones were shown to be able to withstand acoustic fields with intensities up to 1833W.cm-2. The frequency response of the hydrophones was measured using substitution calibration and the features of the response were investigated using a finite difference model (AFiDS). The directional response was also measured, and the hydrophones were found to be most sensitive to acoustic fields of non-normal incidence. The sensors were shown to not be subject to self-heating when interrogated by light of optical power below 3.78mW and to have a noise equivalent pressure of 30.9kPa (at 3.5MHz over a 20MHz noise bandwidth). A 3MHz frequency component was observed during the acoustic characterisation and after extensive experimentation the transduction mechanism was found to be optically sensitive and not due to the Fabry-Pérot interferometer. A second form of Fabry-Pérot fibre optic hydrophone was introduced and has previously been reported to not be able to operate at temperatures in excess of 70°C. The cause for this limitation was investigated and found to be due to the annealing of the Parylene-C spacer within the Fabry-Pérot interferometer when elevated to temperatures exceeding 56°C. The effects of annealing on the interferometer transfer function, and acoustic response were investigated. Annealing was found to produce an irreversible shift in the reflectance minimum of the interferometer transfer function, as well as a change in the frequency response of the sensor, with the features moving to higher frequencies.

Electromagnetic (EM) threats are a specialised subset of offensive threats to the confidentiality, integrity, and availability of information and to information security (INFOSEC) in general. Two broad classifications of threat types; EM interceptors (interception of compromising Radio Frequency (RF) emissions) and EM disruptors (The use of high power RF to disrupt electronics) have been considered and the technical aspects of these threats have been assessed. The technical complexity amongst other factors required to mount an EM interceptor based attack has been shown by analysis to be significant. The hypothesis of this thesis has therefore been focussed on the development of detection and diagnostic concepts analogous to those used to defend against conventional cyber or Computer Network Attack (CNA) threats for EM disruptive attacks. EM Disruptors which have been the focus of this study are likely to have a large impact on the availability of information systems but it has been shown that the effectiveness of the threat and therefore the risk posed to INFOSEC is extremely difficult to quantify. Nonetheless, it has also been shown through analysis and discussion that the `Low Tech' perpetrator (well funded amateur) would be likely to be capable of building an effective EM disruption system. Whilst effective countermeasures for EM disruptors exist it is suggested that it is difficult for INFOSEC professionals to recommend their installation for risk mitigation because the risk is poorly quantified. A series of rigorous EM susceptibility experiments were conducted on computer systems to identify susceptibility trends and to assist with understanding the risk. A prototype Electromagnetic Disruption Detection System (EMDDS) has also been designed and developed. This detector uses similar principles to cyber type Intrusion Detection Systems (IDS) and should therefore be understandable to non EM specialists. It has been shown that the EMDDS system can be used for responding to incidents and as part of a forensic evidence gathering process. The results of this thesis have supported the hypothesis.

Wireless Personal Area Networks are increasingly widespread In their application. These range from simple remote controls to highly complex networks that allow the communication of heterogeneous data-collecting devices and actor devices via routing nodes and gateways, to networks such as wired JP networks. Key performance aspects for wireless personal area networks are the large number of nodes capable of working within the same reception area and the energy consumption of such nodes. This work shows how equalisation for wireless personal area networks can be realised with a small increase in hardware, in order to achieve performance improvements in highly dispersive environments. The proposed ideas are extended to a multiple-antenna solution which can be made downwards-compatible to the appropriate IEEE standard. In this manner, the performance in terms of either bit rate or robustness can be improved. For both equalisation and multiple-antenna procedures it is shown how the existing pre-amble can be used to find an estimate of the channel impulse response. This processing is performed in the time domain. Equalisation as well as multi-antenna channel separation is achieved by one set of cyclic filters operating at the chip rate. This allows a simple hardware design for integration. Selected methods are tested in real-time in a testbed for wireless personal area networks and.are shown to offer real performance improvements.