This work aims at investigating the avalanche excess noise characteristics of the AlxGa1-xInP (x from 0 to 1) avalanche photodiode (APDs) for practical applications such as underwater detection, oil well logging and space exploration. Normally, Si APDs are used to operate in the visible part of the spectrum, however this has a broad spectral response and requires optical band-pass filters to avoid the detection of extraneous light sources at other wavelengths, which adds cost and complexity to the system. To enable these measurements to be undertaken, a high sensitivity system for measuring multiplication and the excess noise is described. The system is capable of measuring the multiplication and excess noise power of devices, even when the photocurrent is low (approximately 10 nA). The signal to noise ratio (SNR) of the system is more than two orders of magnitude better than previously reported systems. The ability to characterise APD performance with such low photocurrents enables the use of low power light sources such as light emitting diode (LED) rather than lasers to investigate their noise performance. Multiplication and avalanche excess noise measurements have been undertaken on a series of AlInP homo-junction PIN and NIP diodes with i region widths ranging from 0.04 μm to 1 μm, using 442 nm (laser) and 460 nm (LED) wavelength light. Low dark currents of < 170 nA cm-2 at 95% of breakdown voltage were obtained in all the devices because of its wide bandgap and there was no tunneling dark current present even at high-fields > 1000 kV/cm. For a given multiplication factor, the excess noise decreased as the avalanche width decreased due to the ‘dead-space’ effect. Avalanche excess noise measurements were also performed on a separate absorption multiplication avalanche photodiode (SAM-APD) with a nominal multiplication region width of 0.2 μm at 460 nm wavelength and it had an effective k (hole to electron ionization coefficient ratio) of approximately 0.3. There is also a requirement for photodetectors capable of high temperature operation. A high temperature measurement system is described. The system can measure temperature dependence of the noise characteristics in AlInP devices. AlInP devices are found to be suitable for operating in high temperature environments. Avalanche multiplication and excess noise from 298 K to 475 K have been measured on AlInP diodes under 460 nm LED illumination. These devices have i region widths ranging from 0.04 μm to 1 μm. The impact ionization coefficients have been extracted as the function of the temperature. The experiment results show the excess noises of the AlInP diodes are very similar at different temperatures and vary only with the avalanche width and the value of multiplication.

High speed permanent magnet machines have been widely adopted for their ability to achieve high power densities while also retaining high efficiencies. However, operation at high speeds introduce several design and analysis challenges which encompass electromagnetic, thermal and mechanical considerations. Arguably the most challenging aspect of high-speed machine design is the reliable prediction of losses in the machine, particularly if the influence of the converter is accounted for. This thesis is focussed on the design and loss modelling of high speed permanent magnet machines, with a particular emphasis on establishing a detailed understanding factors that result in the torque density of machines decreasing with increasing speed. The thesis reports on a systematic investigation to establish the variation of torque density and power density with machine speed by way of series of design studies for a 250kW surface-mounted permanent magnet. The torque density is shown to reduce with machine speed rating at different rates depending on the constraints applied. In several cases, an optimum speed for achieving maximum power density is observed, beyond which the power density begins to reduce. The thesis then considers in detail the influence of ripple currents generated from hysteresis closed loop control, on machine torque output and iron losses. A novel post-processing method is developed for iron loss calculation to accommodate with the large number of data points required to fully capture the effect of high frequency current ripple. A series of analytical derivations are developed to illustrate that high frequency iron losses due to switching are largely independent of the exact nature of the switching behaviour and governed by steady-state machine parameters. The rotor eddy current losses in rotor magnets and a metallic containment sleeve are then calculated, using a novel three-dimensional analytical model for field, current, and loss prediction. Good agreement is achieved between the analytical model and finite element simulations.

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With the increasing demand for high speed wireless network services, the next generation wireless networks are proposed to use advanced wireless communication technologies. These technologies include massive MIMO, mmWave and distributed MIMO. In order to deploy wireless networks equipped with these technologies, channel models capturing the channel features and characteristics of these wireless technologies are essential in the planning and optimisation of networks. However, conventional channel models lack the capability to support these next generation network technologies. In this PhD thesis, I investigated the channel models for the next generation wireless technologies, including massive MIMO, mmWave communications and distributed MIMO. I developed channel models for network planning and optimisation based on conventional ray launching algorithms for these wireless technologies. The models have been validated and applied to optimise network performance. The existing challenge in wireless channel modelling is the improvement of modelling accuracy without increasing modelling complexity. In order to achieve this goal, a new calibration method is developed to improve the accuracy of the predication model when measurements are available. Moreover, in order to use the channel models as an effective tool in wireless network planning and optimisation, a new wireless capacity definition from radio propagation perspective is also investigated. It provides insight to the physical limit of wireless channel capacity from a radio propagation perspective.

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