Cooperative relaying has been proposed as a promising solution to mitigate and combat the deleterious effects of fading by sending and receiving independent copies of the same signal at different nodes. It has attracted huge attention from both industry and academia. The purpose of this thesis is to provide an analytical performance evaluation of the cooperative wireless systems while taking some realistic conditions into consideration. To achieve this, first, performance analysis of amplify-and-forward (AF) relaying using pilot-aided maximum likelihood estimation is studied in this thesis. Both disintegrated channel estimation (DCE) and cascaded channel estimation (CCE) are considered. Based on this analysis, optimal energy allocation is proposed. Then, performance analysis for AF relaying corrupted by interferers are investigated. Both randomly distributed and fixed interferers are considered. For random interferers, both the number and the locations of the interferers are random while for fixed interferers, both the number and the locations are fixed. Next, multihop relaying and multiple scattering channels over α - μ fading are analyzed. Channels with interferences and without interferences are considered. Exact results in the form of one-dimensional integral are derived. Also, approximate results with simplified structure and closed-form expressions are provided. Finally, a new hard decision fusion rule that combines arbitrary numbers of bits for different samples taken at different nodes is proposed. The best thresholds for the fusion rules using 2 bits, 3 bits and 4 bits are obtained through simulation. The bit error rate (BER) for hard fusion rule with 1 bit is provided. Numerical results are presented to show the accuracy of our analysis and provide insights. First, they show that our optimal energy allocation methods outperform the conventional system without optimal energy allocation, which could be as large as several dB’s in some cases. Second, with the increase of signal-to-interference-plus-noise ratio (SINR) for AF relaying with interference, the outage probability decreases accordingly for both random and fixed interferers. However, with the change of interference-to-noise ratio (INR) but with the SINR fixed, the outage probability for random interferers change correspondingly while the outage probability for fixed interferers remains almost the same. Third, our newly derived approximate expressions are shown to have acceptable performances in approximating outage probability in wireless multihop relaying system and multiple scattering channel considering interferences and without interferences. Last, our new hard decision fusion rule is shown to achieve better performance with higher energy efficiency. Also they show that there is a tradeoff between performance and energy penalty in the hard decision fusion rule.

This thesis provides an in-depth investigation and evaluation of infrared optical wireless MIMO communication systems to be applied in both indoor and outdoor environment. The principle objective of the research is to demonstrate both the advantages and disadvantages of the optical wireless MIMO systems using different modulation types. The first part provided analyses of important OW configurations using APD receivers using WMC model and SISO, MISO, SIMO and MIMO configuration. Thus, an analytical expression for 2-1 MISO, 1-2 SIMO and MIMO was successfully developed. This part also illustrates the coding gains possible using diversity schemes for APD OW systems. In the presence of strong fading, the SISO approach is rendered virtually useless, whereas diversity offers acceptable BER values. The results underpin the approach of this thesis, where indoor PIN diode based experimental measurements confirm the gains offered by diversity. In the second part of the work, several optical wireless MIMO systems applicable for the indoor environment are developed for three different modulation types, OOK modulation, PPM modulation and SIR-RZI modulation. These modulations are used in optical MIMO systems are studied for which, mathematical models that evaluate the BER performance of the MIMO system for different axis displacement and for different distances between transmitters and receivers. Based on the results, the PPM system has been shown to present the best BER performance, including high interference-resistance capability. A group of new mathematical models have been evaluated, which demonstrates a high level of correlation with the results derived from empirical models at 93%. Thus, the mathematical models developed and used for the specified evaluation appear to correspond reasonably well, and can be applied in future research on these aspects.

Despite the excellent electrical and thermal properties of 4H-silicon carbide (SiC), the fabrication of high-voltage SiC power devices is still proving problematic, being hindered by material defects resulting in low carrier lifetimes and forward voltage drift, and suboptimum ohmic contacts to p-type material. The PiN diode is one such device that suffers from the aforementioned problems, though at the same time is sought after for high voltage power electronics applications due to the prospect of greatly reduced power losses and increased power handling capability than the Si devices currently in use. As such, this thesis is focussed on the development of these devices, investigating various device structures to achieve high reverse blocking voltages as well as developing novel fabrication processes to improve the electrical performance of the devices. Electrical characterisation of ohmic contacts to p-type 4H-SiC showed that Ti/Al-based metal schemes offered the lowest specific contact resistivity of approximately 2.2 x 10-6 Ω-cm2, which was achieved after annealing at 1000°C for 2 minutes. Physical analysis showed that these annealing conditions were optimum for formation of the Ti3SiC2 alloy at the metal-semiconductor interface, the presence of which was found to correlate with lower specific contact resistivity values. Electrical characterisation of first generation PiN diodes designed for blocking 3.3 kV showed that the fabricated devices had a differential on-resistance (Ron,dif f) of 17 Ωm -cm2 at 100 A/cm2 and 25°C, and near-ideal (η = 1.3) characteristics in the diffusion current regime. Based on the measured reverse saturation currents, the carrier lifetime of the fabricated devices was estimated to be 480 ns. Reverse leakage currents were found to vary significantly across the devices, from 5 nA/cm2 up to 200 μA/cm2 at 100 V reverse bias and 25°C. Second generation 3.3 kV PiN diodes, which featured a B-implanted JTE structure, were found to block a maximum reverse voltage of 2.8 kV, which was around 85% of the target value. PiN diodes fabricated with a drift region designed for blocking 10 kV underwent thermal oxidation processes at temperatures ranging from 1400°C to 1600°C in order to increase the carrier lifetime. Devices having undergone no lifetime enhancement treatment were found to have a Ron,dif f of 11.6 mΩ-cm2 at 100 A/cm2 and 25°C, and an ideality factor η = 1.5 in the diffusion current regime. PiN diodes that had undergone thermal oxidation were found to have improved forward characteristics, with devices oxidised at 1500°C exhibiting a Ron,dif f of around 9 mΩ-cm2 at 100 A/cm2 and 25°C, an improvement of nearly 25%. A novel combined thermal oxidation and annealing process was developed and applied to second generation 10 kV PiN diodes; a mean Ron,dif f of 4.45 mΩ-cm2 was achieved, and a carrier lifetime of 1.21 μs was extracted from reverse recovery characteristics; these were both significant improvements on both the second generation control sample and the first generation thermally oxidised PiN diodes.

In a bid to cope with challenges of increasing demand for higher data rate, better quality of service, and higher network capacity, there is a migration from Single Input Single Output (SISO) antenna technology to a more promising Multiple Input Multiple Output (MIMO) antenna technology. On the other hand, Orthogonal Frequency Division Multiplexing (OFDM) technique has emerged as a very popular multi-carrier modulation technique, thus it is considered as a promising solution to enhance the data rate of future broadband wireless communication systems. The first contribution of this thesis is the development of a low complexity adaptive algorithm that is robust against slow and fast fading channel scenarios, in comparison to the conventional individual parameter estimation by E. Teletar in his famous paper of 1999. Implementing the Adaptive MMSE Receivers in MIMO OFDM systems which I refer to (AMUD MIMO OFDM), combines the adaptive minimum mean square error multiuser receiver's scheme with prior information of the channel and interference cancelation in the spatial domain, achieves enhanced joint channel estimation and signal detection which makes the new technique effectively mobile. A mathematical analysis and simulation results to estimate the Information Capacity of Mobile Communication system with MMSE DFE and OFDM receivers were investigated. The capacity of a stationary channel with ISI is achievable by both the single carrier MMSE DFE and multicarrier modulation over narrow sub channels with OFDM receivers. The achieved capacity result shows that in both techniques single carrier and multicarrier, apart from different implementations are essentially identical when it comes to achievable criteria for information channel capacity. Lastly, AMUD MIMO OFDM were compared with both adaptive vector pre-coding and iterative system and their performance were fantastic, results shows that it will assure transmission over a high channel capacity.

At high-temperature operations of electronic control devices, Tin-Silver-Copper (SnAgCu) alloy solder joints used to assemble the component of the devices are functioning at homologous temperature above 0.8. In such ambient temperatures, solder alloys have limited mechanical strength and will be sensitive to strain rate. The sensitivity of solder properties to creep/visco-plastic deformation increases the rate of accumulation of plastic damage in the alloy and decreases the number of cycles to failure (Nf) of the joints. Most untimely rupture of solder joints in high-temperature electronics (HTE) system usually culminates in colossal loss of resources and lives. Typical incidences are reported in recent automotive and aircraft crashes as well as the collapse of oil-well logging equipment. To increase the mean time to failure (MTTF) of solder joints in HTE, an in-depth understanding and accurate prediction of the response of solder joints to thermally induced plastic strain damage is crucial. This study concerns the prediction of the reliability of lead-free solder joints in a flip chip (FC) model FC48D6.3C457 which is mounted on a substrate and the assembly subjected to high-temperature excursions. The research investigates the effect of the high-temperature operations on reliability of the joints. In addition, the investigation examines the impact of control factors (component stand-off height (CSH), inter-metallic compound (IMC) thickness, number of thermal cycle and solder volume) on Nf of the joints. A model developed in the course of this investigation was employed to create the assembly solder joints architecture. The development of the model and creation of the bump profile involve a combination of both analytical and construction methods. The assembled package on a printed circuit board (PCB) was subjected to accelerated temperature cycle (ATC) employing IEC standard 60749-25 in parts. The cycled temperature range is between -38 oC and 157 oC. Deformation behaviour of SnAgCu alloy solder in the joints is captured using Anand’s visco-plasticity model and the response of other materials in the assembly were simulated with appropriate model.

Solder joints play a very important role in electronic products as the integrity of electronics packaging and assembly rests on the quality of these connections. The increasing demands for higher performance, lower cost, and miniaturisation in hand-held and consumer electronic products have led to the use of dense interconnections. This miniaturization trend means that solder joint reliability remains an important challenge with surface mount electronics assembly, especially those used in hostile environments, and applications such as automobile, aerospace and other safety critical operations. One of the most important factors which are known to affect solder joint reliability is the thickness of intermetallic compound (IMC) layer formed between the solder and the substrate. Although the formation of an IMC layer signifies good bonding between the solder and substrate, its main disadvantage is that it is also known to be the most brittle part of the solder joint. Thus as the miniaturisation trend continues, and solder joints become even smaller in size, the nature and impact of IMC layer thickness on solder joint reliability becomes even more of a concern with the introduction of new lead-free soldering. Other factors which are known to affect solder joint reliability include the bonding strength, the voiding percentage in joints, the size of the voids and their location within the joint. The work reported in this thesis on formation and growth of intermetallic compound layer, and evaluation of the shear strength of lead-free solder joints is divided into four main parts. The first part of the study is concerned with understanding of the effect of pad sizes on Inter-metallic compound layer formation and growth for lead-free solder joints. The second part concerns the study of the effect of temperature cycling and reflow profiles on intermetallic growth between Sn-Ag-Cu alloy and Cu substrate. The third part of the study concerns the investigation of the effect of reflow soldering profile optimization on solder volumes using design of experiment technique. The focus of the final part of the study is the investigation of the effect of Inter-metallic Compound thickness on shear strength of 1206 surface mount chip resistor. The results from the experimental work showed that the pad size has very little influence on the growth of the IMC. The result also shows that the growth of IMC depends on diffusion rate, temperature and time according to the power-law model; and that the IMC layer thickness is independent of pad size. The significance of this result is that with further reductions in joint size (with IMC layer thickness remaining the same), the ratio of the IMC layer thickness to solder joint size will increase and adversely impact the joint reliability. The work carried out on ageing temperatures and reflow profiles of Sn-Ag-Cu alloy and Cu substrate also showed the reaction-diffusion mechanism of intermetallic compound formation and growth in solder joints. The study also showed that the most significant factor in achieving lower IMC layer thickness and fine microstructures is the time to peak temperature of the reflow soldering process. The effect of IMC layer thickness on the shear strength of Sn-Ag-Cu solder joints was investigated. The relationship of shear strength, interfacial microstructures and fracture surfaces was considered. It is clear that formation of continuous Cu-Sn and SnNiCu layers are the reason for the weak interface strength. The results show that the shear strength of solder joints decreases with increasing ageing time. The results of this study have been disseminated through journal and conference publications and will be of interest to R&D personnel working in the area of high temperature electronics and in particular those working in the field of automotive electronics.

Solder pastes are complex materials whose properties are governed by many factors. Variations exhibited in solder paste characteristics make it increasingly difficult to understand the correlations between solder paste properties and their printing process performance. The recent EU directives on RoHS (Restriction of Hazardous Substances – enacted by UK regulations) and WEEE (Waste from Electrical and Electronic Equipment) has led to the use of lead-free soldering in the SMA (surface mount assembly) process, and an urgent need for better understanding of the characteristics and printing performance of new solder paste formulations. Equally, as the miniaturisation of hand-held and consumer electronic products continues apace, the solder paste printing process remains a real challenge to the electronics assembly industry. This is because the successful assembly of electronic devices at the ultra-fine pitch and flip-chip geometry requires the deposition of small and consistent paste deposits from pad to pad and from board to board. The paste printing process at this chip-scale geometry depends on conditions such as good paste roll, complete aperture filling and paste release from the apertures onto the substrate pads. This means that the paste flow and deformation behaviour, i.e. the paste rheology, is very important in defining the printing performance of any solder paste. Rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. In addition, the rheological measurements can also be used as a quality control tool in the paste production process for identifying batch-to-batch variation, and to reduce the associated printing defects in the paste printing process. The work reported here on the characterisation of lead-free solder pastes and their correlation with the stencil printing process is divided into five main parts. The first part concerns the study of the effect of variations in flux and particle size distribution (PSD) on the creep recovery performance of lead-free solder pastes used for flip-chip assembly. For this study, a novel technique was calculating the extent of paste recovery and hence characterising the slumping tendency in solder pastes. The second part of the study concerns the influence of long-term ageing on the rheology and print quality of lead-free solder pastes used for flip-chip assembly, and the main focus of the work was to develop methodologies for benchmarking new formulations in terms of shelf life, rheological deterioration and print performance. The third part of the work deals with a rheological simulation study of the effect of variation in applied temperature on the slumping behaviour of lead-free solder pastes, and the fourth part considers the rheological correlation between print performance and abandon time for lead-free solder paste used for flip-chip assembly. The final part of the study concerns the influence of applied stress, application time and recurrence on the rheological creep recovery behaviour of lead-free solder pastes. The research work was funded through the PRIME Faraday EPSRC CASE Studentship grant, and was carried out in collaboration with Henkel Technologies, Hemel Hempstead, UK. The extensive set of results from the experimental programme, in particular relating to the aspect of key paste performance indicators, has been adapted by the industrial partner for implementation as part of a quality assurance (QA) tool in its production plant, and the results have also been disseminated widely through journal publications and presentations at international conferences.

In conventional cellular wireless communication system, interference modelling has focused on the six primary co-channel interfering cells (first tier co-channel cells). In the current accepted interference model, co-channel interfering cells beyond the first tier (subsequent tier co-channel cells) are neglected. This currently accepted interference models is suitable for cellular wireless communication systems operating at carrier frequencies, f c = 0.9 and 1.8 GHz, cell size radii R > 1 km and basic path loss exponent α ≥ 2. The future and emerging wireless communication systems are expected to be operating at frequencies f c > 2 GHz (3.35 - 15.75 GHz), cell size radii R≤ 1 km and basic path loss exponent α ≤ 2. This, makes the current acceptable co-channel interference model unsuitable for information capacity analysis of the future cellular systems. Therefore, a co-channel interference model suitable for future and emerging wireless communication system becomes necessary. In this thesis a new and modified interference model is proposed. The proposed interference model includes the first and subsequent tier co-channel interfering cells. The proposed interference model will be suitable for cellular wireless communication systems operating at carrier frequencies f c > 2 GHz, cell size radii R≤ 1 km and basic path loss exponent α ≤ 2. A mathematical analysis, supported by computer simulation is used, to study the uplink information capacity performance for the conventional and proposed interference model. The analysis and simulation results of the proposed interference model show that at carrier frequencies f c > 2 GHz, co-channel interfering cells beyond the first tier become active as cell size radius R, reduces. As an example for a carrier frequency f c = 15.75 GHz, cell size radius R = 100 m at a normalized reuse distance Ru = 4, there was a 15.32 % decrease in the information capacity between the conventional and proposed interference model. An information capacity - cost analysis is used to find a minimum cell size for information capacity increase in cellular wireless network, thus a theoretical limit to cell size reduction. The results show that as the cell size radius R reduces to 300 m and less, the proposed interference model show a 5.76 - 18.89 % decrease in the information capacity per unit cost (£, $, etc) at microwave carrier frequencies f c > 3.35 GHz. This result illustrates that there is a theoretical limit to cell size reduction in relation to information capacity performance and cost. An inductive approach is used to generate a formula for calculating the number of co-channel interfering cells Nn in a cellular wireless site layout. Such a formula allows one to calculate the number of co-channel interfering cells in subsequent tiers of a cellular wireless site layout. The geometric derivation shows that the number of co-channel interfering cell Nn in a subsequent tier is the product of the number of co-channel interfering cells in the first tier NI and the tier number n. Thus, the number of co-channel interfering cell in a subsequent tier Nn = NI × n. This formula enables subsequent tier co-channel interference to be included in the information capacity analysis of future and emerging, and finding the minimum cell size for information capacity increase in a cellular wireless communication system.

The need to maximise thermal performance of electronic devices coupled with the continuing trends on miniaturization of electronic packages require innovative package designs for power devices and modules such as Electronic Control Unit (ECU). Chip scale packaging (CSP) technology offer promising solution for packaging power electronics. This is as a result of the technology’s relatively improved thermal performance and inherent size advantage. In CSP technology, heat removal from the device could be enhanced through the backside of the chip. Heat dissipating units such as heat spreader and/or heat sink can be attached to the backside (reverse side) of the heat generating silicon die (via TIM) in an effort to improve the surface area available for heat dissipation. TIMs are used to mechanically couple the heat generating chip to a heat sinking device and more crucially to enhance thermal transfer across the interface. Extensive review shows that solder thermal interface materials (STIMs) apparently offer better thermal performance than comparable state-of-the-art commercial polymer-based TIMs and thus a preferable choice in packaging power devices. Nonetheless, voiding remains a major reliability concern of STIMs. This is coupled with the fact that solder joints are generally prone to fatigue failures under thermal cyclic loading. Unfortunately, the occurrence of solder voids is almost unavoidable during manufacturing process and is even predominant in lead (Pb)-free solder joints. The impacts of these voids on the thermal and mechanical performance of solder joints are not clearly understood and scarcely available in literature especially with regards to STIMs (large area solder joints). Hence, this work aims to investigate STIM and the influence of voids on the thermo-mechanical and thermal performance of STIM. As previous results suggest that factors such as the location, configuration (spatial arrangement) and size of voids play vital roles on the exact effect of voids, extensive three dimensional (3D) finite element modelling is employed to elucidate the precise effect of these void features on a Pb-free STIM selected after thermo-mechanical fatigue test of standard Pb-free solder alloys. Finite element analysis (FEA) results show that solder voids configuration, size and location are all vital parameters in evaluating the mechanical and thermal impacts of voids. Depending on the location, configuration and size of voids; solder voids can either influence the initiation or propagation of damage in the STIM layer or the location of hot spot on the heat generating chip. Experimental techniques are further employed to compare and correlate levels of voiding and shear strength for representative Pb-free solders. Experimental results also suggest that void size, location and configuration may have an influence on the mechanical durability of solder joints. The findings of this research work would be of interest to electronic packaging engineers especially in the automotive sector and have been disseminated through publications in peer reviewed journals and presentations in international conferences.

Turbo coding, a forward error correcting coding (FEC) technique, has made near Shannon Limit performance possible when Iterative decoding algorithms are used. Intersymbol interference (ISI) is a major problem in communication systems when information is transmitted through a wireless channel. Conventional approaches implement an equalizer to remove the ISI, but significant performance gain can be achieved through joint equalization and decoding. In this thesis, the suitability of turbo equalization as a means of achieving low bit error rate for high data communication systems over channels with intersymbol interference was investigated. A modified decision feedback equalizer algorithm (DFE) that provides significant improvement when compared with the conventional DFE is proposed. It estimates the data using the a priori information from the SISO channel decoder and also a priori detected data from previous iteration to minimize error propagation. Investigation was also carried out with Iterative decoding with imperfect minimum mean square error (MMSE) decision feedback equalizer, assuming soft outputs from the channel decoder that are independent identically distributed Gaussian random variables. The prefiltering method is considered in this thesis, where an all-pass filter is employed at the receiver before equalization to create a minimum phase overall impulse response. The band limited channel suffers performance degradation due to impulsive noise generated by electrical appliances. This thesis analysed a set of filter design criteria based on minimizing the bit error probability of impulse noise using digital smear filter.