Visible light communication (VLC) systems have become promising candidates to complement conventional radio frequency (RF) systems due to the increasingly saturated RF band and the potentially high data rates that can be achieved by VLC systems. Over the last decade, significant research effort has been directed towards the development of VLC systems due to their numerous advantages over RF systems, such as the availability of simple transmitters (light emitting diodes, LEDs) and receivers (silicon photo detectors), better security at the physical layer, improved energy efficiency due to the dual functionally (i.e., illumination and communication) and hundreds of THz of license-free bandwidth. However, there are several challenges facing VLC systems to achieve high data rates (multi gigabits per second). These challenges include the low modulation bandwidth of the LEDs, co-channel interference (CCI), inter symbol interference (ISI) due to multipath propagation and the light unit (i.e., VLC transmitter) should be ‘‘ON’’ all the time to ensure continuous communication. This thesis investigates a number of techniques to overcome these challenges to design a robust high-speed indoor VLC system with full mobility. A RGB laser diode (LD) is proposed for communication as well as illumination. The main goal of using LD is to enable the VLC system to achieve multi-gigabits data rates when employing a simple modulation technique (such as on-off keying (OOK)), thus adding simplicity to the VLC system. A delay adaptation technique (DAT) is proposed to reduce the delay spread and enable the system to operate at higher data rates (10 Gb/s in our case). The thesis proposes employing angle diversity receivers (ADR) and imaging diversity receivers to mitigate the impact of ISI, CCI, reduce the delay spread (increase the channel bandwidth) and increase the signal to noise ratio (SNR) when the VLC system operates at high data rates (5 Gb/s and 10 Gb/s) under the effects of mobility and multipath dispersion. Moreover, the work introduces and designs three new VLC systems, an ADR relay assisted LD-VLC (ADRR-LD), an imaging relay assisted LD-VLC (IMGR-LD) and a select-the-best imaging relay assisted LD-VLC (SBIMGR-LD), which are modelled and their performance is compared at 10 Gb/s in two VLC room sizes (5m × 5m × 3m and 4m × 8m × 3m). As well as modelling in two different room scenarios: an empty room and a realistic environment were considered. The work also introduces and designs a high-speed fully adaptive VLC system that employs beam steering and computer generated holograms (CGHs), which has the ability to achieve 20 Gb/s with full receiver mobility in a realistic indoor environment. Furthermore, a new high-speed fast adaptive VLC system based on a divide-and-conquer methodology is proposed and integrated with the system to reduce the time required to identify the optimum hologram. The new system has the ability to achieve 25 Gb/s in the worst case scenario. This thesis also proposes four new infrared (IR) systems to support VLC systems when the light is totally turned off. In addition, it introduces the concept of a collaborative VLC/IR optical wireless (OW) system and investigates the impact of partial dimming on the VLC system performance. An adaptive rate technique (ART) is proposed to mitigate the impact of light dimming. Finally, an IROW system (cluster distributed with beam steering) is introduced to collaborate with a VLC system to maintain the target data rate in the case of partial dimming.

The move to increasingly high resolution numerical weather prediction models has created a demand for high resolution observations over wide areas, for model initialisation. The weather radar network is a valuable source of such observations. To address this requirement and increasing concerns of obsolescence in the UK Weather Radar network a program of renewal was undertaken. This resulted in the creation of the Cyclops-DP, dual polarisation weather radar processing and control system, which was used as a platform from which investigation of novel radar observables could be undertaken. The retrieval of near-surface refractivity changes, using fixed clutter targets is investigated. It is shown that by combining dual polarisation measure- ments, an improvement in the correlation with surface observations is obtained. A novel method of determining the target location within the range gate, with the aim of reducing the bias and error in the refractivity retrievals, is tested but not shown to give benefit. The development of, what is thought to be, the first combined dual polar- ization weather radar and radiometer is described. It is shown that useful radiometric measurements can be made using conventional radar hardware and with relatively minor changes, the radar radiometric sensitivity is im- proved by a factor of 3.5. The sensitivity of the atmospheric background noise temperature to changes in temperature and pressure, as a function of elevation, at C band is investigated for the purposes of radiometric calibra- tion. It is shown that a climatological profile can be used in calibrations with certain caveats. A comparison of different methods of estimating the path integrated attenuation is presented and demonstrates that the radiometry- based estimates have skill and benefits compared to other techniques. The potential for using radiometric measurements to estimate partial beam blocking is demonstrated.

Space weather presents a threat to human activities such as Global Navigation Satellite System (GNSS) positioning and timing, power systems, radio communications and transpolar aviation. Nowcasts and forecasts of the ionosphere could help mitigate some of these damaging effects. In this thesis, state-of-the-art ionospheric specification techniques are assessed in a long-term study. That study shows that Global Positioning System (GPS) derived tomographic images specify monthly median ionospheric Total Electron Content (TEC) accurately in Europe and North America throughout the twelve-year test period. Following this assessment, developments are presented in three key areas. The resolution of horizontal structures in ionospheric images over Africa is assessed. The accuracy gains from adding receivers are quantified using a simulation approach, showing that an extended GPS network reduces Root-Mean-Square (RMS) error from 9.5 TEC units for the currently operational network to 4.5 TEC units. A fictional, ideal network is demonstrated to produce images with RMS errors of 3.0 TEC units. Images of the vertical electron density distribution, vital for High Frequency (HF) radio operators, are greatly improved by adding observations of the ionospheric vertical profile to an imaging algorithm that relies on GPS observations. The peak electron density is resolved to an RMS accuracy of 0.5 x 1011 electrons/m3, compared to an RMS accuracy of 1.0 x 1011 electrons/m3 for the standard approach. A novel experimental method is employed to show that forecasts of ionospheric storms could benefit significantly from accurate specification of the initial neutral composition, in particular the ratio of O to N2 . A theoretical experiment shows that an ideal assimilation of the thermospheric composition can improve storm-time forecasts by at least 10% for over 19 hours, whilst an ideal ionospheric assimilation improves forecasts for less than four hours. This finding will aid the development of a coupled thermosphere ionosphere forecast system.

The radio sky can exhibit dynamic changes that provide an excellent opportunity to study the most exotic events in the Universe. In recent years the development and completion of advanced radio facilities has enabled the transient radio sky to be probed to unprecedented levels, especially at low frequencies with telescopes such as the Low-Frequency Array (LOFAR) revolutionising observations in this regime. This work primarily focuses on the detection of radio transients both at low and high frequencies. I describe the analysis of a 60 MHz LOFAR monitoring campaign centred at the North Celestial Pole, and report the discovery of the first LOFAR transient source. I also present the initial transient search of the CHILES survey that was performed with the VLA at 1.4 GHz, supported by simultaneous optical observations. Three strong transient candidates are discussed along with identifying a further 72 possible candidates. Furthermore, I investigate the relation between the radio and optical flux properties of known transient and variable sources, with the aim of developing a classification method for these types of events. I show that from just radio and optical flux measurements, it is possible to separate certain classes of objects, with additional information provided by analysing how individual classes evolve in the radio and optical parameter space over time. Lastly, I explore the future prospects of the LOFAR and CHILES transient searches, detailing what techniques could be used to further define the transient parameter space. I also discuss how the radio and optical classification method can be advanced and incorporated into automatic transient pipelines, which may be a necessity in the future Square Kilometre Array era.

In this thesis, the process of optimizing Cooperative Spectrum Sensing in Cognitive Radio has been investigated in fast-fading environments where simulation results have shown that its performance is limited by the Probability of Reporting Errors. By proposing a transmit diversity scheme using Differential space-time block codes (D-STBC) where channel state information (CSI) is not required and regarding multiple pairs of Cognitive Radios (CR’s) with single antennas as a virtual MIMO antenna arrays in multiple clusters, Differential space-time coding is applied for the purpose of decision reporting over Rayleigh channels. Both Hard and Soft combination schemes were investigated at the fusion center to reveal performance advantages for Hard combination schemes due to their minimal bandwidth requirements and simplistic implementation. The simulations results show that this optimization process achieves full transmit diversity, albeit with slight performance degradation in terms of power with improvements in performance when compared to conventional Cooperative Spectrum Sensing over non-ideal reporting channels. Further research carried out in this thesis shows performance deficits of Cooperative Spectrum Sensing due to interference on sensing channels of Cognitive Radio. Interference Alignment (IA) being a revolutionary wireless transmission strategy that reduces the impact of interference seems well suited as a strategy that can be used to optimize the performance of Cooperative Spectrum Sensing. The idea of IA is to coordinate multiple transmitters so that their mutual interference aligns at their receivers, facilitating simple interference cancellation techniques. Since its inception, research efforts have primarily been focused on verifying IA’s ability to achieve the maximum degrees of freedom (an approximation of sum capacity), developing algorithms for determining alignment solutions and designing transmission strategies that relax the need for perfect alignment but yield better performance. With the increased deployment of wireless services, CR’s ability to opportunistically sense and access the unused licensed frequency spectrum, without causing harmful interference to the licensed users becomes increasingly diminished, making the concept of introducing IA in CR a very attractive proposition. For a multiuser multiple-input–multiple-output (MIMO) overlay CR network, a space-time opportunistic IA (ST-OIA) technique has been proposed that allows spectrum sharing between a single primary user (PU) and multiple secondary users (SU) while ensuring zero interference to the PUs. With local CSI available at both the transmitters and receivers of SUs, the PU employs a space-time WF (STWF) algorithm to optimize its transmission and in the process, frees up unused eigenmodes that can be exploited by the SU. STWF achieves higher performance than other WF algorithms at low to moderate signal-to-noise ratio (SNR) regimes, which makes it ideal for implementation in CR networks. The SUs align their transmitted signals in such a way their interference impairs only the PU’s unused eigenmodes. For the multiple SUs to further exploit the benefits of Cooperative Spectrum Sensing, it was shown in this thesis that IA would only work when a set of conditions were met. The first condition ensures that the SUs satisfy a zero interference constraint at the PU’s receiver by designing their post-processing matrices such that they are orthogonal to the received signal from the PU link. The second condition ensures a zero interference constraint at both the PU and SUs receivers i.e. the constraint ensures that no interference from the SU transmitters is present at the output of the post-processing matrices of its unintended receivers. The third condition caters for the multiple SUs scenario to ensure interference from multiple SUs are aligned along unused eigenmodes. The SU system is assumed to employ a time division multiple access (TDMA) system such that the Principle of Reciprocity is employed towards optimizing the SUs transmission rates. Since aligning multiple SU transmissions at the PU is always limited by availability of spatial dimensions as well as typical user loads, the third condition proposes a user selection algorithm by the fusion centre (FC), where the SUs are grouped into clusters based on their numbers (i.e. two SUs per cluster) and their proximity to the FC, so that they can be aligned at each PU-Rx. This converts the cognitive IA problem into an unconstrained standard IA problem for a general cognitive system. Given the fact that the optimal power allocation algorithms used to optimize the SUs transmission rates turns out to be an optimal beamformer with multiple eigenbeams, this work initially proposes combining the diversity gain property of STBC, the zero-forcing function of IA and beamforming to optimize the SUs transmission rates. However, this solution requires availability of CSI, and to eliminate the need for this, this work then combines the D-STBC scheme with optimal IA precoders (consisting of beamforming and zero-forcing) to maximize the SUs data rates.

The hype surrounding the 5G mobile networks is well justified in view of the explosive increase in mobile traffic and the inclusion of massive “non-human” users that form the internet of things. Advanced radio features such as network densification, cloud radio access networks (C-RAN), and untapped frequency bands jointly succeed in increasing the radio capacity to accommodate the increasing traffic demand. However, a new challenge has arisen: the backhaul (BH), the transport network that connects radio cells to the core network. The BH needs to expand in a timely fashion to reach the fast spreading small cells. Moreover, the realistic BH solutions are unable to provide the unprecedented 5G performance requirements to every cell. To this end, this research addresses the gap between the 5G stipulated BH characteristics and the available BH capabilities. On the other hand, heterogeneity is a leading trait in 5G networks. First, the RAN is heterogeneous since it comprises different cell types, radio access technologies, and architectures. Second, the BH is composed of a mix of different wired and wireless technologies with different limitations. In addition, 5G users have a broader range of capabilities and requirements than any incumbent mobile network. We exploit this trait and develop a novel scheme, termed User-Centric-BH (UCB). The UCB targets the user association mechanism which is traditionally blind to users’ needs and BH conditions. The UCB builds on the existing concept of cell range extension (CRE) and proposes multiple-offset factors (CREO) whereby each reflects the cell's joint RAN and BH capability with respect to a defined attribute (e.g., throughput, latency, resilience, etc.). In parallel, users associate different weights to different attributes, hence, they can make a user-centric decision. The proposed scheme significantly outperforms the state-of-the-art and unlocks the BH bottleneck by availing existing but misused resources to users in need.

Massive multiple-input multiple-output (MIMO) technology is considered as one of the enabling technologies to scale up the data rates for the future communication systems. Traditional MIMO systems employ digital beamforming where each antenna element is equipped with one radio frequency (RF) chain. When the number of the antennas are scaled up, the cost and power consumption of massive MIMO systems also increase significantly. Recently, hybrid analog-and-digital beamformers have attracted a lot of attention as a cost effective approach to benefit from the advantages of massive MIMO. In hybrid structure, a small number of RF chains are connected to a large number of antennas through a network of phase shifters. The optimal hybrid beamforming problem is a complex nonconvex optimization due to the nonconvex constraint imposed by phase shifters. The overall objective of this thesis is to provide simple and effective hybrid beamforming solutions for narrowband point-to-point and multiuser massive MIMO scenarios. Firstly, hybrid beamforming problem for a point-to-point communication system with perfect channel state information (CSI) is investigated, and an effective codebook based hybrid beamforming with low resolution phase shifters is proposed which is suitable for sparse scattering channels. Then, by leveraging the properties of massive MIMO, an asymptotically optimal hybrid beamforming solution as well as its closed-form formula will be presented. It will be shown that the proposed method is effective in both sparse and rich scattering propagation environments. In addition, the closed-form expression and lower-bounds for the achievable rates are derived when analog and digital phase shifters are employed. Secondly, hybrid beamforming problem to maximise the total sum-rate for the downlink of multiuser MIMO is investigated, and an effective solution as well as its closed-form expression for this system is proposed. The presented solutions for the single-antenna and multiantenna scenarios are shown to be effective as they can achieve a similar sum-rate as digital beamforming can reach. In addition, it is shown that the proposed technique with low-cost low resolution phase shifters at the RF beamformer demonstrates a comparable performance to that of a hybrid beamformer with an expensive analog beamformer. Finally, two novel hybrid beamforming techniques are proposed to reduce the power consumption at the RF beamformer. Defining a threshold level, it is shown that half of the phase shifters could be turned off without a performance loss when the wireless channel matrix is modeled by Rayleigh fading. Then, we reduce the number of the phase shifters by using a combination of phase shifters and switches at the RF beamformer. The proposed methods can significantly reduce the power consumption as switches, in general, have lower power consumption compared to phase shifters. It is noted that the presented algorithms and the closed-form expressions of their performance are derived by using the asymptotic properties of the elements of the singular vectors for the rich scattering channel matrix.

Simulation tools are primary means for evaluating and analysing performance of Mobile Adhoc NETworks (MANETS). Different mobility and propagation models have been used in this context. However, simple propagation models have been used heavily in simulation based MANETS routing performance analysis. A range of propagation models (such as ITU-R P.1411-5, ITU-R P1238-6, GOA-LoS, modified TRG, C-Shadowing and WINNER) for various indoor/outdoor and LoS/NLoS scenarios have been added into ns-2 based simulation and results have been analysed with those readily available in the simulator for AODV, DSR and OLSR routing protocols. A variety of synthetic mobility models have been implemented under those pathloss conditions and their impact on routing performance has been observed. Heterogeneous mobility conditions have been introduced for performance analysis of AODV and DSR routing protocols under, TRG, ITU and GOA pathloss conditions. It has been found that the DSR protocol fails to perform where AODV prevails in specific mobility environment. RPGM model have been analysed with variations of mobility model adopted by group-heads and its impact on MANETS have been investigated. A very large adhoc network has been tested through ns-2 simulator with comparison of ITU and TRG channel loss conditions and its results have been compared with other scalable routing performance analysis studies. Impact of corner-loss effects due to typical street movement scenario such as under MG mobility have been experimented through ITU-R based recommendations in ns-2 simulation environment. A small testbed based AODV performance analysis has been compared with ns-2 based simulation results under ITU and Shadowing propagation models and significant difference have been recorded in real vs simulation based results for NRL and Mean. Delay analysis. In summary, MANET performance have been analysed under a range of operational conditions and applications and it has been demonstrated that various factors such as mobility and propagation environment could significantly challenge the deployment of such networks.

In the actual era of the information digitisation, the technological advances are enabling the development of Wireless Sensor Networks for the observation of physical phenomena in environmental, industrial and commercial domains. The proposed applications require ever larger networks in size and number of nodes that involve the use of multi-hop communications in order to collect information in the base stations (sinks) of the network. Due to the limited energy and radio resources of the sensor nodes and, taking into account the dynamics of such networks in terms of topology, application requirements, congestion access and node failures, the interconnection of such networks into effective systems is extremely difficult in order to achieve the exigent lifetime expectation. This research focuses on the design of communication protocols directed to increase the adaptability of the network with the aim of managing these network dynamics, congestion and failure problems. The methodology used consists in the co-design of cross-layer solutions that make use of adaptive techniques like smart antennas, hybrid access mechanism and merged Routing-MAC proactive schemes. The cross-layer developments have been integrated into a flexible communications framework (XLCA) that includes the functionality needed in sensor networks applications. The exercising and evaluation of XLCA has demonstrated the adaptability skills and potential to extend the operational network lifetime of the proposed cross-layer techniques. The results of this research show that the integration of functionality and capabilities of the various communication elements (Routing, MAC and Physical), firstly, allow to adjust the post-deployment nodes operation that enhance the network adaptability and, secondly, optimises the use of radio resources, which consequently increases the longevity of the network.

Modern mobile applications demand a communications network that is universally accessible at any time and able to adapt to the application needs. Meeting these requirements can present a considerable challenge. In order to access information ubiquitously, hardware manufacturers started to integrate many network access technologies in single small, light and low-power portable device. The main objective of this thesis is to exploit the availability of multiple wired and wireless communication channels for multimedia data transmission and location based services.