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Within the innovation of MRI systems, high image quality and less imaging time become the major researching target. Metamaterials have shown a big potential to provide new method to improve the transceiver performance of the MRI system, without changing radio frequency (RF) coil structure or increasing the static field strength of the system. However, most of these designs are based on periodic structures, which will have extra power losses caused by extra lumped elements, effects to the homogeneity due to the surface currents between each unit cell, or increment in E-field which is proportional to the specific absorption rate (SAR) as the metamaterial focus the field. This thesis proposed a metasurface with a non-periodic structure to enhance the magnetic flux density B1 + and signal to noise ratio (SNR) for 1.5T MRI system, when it was placed between the RF coil and the dielectric phantom. In order to obtain the best design, numerical simulations were tested in CST studio. S-parameter, magnetic field, magnetic flux density and E-field were calculated to define the benefits from the metasurface. During the simulation, it was found that the field of MRI transceiver system was improved, due to the metasurface worked as a second source instead of focusing the field. As a result, E-field was not increased obviously compared with RF coil only case. Then, the optimal design was manufactured by PCB etching technique and tested in 1.5T MRI scanner. From the SNR results obtained from the scanner, the SNR on the phantom surface was improved by 133%, and the enhancement covered the whole penetration depth. Compared with previous researches of the field of view, this non-periodic metasurface has much smaller and flexible dimension. From the SNR results in 2D, the SNR was decayed uniformly inside the dielectric phantom, because the metasurface is non-periodic. This will further increase the sensitivity of RF coil. During the measurement, it was found the non-periodic metasurface could provide an extra frequency band, by the coupling between the metasurface and RF coil. Therefore, the dual-band property of the metasurface was investigated to achieve the multi-nuclei image. After simulation and measurement, a new metasurface in nonperiodic structure was proposed to provide two resonances at 60.08MHz and 63.85MHz for the 19F nuclei and 1H nuclei imaging in 1.5T MRI system. From the SNR results obtained from 1.5T MRI scanner, the SNR was enhanced by 37.9% at 60.08MHz and 32.7% at 63MHz, when comparing with RF coil only case. From the methods people used to achieve multi-nuclei imaging, there is no research could image two frequency bands without reducing the RF performance of the transceiver system. Furthermore, this is the first time to achieve multi-nuclei imaging by using non-periodic metamaterial. Its flexible and simple structure open a new and advanced researching area.

In the coming years, the number of connected wireless devices will increase dramatically, expanding the Internet of Things (IoT). It is likely that much of this capacity will come from network densification. However, base stations are inefficient and expensive, particularly the downlink transmitters. The main cause of this is the power amplifier (PA), which must amplify complex signals, so are expensive and often only 30% efficient. As such, the cost of densifying cellular networks is high. This thesis aims to overcome this problem through codesign of a low complexity, energy efficient transmitter through electromagnetic design; and a waveform which leverages the advantages and mitigates the disadvantages of the new technology, while being suitable for supporting IoT devices. Direct Antenna Modulation (DAM) is a low complexity transmitter architecture, where modulation occurs at the antenna at transmit power. This means a non-linear PA can efficiently amplify the carrier wave without added distortion. Frequency Selective Surfaces (FSS) are presented here as potential phase modulators for DAM transmitters. The theory of operation is discussed, and a prototype DAM for QPSK modulation is simulated, designed and tested. Next, the design process for a continuous phase modulating antenna is explored. Simulations and measurement are used to fully characterise a prototype, and it is implemented in a line-of-sight end-to-end communications system, demonstrating BPSK, QPSK and 8-PSK. Due to the favourable effects of spread spectrum signalling on FSS DAM performance, Cyclic Prefix Direct Sequence Spread Spectrum (CPDSSS) is developed. Conventional spreading techniques are extended using a cyclic prefix, making multipath interference entirely defined by the periodic autocorrelation of the sequence used. This is demonstrated analytically, through simulation and with experiments. Finally, CPDSSS is implemented using FSS DAM, demonstrating the potential of this new low cost, low complexity transmitter with CPDSSS as a scalable solution to IoT connectivity.