Contributing Towards Improved Communication Systems for Future Cellular Networks

The rapid growth of wireless communications and upcoming requirements of 5G networks are driving interest in the areas from wireless transceivers to sensor nodes. One of the most vital components of the wireless transmitter is the radio frequency power amplifier. A large-signal device model of the transistor is an essential part of the power amplifier design process. Despite the significant developments in large-signal modelling, the models for commercially available devices from the manufacturers are still under continuous development and often lack accuracy. One of the main objectives of this thesis is the validation and extension of an analytic approach as an alternative to conventional large-signal modelling for power amplifier designing. The first contribution is the derivation of new analytical expressions based on the equivalent circuit model, including the extrinsic parasitic elements introduced by the package, to calculate the optimum source and load impedances and to predict the performance of a radio frequency power amplifier. These expressions allow to evaluate the effects of a package on the optimum impedance values and performance. The second contribution is establishing the accuracy of the analytic approach. Harmonic balance simulation is used as the first benchmark to evaluate the method at various bias points and frequencies. The validity of the analytic approach is demonstrated at a frequency of 3.25 GHz for gallium nitride based high power devices with measurement of prototype radio frequency power amplifier designed for the impedance values obtained from the analytic expressions. The third contribution is extending the analytic approach to determine the optimum impedance values for different criteria of maximum gain, linearity and efficiency. The analytic expressions are utilized to gain an understanding of the relationship among the device performance, the elements of devices and package models and I-V characteristics. The wireless sensor networks are essential elements for the realization of the Internet of Things. Sensor nodes, which are the fundamental building blocks of these networks, have to be energy efficient and able to produce energy to reduce the maintenance cost and to prolong their lifetime. The second main aim of the thesis is designing and implementing an ultra-low power autonomous wireless sensor node that harvests the indoor light energy. The forth contribution of this thesis includes a comprehensive comparison of six different solar cell technologies under a controlled light intensity, carried out to determine the best option for indoor light energy harvesting. The power consumption of the node is reduced by selecting the appropriate hardware and implementing a wake-up receiver to reduce the active and idle mode currents. The low power consumption coupled with light energy harvesting significantly prolong the operating lifetime of the node