Spectrum sharing using cognitive radio system capabilities: Methods to obtain and exploit knowledge of spectrum availability: Dissertation

This thesis presents methods to obtain and exploit knowledge of spectrum availability for cognitive radio systems (CRSs). CRSs can change the way to access the radio spectrum in response to the growing data rate and spectrum demand of the future mobile telecommunication market. A CRS includes capabilities to obtain knowledge of system internal and external state, dynamically and autonomously adjust its operations accordingly, and learn from the results. Future CRSs can enhance spectrum sharing by exploiting temporarily and locally available spectrum while guaranteeing that primary systems remain free from harmful interference. This thesis presents novel directional and distributed spectrum occupancy measurements for the 2.4 GHz industrial, scientific and medical (ISM) band to characterise the current spectrum use and the potential availability of spectrum for CRSs, taking into account the spatial dimension. This is the first study to show that the spectrum occupancy can vary significantly depending on the measurement location even in the same office area at the same time. Knowledge of spectrum availability for CRSs can be accomplished by several methods, including control channels, databases, and spectrum sensing techniques, which all have different capabilities, requirements and performances. In order to use proper methods in different situations, this thesis proposes a novel band-specific approach, where the selection of the method to obtain knowledge of spectrum availability is determined separately for each frequency band based on the deployment characteristics and regulatory requirements of the specific band. Spectrum sensing is studied in more detail by presenting analytical performance evaluation for a selected algorithm, Welch's periodogram, in a Rayleigh fading channel. Fuzzy combining is proposed for cooperative spectrum sensing, where the sensing results from several nodes are combined to improve the sensing reliability in a fading environment. In addition, a novel rule-based decision-making system with a learning mechanism is developed for the selection between different spectrum sensing techniques.This is the first work in the research literature to consider this problem. Finally, in order to exploit the spectrum and assign the available frequency channels to the different users, this thesis presents centralised and distributed channel assignment methods based on a heuristic harmony search algorithm. The presented results can be used in the development of future mobile communication systems enhanced with CRS capabilities to respond to the growing data rate and spectrum demand.