Abstract
Future demands for higher data-rate services is putting pressure on the current fixed radio spectrum assignment scheme adopted by regulators. The Cognitive Radio (CR) approach is one possible solution to these issues by dynamically assigning spectrum between a Primary User (PU) and Secondary User (SU) and thus sharing spectrum and using it more efficiently. In this thesis we consider various forms of CR (interweave, underlay and joint interweave/underlay) applied to OFDM based radio systems. As conventionally used power allocation of the OFDM modulated SU aims to dynamically control the transmit power on each subcarrier in order to maximize the achievable data rate while protecting the PU from the interference caused by the SU. However, assumptions currently made do not account for practical system considerations e.g., Gaussian input and an interference model that assumes an FDM modulated PU with perfect synchronization with the SU plus a static traffic model. This results in higher optimized power levels and lower SU achievable data rates and thus underestimates usefulness of the system. The thesis addresses these issues by deriving a more general interference model which takes into consideration the more realistic waveforms and imperfect synchronization as well as real traffic models. Based on the interference model, the power optimization problem is formulated under the condition of real (Finite Symbol Alphabet) input as used in practical systems, and evaluates, for the interweave and joint schemes, using single and multiple terminal antennas (SISO and MIMO). The more generalised model shows a saving of transmit power and achieves higher data rates compared to the previous Gaussian approximation. Motivated by the power saving results, a theoretical framework is further established for all cases by evaluating average allocated power for the Gaussian and real waveform model. This theoretical analysis provides guidance for the system design and gives a deeper insight into the choice of parameters affecting power saving and rate improvement. In addition, for given channel statistics, the theoretical analysis can be used to estimate the power saving without running time-consuming Monte Carlo simulations. Finally, by taking into account more practical scenarios, optimal power and achievable data rate is also evaluated for the case of the dynamic (considering probability of PU arrival and departure) joint scheme. In comparison to the static scheme, the joint scheme protects the PU in case of PU arrivals and yields improved utilization of spectrum when the PU departures are considered.