Aspects of Design and Analysis of Cognitive Radios and Networks
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Recent survey campaigns have shown a tremendous under utilization of the bandwidth allocated to various wireless services. Motivated by this and the ever increasing demand for wireless applications, the concept of cognitive radio (CR) systems has rendered hope to end the so called spectrum scarcity. This thesis presents various different facets related to the design and analysis of CR systems in a unified way. We begin the thesis by presenting an information theoretic study of cognitive systems working in the so called low interference regime of the overlay mode. We show that as long as the coverage area of a CR is less than that of a primary user (PU) device, the probability of the cognitive terminal inflicting small interference at the PU is overwhelmingly high. We have also analyzed the effect of a key parameter governing the amount of power allocated to relaying the PU message in the overlay mode of operation in realistic environments by presenting a simple and accurate approximation. Then, we explore the possibilities of statistical modeling of the cumulative interference due to multiple interfering CRs. We show that although it is possible to obtain a closed form expression for such an interference due a single CR, the problem is particularly difficult when it comes to the total CR interference in lognormally faded environments. In particular, we have demonstrated that fitting a two or three parameter lognormal is not a feasible option for all scenarios. We also explore the second-order characteristics of the cumulative interference by evaluating its level crossing rate (LCR) and average exceedance duration (AED) in Rayleigh and Rician channel conditions. We show that the LCRs in both these cases can be evaluated by modeling the interference process with gamma and noncentral χ2 processes, respectively. By exploiting radio environment map (REM) information, we have presented two CR scheduling schemes and compared their performance with the naive primary exclusion zone (PEZ) technique. The results demonstrate the significance of using an intelligent allocation method to reap the benefits of the tremendous information available to exploit in the REM based methods. At this juncture, we divert our attention to multiple-input multiple-output (MIMO) CR systems operating in the underlay mode. Using an antenna selection philosophy, we solve a convex optimization problem accomplishing the task and show via analysis and simulations that antenna selection can be a viable option for CRs operating in relatively sparse PU environments. Finally, we study the impact of imperfect channel state information (CSI) on the downlink of an underlay multiple antenna CR network designed to achieve signal-to-interference-plus-noise ratio (SINR) fairness among the CR terminals. By employing a newly developed convex iteration technique, we solve the relevant optimization problem exactly without performing any relaxation on the variables involved.