Interweave/Underlay Cognitive Radio Techniques and Applications in Satellite Communication Systems

The demand for precious radio spectrum is continuously increasing while the available radio frequency resource has become scarce due to spectrum segmentation and the dedicated frequency allocation of standardized wireless systems. This scarcity has led to the concept of cognitive radio communication which comprises a variety of techniques capable of allowing the coexistence of licensed and unlicensed systems over the same spectrum. In this context, this thesis focuses on interweave and underlay cognitive radio paradigms which are widely considered as important enablers for realizing cognitive radio technology. In the interweave paradigm, an unlicensed user explores the spectral holes by means of some spectrum awareness methods and utilizes the available spectral availabilities opportunistically while in the underlay paradigm, an unlicensed user is allowed to coexist with the licensed user only if sufficient protection to the licensed user can be guaranteed. Starting with a detailed overview of the existing techniques, this thesis provides contributions in both theoretical and the practical aspects of these paradigms. This thesis is structured into two main parts as described below. One of the important capabilities required for a cognitive radio transceiver is to be able to acquire the knowledge of its surrounding radio environment in order to utilize the available spectral opportunities efficiently. The higher the level of information it can acquire, the better becomes the spectrum utilization. In this context, the first part of this thesis focuses on spectrum awareness techniques such as spectrum sensing, signal to noise ratio estimation and sparsity order estimation which are useful for realizing interweave and underlay based cognitive transceivers as well as a hybrid cognitive transceiver, which overcomes the drawbacks of both the above approaches. The effects of noise and channel correlations, which are often neglected in the existing literature, are considered in our analysis. In the above context, firstly, we propose new sensing thresholds for the eigenvalue based sensing approach using recent results from random matrix theory in order to achieve the improved sensing in correlated scenarios. Then we study the signal to noise ratio estimation problem for both narrowband and wideband transceivers with the help of a detailed theoretical analysis under the signal plus noise hypothesis for a range of correlated scenarios. Subsequently, we study the problem of compressive sparsity order estimation in order to estimate the sparsity order of the carrier occupancy over the wide spectrum using a compressive sensing approach. In addition, we carry out the performance analysis of a hybrid cognitive transceiver which combines the spectrum sensing approach with a power control-based underlay approach. The second part of this thesis introduces the concept of cognitive satellite communications which is a rather unexplored area in the literature despite its significant benefits to both satellite and terrestrial operators. This concept has been motivated due to the limited availability of the satellite spectrum as well as the continuously increasing demand of broadband multimedia, broadcast and interactive satellite services. Subsequently, we study the applications of various cognitive radio techniques in satellite communication systems focusing on the following two scenarios: (i) Hybrid cognitive satellite communication which deals with the spectral coexistence of satellite systems with the terrestrial systems, and (ii) Dual cognitive satellite communication which deals with the spectral coexistence of two satellite systems operating over the same coverage area. Understanding the characteristics of coexisting systems is of great importance while applying a suitable cognitive radio technique. In this context, this thesis exploits the specific characteristics of satellite systems in order to map a suitable cognitive radio technique to a specific scenario. For hybrid cognitive scenarios, we propose the following enabling techniques: (a) Interference modeling, (b) Harmful interference detection, (c) Cognitive beamforming including both transmit and receive beamforming, and (d) Dual-polarized spectrum sensing. Similarly, for dual cognitive scenarios, we propose the following techniques: (i) Cognitive interference alignment, (ii) Cognitive beamhopping, and (iii) Dual-polarized spectrum sensing. Finally, we provide interesting open research issues in the considered domain