The ubiquitous trend to the “next generation communication” is a symbol of the communication arena’s need for independence, efficiency and flexibility. Cognitive radio was introduced in the late 1990s as a concept to improve efficiency of spectrum use. Cognitive users would sense spectral holes and exploit unused spectrum. The original strategy suggested that spectrum being employed by licensed cognitive users should be strictly avoided in order to ensure no interference to the licensed users. However, this strict “white space” approach is also inherently spectrally inefficient. This Ph.D. thesis focuses its contributions to researching into technologies and solutions intended for cognitive radio networks that may lead to improvements in the coming wireless communication generations. Due to different technical challenges for spectrum sensing, power allocation and security in the physical layer, the contribution of this thesis is the proposition of a realistic scenario for cognitive radio systems. In this thesis, we suggest several strategies that offer limited interference to primary users while significantly improving the throughput of cognitive users. A novel cognitive radio scheme is proposed which exhibits improved achievable throughput levels and spectrum sensing capabilities compared to the conventional opportunistic spectrum access cognitive radio networks studied so far. The proposed cognitive radio strategy can overcome the sensing throughput trade-off problem in the opportunistic spectrum access cognitive radio systems. In addition, it also provide its cognitive users with increased levels of average achievable throughput.