Communication systems have developed an integral place in human society and have paved the way for many scientific and social developments. The diverse range of applications and facilities that wireless communication provides points out that there is still huge potential of use in the future. With the fast changes in technology, the demand to access wireless systems has risen and the achievable data rate has been increasing. To reach the over-arching goals of communications requires significant effort in researching, designing and implementing new models. Hence, it is important to understand and predict trends in designing, improving and deploying communication systems. The book presents the wide-ranging coverage of the fundamental technological advances that have led to development in the region of wireless communication systems in current years. This book starts with primary and secondary users in cognitive radio-based wireless communication systems. The origins of cellular communications can be traced be traced back to 1947 and their first implementation in 1978. Since then, the technology has seen an extensive implementation and use, to the extent that today researchers are working on the standardizing and designing 5G and 6G networks. The implementation of 5G is very near and is set to hit commercial markets by 2020. But many problems still persist, which need to be solved and standardized before the technology is ready for the user. 5G technology is expected to provide faster and higher quality network features to meet the demand of tomorrow. This book presents a summary of these concepts based on discussions with researchers and network architects. It goes on to describe how a model would operate in the complex network environment that exists today. Next, it focuses on principles, applications, and challenges of synchronization in nature for future mobile communication systems. The book follows with a focus on rateless space-time block codes for 5G wireless communication systems and user perspective and security of a new mobile authentication method. The last chapter explains what changes have taken place in standardization processes of mobile telecommunication. It dives into the prevalent theory and designs which are dominantly present. It also constructs a framework which incorporates geographic setting, government and industrial regimes. This book is intended for researchers and students who need to have a wide-ranging understanding of the theory and applications of communication systems.