Although different spectrum bands are allocated to specific services, it has been identified that these bands are underutilized. In this context, cognitive radio (CR) was proposed to address the existing spectrum scarcity and underutilization problems. Under the scenario of CR, spectrum sharing systems allow the coexistence of primary users (PUs) and secondary users (SUs) in the same spectrum as long as the interference from the secondary to the primary link stays below a given threshold. In this book, we propose a number of enabling transmission techniques aiming at improving the performance...