Wireless techlogies grow rapidly and benefit almost every aspect of our daily lives. In a typical multiple-user environment, different users may severely interfere with each other. How to reduce the coordination overhead in order to improve the efficiency of the wireless networks becomes a big challenge. Unlike the wired counterpart, a wireless link is easily affected by environment changes and surrounding wireless activities. Determining the instant link conditions (or qualities) is essential for most protocol designs and application developments in wireless communications. In this book, we introduce several techniques which leverage the physical layer information or cross layer design to enhance the wireless network performance.