Today, the largest and most desirable portion of the radio spectrum is allocated to licensed services, which has resulted in the well-known profound scarcity of this resource for emerging applications. With the rapid growth of wireless technologies, current spectrum scarcity has become a serious problem as more and more wireless applications compete for very little spectrum. On the other hand, the licensed spectrum allocated to applications such as television, cellular telephony and public

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... y show very little usage over time at different geographical locations. This has, therefore, seriously impaired the evolution of newer technologies because of current regulatory constraints on the operation in licensed spectrum, such as TV bands, and is being addressed by FCC through recent rule makings. With the goal of ubiquitous communication in mind, we look into spectrum agile radios as a new technology enabled by such emerging regulatory rulings and study its advantages over conventional radios. Initially, we provide a simple mathematical modeling to understand the utilization that is achievable by spectrum agile radios. Next, we investigate several issues related to spectrum sensing, as it is one of the key pillars to realize spectrum agile radios. Through sensing, the spectrum agile radio identifies the so-called "whitespaces" in the spectrum and then decides whether to occupy those white spaces opportunistically to transmit data. We also discuss the concept of interference temperature introduced by the FCC, and propose a spectrum-aware sensor network as a way to address it. Finally, we extend this spectrum-aware sensor networks to introduce a new sensing architecture to identify and locate white spaces in the spectrum.