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 safety 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 socalled "white-spaces" 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.
A INTRODUCTIONElectromagnetic spectrum is a valuable natural resource, and hence is tightly regulated around the world. In the US, spectrum auction for third-generation (3G) mobile communications yielded USD 17 billion, while it yielded USD 34 billion and USD 46 billion in England and Germany, respectively. The US frequency allocations chart produced by the Office of Spectrum Management, US Department of Commerce, reveals that almost all of the electromagnetic spectrum is allocated; while recent studies indicate most of those allocations are utilized only in low duty cycles. One such spectrum measurement study, conducted by Shared Spectrum Company, indicates occupancy in less than 35% percent of the radio spectrum below 3GHz, even in the most crowded area near downtown Washington DC, where both government and commercial spectrum usage is intensive [1]. Similar results have been obtained when measurements were made in New York City during the republican convention held last year (see Figure 1) [2]. Furthermore, it is noted that spectrum usage varies dramatically in time, geographic locations, and frequency. Figure 1: Occupancy of spectrum in 30 MHz to 3 GHz in NYC over a 24 hour period yielding 13% duty cycle (source: Shared Spectrum Company website).These and other studies [10] have indicated that p...
