Dynamic Spectrum Access in Cognitive Radio Networks
Clancy, Thomas Charles
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Since the 1930s, the Federal Communications Commission (FCC) has controlled the radio frequency energy spectrum. They license segments to particular users in particular geographic areas. A few, small, unlicensed bands were left open for anyone to use as long as they followed certain power regulations. With the recent boom in personal wireless technologies, these unlicensed bands have become crowded with everything from wireless networks to digital cordless phones. To combat the overcrowding, the FCC has been investigating new ways to manage RF resources. The basic idea is to let people use licensed frequencies, provided they can guarantee interference perceived by the primary license holders will be minimal. With advances in software and cognitive radio, practical ways of doing this are on the horizon. In 2003 the FCC released a memorandum seeking comment on the interference temperature model for controlling spectrum use. Analyzing the viability of this model and developing a medium access protocol around it are the main goals of this dissertation. A system implementing this model will measure the current interference temperature before each transmission. It can then determine what bandwidth and power it should use to achieve a desired capacity without violating an interference ceiling called the interference temperature limit. If a system consisting of interference sources, primary licensed users, and secondary unlicensed users is modeled stochastically, we can obtain some interesting results. In particular, if impact to licensed users is defined by a fractional decrease in coverage area, and this is held constant, the capacity achieved by secondary users is directly proportional to the number of unlicensed nodes, and is actually independent of the interference and primary users' transmissions. Using the basic ideas developed in the system analysis, Interference Temperature Multiple Access, a physical and data-link layer implementing the interference temperature model, was formulated, analyzed, and simulated. Overall, the interference temperature model is a viable approach for dynamic spectrum access, and ITMA is a concrete technique implementing it. With the help of cognitive radios, we can reform spectrum policy and have significantly more room to innovate, ushering in a new era of high-speed personal wireless communications.