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Cognitive radio is a potential technology for increasing spectral efficiency in wireless communications systems, where in a hypothetical future overlay policy, secondary cognitive radios temporarily use spectrum that is not utilized, as long as negligible impact is caused to primary licensed users. Dependable spectrum sensing is needed to overcome the hidden node problem that arises when a primary transmitter is in a fade relative to the cognitive radio. Although wavelength-scale spatial diversity can overcome fast-fading due to multipath, overcoming slow-fading (or shadowing) due to obstructions requires larger separation of sensors, possible through collaboration among multiple secondary nodes.
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