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In a network with dynamic spectrum access, secondary users equipped with frequency-agile cognitive radios communicate with one another via spectrum that is not being used by the primary, licensed users of the spectrum. The authors consider a scenario in which a secondary transmitter can communicate with a secondary receiver via a direct communication link or a relay channel, depending on the state of a primary transmitter. They develop a cooperative communications strategy that exploits the presence of spectrum holes both in time and in space by combining cooperative relaying with joint spatial-temporal spectrum sensing. A strategy based on pure temporal sensing alone uses the direct link when the primary transmitter is off, whereas a scheme based on spatial sensing alone uses the relay channel.
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