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Spectrum access is an important function of cognitive radios for detectingand utilizing spectrum holes without interfering with the legacy systems. Inthis paper we propose novel cooperative communication models and show howdeploying such cooperations between a pair of secondary transmitter andreceiver assists them in identifying spectrum opportunities more reliably.These cooperations are facilitated by dynamically and opportunisticallyassigning one of the secondary users as a relay to assist the other one whichresults in more efficient spectrum hole detection. Also, we investigate theimpact of erroneous detection of spectrum holes and thereof missingcommunication opportunities on the capacity of the secondary channel. Thecapacity of the secondary users with interference-avoiding spectrum access isaffected by 1) how effectively the availability of vacant spectrum is sensed bythe secondary transmitter-receiver pair, and 2) how correlated are theperceptions of the secondary transmitter-receiver pair about network spectralactivity. We show that both factors are improved by using the proposedcooperative protocols. One of the proposed protocols requires explicitinformation exchange in the network. Such information exchange in practice isprone to wireless channel errors (i.e., is imperfect) and costs bandwidth loss.We analyze the effects of such imperfect information exchange on the capacityas well as the effect of bandwidth cost on the achievable throughput. Theprotocols are also extended to multiuser secondary networks.

Beacon-Assisted Spectrum Access with Cooperative Cognitive Transmitter and Receiver