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Opportunistic spectrum access (OSA) allows unlicensed secondary networks to share licensed spectrum in space and time, but only when licensed users are not using the spectrum. Thus this novel spectrum management technique started drawing the attention of researchers recently. Although many interesting approaches have been proposed, most techniques are early proposals that often only cover a subset of the problems related to OSA. In this paper, we give an extensive overview of medium access control design challenges specific to OSA, while discussing the main approaches proposed so far in the literature. We give an extensive survey of protocols proposed and discuss which features are not explored yet and which one need to be looked at more carefully.