  Cognitive radios have been proposed as a means to implement efficient reuse
of the licensed spectrum. The key feature of a cognitive radio is its ability
to recognize the primary (licensed) user and adapt its communication strategy
to minimize the interference that it generates. We consider a communication
scenario in which the primary and the cognitive user wish to communicate to
different receivers, subject to mutual interference. Modeling the cognitive
radio as a transmitter with side-information about the primary transmission, we
characterize the largest rate at which the cognitive radio can reliably
communicate under the constraint that (i) no interference is created for the
primary user, and (ii) the primary encoder-decoder pair is oblivious to the
presence of the cognitive radio.
