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In this paper, we consider a communication scenario in which the primary and the cognitive radios wish to communicate to different receivers, subject to mutual interference. In the model that we use, the cognitive radio has noncausal knowledge of the primary radio's codeword. We characterize the largest rate at which the cognitive radio can reliably communicate under the constraint that 1) no rate degradation is created for the primary user, and 2) the primary receiver uses a single-user decoder just as it would in the absence of the cognitive radio. The result holds in a "low-interference" regime in which the cognitive radio is closer to its receiver than to the primary receiver. In this regime, our resuits are subsumed by the results derived in a concurrent and independent work (Wu et al., 2007). We also demonstrate that, in a "high-interference" regime, multiuser decoding at the primary receiver is optimal from the standpoint of maximal jointly achievable rates for the primary and cognitive users. [ABSTRACT FROM AUTHOR]