Cognitive radio transmission strategies for primary erasure channels

A fundamental problem in cognitive radio systems is that the secondary user is ignorant of the primary channel state and the interference it inflicts on the primary license holder. We consider a secondary user that can eavesdrop on the ACK/NACK Automatic Repeat reQuest (ARQ) fed back from the primary receiver to the primary transmitter. Assuming the primary channel states follow a Markov chain, this feedback gives the secondary user an indication of the primary channel quality. Based on the ACK/NACK received, we devise optimal transmission strategies for the secondary user so as to maximize a weighted sum of primary and secondary throughput. We fully characterize an optimal achievable scheme that spans the boundary of the primary-secondary rate region for a two state erasure primary channel. Our scheme maximizes a weighted sum of primary and secondary throughput. The actual weight used during network operation is determined by the degree of protection afforded to the primary channel. Moreover, we study a three-state model where we derive the optimal transmission strategy using dynamic programming.