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A fundamental problem in cognitive radio systems is that the cognitive radio is ignorant of the primary channel state and the interference it inflicts on the primary license holder. In this paper, the authors assume that the primary transmitter sends packets across an erasure channel and the primary receiver employs ACK/NAK feedback (ARQ) to retransmit erased packets. The cognitive radio can eavesdrop on the primaries ARQs. Assuming the primary channel states follow a Markov chain, this feedback gives the cognitive radio an indication of the primary link quality. Based on the ACK/NACK received, they devise optimal transmission strategies for the cognitive radio so as to maximize the weighted sum of primary and secondary throughput.
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