  A fundamental problem in dynamic frequency reuse is that the cognitive radio
is ignorant of the amount of interference it inflicts on the primary license
holder. A model for such a situation is proposed and analyzed. The primary
sends packets across an erasure channel and employs simple ACK/NAK feedback
(ARQs) to retransmit erased packets. Furthermore, its erasure probabilities are
influenced by the cognitive radio's activity. While the cognitive radio does
not know these interference characteristics, it can eavesdrop on the primary's
ARQs. The model leads to strategies in which the cognitive radio adaptively
adjusts its input based on the primary's ARQs thereby guaranteeing the primary
exceeds a target packet rate. A relatively simple strategy whereby the
cognitive radio transmits only when the primary's empirical packet rate exceeds
a threshold is shown to have interesting universal properties in the sense that
for unknown time-varying interference characteristics, the primary is
guaranteed to meet its target rate. Furthermore, a more intricate version of
this strategy is shown to be capacity-achieving for the cognitive radio when
the interference characteristics are time-invariant.
