Prior study on the cognitive radio (CR) has mostly adopted the passive sensing protocol, where the CR keeps monitoring the spectrum of interest, and decides to transmit when the sensing result indicates that the primary radio (PR) is inactive. In this paper, we propose an alternative active sensing protocol where, prior to the sensing, the CR generates a temporary jamming signal to deliberately interfere with the PR. The jamming signal is aimed to trigger an interesting interaction between the CR and PR, named as the hidden power-feedback loop: If the PR is indeed active and reacts upon receiving the jamming signal by increasing its transmit power, the CR will receive a power-boosted PR signal that is more easily detectable. Moreover, this paper demonstrates the usefulness of exploiting the power-feedback loop for the CR to effectively estimate the channel from the CR to the PR. This is well-known as a crucial task to implement for the CR operating under the so-called interference temperature constraint, where the CR is allowed to transmit regardless of the PR's on/off status, provided that the resultant interference power level at the PR is kept below some predefined threshold.
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