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Abstract-In this paper, we investigate distributed control of multiple secondary users attempting to access the channel of a high priority primary user. Our aim is to maximize the sum cognitive (secondary) user throughput under the constraint of primary user's queue stability. We consider the effect of primary user link adaptation that allows the primary transmitter (PTx) to adapt its transmission rate in response to the secondary interference-level at the primary receiver (PRx). To control the sum secondary interference to PRx beyond the traditional collision-avoidance paradigm, we propose a novel power-control algorithm for secondary nodes to function. To develop such a distributed algorithm and to improve secondary user adaptability, we allow secondary nodes to monitor primary's radio link control information on the feedback channel. We present practical schemes that approximate the optimum solution without relying on global channel information at each secondary node.

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Cognitive radio systems capable of opportunistic spectrum access represent a new paradigm for improving the efficiency of current spectrum utilization. In this work, we present a novel cognitive channel access method based on learning from both primary channel transmissions and the receiver ARQ feedback signals. This new sensing-plus-confirmation scheme constitutes a non-trivial generalization of the more traditional "Listen-Before-Talk" (LBT) strategy that merely listens to and yields to primary transmissions regardless of primary receiver responses. Our new method exploits the bi-directional and interactive nature of most wireless communication links to facilitate better opportunistic secondary access while achieving primary receiver protection. By allowing the secondary users to learn from both primary transmissions and the corresponding receiver confirmations, our approach allows secondary cognitive users to exploit critical information that primary receivers regularly send to their transmitters. We show that, by monitoring both primary transmissions and receiver feedback signals, secondary radio access can improve throughput over the traditional LBT while limiting the probability of collision with primary user signals.

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