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We consider an opportunistic cognitive radio (CR) system consisting of aprimary user (PU), secondary transmitter (SUtx), and secondary receiver (SUrx),where SUtx is equipped with an electrically steerable parasitic array radiator(ESPAR) antenna with beam steering capability for sensing and communication,and there is a limited feedback channel from SUrx to SUtx. Taking a holisticapproach, we develop a framework for integrated sector-based spectrum sensingand sector-based data communication. Upon sensing the channel busy, SUtxdetermines the beam corresponding to PU's orientation. Upon sensing the channelidle, SUtx transmits data to SUrx, using the selected beam corresponding to thestrongest channel between SUtx and SUrx. We formulate a constrainedoptimization problem, where SUtx-SUrx link ergodic capacity is maximized,subject to average transmit power and interference constraints, and theoptimization variables are sensing duration, thresholds of channel quantizer atSUrx, and transmit power levels at SUtx. Since this problem is non-convex wedevelop a suboptimal computationally efficient iterative algorithm to find thesolution. Our numerical results quantify the capacity improvement provided bythe ESPAR antenna and demonstrate that our CR system yields lower outage andsymbol error probabilities, compared with a CR system that its SUtx has anomni-directional antenna.

Beam Selection and Discrete Power Allocation in Opportunistic Cognitive Radio Systems with Limited Feedback Using ESPAR Antennas