  We consider an opportunistic cognitive radio (CR) system consisting of a
primary 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 holistic
approach, we develop a framework for integrated sector-based spectrum sensing
and sector-based data communication. Upon sensing the channel busy, SUtx
determines the beam corresponding to PU's orientation. Upon sensing the channel
idle, SUtx transmits data to SUrx, using the selected beam corresponding to the
strongest channel between SUtx and SUrx. We formulate a constrained
optimization problem, where SUtx-SUrx link ergodic capacity is maximized,
subject to average transmit power and interference constraints, and the
optimization variables are sensing duration, thresholds of channel quantizer at
SUrx, and transmit power levels at SUtx. Since this problem is non-convex we
develop a suboptimal computationally efficient iterative algorithm to find the
solution. Our numerical results quantify the capacity improvement provided by
the ESPAR antenna and demonstrate that our CR system yields lower outage and
symbol error probabilities, compared with a CR system that its SUtx has an
omni-directional antenna.
