  We consider an opportunistic cognitive radio (CR) system in which secondary
transmitter (SUtx) is equipped with a reconfigurable antenna (RA). Utilizing
the beam steering capability of the RA, we regard a design framework for
integrated sector-based spectrum sensing and data communication. In this
framework, SUtx senses the spectrum and detects the beam corresponding to
active primary user's (PU) location. SUtx also sends training symbols (prior to
data symbols), to enable channel estimation at secondary receiver (SUrx) and
selection of the strongest beam between SUtx-SUrx for data transmission. We
establish a lower bound on the achievable rates of SUtx-SUrx link, in the
presence of spectrum sensing and channel estimation errors, and errors due to
incorrect detection of the beam corresponding to PU's location and incorrect
selection of the strongest beam for data transmission. We formulate a novel
constrained optimization problem, aiming at maximizing the derived achievable
rate lower bound subject to average transmit and interference power
constraints. We optimize the durations of spatial spectrum sensing and channel
training as well as data symbol transmission power. Our numerical results
demonstrate that between optimizing spectrum sensing and channel training
durations, the latter is more important for providing higher achievable rates.
