  RF energy harvesting can be used to power communication devices so that
perpetual operation of such devices can be ensured. We consider a RF energy
harvesting underlay cognitive radio system operating in slotted fashion. The
primary user (PU) is equipped with a reliable power source and transmits with a
constant power in all the slots. However, the secondary user (SU) harvests
energy from primary's transmission and simultaneously transmits it's own data
such that interference at the primary receiver (PR) remains below an acceptable
threshold. At the secondary transmitter (ST), each time slot is divided into
two phases: energy harvesting (EH) phase and information transfer (IT) phase.
We formulated the problem of maximizing the achievable secondary sum rate under
primary receiver's protection criteria as a convex optimization problem and
obtained the optimal time sharing between EH and IT phase, and optimal
secondary transmit power under offline setting. The optimal offline scheme is
then compared with an online myopic policy, where the optimal time sharing
between EH and IT phase, and optimal secondary transmit power are obtained
based on instantaneous channel gains only.
