  Energy harvesting is a promising solution to prolong the operation of
energy-constrained wireless networks. In particular, scavenging energy from
ambient radio signals, namely wireless energy harvesting (WEH), has recently
drawn significant attention. In this paper, we consider a point-to-point
wireless link over the narrowband flat-fading channel subject to time-varying
co-channel interference. It is assumed that the receiver has no fixed power
supplies and thus needs to replenish energy opportunistically via WEH from the
unintended interference and/or the intended signal sent by the transmitter. We
further assume a single-antenna receiver that can only decode information or
harvest energy at any time due to the practical circuit limitation. Therefore,
it is important to investigate when the receiver should switch between the two
modes of information decoding (ID) and energy harvesting (EH), based on the
instantaneous channel and interference condition. In this paper, we derive the
optimal mode switching rule at the receiver to achieve various trade-offs
between wireless information transfer and energy harvesting. Specifically, we
determine the minimum transmission outage probability for delay-limited
information transfer and the maximum ergodic capacity for no-delay-limited
information transfer versus the maximum average energy harvested at the
receiver, which are characterized by the boundary of so-called "outage-energy"
region and "rate-energy" region, respectively. Moreover, for the case when the
channel state information (CSI) is known at the transmitter, we investigate the
joint optimization of transmit power control, information and energy transfer
scheduling, and the receiver's mode switching. Our results provide useful
guidelines for the efficient design of emerging wireless communication systems
powered by opportunistic WEH.
