  Unpredictable nature of fading channels and difficulty in tracking channel
state information pose major challenge in wireless energy harvesting
communication system design. In this work, we address relay selection problem
for wireless powered communication networks, where the relays harvest energy
from the source radio frequency signals. A single source-destination pair is
considered without a direct link. The connecting relay nodes are equipped with
storage batteries of infinite size. We assume that the channel state
information (CSI) on the source-relay link is available at the relay nodes.
Depending on the availability of the CSI on the relay-destination link at the
relay node, we propose two relay selection schemes and evaluate their outage
probability. Availability of the CSI at the relay node on the relay-destination
link considerably improves the performance due to additional flexibility in the
relay selection mechanism. Due to absence of CSI throughout the network at the
time of transmission path selection, the analysis of the problem is not
tractable. Therefore, we relax our assumptions on availability of CSI and
closed-form expressions of the outage probability as a function of the amount
of the available harvested energy are derived for both CSI availability cases.
Finally, we numerically quantify the performance for the proposed schemes and
compare the outage probability for fixed and equal number of wireless powered
forwarding relays.
