  This paper deals with the problem of relay selection in wireless powered
cooperative networks, where spatially random relays are equipped with energy
storage devices e.g., batteries. In contrast to conventional techniques and in
order to reduce complexity, the relay nodes can either harvest energy from the
source signal (in case of uncharged battery) or attempt to decode and forward
it (in case of charged battery). Several relay selection schemes that
correspond to different state information requirements and implementation
complexities are proposed. The charging/discharging behavior of the battery is
modeled as a two-state Markov chain and analytical expressions for the
steady-state distribution and the outage probability performance are derived
for each relay selection scheme. We prove that energy storage significantly
affects the performance of the system and results in a zeroth diversity gain at
high signal-to-noise ratios; the convergence floors depend on the steady-state
distribution of the battery and are derived in closed-form by using appropriate
approximations. The proposed relay selection schemes are generalized to a
large-scale network with multiple access points (APs), where relays assist the
closest AP and suffer from multi-user interference.
