  The use of energy harvesting (EH) nodes as cooperative relays is an emerging
solution for enabling green wireless systems. In this paper, we consider
multiple EH relay nodes harvesting energy from the radio frequency (RF) signal
received from the source and use that harvested energy to forward the source
information to the destination. Unlike conventional relays with fixed power
supplies, EH relays may not be permanently available to assist the source
transmission due to the limited energy conversion efficiency, the mismatch
between the charging and discharging profiles, and the finite energy storage
capacity. We propose the battery-aware relay selection (BARS) scheme, which
jointly considers the channel condition and the battery status for relay
selection. The outage probability of the proposed scheme is analyzed using a
Markov chain model. Simulations are performed to validate the analysis
accuracy. Through numerical results, we show that the proposed BARS scheme can
achieve full diversity order equal to the number of relays without the need of
fixed power cables.
