  Energy harvesting (EH) has recently emerged as a key technology for green
communications as it can power wireless networks with renewable energy sources.
However, directly replacing the conventional non-EH transmitters by EH nodes
will be a challenge. In this paper, we propose to deploy extra EH nodes as
relays over an existing non-EH network. Specifically, the considered non-EH
network consists of multiple source-destination (S-D) pairs. The deployed EH
relays will take turns to assist each S-D pair, and energy diversity can be
achieved to combat the low EH rate of each EH relay. To make the best of these
EH relays, with the source transmit power minimization as the design objective,
we formulate a joint power assignment and relay selection problem, which,
however, is NP-hard. We thus propose a general framework to develop efficient
sub-optimal algorithms, which is mainly based on a sufficient condition for the
feasibility of the optimization problem. This condition yields useful design
insights and also reveals an energy hardening effect, which provides the
possibility to exempt the requirement of non-causal EH information. Simulation
results will show that the proposed cooperation strategy can achieve
near-optimal performance and provide significant power savings. Compared to the
greedy cooperation method that only optimizes the performance of the current
transmission block, the proposed strategy can achieve the same performance with
much fewer relays, and the performance gap increases with the number of S-D
pairs.
