  Distributed antenna systems (DAS) have been widely implemented in
state-of-the-art cellular communication systems to cover dead spots. Recent
studies have also indicated that DAS have advantages in wireless energy
transfer (WET). In this paper, we study simultaneous wireless information and
power transfer (SWIPT) for a multiple-input single-output (MISO) DAS in the
downlink which consists of arbitrarily distributed remote antenna units (RAUs).
In order to save the energy cost, we adopt energy cooperation of energy
harvesting (EH) and two-way energy flows to let the RAUs trade their harvested
energy through the smart grid network. Under individual EH constraints, per-RAU
power constraints and various smart grid considerations, we investigate a power
management strategy that determines how to utilize the stochastically spatially
distributed harvested energy at the RAUs and how to trade the energy with the
smart grid simultaneously to supply maximum wireless information transfer (WIT)
with a minimum WET constraint for a receiver adopting power splitting (PS). Our
analysis shows that the optimal design can be achieved in two steps. The first
step is to maximize a new objective that can simultaneously maximize both WET
and WIT, considering both the smart grid profitable and smart grid neutral
cases. For the grid-profitable case, we derive the optimal full power strategy
and provide a closed-form result to see under what condition this strategy is
used. On the other hand, for the grid-neutral case, we illustrate that the
optimal power policy has a double-threshold structure and present an optimal
allocation strategy. The second step is then to solve the whole problem by
obtaining the splitting power ratio based on the minimum WET constraint.
Simulation results are provided to evaluate the performance under various
settings and characterize the double-threshold structure.
