In this paper, we investigate energy cooperation among sustainable cooperative relay nodes, which adopt decode- and-forward (DF) relaying. The relay nodes work on either information decoding (ID) mode or energy harvesting (EH) mode when receiving the signal from the source. Due to different harvested energy at the relay nodes, effective energy cooperation can benefit the energy efficiency, but suffer from practical energy loss when achieving energy cooperation. By solving the problem using Lagrangian duality, we obtain the optimal energy cooperation policy for the relay nodes and further discuss its two-level waterfilling structure. Then we derive the optimal and low-complexity EH/ID mode selecting algorithm with the two-level waterfilling energy cooperation policy. Finally, we compare the throughput between the systems with and without energy cooperation to show the benefit of energy cooperation.
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