  This paper considers a two-hop network consisting of a source, two parallel
half-duplex relay nodes, and two destinations. While the destinations have an
adequate power supply, the source and relay nodes rely on harvested energy for
data transmission. Different from all existing works, the two relay nodes can
also transfer their harvested energy to each other. For such a system, an
optimization problem is formulated with the objective of maximizing the total
data rate and conserving the source and relays transmission energy, where any
extra energy saved in the current transmission cycle can be used in the next
cycle. It turns out that the optimal solutions for this problem can be either
found in a closed form or through one-dimensional searches, depending on the
scenario. Simulation results based on both the average data rate and the outage
probability show that energy cooperation between the two relays consistently
improves the system performance.
