  In this paper, we propose a model for energy cooperation between cellular
base stations (BSs) with individual hybrid power supplies (including both the
conventional grid and renewable energy sources), limited energy storages, and
connected by resistive power lines for energy sharing. When the renewable
energy profile and energy demand profile at all BSs are deterministic or known
ahead of time, we show that the optimal energy cooperation policy for the BSs
can be found by solving a linear program. We show the benefits of energy
cooperation in this regime. When the renewable energy and demand profiles are
stochastic and only causally known at the BSs, we propose an online energy
cooperation algorithm and show the optimality properties of this algorithm
under certain conditions. Furthermore, the energy-saving performances of the
developed offline and online algorithms are compared by simulations, and the
effect of the availability of energy state information (ESI) on the performance
gains of the BSs' energy cooperation is investigated. Finally, we propose a
hybrid algorithm that can incorporate offline information about the energy
profiles, but operates in an online manner.
