  Harvesting energy from nature (solar, wind etc.) is envisioned as a key
enabler for realizing green wireless networks. However, green energy sources
are geographically distributed and the power amount is random which may not
enough to power a base station by a single energy site. Burning brown energy
sources such as coal and crude oil, though companied with carbon dioxide
emission, provides stable power. In this paper, without sacrificing
communication quality, we investigate how to perform green energy allocation to
abate the dependence on brown energy with hybrid brown and green energy
injected in power networks. We present a comprehensive framework to
characterize the performance of hybrid green and brown energy empowered
cellular network. Novel performance metric "bits/ton\ce{CO2}/Hz" is proposed to
evaluate the greenness of the communication network. As green energy is usually
generated from distributed geographical locations and is time varying, online
geographical power allocation algorithm is proposed to maximize the greenness
of communication network considering electricity transmission's physical laws
i.e., Ohm's law and Kirchhoff's circuit laws. Simulations show that
geographically distributed green energy sources complement each other by
improving the communication capacity while saving brown energy consumption.
Besides, the penetration of green energy can also help reduce power loss on the
transmission breaches.
