  The massive increase of data traffic, the widespread proliferation of
wireless applications and the full-scale deployment of 5G and the IoT, imply a
steep increase in cellular networks energy use, resulting in a significant
carbon footprint. This paper presents a comprehensive model to show the
interaction between the networking and energy features of the problem and study
the economical and technical viability of green networking. Solar equipment,
cell zooming, energy management and dynamic user allocation are considered in
the upgrading network planning process. We propose a mixed-integer optimization
model to minimize long-term capital costs and operational energy expenditures
in a heterogeneous on-grid cellular network with different types of base
station, including solar. Based on eight scenarios where realistic costs of
solar panels, batteries, and inverters were considered, we first found that
solar base stations are currently not economically interesting for cellular
operators. We next studied the impact of a significant and progressive carbon
tax on reducing greenhouse gas emissions (GHG). We found that, at current
energy and equipment prices, a carbon tax ten-fold the current value is the
only element that could make green base stations economically viable.
