Figure 1. Sources of data traffic
However, there is trade-off between data capacity, energy consumption and costs. There are three main developments that the network operators are considering. The first one is the use of heterogeneous networks which consists in both macro- cells, to cover wide areas, and short range small cells. The increase of the number of small cells will allow to cover areas of high traffic demand such as businesses campus or shopping malls and they provide a main advantage in comparison to macro-cell in terms of energy efficiency while keeping high data rates. Base stations is the network element that consumes a 90% of the total energy. We can estimate the energy consumed by several small cells and compare it with the energy consumed by a macro-cell using the online IMEC power model, where we can select several parameters of the base station and see the power consumption of the different components:
Figure 2. IMEC Wireless Power Model
The second development is the use of more radio spectrum to boost capacity and meet the traffic demand. By using a new set of higher frequency bands, the millimetre waves, much higher data rates can be achieved. However, these radio signals do not propagate as far as for existing broadband frequencies, so they would be best suited for small cells.
The third concept relates to efficient management of radio spectrum with a large number of small cells operating. The first challenge in this scenario is the difficult management of vast number of small cells; operators are still debating whether this management would be central or distributed. The second problem is the interference that these small cells can cause which would limit their efficiency. One solution to these problems are the “phantom cells”. The principle of these is to use a separate higher power for the data link to coordinate the mobile users; this will simplify the system by spitting the control plane and the user plane (i.e., the data link).
Figure 3. Phantom cells power off-load
Integrating these three developments will result in a green radio access network by 2020; where the energy consumption will be reduced without compromising the high data rate demand. The use of new spectrum bands will also make possible the implementation of new ways to share bandwidth and power among the users (the so named “scheduling”), which will contribute to achieve the goals of 5G.
 Nokia Siemens, White Paper “Beyond 4G”, 2011
 IMEC, “Power Model for Wireless Base Stations”. http://www2.imec.be/be_en/research/wireless-communication/power-model-html.html
 DOCOMO, 5G White Paper “5G Radio Access; Requirements, Concepts and Technologies”, July 2014.