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Prioritising energy efficiency over spectrum efficiency

By Simon Tonks, wireless technology expert


There has been a focus on energy efficiency in sectors such as transport and domestic use for many years, and more recently this has expanded to cover ICT systems. PA’s analysis of mobile networks reveals that whilst technical solutions to improve energy efficiency already exist, financial incentives work against them.

As with most businesses, operators of telecom networks continually have to look at how to improve their offering at minimum cost. This occurs in both the short term when expanding an existing network and in the long term when considering the next generation of network to deploy.

Increasing the number of base stations or the amount of spectrum used is very costly. So there has understandably been a lot of focus on the third option – increasing the spectrum efficiency. Unfortunately there is a hidden penalty with this option. Higher spectrum efficiency tends to create lower energy efficiency.

Pursuit of good spectrum efficiency at the expense of energy efficiency is a poor trade-off. Energy usage is becoming an increasingly pressing issue for reasons including: climate change; security of supply; and ever increasing production costs.

By contrast, spectrum is like land – there is a finite supply but it can be re-used ad infinitum. In addition, the evolution of technology means that the amount of spectrum that can be used in an economically viable way keeps growing. Looking at the big picture, energy efficiency should be prioritised over spectrum efficiency. Spectrum can be re-used, energy cannot.

The energy efficient RAN – Small Cells, Big Bandwidth

Network capacity can be delivered with lower power consumption by using wider bandwidths and simpler modulation. This would involve re-writing the cellular standards and can only realistically be considered as a long-term option for the industry as a whole.

However in the shorter term individual operators can control their network design. Wide area coverage requires a number of base stations. With large cells, a small number of base stations achieve the necessary coverage by transmitting at high power. Smaller cells require more base stations for the same coverage but their much reduced powers mean that the overall network’s power consumption is lower. PA’s analysis shows that there is still a net reduction in power for networks many more times more dense that at present, even taking account of the increased number of backhaul links.

In addition, the fact that there are more base stations means that the load on each cell has decreased. This gives the additional benefit of reducing the bit rate in the same bandwidth, further reducing the power required. So if the energy efficiency and associated costs would benefit from a dense network of small cells, there must be a reason why it isn’t done currently.

Cost factors work against energy efficient networks at present

The cost of energy already gives a small incentive in the right direction. The introduction of carbon trading has increased the sensitivity of the business case to energy consumption, but not significantly so. The disincentives are the site costs and the spectrum cost, and these are far stronger. The extra capital cost of the sites alone would take more than 20 years to repay in energy savings. To that can be added financing cost and increases in other Opex. In an industry where a new generation of network is deployed approximately every 10 years that does not make commercial sense.

Regulators should prioritise energy efficiency over spectrum efficiency

Regulators are not in a position to have much impact on the site costs but they do determine the spectrum costs. The thrust of many telecoms regulators over the last decade has been to maximise use of the finite radio spectrum. Whilst it is still important not to waste spectrum, linking the spectrum cost to energy usage and not just to the bandwidth would give a much more positive incentive and would help to offset the investment needed to create a more efficient network.

If the dynamics of the spectrum marketplace can be tilted in this way then operators will have a greater incentive to find lower power means of providing their services. This in turn will be passed onto manufacturers and their representatives on the standards bodies. For years researchers have concentrated on maximising bit rate over a given channel. In future, given the right incentives from regulators, this emphasis should move towards minimising the energy per bit rather than maximising bits per Hz.

What is needed is a change of mindset from spectrum efficiency to energy efficiency and a change of financial incentives to encourage it.

Simon Tonks, Expert in wireless technology,  PA Consulting Group

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