Energy Consumption of Femtocells – are they Green?

With growing awareness of climate change, increasing energy prices and close scrutiny of operating costs, there is good reason to consider whether femtocells would reduce overall power consumption or not and add to our carbon footprint.

From the operator’s viewpoint, offloading capacity from the outdoor cellsites will obviously save costs. Reductions in numbers of cellsites, equipment required at cellsites, transmission equipment to connect back through to their core network, all of which require additional energy. Bearing in mind that a typical outdoor cellsite costs around US $4000 per year in energy costs, this can add up. Let’s say a typical cellsite serves 1000 subscribers, this works out at about $4 per subscriber per year.

From the customer’s viewpoint, the femtocell is likely to be powered on 24x7 – whilst it is possible to turn it off overnight, or even when away from home (work or pleasure), it’s much more likely that the equipment would be kept on, ready to make or receive a call at any time of day. As an independent unit, typical consumption is likely to be in the range of a few watts – much less than even a low powered light bulb. Assuming a total consumption of 5W, this would equate to 43.8kWh over a year (5x24x365/1000) – which at today’s prices of US$0.20 per kWh is about $8 per annum. If we estimate that the average femtocell will serve around 2 subscribers, then this works out in the same ballpark $4 per subscriber figure as the macrocellular alternative.

Where femtocells are incorporated into DSL/Broadband modems, the above calculation may be too generous. In the longer term, as further chip development and engineering techniques are applied, the power consumption may be reduced further.

Indeed, several factors can offset the additional energy used:

• If it replaces several cordless phones, which are often individually plugged into their own mains chargers.
• If the femtocell circuitry is added into an existing DSL or cable broadband modem, it is only adding a small additional power load.
• Since phones using the femtocell operate at much lower power levels, and thus do not require such frequent recharging.
• The growing interest in self-generated power, such as from wind turbines or photo-voltaic cells, could enable domestic users to ensure green energy was used to power their femtocells.

Lastly, if the price and convenience of using services via the femtocell is so much better, it could encourage greater use of the equipment. At such low power levels, and with little difference in the power consumption of the femtocell when handling a call compared to idle, the effects can probably be ruled out of the equation.

Thinking a little more "out of the box" - if a femtocell avoids just one car journey per year by encouraging staff to work from home more, then the overall carbon footprint would easily be paid back. Given the rising price of oil, home working is likely to become more popular, justifying this enabling technology.

Thus, we would argue that the overall ongoing effects of femtocell deployment in power consumption is likely to be neutral.

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