UT Bram Nauta

Bram Nauta is a professor of IC design at the University of Twente.

19 November 2019

A long time ago, we already had wireless communication. Like we know from the western movies: people were making a small fire on a hilltop and with a piece of cloth, they shaped the stream of smoke in a sort of on-off-keying modulation.

Since we’re among nerds here anyway, let’s have a look at how power efficient this type of communication was. First, the harvesting of the energy we need: a decent fire has – say – 5 logs of wood, which burn well for half an hour. Extrapolated to one year, 24/7 communication requires 90,000 logs per year. With 500 logs per cubic meter of wood, this is 180 m3 per year. In a typical forest, wood grows with 7 m3 per 10,000 m2 per year, so we need 250,000 m2 of forest to keep this single fire burning.

Now the bitrate: my guess is that with a few smoke symbols and a bit of practice, one can send about 2 bits per second. This is 64 megabit/year.

My mobile subscription today gives me 20 gigabyte/month, which is 1920 gigabit/year. This is equivalent to 30,000 fires in parallel, requiring 7,500 km2 of forest to keep my communication going. And since communication to my ‘base station’ is two-way, we can safely say I need 15,000 km2 of forest.

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The earth has about 200,000,000 km2 of land, so in theory, there’s room for a maximum of only 13,000 people to communicate like me on this planet. So, I fully understand that the people making these kinds of calculations back then freaked out and searched for other ways of wireless communication.

We invented electrical energy, batteries, radio communication and cheap microchips and now billions of people can wirelessly communicate gigabits per day with simple handheld devices at very low cost. What progress! Yes, we want more! We want the Internet of Things, 5G, 6G, more bandwidth! We want to increase our bandwidth with at least a factor 100 in the next decade.

A conservative estimation says that all wireless communication today consumes about 1 percent of the total energy used on this planet. And given that power dissipation in wireless communication scales linearly with bandwidth, without additional measures, the 100 times 1 percent becomes a bit of a problem.

Communication roadmaps predict the migration to “free frequency space” at tens of gigahertz frequencies because that’s where the free bandwidth is. But at those frequencies, communication will be limited to line of sight and relatively short distances. Beamforming techniques to aim those radio beams require a lot of transmitters and receivers per terminal. The 5G terminals being developed at 28 GHz already have a serious cooling problem. Without cooling, they may catch fire! (And we don’t want to go back to smoke signals.)

But we actually may use a modern version of smoke signals: for certain IoT applications, for example, we may actually go back in time and ‘see’ the bits again. If we have a short distance and line of sight anyway, why not make an ultra-low-power monochrome display – like an e-book – tag, on which a type of QR code is visible representing the data. One central optical camera can then see a lot of these tags and receive the information in a massively parallel way at very low power.

Finally, we should re-think radio communication completely. We’re still sending too much radio power, too long, to places where it’s not being used, and that even harms other users. I’m sure we can come up with other ideas if we think energy-centric. We might go back to our old-school IR remote control technology or even throw USB sticks to each other. But anything is better than smoke signals.