Increase in mobile traffic requires higher energy efficiency in networks

When the 4G technology for wireless communication was developed, not many people thought about how much energy is consumed in transmitting bits of information backwards and forwards between mobile devices and wireless masts. This means that a stand-by mode was not built into the 4G technology, and almost the same amount of energy is used, independently of whether the network is in use or not. The base stations frequently transmit signals to state their presence and identity, even if there is not a single mobile unit close by.

“When the 4G network was built, the operators wanted to be certain that the technology would work in all conditions, and thus they included both belt and braces. We now know that this is not necessary”, says Emil Björnson, associate professor of communication systems at LiU.

They took a holistic approach

When it was subsequently time for the 5G technology to be developed, this was one of the first problems the engineers dealt with. The 5G networks consume little energy when the traffic is low. Emil Björnson and some research colleagues, however, started to wonder whether it was also possible to make the networks efficient when the traffic is high, with many simultaneous users.

Massive MIMO involves using hundreds of small antennas, connected together, that gives a strong signal at the intended receiver and only a weak disturbance at all the others. Photo credit Magnus Johansson“We were looking for a holistic approach. Rather than optimising each part individually, we considered the hardware in the masts, the circuits, how the signal processing is done, the number of simultaneous users, how closely the masts are spaced, etc., and then we used mathematical modelling to optimise everything as a single system.”

The calculations showed that a new antenna technology known as “massive MIMO” is one important part of the puzzle of reducing energy consumption per transmitted bit of information. Massive MIMO is an antenna technology that LiU researchers were among the first to study, and they have built up world-leading expertise.

The result has been published in a scientific article that has received hundreds of citations. Emil Björnson and his co-authors last year also received the IEEE Marconi Prize Paper Award in Wireless Communications. This is the most prestigious award that an article can receive within wireless communication.

Open source

“We have also made simulation code available as open source, and this has been very positively received. It has been a problem in our field that everyone has made their own assumptions, and thus it is impossible to compare the different results. Publishing the simulation source code has caused a shift in attitudes in the field, from competitiveness to a desire to help each other make rapid progress. It also leads to more citations, and I’m convinced that open source publishing is advantageous for both the research field and for me as a researcher”, says Emil Björnson.

A three-year research grant from the Swedish Foundation for Strategic Research has enabled Emil Björnson to take on the problem of determining the theoretical limit for how energy-efficient a wireless communication system can become.
“We are far below the theoretical limit today, but if you consider what can be achieved in practice, with acceptable aesthetic properties, we are much closer. The 5G network may be a thousand times more efficient than 4G, and one contribution to this will be the use of massive MIMO”, says Emil Björnson.

5G a marketing issue

He points out, however, that the commercial introduction of 5G technology and telephones that use it is a marketing issue. The technology currently in use is already called “5G” in certain countries.

“We may see 5G telephones towards the end of the year, but it won’t be a dramatic new launch, rather a gradual development. The volume of traffic in the networks increases by 30-40% per year, and there is no reason for operators to replace current technology as long as it’s working satisfactorily.”

The massive MIMO technology is now being tested at several locations worldwide, such as during the World Cup in football in Russia last summer, where many users were concentrated into a relatively small area.

“But here there is also a lot that remains to be done. The software is rather rudimentary, and researchers working in the academic world still have a lot to contribute”, Emil Björnson concludes.

MIMO

is an acronym for “Multiple Input, Multiple Output” and the technology involves using hundreds of small antennas, each with a power of around 10 mW, connected together. All the antennas send a few tens of signals with carefully determined delays. The delays are chosen so that the copies of a signal arrive at the intended receivers at exactly the same instant, but at slightly different times at all other receivers. This gives a strong signal at the intended receiver and only a weak disturbance at all the others. Both Emil Björnson and Professor Erik G Larsson have written books about the technology.

Emil Björnson, Luca Sanguinetti, Jakob Hoydis, and Mérouane Debbah “Optimal Design of Energy-Efficient Multi-User MIMO Systems: Is Massive MIMO the Answer?”, IEEE Transactions on Wireless Communications, Vol. 14, No. 6, pp. 3059-3075, June 2015. DOI: 10.1109/TWC.2015.2400437

Emil Björnson, Erik G. Larsson, “How energy-efficient can a wireless communication system become?”, Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, USA, October 2018.

Emil Björnson and his co-authors received the IEEE Marconi Prize Paper Award in Wireless Communications.