White Space Can Meet Mobile Demand

Imagine a world where wireless devices have unrestricted and free access to any radio spectrum; where we can maximise utilisation of spectrum, but ensure that safety-critical services receive priority. This goal of cognitive radio may seem a dream; but steps toward it have already been made as the industry and forward thinking regulators, such as Ofcom, focus on exploiting the white spaces that exist between licensed transmissions.

My daughter is three, and already has a huge appetite for data. She uses an iPhone to watch her favourite television programmes streamed through the Internet. By the time she is in her teens, she will expect to be able to instantly access enormous amounts of data from anywhere, anytime. In fact, she will demand it. The challenge for politicians, regulators and industry is to ensure more effective and efficient use of spectrum to deliver it.

The spectrum crunch

By 2020, it is conservatively anticipated there will be 50 billion connected devices worldwide. Demand for streaming video is predicted to multiply twenty-fold in the next five years, and already, 24 hours of video are uploaded to YouTube every 60 seconds. These are just a few statistics that highlight the pressure on spectrum.

Demand is starting to outpace advances in technology and spectrum availability. Modern wireless systems such as LTE and Wi-Fi use modulation and transmission schemes that are reaching the limits of what is theoretically possible (Shannon’s Law) in terms of bits per second per Hertz. And while we can overcome this to some degree by using smaller, lower power cells and more effective data compression, these techniques have their limits too.

Usable and prime radio frequency spectrum is a scarce resource and yet, many licensed legacy systems that have exclusive access to large chunks of this spectrum are not making full and efficient use of it.

While the growth in demand for mobile data is the main pressure on spectrum, there are many other potential issues such as more effective and innovative data communications systems for our emergency services; robust wireless healthcare systems; and wireless systems that can help manage our electricity grids more effectively.

What is white space spectrum?

The words you are reading right now are separated by ‘white space’. Although these spaces contain no information, they ensure the words do not interfere with each other. Without these spaces, this article would become extremely difficult to read. But imagine that a machine readable message is written within these spaces in a very light grey colour that does not interfere with the readability of the article. Filling the spaces with more information makes more efficient use of the paper, and perhaps the used spaces between the words could now be called ‘grey space’.

Such spaces exist in radio spectrum for the same reason, and occur in the frequency, time and space domains. Radio signals are separated in the frequency domain by guard bands that help minimise interference from adjacent channels. Some systems coordinate the time separation of signals to prevent overlap at the receiver; other systems are used infrequently, such as specialised systems for defence.

Finally, transmissions are naturally separated in geographic space – with national networks reusing the same channels in different locations through co-channel planning. The goal is to maximise the re-use of spectrum while also minimising both interference from neighbouring co-channel transmission sites and overall system cost.

All of these spaces exist to minimise interference, but they are wasteful. It is technically possible to deploy low power transmissions inside these spaces without interfering with the primary users. The challenge is to convince licence holders and regulators to provide access on an unlicensed basis.