White Space Can Meet Mobile Demand

What is TV white space spectrum?

Some of the most abundant and valuable white space spectrum comes from geographic co-channel planning of terrestrial broadcast television. In the UK, each television channel uses 8MHz of spectrum and the empty channels in a given location soon add up to large swathes of prime spectrum that could be put to good use. Today, some of this spectrum is used in the program-making and special events industry (PMSE) for low power wireless microphones on a licensed basis; but this industry tends to use only a fraction of the available spectrum.

Why all the fuss about TV white space spectrum? Firstly, not all spectrum is created equal. While lower frequencies have excellent propagation characteristics, their ability to carry large amounts of information is limited. Higher frequencies tend to have worse propagation characteristics at a given power level, but are able to carry larger amounts of information due to increased bandwidth.

Part of the natural sweet spot – where the bandwidth to carry large amounts of information is combined with good propagation characteristics – is precisely where the majority of worldwide TV transmissions occur.

Another characteristic of TV transmission is the relatively small number of primary TV transmission sites operating at very high power levels. This leads to coarse co-channel planning and large amounts of geographic white space spectrum.

Finally, the switch to digital TV is ensuring a more efficient use of the spectrum. Some of the liberated spectrum above 800MHz will be auctioned to operators of cellular mobile phone networks, while the remaining white space can be made available for unlicensed use. Led by announcements made by the FCC in 2010 and the innovative work by Ofcom, several other governments and regulators across the world are considering TV white space for unlicensed use.

The majority of radio frequency spectrum is licensed/auctioned by regulators for mobile phones, aviation, TV, defence, etc. Some spectrum is available for general unlicensed use, most notably the 100MHz of spectrum centred at 2.4GHz that is also shared with your microwave oven. The availability of this unlicensed spectrum has created ecosystems of valuable industries including those based on the Wi-Fi and Bluetooth standards.

There is a clear relationship between the availability of unlicensed spectrum and fast-paced innovation in wireless technology. Despite the unlicensed nature of the spectrum used in Wi-Fi, which is prone to interference from other users and technologies, Wi-Fi carries the dominant proportion of data transmitted wirelessly and uses some of the most advanced transmission schemes available. However, the spectrum at 2.4GHz has relatively poor propagation characteristics when compared to TV spectrum at the same power levels, which is why Wi-Fi only has an effective range of a few meters within a building.

What if..?

Imagine that there was spectrum available for unlicensed use that had far superior propagation characterises – capable of travelling several miles. The innovative applications that could make use of this spectrum are endless, and include connecting rural communities to the Internet; new augmented reality applications for emergency services; machine to machine communications including smart metering and healthcare; longer range Wi-Fi; smart city communications and so on. The value to industry and society would be far greater than could be realised by auctioning the spectrum off to specific companies.

But there is a catch. White space devices can’t interfere with primary users such TV receivers. This means using sophisticated cognitive radio techniques including the deployment of Internet-based geographic databases. These show white spaces in a defined location and provide essential transmission parameter limits such as power level.

Such databases are initially populated using computer modelling, by knowing the location of the primary user transmission sites, geographic terrain, propagation characteristics and general receiver characteristics of primary users.

This data will change dynamically, so white space devices must regularly check back. In the future, sophisticated feedback and learning mechanisms could be built-in and ask white space devices to report which channel(s) they have chosen, and for which applications.

There are white space trials running in several countries, but the biggest by far is in Cambridge, UK. It involves a consortium of international technology media companies, which are testing a range of applications. As a consortium member, TTP is primarily focused on rural broadband and innovative applications for emergency services and machine to machine communication.

In partnership with Neul, TPP has a 5.6km broadband link in rural Cambridgeshire which has already delivered speeds of 8Mbps using a single TV channel. We expect to exceed 12Mbps very soon. Compare this to wired ADSL broadband, that struggles to achieve 2Mbps at a 4km range.

The goal for rural broadband is to serve the unconnected and poorly connected cost-effectively at over 2Mbps. We estimate over 600,000 premises in the UK are poorly served by wired ADSL and could be efficiently connected using cheap hardware operating in unlicensed TV white space. Optical fibre connections to these premises are prohibitively expensive or impractical. Assuming that Ofcom gets the key legislation in place during the first half of 2012, we could expect deployment of an innovative array of white space systems and applications during 2013.

The key technical problems are now being solved, and the big challenge for governments and society is to change the way we manage and use spectrum on a global basis. Whilst the goal of true cognitive radio is likely to take longer than my lifetime, significant steps are being taken now that will help deal with the growing demand for instant access to data. By the time my daughter is my age, she will be able to search the Internet for this article on her cognitive wireless device and reminisce of the time when her dad still had a brain.

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