By Richard Walker
                          
                         

                                          
Richard Walker: The key technical challenges are now being
solved and the big challenge for governments and society is to
change the way we manage and use spectrum 
                            

                              
The UK regulator expects that white space technology could
start in 2013 on TV channels and it is looking at white spaces
in FM radio bands 
                                      
                             
Technology innovators and regulators around the world are looking at the possibilities of adopting unused spectrum to meet growing demands for wireless internet access.
Imagine a world where wireless devices have unrestricted and free access to any radio spectrum; where we can maximise use of spectrum but ensure that safety critical services receive priority.
This goal of cognitive radio may seem a dream; but there have already been steps towards it as industry and forward-thinking regulators such as the FCC in the US and the UK’s Ofcom focus on exploiting the white spaces that exist between licensed transmissions.
There is a growing appetite for data and we increasingly expect to be able to instantly access enormous amounts of content from anywhere, anytime. The challenge for politicians, regulators and the industry is to ensure more effective and efficient use of spectrum to deliver it. 
                             
                             
The spectrum crunch 
                             
Some think that by 2020 there will be 50 billion connected devices worldwide. Demand for streaming video is predicted to multiply by over twenty fold in the next five years and 24 hours of YouTube video are already uploaded every 60 seconds. These are just a few statistics that highlight the pressure on spectrum.
Demand is starting to outpace technology and spectrum availability. Modern wireless systems such as LTE and wifi use modulation and transmission schemes that are reaching the limits of what is theoretically possible under Claude 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. Useable 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 pressures 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 signals 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 geographic locations through co-channel planning. The goal is to maximise the re-use of spectrum while also minimising both interference from neighbour co-channel transmission sites and overall system cost.
All of these spaces exist to minimise interference but are wasteful. It’s 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. 
                             
                             
What is TV white space spectrum? 
                             
White space spectrum is available globally and some of the most abundant and valuable white space comes from geographic co-channel planning of terrestrial broadcast television.
In the Americas, as well as Japan, South Korea, Taiwan and the Philippines, each television channel uses 6 megahertz of spectrum. In Europe, Africa and most countries in Asia it is 8 megahertz.
But in both cases the empty channels in any given location soon add up to large swaths of prime spectrum that could be put to good use. A fraction of the spectrum is licensed for low power wireless microphone.
Why all the fuss about TV white space spectrum? Firstly, not all spectrum is 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 channels is the relatively small number of primary transmission sites operating at very high power levels. This leaves large amounts of geographic white space spectrum.
Finally, the switch from analogue to digital TV is ensuring a more efficient use of the spectrum. In many countries, some of the liberated spectrum above 800 megahertz will be auctioned to operators of mobile phone networks for LTE, while the remaining white space can be made available for unlicensed use.
Most spectrum is licensed by regulators for mobile phones, aviation, TV, defence and other purposes. Some is available for general unlicensed use, most notably the 100 megahertz around 2.4 gigahertz — where it is shared with your microwave oven — for wifi and Bluetooth.
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, wifi carries the dominant proportion of data transmitted wirelessly and uses some of the most advanced transmission schemes available.
However, the spectrum at 2.4 gigahertz has relatively poor propagation characteristics when compared to TV spectrum at the same power levels, which is why wifi has an effective range of only a few metres within buildings.
Imagine if spectrum was 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 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 wifi; smart city communications and so on.
The value to industry and society would be far greater than could be realised by auctioning the spectrum to specific companies.
But there is a catch. White space devices can’t interfere with primary users such TV services. This means using sophisticated cognitive radio techniques including the deployment of internet-based geographic databases. These show white spaces in a defined geographic location and provide essential transmission parameter limits such as power level.
Such databases are initially populated using computer modelling from 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 will require devices to report which channels they have chosen, for which application and the signal levels of surrounding white space channels.
There are trials in several countries but by far the biggest is in Cambridge in the UK, involving 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. In partnership with Neul, TTP has a 5.5 kilometre rural broadband link and is on track to achieve over 6 megabits a second using a single TV channel — an impressive achievement considering that wired ADSL broadband struggles to achieve 2 megabits at a 4 kilometre range.
The goal for rural broadband is to serve the unconnected and poorly connected cost-effectively at over 2 megabits. Many homes that are poorly served by wired ADSL could be efficiently connected using cheap hardware operating in unlicensed TV white space. Optical fibre connections to these premises are prohibitively expensive or impractical.
The key technical challenges are now being solved and the big challenge for governments and society is to change the way we manage and use spectrum.
The white space wave started in the US, led by announcements made by the FCC in 2010 that already allows the use of white space devices. The FCC requires providers to simply complete a 45-day trial before going into service.
Canada has also started a consultation process to develop a policy and technical framework for the use of applications in the TV broadcasting bands below 698 megahertz.
In the UK, Ofcom is at the forefront of white space innovation by freeing up TV spectrum as part of the digital dividend. Ofcom also recently announced that it would make white space devices licence-exempt and allow third-party providers to develop white space databases.
But it is important that the rest of Europe gets on the white space wave. Finland and Sweden are already involved in studies and trials, but Germany and France need also to embrace white space opportunities. Ofcom and other bodies are working hard to promote a unified approach in Europe.
In Asia, South Korea, Singapore and China are likely to be looking closely at the use of unlicensed white space spectrum. Singapore for example, recently published a framework document for trials to explore the various spectrum environments and regimes that white space technology could operate in.
Ofcom expects that white space technology could be launched in the UK as early as 2013 and is already considering the future use of other white spaces such as those in the band used by FM radio services.
The goal of true cognitive radio is likely to take a little longer, these significant steps happening play an important part in dealing with the growing demand for instant access to data. GTB
                             
Richard Walker is head of wireless at TTP, an independent technology development company
www.ttp.com