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Whitespace heats up

E&T looks into the unique properties of the whitespace radio frequency band may solve some frustrating problems seen with existing wireless devices.

Whitespace frequencies can cause political ructions - just ask former Prime Minister Gordon Brown. The wireless microphone which he unfortunately forgot to turn off in Rochdale last month was licensed to use these frequencies. They are so-called because they are the unused bits between the various digital and analogue terrestrial TV signals on the UHF (ultra high frequency) bands.

The new frequencies, recently authorised in the US by the Federal Communications Commission (FCC), greatly increase the overall wireless bandwidth available to computers, set-top boxes, laptops, Wi-Fi hot-spots and other radio devices that use the unlicensed band around 2.4GHz. But the primary incumbent users of these TV band frequencies are concerned about the potential for interference caused by the influx of new users.

'Whitespace devices have the potential to enable a vast range of new and innovative applications - from broadband access for rural communities, to innovative personal consumer applications - each benefiting from improved signal reliability, capacity and range offered by unused TV frequencies,' says Professor William Webb, head of research services at UK regulator Ofcom.

'However, this technology remains largely unproven and a significant amount of work needs to be done before claims can be tested,' adds Webb.

Needless to say, consumers do not particularly care about how their data arrives or departs. The existing TV band, centred round 600MHz, has excellent propagation characteristics, which is why broadcasters chose it in the first place. The signals can travel a long way and pass easily through walls.

Basically, a data transmitter using whitespace technology would be able to provide enough coverage to cover ten times the area that a typical 2.4GHz transmitter can. The upside is obvious, but the downside is that, because the information would be able to go much further, greater congestion could potentially be encountered.

Therefore, the new radios have to incorporate new technology to ensure that they do not interfere with existing TV broadcasts and wireless microphones. This will increase the cost of these devices when compared with existing consumer transmitters that operate in the unlicensed 2.4GHz spectrum band.

Spectral sensing

There are two methods for finding spectrum in the TV band. The first one, generally favoured by consumer tech companies, involves spectral sensing. This is a cognitive radio technique that uses a sensitive radio receiver to listen for transmissions on a particular channel. If the sensing algorithm discovers a signal is busy, it reports this.

It continually monitors for other users of the channel - even while using the spectrum itself. This is particularly useful because, although TV stations do not change frequency that often in a particular place, other devices such as wireless microphones can be turned on and off at any time and in any location.

Certainly, design engineering group Cambridge Consultants sees it this way. Last year it announced the development of InCognito, a low-cost spectral sensing cognitive radio technology platform that promises to allow any radio product to transmit without interference over whitespace frequencies.

'Based on highly complex cognitive 'spectral sensing' radio technology, which until now, has only been used in defense and security applications, the InCognito platform enables 'whitespace' radios to quickly and accurately detect and avoid other broadcasts,' says Luke D'arcy, head of cognitive radio at Cambridge Consultants.

But this might make the entire process of licensing spectrum frequencies to specified users obsolete (an important revenue earner for cash strapped governments). Taken to its logical conclusion, any user with a cognitive radio would be able to use any part of the spectrum at any time - effectively declaring all spectrum to be unlicensed, subject only to rules regarding interference-avoidance protocols. But this also assumes that all radios are smart and everyone plays by the same rules.

Database search

The other system, generally favoured by government regulators, involves some form of database lookup - where the radio determines its location, perhaps using GPS, and consults an online database. The database contains dynamically updated information about free channels in all locations of the region. It then communicates this to the radio allowing it to choose a free channel.

The problem with this system is that GPS will not work in all areas. In a shopping mall, an airport or a mainline train station, GPS is unlikely to work due to the lack of line-of-sight - even with the latest GPS chips. Additionally, you still need to access the database online before switching on the whitespace radio. Therefore, you will still need an alternative way of accessing the Internet. Thus the need to get rid of the old 2.4GHz transmitter frequency may not be possible - where a physical tether to the Internet is not viable.

The long range of whitespace frequencies should make it ideal for connecting wirelessly to the Internet. It could even mean that unlicensed networks could seriously rival cellular networks for coverage in towns and cities.

Currently, there are several advertising supported public access Wi-Fi networks in several cities around the world. These have, at best, been a qualified success in just a few areas. While spectrum cost in 2.4GHz is free, the need for hundreds (if not thousands) of transmitters due to low geographical reach means that site rental, backhaul and base station equipment is a significant cost.

Even so, trying to obtain a signal indoors is a very hit-and-miss affair without urban dwellers having to fit a high gain antenna to obtain a reliable indoor signal. At the 600MHz, signals will be able to travel through walls and go much further than your typical access router - with the possibility that Starbucks could compete with Vodafone in urban areas.

Home networking companies have been working for years to make the digital multimedia home a reality for wireless users. Use of whitespace spectrum should make it possible to reliably deliver high-quality video, audio, games and rich data anywhere throughout the home while extending the reach of Wi-Fi transmissions.

Consumers would be able to overcome environmental challenges such as interference from surrounding wireless devices and networks, or bandwidth contention from concurrent applications such as email downloads and Web surfing, to receive uninterrupted transmission of delay-sensitive media traffic in all parts of the home.

Existing 802.11n systems work by automatically and continuously steering wireless signals around sources of interference while intelligently managing all voice, video and data traffic to ensure the most reliable wireless transmissions possible.

Triple play

A growing number of service providers are bundling IP voice, video and data services - commonly referred to as 'triple play' - over a single broadband connection in order to cost-effectively reduce customer churn, slash deployment costs and increase the average revenue per user through innovative services such as IPTV. However, distributing IP multimedia traffic within the home has been a problem, requiring costly and cumbersome cabling between the broadband modem and the receiving device such as an IP set-top box or a media adapter.

While Wi-Fi is the preferred method for data distribution within the home, conventional technology has, until now, been unable to provide the reliable and quality transmission essential to support all three traffic types concurrently. Off-the-shelf Wi-Fi products only stream video over a short distance before it becomes unwatchable.

Yet there is opposition from the microphone vendors and users. In the UK and US, these devices use the same whitespace frequencies that the new devices intend to use. This has thrown up an alliance of 'luvvies' across the channel - such as Sir Andrew Lloyd Webber, Dolly Parton and Bono to oppose the opening of these frequencies.

The regulators and the industry are keen to allay the fears. Certainly, the manufacturers such as Sennheiser and Sony are keeping an open mind on developments. Ofcom are yet to report on their findings.

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