Gaps of unused spectrum, or ‘white space’, in the band normally used by TV have been identified as a possible solution to capacity demands on already crowded cellular networks. UK communications regulator Ofcom’s decision last month to allow unlicensed use of this TV ‘white space’ spectrum has opened the door for future wireless communications in a band where signals can span large distances and easily penetrate walls.
Analogue and digital television are broadcast in the band between 450 and 800MHz. Not all frequencies in this band are used in all parts of the UK, though every frequency is in use somewhere, leaving gaps which could be used for transmitting data for a variety of different applications. Ofcom anticipates the amount of white space available to be comparable to spectrum that is currently available for 3G services, with approximately 150MHz of bandwidth available in most locations. This TV white space could potentially be used as a cost-effective way to implement rural broadband provision, for capacity needed by M2M communications in the much-talked about “Internet of Things” and for additional services such as extending the range of WiFi.
However, unlicensed wireless
networks coexisting with users of a licensed spectrum band face a very important challenge – interference. Devices using white space must not interfere with TV transmissions, or wireless microphones which already use the TV band.
There are two cognitive techniques that hold potential for mitigating interference. The first is to use a managed system whereby a database of available spectrum by location is maintained, so that devices using white space can ask the database what spectrum they can use before transmitting. The second is for each white space device to listen and determine what parts of the spectrum are in use in its location.
Ofcom has decided on the database approach - third-party databases will list which frequencies are already in use by licensed users in each part of the country. Since TV transmitters are static and the channels they use are consistent for long periods of time, it should possible to maintain a database of this information.
According to Tim Fowler, Commercial Director of the Wireless division at Cambridge Consultants, it is widely understood that the database-only approach has some drawbacks, and it is expected that with time, device’s sensing capabilities will complement database information.
Left -Tim Fowler, Commercial Director, Cambridge Consultants
“In reality, whitespace is a misnomer, as the spectrum is not simply busy or free,” he said. “Greyspace might be a better term, as users both close and distant leave the spectrum with some residual noise or interference in it. Understanding this is essential in getting the best use of the spectrum.”
As a member of the Cambridge TV White Spaces Consortium, Cambridge Consultants trialled its white space network this summer, in one of the technology’s most anticipated applications: rural broadband provision. The TV band’s long range makes it a great solution for rural broadband, and the successful trial provided wireless internet access to the village of Cottenham, Cambridgeshire, from Cambridge Consultants’ HQ in Cambridge 6km away. The company sent the first tweet over white space on 28th June. Their network uses a combination of cognitive radio techniques - geolocation database information alongside spectral sensing technology - to avoid interference.
Right-The first white space tweet
Their spectral sensing platform, called InCognito, is a combination of novel cognitive radio hardware and software algorithms which the company believes could provide a suitable sensing capability to supplement the database approach. Cambridge Consultants has also developed its own database models to predict the availability of spectrum in any location.
“The word database is something of a simplification,” says Fowler. “While there is considerable data in the database, there are also models that allow prediction and extrapolation of known data to provide an understanding of usability in any location. Future sensing algorithms will help to refine the models in the “databases” and improve the quality with which spectrum can be “filled” maximising both the chance of having access to spectrum and not interfering with primary users.”
Fowler also pointed out that since cellular spectrum is licensed to an operator on a monopolistic basis, it’s their call whether they choose to address the rural broadband market. White space, being unlicensed, does not have this problem and could be shared by many different applications.
Another member of the Cambridge TV White Spaces Consortium has already gone to production with its white space radio system. Neul has launched NeulNET for white space networks, which includes a dual-antenna basestation unit and a battery powered terminal device. NeulNET can send data over a distance of up to 10km at a maximum data modulation rate of 16Mbps. This technology is designed primarily for the world of M2M communications, which has been testing the capacity of 3G and 4G networks for some time.
Luke D’Arcy, VP Marketing at Neul, explained that it’s not necessarily the small amounts of data that are sent in typical M2M communications, but rather the number of devices that want to be connected to the network that cause the capacity problems.
Left-Luke D'Arcy, Neul's Marketing VP
“M2M connections are widely projected to reach 100 billion by 2020, with a progressive ramp between now and that date - in other words, massive growth over the next decade,” he said. “This order of magnitude increase in capacity requirements simply cannot be accommodated by traditional cellular technologies.”
The number of terminals that can be connected to a cellular base station at any given time is in the order of 1000, causing growing problems when every previously taciturn electronic device from electricity meters to fridges wants to connect.
D’Arcy gave several other reasons that the cellular network is unsuitable for M2M communications. For starters, cellular networks are designed for voice, requiring sophisticated modems to handle voice’s bandwidth requirements. This is expensive technology - several tens of dollars per modem. M2M applications do not typically require the same bandwidth and so a modem designed specifically for M2M, like Neul’s, can be considerably less complex and expensive to design and manufacture. Also, white space spectrum is not subject to a license fee.
“This does not only make M2M services lower cost, it actually facilitates M2M applications that would not otherwise be commercially feasible and so opens up entirely new markets and will drive M2M growth,” D’Arcy says. So how does NeulNET avoid interference?
“Neul’s radio leaks virtually no power into adjacent channels,” D’Arcy says. “This ensures that it meets the FCC/Ofcom spec, is able to use more of the available whitespace spectrum, and eliminates the possibility of interference to existing licensed users.”
In fact, NeulNET’s transmitter has been specifically designed to concentrate virtually all energy in a single white space channel (see figure 1). NeulNET achieves an ACP figure of better than -55dBc by using a single carrier modulation system; it can be tightly filtered without losing signal integrity. It also results in a transmit signal that has a low peak to average power ratio allowing the system power amplifier to operate entirely in its linear range, greatly reducing spectral splatter.
The NeulNET receiver also uses a MIMO technique to reduce static cochannel interference from TV transmitters by more than 20dB, plus frequency hopping across all of the available white space channels to ensure reliable transmission without interference.
Emerging white space technology offers so many benefits that it is difficult to imagine the future of wireless communications without it. Certainly, some sort of solution is needed to deal with spectrum shortages, and the cognitive techniques TV white space networks will have to employ could be a model for future wireless networks outside the TV band. Ofcom has said it is investigating potential white spaces in the FM spectrum, and that it predicts widespread use of the TV white space in the UK by 2013.
Tim Fowler compared white space to the 2.4GHz ISM band – when it was opened in 1985 it was considered useless spectrum but today has WiFi, Bluetooth, ZigBee and many more wireless communications technologies operating within it. These are multi-billion dollar industries and enable a wide array of services that we now take for granted.
“If the last 25 years of evolution in the 2.4GHz band teaches us anything, it is that we probably don’t know what will be thriving 25 years from now in the TV white space band,” he said. “But there is potentially much more spectrum available in the TV white space band than is available in the 2.4GHz band, and it has some very favourable characteristics such as improved range in indoor performance. It looks on that basis that the band has a very favourable future.”