Cambridge gets a white (space) wash
Industry gangs up to prove licence-free radio
A consortium of Cambridge-based companies is to start testing white space radios in the UK, to see if they can coexist with each other, and everyone else.
The trials are permitted under a multi-site test licence issued by Ofcom, and will allow the likes of Microsoft, BT, Nokia, Samsung, Spectrum Bridge and BSkyB to set up radio links using TV channels that are locally empty.
The idea is to demonstrate how amazing such a technology is in the hope of getting Ofcom to approve national, and licence-free, use of the white spaces.
Microsoft et al would like us to think that using white space is a risk-free resource only limited by the regulator's imagination, and that the white space model of cognitive radios jumping between empty frequencies should be applied to the rest of the radio spectrum as quickly as possible, as shown in the company's promotional video.
But radios designed to find and use empty frequencies simply don't work, so white space proponents have been forced to use online databases of empty channels based on the location of the user. That means every white space access point will have to have GPS or similar built in, and be able to see the sky too; they'll also have to periodically check back with the database to see if things have changed.
The approach is fraught with technical difficulties, not least the way that radio propagation can change enormously depending on atmospheric conditions. There's also the matter of transmissions bleeding into neighbouring frequencies, which can be mitigated with good filters and clever radios, but not eliminated.
Large margins of error are supposed to avoid knocking out television transmissions, but avoiding interference with other white space users will be more difficult. Proponents of the technology love to compare white space radio with Wi-Fi, and Wi-Fi has certainly proved more robust than anticipated in sharing radio channels with nearby users - both users see a drop in speed, but are prepared to tolerate it.
And that's what the Cambridge trials are all about: to see if the radios can cope with interference generated by other white space users, as well as making sure the online database can provide accurate and timely information about the frequencies available.
Once that's proven then Ofcom can start wrestling with the question of who gets to run the database. In the USA the FCC has been trying to get a handle on that question for a year or so now, while the shortlisted companies continue flinging mud at each other: we can only hope the UK process won't be quite so pugnacious, assuming the trials prove successful. ®
The problem is that simply because you can't hear signals on a given frequency doesn't mean that you can use it without problems.
One such situation is the 'hidden transmitter" problem. Consider a transmitter "A" on one side of a hill, being received by someone on top of that hill, or off to one side of the hill. At a site "B" on the opposite side of the hill the frequency will seem to be clear, since the hill blocks the signal from "A", but anyone broadcasting on that frequency from site "B" will interfere with the signal being received from site A, since the receiver can 'see' both transmitters.
It is less of a problem for Bluetooth because it's very short range.
Not only that ...
>> The problem is that simply because you can't hear signals on a given frequency doesn't mean that you can use it without problems.
Apart from the hidden transmitter problem, you also have the issue of widely differing signal strengths. Consider someone far from the TV transmitter and trying to get a signal on a portable set. The signal is lo win strength, and he is likely to be using an antenna that has poor directionality (or is even omnidirectional). Then you fire up a bit of wireless kit that can't detect the TV signal and starts spewing forth on the same channel - due to differences in distance, it's going to completely swamp the TV signal.
As an analogy, it would be like going to the Opera (or whatever your favoured entertainment is - but not the megawatt blast your eardrums out gigs) and finding yourself at the back where it can be hard to hear the quiet bits. Then someone sat next to you starts SHOUTING DOWN THEIR MOBILE - "YES, I'M AT THE OPERA, CAN'T HEAR A THING THOUGH".
And as for the comparison with WiFi and Bluetooth, words almost fail me ! Both WiFi and Bluetooth are designed on the basis that the space is crowded and all parties can hop around and/or delay their transmissions so that everyone get get a word (or packet) in edgeways. TV isn't. You fire up something else on the same frequency and the TV transmitter doesn't suddenly hop to another channel - no, the receiver just stops getting a usable signal.
And for the first poster asking for evidence that "it doesn't work". IIRC it was reported here by TheReg that trials in the states were quite spectacular in their demonstration of that.
I have observed it work (as a commercial experiment) - external technical auditor for potential investor - and I think I understand how it does and also how it could be improved. The blanket "it dont work guv" is not good enuff.