It's called white space' spectrum because white space is what appears on a spectrum map where guard bands have been inserted between analogue TV channels, or where spectrum is unused in specific regions for all sorts of reasons.

The good news is that the unused spectrum is down in the 50 to 700 MHz zone, so signals at that frequency are good at going through walls; the bad news is that the spectrum is scattered all over both spectrum and the geographical maps: a particular free block may only be available one or two specific areas.

Making use of it therefore means re-thinking the whole approach to licensing and commercialising radio services - in particular the allocation of those little chunks of spectrum to particular networks or even uses, must become more dynamic and flexible.

So a key component in any white space allocation will be the database-based spectrum sharing mechanism.

Several companies, including Google, Nominet, Fairspectrum and Spectrum Bridge are known to be keen to test their spectrum-sharing solutions as part of the just-announced Ofcom trial.

The radio technology itself is already well advanced. One of the lead technology participants, Neul, has developed a white space chip in preparation for mass market adoption (see - White space pilot announced. Hopes high for 2014 service launch).

The technology companies taking part in the Ofcom trial include Google, BT and Microsoft, but there are also about 20 other organisations pitching in to test applications like smart city sensors (traffic and transport system monitoring). That one is being undertaken by BT and Neul, while Microsoft is testing a WiFi application in the Scottish city of Glasgow (which is very low on broadband uptake).

Also important, and oft-mentioned with regard to white space applications, is machine-to-machine communications which is thought to be tailor-made for the approach. Click4internet is gong to test white spaces in rural broadband applications.

The UK regulator has made it clear that it thinks the concept of spectrum sharing could and should be applied more broadly - that is, beyond just the current white space opportunity. Regulators see spectrum sharing (rather than auctioning) as potentially a better way to get much-need competition into the wireless market. So white space will be an overall tester for the approach.

It's interesting that BT (which almost alone amongst European incumbents, doesn't have a mobile arm) is active in the trials and can clearly see some opportunities, while the UK mobile operators appear to be absent.