Sunday, 25 November 2012

A white spectrum to you!

I've been thinking about spectrum this week. Not rainbow colours, and (for once) not even that particular part of light that lies just beyond what we can see and that is called infrared.

My thoughts started with 2.4 gigahertz (or as engineers and regulators sometimes refer to it 2400 megahertz). Apparently, if eight or more people using Wi-Fi on their laptops, phones or other devices should happen to come together on a train on the Shenzen Metro ... the train will stop. This is because this particular frequency band is used by the signalling system.

Unfortunately it's also used by a lot of other things as this band is unregulated ... everywhere. Microwave ovens use it (because water absorbs energy at this frequency and so will heat up), wi-fi uses it and in some parts of the world wireless phones in the home (as opposed to cellular phones) use it ... amongst other things. This freedom of use around the world makes for economies of scale and brings down the prices of kit.

I am surprised that an unregulated band is used for railway signalling. Apparently it's to save time, with the Caijing web site saying line development is faster in China than elsewhere but that the price is insufficient testing. There is also a difficult regulatory environment.

It reminds me of a BBC training course I was on a long time ago where we were told not to use portable tape recorders on the London Underground Victoria Line as they interfered with the trains!

Such stories as this should be borne in mind as engineers and regulators in the UK move towards using so-called white space, which in this case means frequencies that are currently reserved for television but actually have substantial geographical gaps where they are not used. This 'extra' spectrum can be used to extend existing services, such as broadband in rural areas, and provide new ones. The first stage of a consultation, on white space devices, has been published by Ofcom and The Register has a good summary. White space frequencies are different to those used by Wi-Fi and the Shenzen Metro, so they behave slightly differently, especially in how they pass through walls. They are also not the TV frequencies that are being cleared for next-generation mobile phones.

White space devices will need to be careful about which frequencies they try to use, since what is free in one part of the country is carrying BBC One in another. So there will be several databases set up that the devices will have to consult before they start transmitting. Despite there being only one spectrum, Ofcom want several databases; a conundrum devised in the name of competition.

If you would really like to know more about spectrum, and how it is allocated, I suggest you take a deep breath and look at Ofcom's allocation chart. There you will find that '500 kHz is an international distress and calling frequency for Morse radiotelegraphy', as it was when the Titanic went down a century ago, that 2400 megahertz is for 'Spread Spectrum devices, including Radio LANs', and that between 100 and 105 gigahertz lies spectrum reserved for 'Passive research carried out in some countries for intentional emissions of extraterrestrial origin'.

Now there must be some service opportunities in that.