The search for spectrum
Peter Cochrane's Blog | silicon.com | Monday 10 October 2005
All the free spectrum we need for a wireless world is there to go get!
Written at Chatham House, London. Pics compiled on the road between Zurich and Southampton. Copy dispatched via Wi-Fi from a London coffee shop.
Every time I look at the allocation of radio spectrum channels I go to a brightly coloured wall chart (see Fig 1 below) that gives the impression that it is a resource of great scarcity.
However, every time I look at the spectrum using an analyser and antenna I find a desert, just a few signals and a lot of random noise. The reality is that the radio spectrum has, at a modest estimate, a utilisation of less than 10 per cent. What is happening?
Unfortunately, analogue systems and analogue thinking still rules! Dumb transmitters pump out high power AM, FM and PM signals, in many different guises. Then equally dumb receivers listen, plucking out a fraction of the energy from the ether. This is the world I grew up in as a young engineer, where interference, multi-path, scatter and signal-to-noise ratio dominated all design and operational considerations. This automatically leads you to a world of bands and channels - a carving up of the spectrum for individual applications and uses.
But we have a new raft of digital systems being rolled out that have intelligence built into the transmitter and receiver. They talk to each other to minimise the energy used and problems associated with interference and signal scatter. Also, their signal processing aids consistent performance by adjusting the modulation mode. So in contrast to the analogue world, we can reuse time, frequency and space like never before. This gives us a whole raft of new freedoms.
We have already adapted our work patterns to sit in a room full of people with laptops using Wi-Fi, or mobile phones, all operating interference-free. So none of this should come as a surprise but the old analogue history still seems to dog our progress. What we do today was unthinkable 20 years ago when interference was still a problem with the analogue technologies that dominated.
So what happens next? We have hundreds of radio and TV broadcast transmitter masts surpassed by the thousands installed for mobile networks, which in turn are eclipsed by the millions of mobile terminals (see Fig 2 below).
Soon we will see billions of RFID tags and sensors also configured into wireless networks. Will interference be a big problem? No! Will spectrum be limited? No! So what will be the big deal? Migrating from the old to the new - as ever!
We have to find a way of using those bands and channels that are dormant or hoarded, and totally wasted. It could be hard to work out the details but I don't see why unused broadcast radio and TV channels cannot be used for WLAN applications. Or indeed, how about all the channels held in reserve by the military? All it needs is a change of mindset - the technology will do the rest.