Compiled during a visit to Amble in Northumberland, edited on the A1 heading home, and dispatched to silicon.com via 3G from Norwich Airport two days later
I keep hearing presentations and reading reports and articles that paint a worrying picture of a world running short of radio spectrum. But in the back of my mind I have the recollection of being on the periphery of a spectrum study of Chicago by the Illinois Institute of Technology some 15 or so years ago that revealed a total spectrum occupancy of only 17 per cent spanning some 10GHz.
More recently a snapshot survey of Chicago in the range up to 3GHz has confirmed that nothing much has changed.
This is an interesting outcome as a much lower figure had been anticipated for such a radio-dense city. I therefore suspected that the UK would realise similarly surprising results and decided to conduct my own survey as I travelled through city, urban and country terrains.
Anyone working in the radio field will understand that such a task is not to be undertaken lightly and involves a considerable amount of equipment and time. For the purposes of my study I focused on the use of a single handheld scanner of limited span, and a really useful rule of thumb:
Such a rule comes about due to the distance signals can propagate and the actual usage in each grouping of frequencies. Roughly speaking radio signals in the 10kHz to 30MHz region can traverse the entire planet, while above 30MHz they are line-of-sight only.
By the time we get above 1GHz we see the impact of moisture in buildings and trees, and above 10GHz, rain, snow and fog can be problematic. Above 30GHz molecular resonances in the atmosphere start to show up as limiting features.
It’s not surprising that the signal space below 2GHz is very much coveted by mobile phone operators, broadcasters, public mobile radio users and the military. They get line of sight signal propagation with very little impact from weather conditions and other forms of signal deterioration experienced at higher frequencies.
So, for weeks I have been scanning 150kHz to 1GHz and counting the 25kHz channels containing some form of signal energy. In dense cities I found an occupancy rate maxing out in the region of only 30 per cent, while urban areas came in at about 25 per cent and rural around 20 per cent or much less.
We can therefore reasonably assume the worst to best occupancy in the range up to 1GHz to be 30 to 20 per cent. Using our rule of thumb, we see roughly the same figure for 1 to 10GHz, and again for 10 to 100GHz. So in this total signal space up to 100GHz there is an unused availability of 70 to 80 per cent.
Given our overriding interest in the signal space up to 30GHz we would see an even greater availability with and estimated occupancy of less than 15 per cent max.
Looking at any chart of spectrum allocations, you can easily be deceived into thinking that everything is spoken for and there is no slack whatsoever. I think that is exactly what most people do. The truth is, however, that not all allocations are fully utilised – and the vast majority are unused or partially empty.
Now comes another interesting feature – time! While broadcast stations, telemetry and data links tend to be transmitting all the time, that is certainly not the case for air traffic control, maritime, military, emergency services and public mobile radio users. Listening to any of these sources reveals extremely low utilisations – very often in the 10 per cent or lower region.
When I take my estimates of channel occupancy and apply the time factor, I see a dramatic reduction to less than 10 per cent in the range up to 1GHz. Satellite broadcasting combined with microwave radio point-to-point links tends to see a similar figure in the 1 to 10GHz region, but beyond that the utilisation falls even faster with estimates in the range of five per cent from 10 to 30GHz, and I can only guess what happens above that!
So, far from feeling depressed that we are running out of spectrum, I have confirmed my suspicion that we have capacity in abundance. And if we take into account our overriding need for more short range, and very short-range low power applications then I feel really optimistic.
I have, for a long time, harboured a suspicion that a lot of people engaged in the spectrum debate have an always on, point-to-point, long range view of the application space. But this is the wrong direction! Even mobile phones are now being serviced by home hubs on broadband pipes powered by wi-fi or 2.5/3G home repeaters over distances of 10 metres or less.
The slicing and dicing of applications in the wireless and mobile space isn’t going to go away or slow down, it will accelerate, and there are vast tracts of empty, or little used spectrum. Even without smart or cognitive radios I reckon we should have no problem accommodating everything we need for some considerable time to come.
But should we hit the end stop, then of course there are frequencies in the 200 to 300GHz range, plus of course the infra-red optical region. I just can’t see any real problems with growth in this area, other than human ones!