Where do the really big gains in wireless capacity come from?

There are many on-going initiatives seeking to gain additional wireless capacity. In the US much effort is being expended into the “white space” activity to gain access to under-used TV broadcast spectrum while in the UK Governmental bodies are working hard to enable sharing or rental of some of their spectrum. TV digital switch-over is consuming much regulatory effort in order to free up UHF spectrum and so on.

 

It is interesting to do some quick “rule of thumb” analysis on these initiatives. As a starting point recall that the cellular operators in the UK currently have some 450MHz of spectrum available to them now adding up all the spectrum at 900MHz, 1800MHz, 2GHz and including TDD allocations, with another 190MHz being auctioned soon at 2.6GHz making around 650MHz. Licence-exempt or unlicensed usage has around 750MHz below 6GHz, although much of this is in the 5GHz band.

 

The white space work in the UK suggests that in most areas there is around 100MHz of spectrum available for cognitive access. The assumptions are that this will likely be licence-exempt usage hence adding around 14% to total allocations, albeit in a frequency range where the propagation is considered to be good. Government usage is typically between 40% and 50% of the total spectrum under 5GHz although much of their usage is shared so the true figure may be lower. In the extreme case of all Government usage ceasing this would double the spectrum available for civil applications, in practice a dramatic 20% reduction in Governmental use would return around 200MHz in the key 1-3GHz bands, around the same amount as is being auctioned at 2.6GHz (but the Governmental spectrum would likely not have the advantage of being harmonised across Europe for cellular applications). Digital switch-over is set to liberate around 100MHz of spectrum, of which perhaps 50MHz might be used for cellular applications, a mere 7% of the cellular spectrum that will be available by then.

 

Now come at this from a different angle. A useful “law” by Marty Cooper of Arraycom is that wireless voice traffic doubles every 30 months. Growth may no longer be occurring in voice, but it seems now to be happening dramatically in 3G wireless data. Extrapolations and laws are highly approximate, and operators will tend to control traffic growth to exploit the available capacity, but nevertheless over the next decade, if we see anything like the growth in usage of the previous one, we will need a 10-fold increase in capacity (Cooper’s law would suggest a 16-fold increase).

 

All of this gain is clearly not going to come from additional spectrum. Even in the best case the cellular operators might gain 190MHz at 2.6GHz, 200MHz from Governmental use and 50MHz from digital switchover making 450MHz (while licence-exempt usage might gain another 100-200MHz). This would double their current allocation, which would be welcome and worthwhile, but still far short of an order of magnitude increase. History shows us that the big increases in capacity come from ever smaller cells, and with femtocells and Wi-Fi hotspots becoming increasingly available it is clear that the technology already exists to enable an order of magnitude increase in “base station” numbers. The biggest constraint, though, on small cells is typically finding a low-cost but high capacity backhaul mechanism. That is why the most important advance for wireless is not more spectrum but “more wires”, or ideally “fibres”. A widespread roll-out of fibre deep into the network would dramatically improve backhaul availability enabling the deployment of countless micro, pico and femtocells.

So perhaps all those manufacturers, operators and other stakeholders working hard on initiatives such as white space might reconsider whether they could better invest their time and resources in speeding a widespread fibre deployment? Not only would this improve home broadband speeds, it would enable much greater wireless speed and capacity.