Feature Article

September 08, 2009

Spectrum is a Non-Renewable Resource

At the introduction of the automobile, petroleum products were considered a near infinite resource. If you asked my father about the shortage of fresh water, he would have looked at you without comprehension. No one in today’s world would consider either oil or water to be resources that are not in need of conservation and careful management. In the same way, spectrum for wireless applications is a large, but non-renewable resource. This is important to keep in mind, both on the Radio Access Network (RAN) side as well as on the backhaul side.
 
The available spectrum is huge – don’t get me wrong. We have roughly 100 GHz available to us for access and backhaul with today’s technology, however not all spectrum is created equal. The lower frequency spectrum is the most desirable for the RAN due to its ability to penetrate walls and bend around corners, coupled with the ability to integrate the lower frequency radios into low cost CMOS devices for handsets and Customer Premises Equipment (CPE). The higher frequencies are best suited to the demanding bandwidth requirements of base station backhaul; these systems leverage highly directional antennas in order to achieve a significant degree of spectral re-use.
 
In all geographies, the RAN spectrum, which is auctioned off as an area license, is highly valued as it is seen to directly generate revenue by allowing subscribers to get access to the network. Recent spectrum auctions have generated billions of dollars for the governments who sold the spectrum. Backhaul spectrum, on the other hand, tends to be held by a central agency, such as the FCC, and licensed on a link by link basis at a much lower cost. This allows many different users to re-use the same spectrum for different links; the licensing body co-ordinates the process to ensure no interference occurs. The cost for these licenses depends on the scarcity of the resource.
 
In the U.S., where microwave has not been as widely used, spectrum is still perceived as plentiful and the license costs are low (less than $200 per year per link). In Europe, where microwave is used much more intensively, the backhaul spectrum costs are easily 10 times higher. This relative availability of spectrum is driving different behavior when it comes to the development of new spectrum assets. For example, U.S. regulations permit spectrum wasting, low efficiency (but high bandwidth) radios to operate in the 80 to 90 GHz range. In most European jurisdictions, these same frequency bands require more efficient radio technology to be used.
 
So, if we are to draw parallels between fossil fuels and spectrum as resources, what are the wireless equivalents of hybrid cars? This is the target of a variety of signal processing techniques, higher order modulation, compression, Cross Polarization Interference Cancellation (XPIC), software defined channel width and adaptive modulation. The main goal of all these techniques is to obtain the RF equivalent of an improvement in miles per gallon. In RF terms, it is Mbps per MHz. This figure of merit not only reduces the consumption of the non-renewal resource – spectrum – but it also reduces the capital cost since it also equates to more Mbps per radio, and enables the evolution of networks to higher capacity without the need to obtain new (and sometimes unavailable) spectrum resources.
 
Given that spectrum is a non-renewable resource, and that bandwidth only goes one way – up, the Mbps per MHz metric becomes as important as other metrics on power, capacity, cost and size. It would not be unreasonable to expect mandated minimums for spectral efficiency, just the same way that we have such requirements for fuel efficiencies in the auto industry. There are already some requirements in some frequency bands, but you can expect to see this tighten up and become more widely spread in the future. Conserving, managing and optimizing our non-renewal resources is not only fashionable, it makes good business sense. A new definition of Green for the wireless industry just might be spectral efficiency.

Dr. Alan Solheim, Vice President of Product Management at DragonWave, is author of MobilityTechzone�s The Middle Mile column. To read more of Alan’s articles, please visit his columnist page.

Edited by Erik Linask


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