Can White Spaces Save Wireless?

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As most you probably know, there is a general consensus within the wireless industry that there is a spectrum crisis.  The problem is that there is not enough spectrum available for future expansion of the cellular and other wireless businesses.  That is why the mobile carriers are always jockeying for business deals to acquire more spectrum.  AT&T, Verizon, and others pay billions to get new spectrum in FCC auctions or in the acquisition of rival companies.  The recent acquisition of MetroPCS by T- Mobile is a good example. This is a very real crisis and it will have a devastating effect on the wireless business in the years to come if some solution is not found.

There are some who say that there is no spectrum crisis.  These naysayers believe that the cellular carriers are just imagining the spectrum shortage to boost their own fortunes.  While this is probably true, it is also true that the carrier’s engineers are still struggling with how to allocate their splotchy spectrum holdings to implement the best LTE system in response to the growing smartphone bandwidth demand.  That is not easy to do in FDD systems with some spectrum in the 700 MHz range and another chunk in the 2.6 GHz range for example.  Think about it.  Most big network operators probably do not have enough spectrum for what they really want to do.

As it turns out, there is actually lots of spectrum.  The problem is that most of it is not suitable for use in today’s cellular business, broadband access and other wireless businesses.  The best spectrum is in the VHF, UHF and low microwave ranges, roughly 400 MHz to 4 GHz.  It is a line of site (LOS) spectrum where the wavelengths are short and so are the antennas.  That spectrum block is almost totally full and assigned.  Spectrum is like land, there is only a finite amount of it and when we use it up, there is no more.   So what is the solution? 

One useful idea that emerged in recent years is to implement a spectrum sharing approach where spectrum can be repurposed or used by multiple services.  The big example of this is to deploy the unused TV channels assigned broadcasters for wireless data services like broadband Internet access or backhaul.  As it turns out there are a good many unused TV channels.  These channels vary from location to location but in total make up a huge chunk of useful spectrum, especially the channels from 470 to 698 MHz.

These unused 6 MHz channels are referred to as television white space (TVWS) or just white space.  In 2009, the FCC approved the white space idea and issued some regulations for implementation and usage.   A small white space industry is slowly emerging.  The most likely uses will be wireless broadband for underserved rural areas, Wi-Fi hot spot backhaul, and remote monitoring and control. 

The real beauty of these lower frequencies (470-698 MHz) is that the transmission range is huge and can be up to 60 miles or so depending on terrain and other factors.  Wi-Fi and most other wireless technologies cannot do that.  Signals in that frequency range are also capable of penetrating obstacles like some buildings and trees.

The white space equipment is unique in that it uses cognitive radio techniques to avoid interference.  It uses frequency-agile tuners and dynamic spectrum access (DSA) methods to find a piece of spectrum it can use.  It listens to the assigned channels and the selects one based on its lack of activity.  It also relies on a national data base that knows the location and frequency of all the nearby TV channels and other potential interfering devices like wireless microphones.  This is a fully automated approach that uses any channel that is not occupied.

While white space wireless is in its infancy, it appears to be working well in the many trials that are in progress.  Commercial equipment is now available as are a number of data bases.  It is a niche now but could grow as it is further developed.  It could have some unstable moments as the FCC plans to offer up some of that white space spectrum at a forthcoming auction.  It would not take away the spectrum completely but would relegate the white space users to some dedicated segments of the spectrum.   The details are not clear.

The idea of spectrum sharing is now being further explored for other selected portions of the spectrum.  Recently in a report from the President’s Council of Advisers on Science and Technology (PCAST), the concept of frequency sharing was mentioned and recommended.  Specifically, it proposed ways to share the government’s spectrum used by all departments and the military.  An example target is the 1755 to 1850 spectrum used by the military.  The services are such that this spectrum could be re-farmed and shared.  There are other spectrum targets.

The whole key to spectrum sharing is to manipulate and control the factors involved in wireless transmissions.  These are time, space, frequency and transmit power.  This means that there are many ways to share spectrum by selecting the right time, frequency and power level to avoid interference.  This works well in the cellular industry where the same channels are used multiple times in a given area.  Called frequency reuse, this method spaces the cell sites and controls power and range with antenna heights and orientations so cell sites operating on the same channel do not bother one another.

Another factor is receiver sensitivity and selectivity.  Right now, the FCC does not regulate receiver specifications.  It does tightly control transmitter specs like frequency, stability and power level and antenna gains.  By ensuring that receivers have a desired level of sensitivity and filtering, considerable interference can be avoided.  This could become an FCC priority as more frequency sharing is implemented.

Frequency sharing is a complex and messy business.  It requires super technology and most likely more regulations.  However it is a solution to the spectrum problem that will only become more important in the years to come.  The big remaining problem is security.  Solve that and the frequency sharing idea can move forward.

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Blogs on topics such as wired and wireless networking.

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Lou Frenzel

Lou Frenzel is the Communications Technology Editor for Electronic Design Magazine where he writes articles, columns, blogs, technology reports, and online material on the wireless, communications...
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