Spectrum: Sharing Nicely Can Go a Long WayReading Time: 4 minutes
Sharing has been part of U.S. spectrum policy from the beginning. When the government started handing out AM radio licenses in the 1920s and 30s, a relative handful of stations were assigned “clear channels” that they did not share with any other broadcaster in North America. There were allowed to operate at up to 50 kW and on nights when the atmospheric conditions were right, could be heard hundreds of miles from their transmitters. The rest of the stations got just a local monopoly on their frequencies. This worked fine in daytime, but some broadcasters had to shut down as soon as the sun set to avoid interfering with neighbors.
Still, the model for spectrum use in the U.S. and the rest of the world has been exclusivity. If you had a license, whether you were a TV station, a taxicab company, or a wireless phone operator, no one else within range to interfere was allowed to operate on your patch of spectrum. This worked fine as long as spectrum was relatively plentiful, But as noted in the earlier articles int his series, demand for wireless bandwidth is rising fast and we have run out of spectrum to assign. The tendency has been to view spectrum allocation as a zero-sum game: Anyone’s gain had to be someone else’s loss.
But this may well be a self-defeating process. If every megahertz of bandwidth assigned to wireless data has to be pried from the hands on an incumbent, it’s going to be a very slow and painful process. As the President’s Council of Advisors on Science & Technology (PCAST) put it: ”
PCAST finds that clearing and reallocation of Federal spectrum is not a sustainable basis for spectrum policy due to the high cost, lengthy time to implement, and disruption to the Federal mission. Further, although some have proclaimed that clearing and reallocation will result in significant net revenue to the government, we do not anticipate that will be the case for Federal spectrum.
The saving grace is that much of the spectrum currently assigned is not used very intensively. Some, for example, is assigned nationwide, but used in only specific locations. The government reserves a chunk of spectrum in the 3550 megahertz band for radar use, but it is generally used only in locations along the Atlantic and Pacific coasts. One approach to freeing this spectrum for wireless use would be to set up large coastal exclusion zones and issue wireless data licenses for the middle of the country. Unfortunately, this would exclude the most densely populated parts of the country. A better approach, endorsed by the and being actively pursued by the Federal Communications Commission is to take advantage of advances in technology to allow much finer grained sharing by allowing wireless data operations where the spectrum is not being used for rader. There are two possible “smart radio” approaches: One is to have a mobile device check its location against a database and operate on those frequencies only in areas known to be safe. Another is to actively seek out the radar signals and back off if they are detected. The 3500 MHz shared spectrum is likely to be used primarily for small cells, and idea I will explore in the next installment of this series.With those who hold spectrum fighting hard not to give it up, sharing must play a key role in meeting growing demand.
Another form of sharing is utilization of locally unused channels in the large swath of 600-800 MHz spectrum reserved for broadcast television. The FCC hopes to scavange spectrum for Wi-Fi-like unlicensed use in two different way. One, an idea that has been around for several years, is to allow the use of “white spaces”–televisions channels that are unassigned in a given location. The problem is that different channels are free in different places. The FCC is an fairly advanced development of rules for the use white spaces. However, base stations and and devices will be responsible for checking which frequencies they can safely use. White spaces are not likely to do much in the biggest cities, where dense channel assignments leave little spectrum available for sharing. In the end, the most important contribution of white spaces is to provide high speed broadband to rural areas, where TV channel allocations are sparse and good alternatives are few.
A second source of shared spectrum is part of the FCC’s plan to consolidate and sell off unused broadcast spectrum. The analog tuners used for many years in TV sets had a poor ability to reject signals in adjacent channels, so the original channel assignments set up wide “guard bands” to protect signals from interference (these are common throughout spectrum assignments.) New digital tuners are much more precise and the FCC proposes to free TV guard band channels for unlicensed use. Again, exactly which frequencies will be available will vary from market to market.
(A Washington Post story created much excitement around the internet by suggesting that the FCC has a plan to turn this new unlicensed spectrum into a nationwide free Wi-Fi service. The FCC has proposed nothing of the sort. There may be more Wi-Fi-like service available–it would not technically be Wi-Fi and would not work with existing Wi-Fi devices–but it won’t be national and it most likely won’t be free.)
The incumbent carriers continue to prefer exclusive spectrum assignments. It’s the way they are used to operating and besides, their ability to controls lots of bandwidth forms a powerful barrier to entry for potential competitors. They also remain deeply ambivalent about Wi-Fi and unlicensed spectrum schemes, not being quite sure whether they are threats or potential saviors for overcrowded networks. As Joan Marsh, AT&T vice-president, federal regulatory, wrote in response to the PCAST recommendations:
The Report’s core recommendations, however, have generated significant controversy. The Report found that the new norm for spectrum use should be sharing, not exclusive licensing. While we agree that sharing paradigms should be explored as another option for spectrum management, sharing technologies have been long promised but remain largely unproven. The over-eager pursuit of unlicensed sharing models cannot turn a blind eye on the model proven to deliver investment, innovation, and jobs – exclusive licensing. Industry and government alike must continue with the hard work of clearing and licensing under-utilized government spectrum where feasible.
Notwithstanding these misgivings, AT&T, T-Mobile, and Verizon have agreed to cooperate with the Defense Dept. in studying approaches to sharing spectrum in the 1750 MHz band, prime wireless real estate adjacent to frequencies currently used for wireless data. Since no one is willing to part with spectrum they currently hold, one way or another spectrum sharing has to be a key component of any plan to meet growing demand.