Unlicensed Cognitive Sharing of TV Spectrum: The Controversy at the Federal Communications Commission

CIRULE4.GIF (100 bytes)

Michael J. Marcus


The views expressed herein are solely those of the author.

      As societies become increasingly mobile and more dependent on information technologies, their radio spectrum needs change. The basic spectrum management structure in all countries was developed over a long period, and much of the basic framework has remained stable for decades. Much of this framework was also developed when the available frequency range for commercial systems was much lower than it is now, and the bands used had different propagation characteristics than most bands used today. (After all, 3–30 MHz is called high frequency because it was considered "high" in the early days of radio.)
      The Federal Communications Commission (FCC) has been reviewing basic assumptions of spectrum policy in the past few years, and its landmark Spectrum Policy Task Force (SPTF) report in 2003 outlined a variety of possible new initiatives [1]. In May 2004 the FCC followed up on this report and released its proposals in Docket 04-186 [2] dealing with the possible use of cognitive radio technology for low-power unlicensed devices to share spectrum in the VHF and UHF television bands.
     Cognitive radio technology refers to a new type of radio that uses real-time interaction with its environment to determine transmitter parameters such as frequency, power, and modulation. Simple versions of this technology have been in use for several years. For example, many cordless telephones select the frequency to use at a given time based on sensing other users. However, in the past, use of such technology has been limited to bands occupied only by unlicensed devices. If the FCC allows this technology to be used where television broadcasting is unambiguously "primary" with respect to other users, this would be a significant change.
     The FCC was interested in making more spectrum available to unlicensed devices because of the explosive growth of IEEE 802.11/Wi-Fi unlicensed wireless LAN technology, and because the FCC recognizes that there is a great deal of "white space" [2] that could be exploited by unlicensed devices. That is, at a given time and place, monitoring usually shows much apparently unused spectrum that could, in theory, be used by new devices without adversely impacting existing users — even though frequency assignment data shows there is little or no unassigned spectrum in most bands of interest.
     There are many reasons for this white space. Some is due to the large peak-to-average use ratio of many systems that have dedicated spectrum, such as those used for public safety mobile users that have dedicated spectrum. Some white space is due to spectrum assignments designed to accommodate practical receiver limitations, such as limited adjacent channel and image frequency rejection. Finally, some white space is simply due to the fact that population and hence demand for spectrum is not spatially uniform.
     A measure of this white space can be seen in a survey funded by the National Science Foundation in New York City during the 2004 Republic National Convention. Even though the convention led to higher than average spectrum use, there was still significant white space.1
     The FCC proposal selected the TV bands as an initial home for cognitive radio based on several factors. First, the frequencies used for TV would give greater range in rural areas than the microwave frequencies used for Wi-Fi, and would also give better building penetration than Wi-Fi in all areas. Second, TV broadcast systems usually use high antennas, and the intended receivers need greater than 10 dB signal-to-noise ratio (SNR) to function (higher for analog NTSC). These large SNR ratios simplify the technology needed to detect whether a channel is currently in use. Finally, TV transmitters are left on more or less continuously, and infrequently change location or frequency. Thus, it would appear that it would be simpler to use cognitive radio in TV bands than in any other band. While these factors would also apply to AM and FM broadcasting, the 6 MHz bandwidth of TV channels makes them particularly attractive for this use. Since only a minority of households depend on over-the-air TV reception,2 the impact of any errors in selecting unused frequencies would be very small.
     The FCC's recent proposal discussed three possible techniques unlicensed devices might use to determine whether white space spectrum is available for use at a given location:

      As this article is written, the FCC proposal has received almost 200 comments [3] from the public, and more are expected before the issue is resolved. Not surprisingly, the views expressed vary greatly. While the author is a proponent of the FCC proposals, this discussion is intended to be balanced. This type of controversy need not be viewed as a "spectator sport" by readers of this magazine. The FCC welcomes participation from the technical community and provides a Website for submitting comments [4].
     A variety of controversial issues have emerged from public comments. One concern is that confusion resulting from the proposal might discourage purchase of new digital televisions and hence delay the transition from analog to digital televisions. This would also delay the transfer of some current TV spectrum to new mobile uses, including public safety.
     Some have questioned whether available technology can solve the "hidden node problem,"3 which complicates the task of using measurement to determine whether spectrum is idle. Supporters of this proposal have pointed out that either narrowband receivers tuned to narrowband peaks in TV transmissions, or feature/cyclostationary detectors [5] can detect the presence of TV transmissions with a threshold tens of dB greater than a TV receiver could demodulate, and thus could detect the presence of a TV channel even if they were in a propagation null. Opponents so far have been silent on this specific issue.
     There have also been concerns about using geolocation technology to identify white space, as there are problems using geolocation systems such as GPS indoors, and there may be problems of timeliness and accuracy of the FCC databases that show which channels are in use. Proponents have replied that fail-safe systems could be built that just do not transmit if geolocation is not possible, and proposed geolocation systems would have better indoor performance. Proponents have also conceded that improvements in this database are necessary and would have benefits beyond the unlicensed issue in this discussion. Indeed, proponents have indicated that better record keeping of TV translator input frequencies and cable TV headends might be needed.
     Finally, there is the possibility that unlicensed transmitters in the TV band might leak into TV receivers or in-home video cables and cause desensitization of the receivers. Proponents dismiss this as an unrealistic prediction from limited laboratory tests.
     For contrasting views on the many other issues, the joint comments [6] of the Association for Maximum Service Television and the National Association of Broadcasters contrast well with technical comments [7] of a coalition led by the New America Foundation, which includes Microsoft and several academics,4 as these two public comments to the FCC present pro and con arguments on the issues discussed above and several other ones.
     Clearly the two sides of this issue are far apart on most of the technical points. The recent announcement that FCC Chairman Powell is leaving throws further uncertainty on this proposal as it is unclear whether his still unnamed successor will be as enthusiastic about exploring such bold approaches to increasing spectrum use. Additional input from the technical community would be helpful in clarifying the issues associated with the proposal.

References
[1] http://www.fcc.gov/sptf/reports.html
[2] http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-247169A1.pdf
[3] http://gullfoss2.fcc.gov/cgi-bin/websql/prod/ecfs/comsrch_v2.hts?ws_mode=retrieve_list&id_proceeding=04-186
[4] http://gullfoss2.fcc.gov/prod/ecfs/upload_v2.cgi Enter "04-186" in entry box 1, "Proceeding".
[5] http://www.fcc.gov/realaudio/presentations/2003/021203/featuredetection.pdf
[6] http://gullfoss2.fcc.gov/prod/ecfs/retrieve.cgi?native_or_pdf=pdf&id_document=6516883656 and http://gullfoss2.fcc.gov/prod/ecfs/retrieve.cgi?native_or_pdf=pdf&id_document=6516883657
[7] http://gullfoss2.fcc.gov/prod/ecfs/retrieve.cgi?native_or_pdf=pdf&id_document=6516983275

Biography
MICHAEL J. MARCUS [F] received S.B. and Sc.D. degrees from MIT in electrical engineering. He retired from the FCC in 2004 where, among other things, he helped create the unlicensed bands that are home to Wi-Fi and Bluetooth. He now resides in Paris, France, where he does research and consults.