Spectrum Management for Science in the 21st Century (2010) / Chapter Skim
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4 Technology and Opportunities for the Mitigation of Radio Frequency Interference
4 Technology and Opportunities for the Mitigation of Radio Frequency Interference
Pages 135-176
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From page 135... ...
The Earth Exploration-Satellite Service (EESS) and Radio Astronomy Service (RAS)
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From page 136... ...
on a primary basis or a secondary basis, and allocations include details on permitted transmission power levels and operation times. The difference between a primary allocation and a secondary allocation is essentially that the users of a secondary allocation must accept interference from the users of a primary allocation and conversely must not interfere with the users of the primary service.
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From page 137... ...
As noted in Chapters 2 and 3, the spectrum requirements of the radio astronomy and Earth exploration radio science community currently far exceed the spectrum available to the RAS and EESS on 2 National Research Council, Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses, Washington, D.C.: The National Academies Press, 2007.
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. Finding: Owing to their receive-only nature, the passive Earth Exploration-Satellite Service and Radio Astronomy Service, operating from 10 MHz to 3 THz, are incapable of interfering with other services.
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. More often, however, the situation is intermediate in the sense that significant interference is observed but can sometimes be managed through a combination of interference mitigation techniques (see §4.3 and §4.4)
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As explained later in this chapter, this is true even when taking into account the capabilities of existing and emerging techniques for the mitigation of interference. For these reasons, passive-sensing scientific users of the radio spectrum are greatly concerned with the utilization of spectrum both within the bands allocated to the RAS and EESS and in all other bands accessible to current and planned instruments.
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From page 141... ...
This has resulted in a number of studies reporting measurements of the utilization of the spectrum.7 Typically, the results of such studies report results in terms of spectral occupancy, 3 See "VLA Radio Frequency Interference" at http://www.vla.nrao.edu/cgi-bin/rfi.cgi; accessed Janu ary 13, 2009. 4 See "Green Bank Interference Protection Group" at http://www.gb.nrao.edu/IPG/; accessed Janu ary 13, 2009.
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From page 142... ...
For example, a recent study performed by the Shared Spectrum Corpora tion reported 13.1 percent occupancy for New York City and 1 percent at Green Bank, West Virginia, inside the NRQZ.8 By contrast, a study of occupancies in terms somewhat more relevant to radio astronomy applications finds occupancy greater than 30 percent even in the relatively rural areas of Westford, Massachusetts, and Hancock, New Hampshire.9 Both studies are probably internally consistent but cannot be compared because of their different assumptions about the appropriate time-frequency resolutions, thresholds of detection, and tolerable levels of out of-band (OOB) interference.
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From page 143... ...
Detailed, real-time characterization of the spectrum uses provides Figure 4-2 an opportunity to prevent unauthorized uses of the spectrum from potentially causing catastrophic R01628 interference, as well as the capacity for opportunistically using unused spectrum for enhancing mea surements. The Arecibo Observatory is part le the National Astronomy and Ionosphere Center, which is uneditab of bitmapped image operated by Cornell University under a cooperative agreement with the National Science Foundation.
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Ellingson and G.A. Hampson, "Mitigation of Radar Interference in L-Band Radio Astronomy," Astrophysical Journal Supplement Series, 147: 167-176 (July 2003)
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4.2 MAJOR DRIVERS OF SPECTRUM USE Current measurements of spectral utilization and its impact on passive systems may not be indicative of future spectral use. The drivers for additional spectral bands for intensive use, the allocation of additional bands, and the development of "smart" flexible radio technology will have a profound impact on future use.
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This infrastructure is not available in developing nations, but unused radio spectrum is readily available. Therefore, most commercial deployments, including backhaul, are made entirely out of wire less systems.
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Treasury $13.7 billion, which is equivalent to $0.50 MHz-pop.15 This cost is usually called the opportu nity cost for the spectrum. High opportunity costs motivate the licensee to use the spectrum quite efficiently to leverage the already-"sunk costs." This is why cellular operators are very conscious of their spectral efficiency and thus exploit spectral 14"Global Mobile Phone Users Top 3.3 Billion by End-2007 -- Study," Cellular News, May 24, 2008, available at http://www.cellular-news.com/story/31352.php; accessed January 14, 2010.
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From page 148... ...
The public-safety services are less efficient spectrum users than are the cellular telephone services, since the technology of the former lacks any spectral reuse. Third-Generation and Fourth-Generation Systems The 2008-2015 period will see the deployment of new cellular-based services in bands that were allocated in the 2002-2007 period: the 700 MHz band, the AWS bands, and the Broadband Radio Service and Educational Broadband Service (BRS/EBS)
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From page 149... ...
Unlicensed Uses of the Radio Frequency Spectrum There has been a phenomenal proliferation of unlicensed devices over the past decade, to the great benefit of society.18 Over the past 5 years there have also been significant additions to the types of unlicensed devices and spectral bands available for unlicensed uses. The ultrawideband types of unlicensed devices, the expansion of unlicensed use in the Unlicensed National Information Infrastructure (U-NII)
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These emission limits are consistent with those granted by the FCC for personal computer emissions and intentional emissions for unlicensed devices. Unlicensed National Information Infrastructure at 5 GHz The FCC established a schedule for new unlicensed devices that are dynamic frequency selection (DFS)
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The recent changes increased sophistication with both spectral and spatial characterization. It may also be possible to extend regulations to include temporal characterizations that will be useful for developing new interference mitigation techniques.
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Derome, "Interference Temperature Measurements from 70 to 1500 MHz in Suburban and Rural Environments of the Northeast," First IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Networks, pp. 119-123, (November 8-11, 2005)
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The same flexibility poses challenges for certification and the associated liability through potential misuse. SDR provides software control of a variety of modulation techniques, wideband and narrowband operation, communications security functions (such as hopping)
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Digital Modulation and High-Efficiency Modulation Schemes The signals received by radio astronomy and passive Earth remote sensing are random. The signals follow Gaussian statistics within a given bandwidth and 24 International Telecommunication Union, "Techniques for Mitigation of Radio Frequency Inter ference in Radio Astronomy," Document 7D/142-E, January 23, 2007.
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From page 155... ...
However, the spectrum that the radio astronomy and Earth exploration radio science community currently use far exceeds these allocations. The use of new techniques requiring other spectral bands or additional spectrum is driven by the scientific requirements and made possible by continual technical developments, as discussed in Chapters 2 and 3.
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This is the most common situation, but at present the performance achieved by the majority of the unilateral methods has been documented only anecdotally and so remains to be completely quantified.25 Following the review of specific unilateral mitigation 25 International Telecommunication Union, "Techniques for Mitigation of Radio Frequency Inter ference in Radio Astronomy," Document 7D/142-E, January 23, 2007; A.J. Boonstra, "Radio Frequency Interference Mitigation in Radio Astronomy," PhD Thesis, Delft University of Technology, Dept.
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Bettenhausen, and W Johnston, "WindSat Radio-Frequency Interference Signature and Its Identification over Land and Ocean," IEEE Transactions on Geoscience and Remote Sensing, 44(3)
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Thus it can be expected that as RFI becomes more prevalent, the need for interference mitigation in order to do useful science will increase, but at the same time the efficacy of mitigation efforts will decrease and the quality of the resulting science will suffer accordingly. Detection Techniques Various RFI detection techniques are available, with each typically oriented toward a particular class of RFI sources.
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White, "Radio Frequency Interference Excision Using Spectral Domain Statistics," Publ. Astron.
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When the pulses from the radar are received, the radio astronomy receiver electronically triggers a data masking or data flagging process. The limitations of this method are primarily that it requires excellent sensitivity on the unwanted transmission, and secondly that it may be hard to accommodate transmissions arriving with different delays from different directions, such as multipath propagation with multiple reflec tions from surrounding mountains.30 In this case active receiver blanking using a beacon transmission on some carefully chosen frequency could be used at the radio astronomy site to blank the radio astronomy receiver.
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Johnson, and N Niamswaun, "Time and Frequency Blanking for Radio Frequency Interference Mitigation in Microwave Radiometry," IEEE Transactions on Geoscience and Remote Sensing, 45: 3672-3679 (2007)
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Chatterjee, and T.J.W. Lazio, "Radio Frequency Interference Identification and Mitigation Using Simultaneous Dual Frequency Observations," Radio Science, 40(5)
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Weaker RFI sources can be more difficult to detect, however. Interferometric detection strategies can be combined with the spatial excision suppression techniques discussed below.
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The performance of temporal excision in suppressing RFI source contributions is limited solely by the performance of the associated detection algorithm, which determines the false alarm probability and probability of detection for specific RFI types. When measurements in multiple frequency subchannels are available, RFI contributions detected in a particular subchannel can be removed by discarding data from that subchannel in computations of average powers or other averages across frequency.
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Two strategies for improving RFI 38A.J. Boonstra, "Radio Frequency Interference Mitigation in Radio Astronomy," PhD Thesis, Delft University of Technology, Dept.
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[3] Jeffrey Piepmeier, Priscilla Mohammed, and Joseph Knuble, "A Double Detector for RFI Mitigation in Microwave Radiometers," IEEE Transactions on Geoscience and Remote Sensing, 46(2)
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Leuski, "Airborne Radio-Frequency Interference Studies at C-Band Using a Digital Receiver," IEEE Transactions on Geoscience and Remote Sensing, 44(7, Part 2)
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Unilateral Mitigation Successes and Limitations Table 4.2 above provides a short summary of the successes and limitations of the unilateral mitigation methods that have been employed to date by the EESS. Finding: While unilateral radio frequency interference mitigation techniques are a potentially valuable means of facilitating spectrum sharing, they are not a substitute for primary allocated passive spectrum and the enforcement of regulations.
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From page 169... ...
Conceivably, these systems already represent most -- if not all -- of the needed infrastructure for cooperative mitigation. The only miss ing elements are the agreed-on standards and the software that would allow these systems to momentarily relinquish assigned spectral bands in response to science requests.
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The above arguments and statistics strongly suggest that better time manage ment of the available spectrum could result in significantly more time-bandwidth product being made available to passive services without impacting active services to any appreciable degree. To simplify the implementation, cooperative strategies are best implemented in bands used by fixed, registered transmitters -- rather than unlicensed devices -- although most new Internet-connected and GPS, cellular, or Wi-Fi devices could readily be required to contain simple software for cooperative mitigation.
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ET 99-255, Amendment of Parts 2 and 95 of the Commission's Rules to Create a Wireless Medical Telemetry Service," filed with the Federal Communications Commission on September 30, 1999. Available at http://sites.nationalacademies.
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It is even more complicated to back out from such a calculation the degradation associated with RFI. This very problem is at the heart of spectrum allocation decisions when com mercial services such as cellular telephones have an easily demonstrated market value, but scientific and other public uses of spectrum do not.
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From page 173... ...
Radio Astronomy Service (RAS) The RAS is currently dominated by relatively few large radio observatories located in remote areas that nonetheless are beset by increasing levels of inci dental interference from proliferating consumer-level electronics such as cellular telephones, Wi-Fi and Bluetooth systems, computers, and so on; from emissions from aircraft and satellites; and from over-the-horizon signals arising hundreds of miles away, well outside most protected areas but reflected by aircraft, the tropo sphere, and other means.
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From page 174... ...
These measurements are commonly made in the 22-24 GHz frequency range, but microwave point-to-point com munications and automobile anticollision radars operating in this spectral band are a source of significant RFI that will increase as automotive radar becomes more common. • Another example is sea surface temperature, for which measurements are made at 10 GHz.
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43 Harvey J Levin, The Inisible Resource: Use and Regulation of the Radio Spectrum, Baltimore: Resources for the Future and Johns Hopkins University Press, 1971.
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All RFI poses the potential for loss of information in EESS and RAS observations and data, thus undermining realization of the full societal benefit of Earth and radio astronomy science.
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