INTRODUCTION
On June 30, 2000, the National Telecommunications and Information Administration (NTIA) published a notice seeking public comment on a proposed test plan for developing accurate, repeatable, and practical methods for characterizing the signal of ultrawideband (UWB) systems and collecting information to estimate or measure the potential for UWB systems to interfere with select Federal radio receivers (other than Global Positioning System receivers).(1) NTIA received five comments in response to the UWB Notice.(2) Only three of the five comments specifically addressed the test plan described by NTIA.(3)
As indicated in the UWB Notice, NTIA's tests will provide a baseline assessment of the potential of UWB devices to interfere with systems operating in the bands in which the Federal Communications Commission (FCC) and NTIA have restricted emission levels from intentional radiating devices as defined by Part 15 of Title 47 of the Code of Federal Regulations.(4) Both the FCC and NTIA protect systems authorized to operate in these bands because they are very sensitive or serve critical safety-of-life missions. Thus, in the proposed test plan, NTIA selected conservative protection criteria based on increasing the interference plus noise level in the IF stages of the receiver.(5) NTIA believes that if a UWB device cannot produce an interfering signal level in a receiver's IF output above these protection criteria when located a specified distance away, then that UWB system could probably operate at greater distances with no impact to the system analyzed. If interfering signal levels are observed above the protection criteria, then NTIA would need to undertake a much more detailed assessment of the effect of the interfering signals on the performance of the receiver under test or develop a greater protection distance separation.
NTIA sought comments from the Federal agencies as to the most sensitive and critical receivers authorized to operate in the restricted bands. Because of limited funding and time constraints, NTIA identified only four receivers to be tested in both the open and closed modes for potential interference from UWB systems. These are the ASR-9 Airport Surveillance Radar, the ARSR-4 Air Route Search Radar, the ATCRBS-5 radionavigation beacon system, and a 4 GHz digital earth station receiver. These systems were selected because they operate in the restricted bands, were available for testing, and operate in frequency ranges included within the range of our UWB simulation equipment. In addition, NTIA is measuring the UWB signal levels in a number of different bandwidths in the lab. This will permit the agency to extend analytically the results to different types of receivers including, radar altimeters, landing systems, DME, SARSAT receivers, and Terminal Doppler Weather Radars.
NTIA continues to support deployment of UWB technology in ways that will not adversely impact existing critical uses of the spectrum. NTIA began initial planning for the tests before the Federal Communications Commission (FCC) issued its notice proposing to amend its Part 15 rules to permit the operation of ultrawideband systems on an unlicensed basis.(6) NTIA anticipates, however, that the planned measurements would provide a useful technical underpinning for this proceeding.
The final test plan as revised in response to the comments is posted on NTIA's website along with this document at <http://www.ntia.doc.gov/osmhome/uwbtestplan/>.
DISCUSSION OF THE COMMENTS
As noted above, NTIA received five comments in response to the UWB Notice, three of which provided comments on the draft test plan. Detailed discussions of these comments and NTIA's revisions to the draft test plan in response to these comments follow.
Motorola, Inc. - In its comments, Motorola, Inc. expresses support for NTIA's proposed testing, but notes that the test plan does not include standard terrestrial land mobile equipment. While NTIA agrees with Motorola that terrestrial land mobile equipment is of critical importance to fulfilling the missions of all Federal government agencies, these devices do not operate in any of the Part 15 restricted bands and thus can expect to be exposed to signal levels from conventional narrow band unlicensed intentional radiating devices operating in the same bands. Moreover, since NTIA will measure the interference signal levels in a number of receiver bandwidths as described above, and this information may be useful in the assessing of the impact of UWB on land mobile equipment as may be necessary. Therefore, NTIA has not revised the proposed test plan to include terrestrial land mobile equipment testing.
Zircon Corporation - In its comments, Zircon Corporation appears to infer from the reference in footnote 1 of the Master Plan to a "tuned circuit" that NTIA meant a resonant circuit similar to those found in conventional narrow band radios. For purposes of this test plan, however, NTIA uses the term "tuned circuit" to refer to a circuit whose currents are shaped by transmission line and antenna and semiconductor characteristics, not a high-Q circuit that rings for many cycles. In its comments, Zircon also notes the insignificance of temperature effects on UWB devices. NTIA agrees with Zircon's comments and has deleted this sentence from the final test plan. Zircon also comments on the lack of a readily identifiable center frequency, as further indication of a low-Q circuit. While the use of low-Q circuits in most UWB equipments is also NTIA's expectation, the agency staff has observed UWB impulse generators fired into very respectable 8-10 pole filters to generate a very rectangular bandwidth UWB signal. As is readily apparent, there are many, widely different applications and technologies covered by the UWB rubric.
Zircon disagrees with NTIA's statement in the Master Plan that "UWB signals will be most easily (and, most accurately) measured in the time domain." NTIA's major use of the time domain measurements has been to help understand the characteristics of the time waveforms and whether there are any measurement problems in moving between the time and frequency domains. The major measurement effort was to design procedures using readily available test equipment to make good measurements in the frequency domain, and in the context of time responses in the IF-bandpass of victim receivers. NTIA has been well aware of the very difficult RF measurement problems in the time domain and contracted with a National Institute of Science and Technology impulse calibration facility built especially for these purposes. At no time has the agency ever contemplated the use of time domain measurements at commercial testing labs.
Zircon also disagrees with the statement in the Master Plan that UWB signals in narrow bandwidths have spectrums similar to noise, and states that the only way to measure noise is to use a wide bandwidth. To clarify the statement in the Master Plan, this statement applies only to the appearance of the spectra on a spectrum analyzer, not to the actual measurement of noise. However, the planned Amplitude Probability Distributions (APDs), which compute the distribution of sampled signal levels over time, can be measured in any receiver bandwidth. One important part of the APD testing is to see how similar the dithered- and non-dithered-PRF UWB signal statistics are to white Gaussian noise. Although some UWB devices NTIA has tested to date look fairly Gaussian to the casual observer of the spectrum analyzer screen, the APDs show considerable non-Gaussian statistics.(7) The spectral lines from short code lengths will surely be there, but to a much smaller extent than the lines that would appear with no code generator at all (i.e., non-dithered). NTIA has also observed other types of totally random dithering, where these lines are not present.
Zircon also disagrees with NTIA's assertion that the energy from the pulse exists only during the pulse, citing as evidence that a narrow receiver capable of resolving one of the PRF spectral lines will observe a continuous CW signal. NTIA believes that whether this signal represents an artifact of the receiving process or the presence of a continuous CW signal is not as important as the equivalency of the two constructs. Thus, summing several generated CW signals can produce a pulsed signal just as a pulsed signal can be resolved into several CW signals. As noted in the Master Plan, NTIA needs to understand how a receiver responds to an impulse, not only to the averaged (over time) power spectral density, and, how and under what conditions we can develop and use a bandwidth correction factor.
Finally, Zircon expresses concern that ITS measurements appear to focus on pulsed RF signals rather than UWB signals that are the result of impulse emissions. This is a misunderstanding of the ITS measurement program - all of the measurements ITS makes will be of the impulse variety.
Time Domain Corporation - In its comments, Time Domain first encourages NTIA to discuss the proposed test plan with the FCC staff. NTIA has discussed these measurements with the FCC staff engineers during their preparation and intends to discuss the results with them as appropriate.
Time Domain expresses concern with respect to the lack of detail in the Master Plan. The Master Plan, however, should be read along with the ITS measurement program to understand the more complete picture. The Master Plan shows what the agency will examine rather than how we will examine it. Because of the newness of the UWB technology and because often the test engineers can make measurements in several ways, detailed discussions and decisions were provided in the ITS measurement program.
Time Domain also expresses concern that the testing appears to be limited to closed system testing. While NTIA has planned many of the tests to be closed system so that the results of the measurements can be controlled, the agency has also scheduled measurements of four receivers for open and closed system testing, which will allow us to assess the differences between the two. Time Domain also expresses concern that the Master Plan does not provide for NTIA to test any unintentional or incidental radiating Part 15 devices to compare their spectra and potential impact with those of UWB devices. NTIA agrees with Time Domain on the value of such testing and comparison and has amended the test plan to test several such devices so that the differences can be examined. All of the agency's radiated interference testing is currently planned for an open air test site at Federal Aviation Administration facilities in Oklahoma City, Oklahoma.
Time Domain also notes that the Master Plan does not require the measurement of the effect of UWB signals on the performance of the equipment being tested. Time Domain suggests that a better approach would be to "perform a parametric analysis that measures the impact of UWB emissions from which a separate analytic process could determine the point at which harmful interference might occur." Given limited resources and time constraints, NTIA's testing is limited to determining the interfering signal levels from UWB emitters in comparison to the noise levels in the IF sections of the receivers being tested. NTIA intends to measure the relationships between a number of UWB parameters (such as, width, repetition rate, power (peak and average), dithering, bursting, and gating) and the potential for interference as well as APDs. This will allow the agency to extend the results to other types of UWB devices and to other probabilistic or absolute interference criteria. Moreover, performance-based criteria, such as harmful interference, are not easily determined and agreed upon by the affected parties for most classes of receivers with either digital or analog processors. Thus, NTIA has chosen criteria for interference protection as specified in the International Telecommunication Union R-Sector Recommendations where available and appropriate. If these protection criteria are exceeded, further measurements or analyses will be required to determine if the operation of the UWB devices can be permitted without restricting the ability of the existing systems to operate as originally intended.
Time Domain suggests that using more than three emitters in testing the potential effect of aggregate UWB signals would increase the validity of the testing. While more emitters would be useful for obtaining more points to compare the aggregate model with measured results, NTIA believes that three emitters will be sufficient for purposes of the current testing.(8)
Time Domain also provided comments regarding the ITS measurement program. Specifically, Time Domain appears to assume that in the measurement program, ITS will trigger all three of the UWB signal generators used in the testing of aggregate signals synchronously. In fact, ITS will control them independently. This seems to be a misunderstanding of the use of the term "gated" in the ITS measurement program, which is similar to the way Time Domain uses the term "burst." "Gated" is meant to imply a signal generator that is gated on and off to transmit bursts of UWB high prf signals.
Time Domain is also concerned that the "50 MHz IF bandwidth instrument" discussed in the ITS measurement program is not practical or available as commercial, off-the-shelf equipment. NTIA agrees with Time Domain that a 50 MHz bandwidth spectrum analyzer is not commercially available at this time and notes that the 50 MHz bandwidth discussed in Section 4.1 is with respect to calculating values in that bandwidth based on measured levels in a lesser bandwidth. NTIA intends for its measurement report to describe the test equipment we used and suggest appropriate measurement equipment for use by testing labs.
Time Domain suggests that the equation in paragraph 4.4.1 of the ITS measurement program is an oversimplification because it does not take into effect either static or dynamic antenna beam patterns. However, the equation contains the GR() variable, where refers to the angle between the mainbeam of the antenna and the direction of the UWB transmitter. The angle, , can vary in time and its presence in the equation does not imply the use of the mainbeam gain of the antenna. NTIA will not measure long term probabilistic interference performance because of time and resource constraints, the measurements will generally be made with the mainbeam of the antenna of the equipment under test directed as close as possible toward the UWB transmitter. The measurement of APDs will allow us to extend the measurements by convolving the APD of the interference signal with the antenna probability distribution to obtain the overall distribution of the UWB signal.
Time Domain also notes the difficulty in "backing out" the losses, gains, or phase distortions of the receive antennas, cables, and any amplifiers, if ITS intends to try to derive actual radiated time domain waveforms when performing time domain measurements on UWB signals. NTIA agrees that this is a complicated problem, however, the measurement's report will verify the measured and analytic solutions for this calculation are consistent.
Finally, Time Domain comments that "testing should not start with any preconceived notions of compatibility" and should examine "real world environments and not only the theoretical noise limited environment." We agree and assert that it is precisely because we do not have preconceived notions of interference (but do have a limited budget and severe time constraints) that we intend our efforts to be a triage-like assessment designed to determine under what conditions additional measurements, if any, need to be made to develop rules for UWB systems that provide the benefits of the technology to the public while protecting critical Federal systems currently operating in the environment.
CONCLUSION
NTIA appreciates the commenters' efforts to review and provide comments on the proposed test plan. NTIA has made the changes to the Master Plan and ITS measurement program noted above. The final plan and this document are posted on our website at <http://www.ntia.doc.gov/osmhome/uwbtestplan/>.
FOR FURTHER INFORMATION CONTACT: Dr. William Kissick, Institute for Telecommunication Sciences, telephone: (303) 497-7410; or electronic mail: billk@its.bldrdoc.gov ; or Paul Roosa, National Telecommunications and Information Administration, telephone: (202) 482-1559; or electronic mail: proosa@ntia.doc.gov .
Media inquiries should be directed to the Office of Public Affairs, National Telecommunications and Information Administration, at (202) 482-7002.
1. See "Notice, Request for Comments on Ultrawideband Systems Test Plan" (UWB Notice), National Telecommunications and Information Administration, Department of Commerce, 65 Fed. Reg. 40614 (June 30, 2000). The text of the notice, the proposed test plan (Master Plan) and the ITS measurement program are available on NTIA's website at <http://www.ntia.doc.gov/osmhome/uwbtestplan/>.
2. See Terence W. Barrett, UCI, "History of UltraWideband (UWB) Radar & Communications: Pioneers and Innovators," presented to the Progress in Electromagnetics Symposium 2000 (July 2000); Comments of Motorola, Inc. (July 17, 2000); Comments of Zircon Corporation (July 17, 2000); Comments of Time Domain Corporation (July 17, 2000); Comments of Terry James (July 17, 2000). These comments are available on NTIA's website at <http://www.ntia.doc.gov/osmhome/uwbtestplan/comments_list.html>.
3. The remaining two comments addressed the impact of UWB signals on other forms of communication and the history of the technology itself. Mr. Barrett's discussion of UWB history and cautionary explication of the potential interference effects of UWB devices and the need to fully explore these effects was quite helpful to our developing a better understanding of the technology and its potential impact. Mr. James' discussion of the impact of UWB technology on the Internet's capability to carry the burgeoning amount of communications was interesting, but beyond the scope of the agency's program to measure the impact of UWB device operations on the radio-frequency environment.
4. As indicated in the UWB Notice, this particular test plan has been limited to systems not associated with GPS. See UWB Notice, 65 Fed. Reg. 40614, n. 1; see also "Notice, Request for Comments on Global Position System/Ultrawideband Measurement Plan," National Telecommunications and Information Administration, Department of Commerce, 65 Fed. Reg. 49544 (Aug. 14, 2000). This test plan is limited to critical Federal systems operating in the restricted bands because those operating elsewhere in the spectrum are already exposed to intentionally radiating Part 15 devices emitting signals of the same magnitude.
5. This amount of protection is provided to any existing receiver by another licensed radio transmitter introduced in the field unless the users of the transmitter and receiver reach a negotiated agreement to exceed the protection level.
6. See "Revision of the Rules Regarding Ultra-Wideband Transmission Systems," Notice of Proposed Rulemaking, Federal Communications Commission, Dkt. No. ET-98-153, 65 Fed. Reg. 37332 (June 14, 2000). Comments in this proceeding are due on September 12, 2000 and reply comments on October 12, 2000. The FCC encourages the submission of test results by October 30, 2000.
7. This is quite similar to the confusion between the appearance of the spectra of unintentional radiating devices like hair dryers, razors, electric drills and computing devices and those of UWB devices. Although the long duration spectra of the unintentional radiating devices look nearly the same as that of UWB devices, the distribution of the signals over time is significantly different.
8. Recent increases in funding and loans of simulator equipment from manufacturers may allow us to increase the number of emitters used in this test to nine or twelve if time permits.