D. Interference Protection Criteria
38. We now turn to questions and proposals concerning interference protection criteria to govern the authorization of low power radio services. The types of interference protected against and the means of protection are pivotal issues that would significantly affect the number of LPFM stations that could be authorized and the extent of services provided by these stations. The interference protection criteria selected for LPFM stations could also affect existing and future service of FM radio stations of all classes. We urge commenters to consider carefully these issues.
39. Commercial FM stations traditionally have been authorized to operate on channels allotted for a particular community and class of station. A maximum power and antenna height is specified for each station class, which determines the maximum service area for the class. FM channel allotments are determined on the basis of specified minimum distance separations from other allotments for the same channel and three pairs of adjacent channels.53 FM allotment separations are computed on the basis of maximum class facilities. Distances are derived from a desired-to-undesired signal strength ratio methodology to permit interference-free broadcasts within each station's protected service contour. The use of distance separations for determining channel allotments has proven to be an effective and straightforward means for maintaining the technical integrity of the FM radio service.
40. Minimum Distance Separations Between Stations. If we were to create one or more classes of low power radio service, we would expect to receive a very large volume of applications. The expeditious authorization of such service requires a simple, yet effective, means of controlling interference among stations. With this in mind, we believe minimum distance separations between stations may be the best practical means of governing interference to and from low power radio stations. Appendix B hereto presents several tables which specify minimum distance separations for the LPFM classes described above, including an explanation of how these distances were determined. The tables consider the following interference protections: co-channel, 1st-adjacent channel, 2nd-adjacent channel for reserved band frequencies, 2nd/3rd-adjacent channel for commercial band frequencies (in the event we were to require these protections) and intermediate frequencies. In the tables, protection of LPFM stations to each other includes minimum separations for only co-channel and first-adjacent channel interference. The various tables include minimum distance separations for maintaining existing levels of protection to radio stations of each particular class, as well as station separations for equivalent protection from LPFM stations to other LPFM stations. The appendix also includes tables of distance spacings for stations that would operate within 320 kilometers of the common borders with Canada or Mexico, based on the protection requirements in our agreements with these countries. We recognize that an approach based on distance separations could result in fewer LPFM stations and that additional stations could be "squeezed in" if a contour overlap methodology were employed. However, as the Commission learned from implementing the low power television service, the contour overlap approach is resource intensive and requires, among other things, substantial preparation in advance of receiving applications, including the writing of complex computer programs and preparation of several data bases. A contour protection-based licensing system could also impose substantial additional processing burdens on the staff. We are concerned, therefore, that adoption of this approach could substantially delay the authorization of low power radio service and place a heavy burden on small LPFM applicants.54 In contrast, use of minimum spacings would facilitate not only a streamlined application process, but would also enable a quick automated "self-check" of frequency availability before an applicant files its application (see paragraph 0, below).
41. We seek comment on our proposed use of minimum distance separations and, in particular, on whether the specific values tabulated in Appendix B are appropriate for the different types of interference protections.55 While we believe it is important to afford a level of protected service to low power stations, particularly LP1000 stations, we invite comment on whether low power stations of a particular class should be permitted to accept interference from other stations, including interference from other low power stations. If so, should this be permitted only where there are interference agreements between stations? For secondary LPFM service, should there be a limit to the amount of interference that could be received for a channel to be considered available? While we prefer the simplicity offered by station separation requirements, we realize there may be advantages to using a more sophisticated interference modelling approach. Possible approaches might include the combination of contour protection and reduced station separations (Section 73.215 of the FM radio rules), the contour protection methods used in the low power television service (Sections 74.705, 74.705 and 74.706), or even more elaborate methods involving a terrain-dependent propagation model, such as the point-to-point model proposed in our radio technical streamlining proceeding.56 Depending on our initial experience in authorization of LPFM service, should we later consider a more sophisticated and spectrally efficient approach? We invite comment on these issues, including the effectiveness of alternative approaches for interference protection.
42. Types of Interference Protection Standards. No commenter in this proceeding takes issue with the need to protect stations operating on the same channel or on a 1st-adjacent channel from interference caused by LPFM facilities, and we propose these protections for any LPFM class we would authorize. At issue is the need to protect stations operating on the 2nd- and 3rd-adjacent channels with respect to LPFM stations. Commenters supporting LPFM services generally oppose any requirements for 2nd- or 3rd-adjacent channel protections, contending such interference from low power stations would be, at most, minimal.57 Some commenters, including the NAB, NPR, and New Jersey Broadcasting, Inc., believe these protections should be retained to prevent interference and/or protect future digital terrestrial radio service. 58 As noted below, these protections would limit substantially the number of channels available for low power radio generally and could preclude altogether the introduction of LPFM service in mid-sized and large cities. Therefore, to the extent possible, we are inclined to authorize low power service without any 2nd- and 3rd-adjacent channel protection standards.
43. Third-Adjacent Channel Protection. We believe that a strong case can be made for not requiring 3rd-adjacent protection to or from any of the contemplated classes of LPFM station; i.e., protection to stations operating on channels separated by 600 kHz. We believe that authorizing LPFM service without a 3rd-adjacent channel protection requirement would entail, at worst, little risk of interference to existing radio service. Areas of potential interference would be very small and occur only in the immediate vicinity of the low power transmission facility. An LP1000 station operating with maximum facilities would be predicted, under the current protection ratios, to cause 3rd-adjacent channel interference t- a distance of 1.4 kilometers (0.9 miles) from its antenna, and even this very small predicted interference zone could possibly pose a potential problem to other stations only if the LP1000 station were located at, or very near, the outer edge of the protected station's service contour. 59 The interference potential would be even less for LP100 stations. By comparison, the 3rd-adjacent channel interference contour of a maximum-facilities Class A station is 3 kilometers (1.8 miles), while such a contour for a maximum-facilities Class C station is 14 kilometers (8.7 miles). In 1997, we eliminated the 3rd-adjacent channel protection for full power "grandfathered short spaced stations," including stations that operate at substantially higher power levels than LP1000 stations. That decision was supported by nearly all parties filing comments in that proceeding. 60 Additionally, no comments yet filed in this proceeding provide technical support for including this restriction.
44. Relaxed interference standards for low power FM stations may be the only way to "find" sufficient spectrum in medium and larger markets to create any new viable service of 100 watts or more.61 For example, staff analysis62 suggests that with full protection requirements, no LP100 or LP1000 stations could be authorized in Denver, Colorado. No LP1000 and only three LP100 stations could be authorized in Minneapolis, Minnesota. If there were no 3rd-adjacent channel protection requirement, 1 LP1000 or 4 LP100 stations might locate in Denver and perhaps 1 LP1000 or 9 LP100 stations could be located in Minneapolis.
45. On balance, we believe that creating opportunities for new LPFM service should outweigh any small risks of interference to and from LP1000 and LP100 stations. In choosing potential station locations, LPFM applicants would be advised to take into account spectrum congestion considerations and evaluate the extent to which third-adjacent signals could pose a problem. In most instances, we believe the actual effects of such interference might well be insignificant. We seek comment and analysis on our tentative conclusion not to include 3rd-adjacent channel protection requirements for any LPFM service.
46. Second-Adjacent Channel Protection Standards. FM radio stations protect other stations operating on the 2nd-adjacent channel where the frequency separation is 400 kHz. For example, a Class A station must be located at least 31 kilometers (19 miles) from another Class A station on a 2nd-adjacent channel and 95 kilometers (59 miles) from a Class C station on a 2nd-adjacent channel.63 We previously found in the case of "grandfathered" short-spaced FM stations, that during the period in which they were able to modify facilities without regard to 2nd- and 3rd-adjacent channel spacing (1964-1987), we did not receive any interference complaints as a result of such modifications.64 We found only a small risk of interference in that context, which was outweighed by improved service. Similarly, in the noncommercial service, we have been willing to accept small amounts of potential second- and third-adjacent channel interference where such interference is counterbalanced by substantial service gains. Educational Information Corporation, 6 FCC Rcd 2207 (1991).65 We ask commenters to assess the level of risk of increased interference to stations in existing FM services that would result from permitting LPFM stations to locate without regard to 2nd-adjacent channel spacing for this service and to weigh any costs against the additional service to the public that could result. Commenters should consider the likelihood and potential extent of any harmful effects on current stations and listeners, taking into account the size and location of the areas possibly affected and the interference immunity of the existing receiver population. We also seek comment on the state of receiver technology and the ability of receivers to operate satisfactorily in the absence of 2nd-adjacent channel protection.
47. It is also important, as urged by some commenters, to take into consideration the implications of 2nd-adjacent channel protection for the possible conversion of existing analog radio services to a digital mode. While the Commission has yet to formally advance any specific proposals, it has already expressed its support for a conversion to digital radio.66 One specific proposal was recently submitted in a rule making petition (RM-9395) filed by USA Digital Radio Partners, L.P. ("USADR"),67 a terrestrial digital radio proponent of a technology that uses an in-band-on-channel ("IBOC") technology, in which an FM radio station's analog and digital signals would share portions of the same channel. It is possible that we will consider one or more variations of IBOC proposals that would use the outer "edges" of a channel's specified bandwidth and/or portions of the adjacent channel to transmit a digital signal. 68 This signal configuration would reduce the frequency separation ("guardband") that insulates between channels, and we must examine how this could affect the propensity of LPFM stations to interfere with IBOC digital transmissions centered on second-adjacent channels. In the existing radio environment, USADR suggests that 2nd-adjacent channel interference from analog FM signals would not pose an interference threat to its IBOC signal.69
48. Staff analysis suggests that the current 2nd-adjacent protection standards would be a substantially larger impediment to LPFM service than the 3rd-adjacent standard, especially in large and medium-size cities.70 As examples, two LP1000 stations could be located in Nashville, Tennessee if there were no 3rd-adjacent channel protection requirement and as many as ten might be possible if a 2nd adjacent standard also were not required; in San Francisco, no LP100 station could be located with a 2nd-adjacent standard, but two such stations would fit if there were no need for 2nd- and 3rd-adjacent channel protection standards; at least one LP1000 station could be authorized in Los Angeles and Pittsburgh, but only without 2nd- and 3rd-adjacent channel protection standards. The inclusion or exclusion of 2nd-adjacent channel protection requirements for LPFM stations would greatly affect the extent to which LPFM service could be introduced and, therefore, to the extent possible, we would prefer not to adopt any such requirements for LPFM stations. The low ERP levels proposed for LPFM stations (especially LP100 stations), together with a tight spectral emission mask for such stations and our proposed requirement to certify transmitters,71 should significantly reduce the potential for harmful interference to existing service, even if 2nd-adjacent channel interference protections are not adopted.
49. We are concerned that our understanding of future IBOC systems is preliminary and that we may not be fully aware of any negative impact or restrictions that authorization of low power radio service would have on the transition to a digital IBOC technology for FM stations. Clearly, we need to better understand the potential impact of second-adjacent channel LPFM protection standards on the successful development of an IBOC system. Without a 2nd-adjacent channel protection standard, would analog LP1000, LP100, and microradio stations be likely to adversely affect current IBOC designs and, if so, how and to what extent? What would be the effect of digital LPFM stations operating with IBOC technology? Are measurement results available to inform the analyses of the potential for 2nd-adjacent channel interference? As noted more fully below, we are interested in examining LPFM emissions and bandwidth limitations as possible means of ameliorating some interference concerns. Could a strict spectral emission mask and/or a reduced channel bandwidth for LPFM stations play a significant role in reducing the potential for interference, if there were no minimum station separation requirements for LPFM stations operating on the 2nd-adjacent channels to other FM stations? Conversely, could potential interference to digital radio be minimized by such measures as filters and other digital receiver improvements? Would our proposal to certify transmitters, described at paragraph 0, below, be useful in minimizing interference to digital service? In this regard, we are particularly interested in the views of digital radio designers and manufacturers. At this initial stage of our involvement with digital radio, we ask whether the IBOC signal could be designed to be robust against interference from lower power stations operating without a minimum spacing requirement on the second-adjacent channel. What design tradeoff would be involved and with what implications to the effectiveness of terrestrial digital radio? In this regard, would it be appropriate to consider standards for future digital receivers? Finally, we ask whether it would be appropriate to impose a 2nd-adjacent channel protection requirement on LPFM stations for the purpose of protecting a possible future digital radio technology, considering that creating opportunities for new radio service is also an important Commission goal. In this regard, we also note that, as secondary services, LP100 and microradio stations would not be permitted to interfere with future digital radio stations within their protected service areas.
50. We seek comment on whether we should consider lower interference standards for the LPFM service. A staff study, attached hereto as Appendix D, demonstrates that if LPFM stations are required to comply with current interference restrictions, there will be few or no licenses available in most major markets. This study also shows that we measurably increase the opportunity to engineer in LPFM stations if third-adjacent channel protection standards are eliminated and dramatically increase such opportunities if second-adjacent channel standards are not considered. The paucity of major market LPFM spectrum under our current rules testifies to the aggressive efforts of existing broadcasters to maximize service. Principally for this reason, we are disinclined to extend reduced second- and third-adjacent channel protection standards to full power FM stations. We believe that the relatively low maximum power levels of the LPFM stations under consideration here support this distinction. Such stations could create only very limited areas of harmful interference, especially if we impose additional technical modifications to reduce their interference potential. We also note that if we were to take this step, opportunities for low power stations would diminish as existing broadcasters move quickly to improve their own facilities. We seek comment on these issues.
53 Pursuant to Sections 73.213 and 73.215 of our rules, FM stations may be assigned at locations that do not meet the distance separations of Section 73.207 on the basis of a combination of reduced separations and signal contour protection.
54 For example, a contour overlap approach would involve terrain data and computations of antenna height above average terrain. It would also require applicants to submit data on directional antenna patterns, which the Commission would enter into a directional antenna data base.
55 We include minimum separation distances for protection to and from stations operating on 2nd- and 3rd-adjacent channels, even through the need for these protections for LPFM stations are at issue in this proceeding.
56 Notice of Proposed Rule Making and Order in MM Docket No. 98-93, 13 FCC Rcd. 14849, 14863-65 (1998).
57 For example, the Skinner
Petition at 34 suggests that these protections are no longer necessary because of "vast improvements in receiver technology since the restrictions were created decades ago." CRC asserts that these protections are unnecessary for LPFM facilities operating with less than 50 watts at 100 meters HAAT (or any equivalent combination of power and HAAT). CRC Petition for Rule Making at 3.
58 NAB Comments at 20-25; NPR Statement at 7; New Jersey Broadcasting, Inc. Comments at 2-4. See also USA Digital Radio, Inc. Comments at 7-8 (addressing 2nd-adjacent channel protections only).
59 A protected station's signal is most vulnerable to interference where it is weakest, i.e., at the outer edge of the protected station's coverage area. Within the coverage area, the protected signal increases in strength as the station location is approached, thus tending to mask the effects of interference.
60 Report and Order in MM Docket No. 96-120, 12 FCC Rcd 11840, 11847-49 (1997) (Grandfathering of Short-Spaced Stations R&O). We note that the decision was not supported by NAB.
61 To illustrate, suppose we were to apply minimum distance separations for LPFM stations giving protection equivalent to levels that full service FM stations protect each other. LP1000 stations would then have to be separated from existing 2nd- and 3rd- adjacent channel FM stations by approximately 31 to 96 kilometers (19 to 60 miles), depending on the protected station's class. Minimum separation distances of about 17 to 65 kilometers (10.5 to 40 miles) would be required to protect LP1000 stations from the higher station classes. Appendix A gives the minimum separation distances for second adjacent channel protection among the various FM radio classes.
62 Appendix D suggests the extent of available spectrum in sample cities of different population groupings and under different levels of interference protection.
63 For commercial FM stations authorized under the contour methodology, the required separations are slightly smaller; however, the predicted field strength of a potentially interfering station can be no more than 40 dB stronger than the protected field strength along a station's protected contour. 47 C.F.R. ' 73.215.
64 Grandfathering of Short-Spaced Stations R&O at 11849.
65 We seek comment on the original rationale for 2nd- and 3rd-adjacent channel protections and the extent to which circumstances have changed in such a way to support relaxation of these protections.
66 Report and Order in Gen Docket No. 90-357, 10 FCC Rcd 2310, 2315 (1995). 67 Public Notice, DA 98-2244 (November 6, 1998).
68 For example, in USADR's proposed "FM hybrid mode," digital sidebands would occupy the "70 kHz regions between 129 and 199 kHz from the center frequency on either side of the analog spectrum." USADR Petition at 47. Appendix C also discusses the issue of second-adjacent channel protection to the IBOC signal.
69 For example, USADR states that "an analog second adjacent interferer will have a negligible effect on the performance of the all-digital signal, since it does not overlap in frequency with the desired all-digital signal." USADR Petition, Appendix E at 42.
70 The extent of channel availability for LPFM service with and without a 2nd-adjacent protection standard is depicted in Appendix D.
71 See paras. 0, above, and 0, below