Broadcasters have joined other licensed spectrum users in filing comments opposing the use of an interference temperature metric to allow unlicensed operations in licensed spectrum. The Association for Maximum Service Television, Inc. (MSTV) and the National Association of Broadcasters (NAB) filed joint comments in FCC ET Docket 03-237, Establishment of an Interference Temperature Metric to Quantify and Manage Interference and to Expand Available Unlicensed Operation in Certain Fixed, Mobile and Satellite Frequency Bands. The Society of Broadcast Engineers (SBE) filed comments in the same proceeding outlining technical problems with the plan for unlicensed operations in the 12.75 to 13.25 GHz band described in FCC's Notice of Proposed Rulemaking (NPRM). For a details on and links to the NPRM, see RF Report for December 1, 2003.
The Joint Comments of the Association for Maximum Service Television, Inc., and the National Association of Broadcasters said that "significant technical challenges must be addressed before such a metric can be used as a basis for spectrum management decisions instead of merely serving an informational and monitoring function," adding, "the interference temperature metric should not be used as a justification to introduce unlicensed operations in licensed spectrum bands, especially while the interference temperature metric remains a theory and is not close to being implemented." There was also concern that once unlicensed devices get into the marketplace, they are "difficult if not impossible to locate and then control."
MSTV and NAB noted that existing unlicensed devices operate successfully because their existing operations are different from the proposed unlicensed operations.
"First, the existing equipment authorization process for unlicensed devices involves well-understood technologies, making it somewhat easier to predict the interference impact of existing unlicensed operations," the associations noted. "Second, unlicensed devices currently operate in spectrum bands that are either dedicated for unlicensed operations or are shared with services that use spectrum sporadically and that are prepared for a somewhat uncertain interference environment. The new unlicensed operations that are being contemplated would share spectrum with licensed primary services unaccustomed to sharing spectrum, making worse the impact of potentially interfering unlicensed underlay devices."
Although FCC rules prohibit unlicensed devices from interfering with licensed services. MSTV/NAB warned these rules cannot be enforced once consumers have the devices as they don't appear in any FCC or industry database and cannot be easily detected and shut down. Before such devices are authorized, the FCC should have mechanisms in place to identify unlicensed devices that are causing interference. Registration of these devices should be required as well as incorporation of a unique identification code into the devices' transmission.
Unlicensed operations in TV broadcast spectrum will cause special problems for TV viewers, as broadcasters have no control over the receivers used to receive their services, unlike a closed system where a single operator controls both transmitters and receivers. In this case, the operator can adapt to the unlicensed devices and is in a better position to locate and address interference caused by a new device.
With DTV, where interference is likely to completely disrupt reception, MSTV/NAB told the commission that viewers who lost their picture for even a few moments may be inclined to seek programming from other media.
"Viewers have a low tolerance for interference, and tend to simply switch channels when confronted with interference on their radio and television receivers rather than attempt to pinpoint the source of interference." Such interference from unlicensed devices, MSTV and NAB warn, could ultimately derail the digital transition if, at the margin, fewer viewers choose to view digital television over-the-air. This would prevent the FCC from achieving its primary objective of more efficient use of the broadcast band.
The MSTV/NAB filing focused on the importance of the 12.75 to 13.25 GHz band for short-haul mobile pickup and studio-transmitter links. Due to the intermittent nature and different locations used for the short-haul links, it is extremely difficult to monitor interference. The filing warned, "To the extent that interference in the 12.75-13.25 GHz band increases because of new unlicensed operations, short-haul ENG operations may be forced to shift to other, already-crowded ENG bands. However, the FCC's spectrum policies have already increased pressure on these ENG bands, especially the 2 GHz band.
The associations warned the commission that creating situations that continually reduce the amount and integrity of spectrum for newsgathering is contrary to the public interest.
"It simply is inappropriate for the Commission to launch an interference temperature experiment that further threatens this vitally important service," the associations said. "The newsgathering function of local television stations is too important to national security to be jeopardized by application of a new 'spectrum management' theory."
The Comments of the Society of Broadcast Engineers, Inc. agreed with those of Agere, AT&T, Cingular and Verizon Wireless that a noise temperature criteria would be impractical and unworkable, primarily because the location and number of unlicensed devices "can never be known with certainty." SBE added that, "there is no guarantee that all, or even most, of the favorable-to-Part 15-device assumptions made in the NPRM for the 12.75-13.15 GHz TV Broadcast Auxiliary Services (BAS) band, would in fact be the case. SBE can think of several just as plausible cases showing that interference to licensed stations would be caused."
SBE echoed MSTV/NAB's concerns about the difficulty of correcting interference problems after devices were on the market, saying that once these unlicensed devices had been sold to the public, "there would be no practical recourse available to the users of the licensed TV BAS stations entitled to protection from interference from unlicensed, Part 15 devices. The damage would have been done."
SBE said the NPRM relies on several assumptions, none of which can be guaranteed, when it concludes that the proposed higher power (up to 36 dBm EIRP) 13 GHz Part 15 devices would not cause interference to BAS links. These incorrect assumptions are:
* "The unlicensed Part 15 device would be at least 100 meters (328 feet) from the receiving antenna of a licensed Part 74 TV BAS station.
* The licensed Part 74 TV BAS station will be using a highly directive receiving antenna.
* The unlicensed Part 15 device would be at least 20 degrees off-axis to the main beam of this highly directional receiving antenna.
* Even if condition 4c does not apply, the unlicensed Part 15 device would be inside a structure that would provide 'sufficient attenuation' to nevertheless ensure that no interference to the licensed facility is caused.
* That a co-channel desired-to-undesired (D/U) signal ratio of '30 dB to 50 dB' will be adequate to ensure that harmful interference is not caused to the licensed TV BAS station."
The SBE comments note that Section 101.105(c), which applies to the prior coordination rules now mandatory for 13 GHz TV BAS stations, stipulates a 90 dB D/U co-channel interference criteria ratio "where the development of the carrier-to-interference (C/I) ratio is not covered by generally acceptable procedures, or where the applicant does not wish to develop the carrier-to-interference ratio."
SBE points out that Part 15 users would have a wide choice of modulation methods and therefore, at a minimum, a D/U ratio of at least 60 dB should be used. Refer to the Comments of the Society of Broadcast Engineers, Inc. for an analysis showing how easily this D/U ratio could be violated in several situations by Part 15 devices operating under the proposed "interference temperature" rules.
SBE recommended that a better choice of spectrum to test this concept would be in the 7,900 to 8,025 MHz Earth-to-space federal government fixed satellite band. SBE notes that, "The 36 dBm EIRP of an 8 GHz Part 15 device should be inconsequential at the input terminals of a spacecraft-based receiver compared to the signal that is received from the typical 100 dBm to 115 dBm EIRP of an Earth-to-space uplink station. This would give D/U ratios at the spacecraft receiver on the order of 64 to 79 dB, which should be sufficient to ensure no interference."
The SBE continued, "if the Commission is unwilling to propose to NTIA or IRAC placing such a risk on the users of federal government spectrum, or if NTIA or IRAC do not agree to the sharing of federal government spectrum with unlicensed, Part 15 devices, then SBE asks the Commission why it should expect civilian licensees to accept a risk that the federal government would not be willing to accept."
SBE concludes, "While improvements in RF devices are real and ongoing, they do not at this stage of development remotely justify the proposed 77 dB increase in the EIRP allowed for unlicensed, Part 15 devices at 13 GHz. In the interference debate above, SBE would have to declare Part 74 TV BAS licensees as winners of the debate battle, but losers in the interference war if this destructive rule relaxation is adopted. SBE accordingly urges the Commission to terminate this ill-conceived 'noise temperature' rulemaking forthwith, or, lacking that, to not conduct the dangerous experiment in the 12.75-13.25 GHz TV BAS band."
In addition to the Comments of the Society of Broadcast Engineers, Inc. and the Joint Comments of the Association for Maximum Service Television, Inc., and the National Association of Broadcasters, also see the following articles for a glimpse of how other licensed spectrum users are viewing the interference temperature metric NPRM: Time to Take 'Noise Temperature' At FCC - Regulating interference based on noise temperature may not be wise for wireless and ARRL Says 'Interference Noise Temperature' Concept 'Highly Premature'.