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FCC Rejects Concerns Over UWB Interference to C-band Downlinks

The FCC has modified Part 15 of its rules to provide more flexibility for introducing new wide-bandwidth devices and systems. It also issued a Memorandum Opinion and Order rejecting a Petition for Reconsideration filed by the Satellite Industry Association (SIA) and comments and a study from the Coalition of C-band Constituents (Coalition) that asserted that ultra-wideband (UWB) operation under the rules would interfer with C-band reception.

Under Part 15 rules, the wide bandwidth emissions from unlicensed UWB devices can spread into licensed frequency bands allocated to both U.S. Government and non-government operations. To accommodate UWB, the FCC had to remove the Part 15 restriction prohibiting operation in the restricted frequency bands. In this Report and Order, the FCC agreed to permit the use of peak emission levels, similar to the levels applied to UWB devices, for wideband emissions in the 5925-7250 MHz, 16.2-17.7 GHz and, with some restrictions, in the 23.12-29.0 GHz band. The Report and Order noted, "This action will facilitate the introduction of some of the operating systems sought by the commenting parties, including radar systems that would be used to improve automotive safety and tracking systems that could be employed for personnel location..."

The FCC took steps to protect users of this spectrum from interference. For example, the rules prohibit the use of fixed outdoor infrastructures in the 5925-7250 MHz band, which includes TV broadcast auxiliary service links, although it does not consider operation onboard a ship or within a "terrestrial transportation vehicle" as a fixed infrastructure.

As noted in another article in this week's RF Report, there is concern that allowing these devices to operate in the 23.6 to 24 GHz band will interf with Earth Sensing Satellites (ESS). The Second Report and Order and Second Memorandum Opinion and Order addresses these concerns.

Siemens VDO asked the FCC to allow its frequency-hopping vehicular radar system to operate in this band, stating its system would cause no more interference than the UWB devices allowed in this band. NTIA said that frequency-hopping transmitters should not be allowed in this band. The FCC, working with NTIA, determined that because frequency-hopping systems can be programmed to avoid specific frequencies, the Siemens VDO system could maintain the -10 dB bandwidth of the system between 24-29 GHz and avoid the ESS band. As such, the system would be authorized as a non-UWB device and fundamental emissions would not be permitted in the 23.6-24 GHz band.

C-band satellite operators weren't as fortunate. The FCC considered SIA's current Petition for Reconsideration "repetitious of its earlier Petition for Reconsideration of the 1st R&O" and dismissed it. The Coalition of C-band Constituents submitted a study conducted under contract by Alion Science and Technology to see how much interference C-band satellite receivers would incur from UWB operation. The Alion study concluded that satellite receivers "will experience complete reception failure at currently regulated UWB power levels assuming emitter densities currently found in the environment of common wireless-based consumer units" and asked the FCC to reduce allowed emission levels in the 3700-4200 MHz band by 21 dB.

The FCC said that the Alion study is based on multiple worst-case assumptions, "most of which simply are not realistic." It added that "this study is founded on the premise that a large number of UWB devices will be near the FSS receiver and will contribute significantly to the aggregate interference level. Specifically, the study assumes that 90 percent of the UWB devices distributed within 1,000 meters of the FSS station, 70 percent between 1 and 2 km, 50 percent within two and three km, etc. have a direct line-of- sight propagation path into the FSS receive antenna without attenuation from any intervening objects.236 In addition, it is assumed that UWB transmitters may be located as close as 30 meters to the FSS antenna, that all UWB emitters within a 5 km radius are pointed directly at the FSS antenna, and that all UWB emitters operate at a 100 percent activity factor.

"Further, the Alion analysis assumes that every UWB device operating within 5 km of the FSS station produces the maximum permissible power spectral density (PSD) level within the pass band of the FSS receiver. The Coalition has further exacerbated the worst-case results from the Alion study by applying unsupported and unreasonable projections with respect to UWB device proliferation. The Coalition assumes that UWB devices will replace all existing Part 15 cordless telephones, wireless security applications, and wireless data communications and will be used for communications within and between vehicles, resulting in an estimated 64 UWB devices per acre or 1.24 million UWB devices, all operating continuously, within 5 km of an FSS receiver."

The FCC said it was able to duplicate the Alion study and obtain similar results using the same assumptions. However, when it recalculated the analysis based on more realistic operation conditions, such as assuming many UWB devices would be located indoors, it obtained results that more than offset the 21dB of attenuation requested in the Coalition's filing.

The FCC Second Memorandum Opinion and Order concluded, "Based on these two offsets to the Coalition's requested protection criteria, we find no justification to reduce the UWB emission levels in the FSS frequency band. However, we will continue to monitor this situation and will take whatever appropriate action is necessary to ensure that UWB operation does not result in harmful interference to FSS receivers. If a Part 15 device causes interference it is required to remedy the interference or to cease operation. The remedy could be as simple as relocating the UWB device away from a window.."

There is much more to this action than I have reported. Please refer to the full text of the Second Report and Order and Second Memorandum Opinion and Order to see the technical details behind the arguments. For a short summary, refer to the FCC news release FCC Amends Rules to Permit New Wideband Unlicensed Devices and Affirms Rules to Authorize the Deployment of Ultra-Wideband Technology/