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'Widely Different Conclusions'

DOT Raises Concerns on Part of FCC's Proposed Changes to 6 GHz Rules

With FCC commissioners set to vote Oct. 19, the Department of Transportation questioned whether the -27 dBm/MHz out-of-band emission limit allowed by the FCC for very-low power portable devices and mobile access points in the 6 GHz band will be harmful to cellular vehicle-to-everything operations in the adjacent 5.9 GHz band. The FCC’s draft order addresses the C-V2X concerns raised by DOT.

DOT has been expressing concern about protecting auto safety use of the 5.9 GHz band, and opposed a 2020 order reallocating the band (see 2011170058). Those concerns continued during the Biden administration (see 2103250071). The FCC didn’t comment Wednesday on the DOT complaint.

The NTIA letter includes a report on tests by DOT. “Based upon this testing, NTIA and DOT urge the Commission to set the out-of-band emission limit for VLP portable devices at -37 dBm/MHz (at 5.925 GHz) and to prioritize unlicensed operations in channels above 6 GHz,” said a filing Tuesday in docket 18-295. “This proposal is similar to one previously submitted by industry stakeholders,” NTIA said, citing a 2021 letter by Broadcom, Facebook, Intel, Cisco and Qualcomm.

The Alliance for Automotive Innovation also raised concerns. The alliance, in a series of recent meetings at the FCC, “reiterated concerns that, as unlicensed devices proliferate throughout the 6 GHz band over time and operation in the lower portion of the U-NII-5 band becomes likely, the OOBE limits put forth by the Commission will not sufficiently protect V2X operations,” said a filing posted Wednesday. The alliance urged the commission to “adopt an OOBE limit for the lower portion of the U-NII-5 band that is at least as restrictive as the -37 dBm limit previously proposed by a collection of VLP proponents and V2X proponents.” The filing also cites the 2021 letter.

The limits in the order “are intended to protect” C-V2X “operations below the 6 GHz band and federal operations above the band,” said the draft item circulated by FCC Chairwoman Jessica Rosenworcel (see 2309270064). “The Commission previously determined that the -27 dBm/MHz limit will sufficiently protect C-V2X operations from harmful interference from U-NII devices operating in other bands,” the draft says: “We remain convinced that the -27 dBm/MHz out-of-band emission level at the lower edge of U-NII-5 will protect C-V2X operations below 5925 MHz and adopt that level for VLP devices.” The limits “will create a consistent out-of-band limit for all 6 GHz unlicensed devices throughout the 6 GHz band,” the draft concludes.

The auto alliance also urged the FCC to act on an order more broadly allowing early C-V2X deployments, beyond an April waiver (see 2304240066). “While the recent vehicle-to-everything waiver grants have been helpful, the auto industry requires certainty,” the alliance said.

Also posted Wednesday was a joint letter from the alliance, the 5G Automotive Association and ITS America raising concerns about the -27 dBm/MHz limit. A limit of -37 dBm/MHz is “technically achievable, consistent with protections adopted in other countries, and necessary to ensure continued protection of C-V2X as unlicensed VLP devices proliferate,” the groups said.

The Open Technology Institute at New America questioned why the order reduces the proposed maximum power level of VLP devices from 1 dBm/MHz power spectral density (PSD) to -5 dBm/MHz in the U-NII-5 and U-NII-7 portions of the band. The group questioned whether that limit “is necessary to protect high-power fixed link incumbent operations from actual harmful interference.” A representative spoke with aides to Rosenworcel and Commissioner Geoffrey Starks.

The draft cites but rejects arguments by Wi-Fi advocates that VLP devices require a 1 dBm/MHz PSD limit “to meet expected consumer use cases, overcome on-body loss, and meet minimum throughput, latency, and power efficiency requirements.” The draft notes “the record is replete with many analyses and tests that come to widely different conclusions.”