In May, the FCC issued a comprehensive incentive auction framework order that represented a substantial step toward a successful auction. While the order resolved many open issues, it also set the stage for a series of new ones and the published list of follow-on proceedings made clear that significant work remains to be done.
Of the open issues, the one that is of greatest concern for auction success is the service rules that will be adopted for the unlicensed services that the FCC has concluded should be permitted in the 600 MHz duplex gap. In the order, the FCC expressed confidence that unlicensed TVWS (TV white space) devices can operate in a 6 MHz channel in the duplex gap adjacent to a 4 MHz licensed wireless microphone channel without causing interference to neighboring licensed allocations. The FCC further tentatively concluded that unlicensed devices operating at 40 mWs were viable, even with no separation from licensed wireless uplink. Importantly, the order contained no technical analysis supporting this conclusion, leaving open the question of how these proposals were justified.
The Commission, however, correctly concluded that, consistent with the Spectrum Act, use of the guard bands would be subject to the Commission’s ultimate determination that such use will not cause harmful interference to licensed services. We agree that such a determination is essential – interference issues that were left unresolved in the 700 MHz band plan created significant post-auction deployment challenges. Indeed, the 700 MHz A block remains significantly under-deployed even today, six years after the auction.
We recognize that the Commission has yet to adopt the final technical rules that will govern the proposed unlicensed uses in the 600 MHz band and a draft NPRM on this issue just circulated for a vote. But I must be honest; we do not, at this time, share the Commission’s confidence that unlicensed devices in the duplex gap in the configuration and at the power levels identified in the order can operate without creating interference to the adjacent licensed allocations. Indeed, at least one technical analysis has concluded that a 600 MHz LTE device would be unable to properly receive if it was within 20 meters of an adjacent unlicensed device operating at 40 mWs.
Adding to our concern is a recent filing by Broadcom. Broadcom claims that an unlicensed device can operate within the duplex gap at a transmit power more than double that contained in the FCC’s tentative conclusion. To support this, Broadcom offered a detailed technical analysis that was quickly challenged by Qualcomm.
I’m not going to repeat the various technical flaws and contradictions that Qualcomm identified, but I think the two filings are important in that they are a preview of the complicated and challenging interference issues the Commission will face in the unlicensed proceeding to come.
Let’s look at just a single issue in dispute. In its analysis, Broadcom used an assumed separation radius around 600 MHz Wi-Fi access points of three meters – a distance that neither the FCC nor the industry has ever endorsed. This essentially means that a 600 MHz LTE device and a 600 MHz unlicensed device could not co-exist in the same room.
Qualcomm correctly points out just how flawed that assumption is. LTE and Wi-Fi devices co-exist today in homes, offices, schools, airports, convention centers and stadiums. Indeed, LTE and Wi-Fi devices often co-exist on a single person – it’s not hard these days to find someone working on a Wi-Fi tablet while simultaneously talking on a LTE smartphone.
These filings are indicative of why it is essential that the Commission move cautiously as it embarks on the proceeding to establish service rules for unlicensed use in the 600 MHz duplex gap. If interfering services are ultimately permitted in the duplex gap, the adjacent licensed blocks (and their associated paired channels) will not be fungible. They will instead be impaired licenses.
Some license impairments may be unavoidable – particularly in the border areas. But as the 700 MHz band plan taught us, the final 600 MHz band plan adopted by the FCC should seek to avoid interference impairments wherever possible. Introducing interference into the band plan where it need not exist would be a significant step in the wrong direction.