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Before the Federal Communications Commission Washington DC 20554

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Before the Federal Communications Commission Washington DC 20554 Before the Federal Communications Commission Washington DC 20554 In the Matter of ) ) Unlicensed Use of the 6 GHz Band ) ET Docket No. 18-295 ) Expanding Flexible Use of the Mid-Band ) GN Docket No. 17-183 Spectrum Between 3.17 and 24 GHz ) COMMENTS OF THE FIXED WIRELESS COMMUNICATIONS COALITION Cheng-yi Liu Mitchell Lazarus FLETCHER, HEALD & HILDRETH, P.L.C. 1300 North 17th Street, 11th Floor Arlington, VA 22209 703-812-0400 Counsel for the Fixed Wireless February 15, 2019 Communications Coalition TABLE OF CONTENTS A. Summary .................................................................................................................................. 1 B. 6 GHz FS Bands and the Public Interest ................................................................................. 6 C. Fallacies on RLAN/FS Interference ........................................................................................ 9 1. High-off-the-ground FS antennas .................................................................................... 9 2. Indoor operation ............................................................................................................. 10 3. Statistical interference prediction .................................................................................. 10 D. Automatic Frequency Control Using Exclusion Zones ......................................................... 13 E. Multipath Fading and Fade Margin ....................................................................................... 15 1. Momentary interference ................................................................................................. 18 F. Indoor Operation.................................................................................................................... 18 G. Details of Automatic Frequency Control .............................................................................. 22 1. Protection Criterion ........................................................................................................ 22 2. Propagation models ........................................................................................................ 23 3. Need for guard band ....................................................................................................... 25 4. Choice of database ......................................................................................................... 28 5. RLAN location accuracy ................................................................................................ 29 6. RLAN elevation ............................................................................................................. 29 7. Client devices ................................................................................................................. 30 8. Initial request for frequencies ........................................................................................ 30 9. AFC security, reliability, and testing ............................................................................. 31 10. Controlled AFC rollout .................................................................................................. 32 11. Allocation of costs ......................................................................................................... 32 H. Other Issues ........................................................................................................................ 33 1. Rural and underserved areas .......................................................................................... 33 2. Moving vehicles and drones .......................................................................................... 34 3. Digital Identifying Information ...................................................................................... 34 4. Interference resolution ................................................................................................... 35 CONCLUSION ............................................................................................................................. 36 Attachment A: Determining the Impact of Non-Coordinated Indoor 6 GHz RLAN Interference on Fixed Service Receivers Attachment B: Need for Adjacent Channel Interference Protection Attachment C: RLAN/FS Guard Band Analysis Before the Federal Communications Commission Washington DC 20554 In the Matter of ) ) Unlicensed Use of the 6 GHz Band ) ET Docket No. 18-295 ) Expanding Flexible Use of the Mid-Band ) GN Docket No. 17-183 Spectrum Between 3.17 and 24 GHz ) COMMENTS OF THE FIXED WIRELESS COMMUNICATIONS COALITION The Fixed Wireless Communications Coalition, Inc. (FWCC”)1 files these comments in response to the Notice of Proposed Rulemaking in the above-captioned proceeding.2 A. SUMMARY These comments are directed solely to protecting the 96,604 fixed service (FS) links in the 6 GHz bands from interference due to unlicensed devices.3 1 The FWCC is a coalition of companies, associations, and individuals actively involved in the fixed services—i.e., terrestrial fixed microwave communications. Our membership includes manufacturers of microwave equipment, fixed microwave engineering firms, licensees of terrestrial fixed microwave systems and their associations, and communications service providers and their associations. The membership also includes railroads, public utilities, petroleum and pipeline entities, public safety agencies, cable TV providers, backhaul providers, and/or their respective associations, communications carriers, and telecommunications attorneys and engineers. Our members build, install, and use both licensed and unlicensed point–to–point, point–to–multipoint, and other fixed wireless systems, in frequency bands from 900 MHz to 95 GHz. For more information, see www.fwcc.us. 2 Unlicensed Use of the 6 GHz Band, ET Docket No. 18-295, GN Docket No. 17-183, Notice of Proposed Rulemaking, FCC 18-147 (released Oct. 24, 2018) (Notice). 3 All link data are courtesy of Comsearch, current as of January 21, 2019, except as noted. Terminology: A “link” is a licensed channel on a physical path. There may be multiple links on a single license. The term “fixed service” (FS) includes the Part 101 Common Carrier and Private Operational Fixed Services, but not the Part 74 Broadcast Auxiliary Service. The bands at issue are: Frequencies Band Name FS Usage (MHz) U-NII-5 5925-6425 66,324 FS links U-NII-6 6425-6525 no FS links U-NII-7 6525-6875 30,280 FS links U-NII-8 6875-7125 194 FS links We use the term radio local area networks (RLANs) for 6 GHz unlicensed devices generally. The Commission contemplates two RLAN categories: access points that receive frequency permissions from a database system, and client devices that receive frequency permissions from a nearby access point. We take no position on the NPRM proposal to allow indoor operation in the U-NII-6 and U-NII-8 bands without frequency coordination. RLANs in the U-NII-5 and U-NII-7 bands, however, will have to protect FS links at their current levels of reliability. This is a demanding standard. Many FS systems at 6 GHz carry critical services: controlling pipelines, railroad trains, and the electric grid, and providing backhaul for public safety communications. Some links operate at reliability levels of 99.9999%: downtimes of one second out of every million, or thirty seconds per year. Most others operate at 99.999%, a limit of five minutes’ outage per year. A system served by a networked FS receiver—most receivers are networked—may need fifteen minutes to resynchronize after even a short interruption, thus magnifying the effects of a transitory interference event. To keep 96,000 2 FS systems operating at their present reliability, RLAN interference will have to be extremely improbable. The needed degree of protection is feasible, but the measures to achieve it will have to go beyond the RLAN proponents’ proposals, and in some respects, beyond the Commission’s proposals as well. A study in the record, submitted by RLAN proponents, underestimates interference into the FS by using the shortcut of statistical modeling. The approach uses a computer program that populates an area with virtual FS receivers. It scatters RLANs at random, under some assumed distribution of locations and powers. The program tests to see how much RLAN/FS interference occurs. It then re-scatters the RLANs and tests again. Running a large number of such cases yields an overall estimated average probability of RLANs causing interference to FS receivers. Those estimates, unsurprisingly, depend on the assumed numbers: likely locations of RLANs, distributions of RLAN power levels, expected propagation losses, and so forth. The approach has important limitations: it can successfully predict the incidence of interference only from RLANs whose placements match the study designers’ expectations, and only where signal losses along the RLAN-to-FS path match the study’s assumptions. But FS interference rarely arises from the expected. Decades of experience show that interference is rare, but when it occurs, most often comes from a single source at an unlikely location and atypical path loss—precisely the cases that statistical modeling is most likely to miss. Statistical studies typically use propagation models that assume ambient clutter and local terrain will attenuate interference by X dB along part of every interference path between an RLAN and an FS receiver. If the model is properly chosen, then indeed, some large fraction of interference
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