FCC: Laying Down the UNII Law

FCC: Laying Down the UNII Law

By CWNP On 08/03/2010 - 31 Comments

The word on the street is that the FCC is cracking down on wireless usage in parts of the UNII 2e band. The FCC released a memorandum last week to encourage new wireless deployment practices and vendor marketing emphasis for avoiding interference with certain kinds of radar. I’ll explain the purpose and message of the memorandum here, but if you’d like to read for yourself, here’s a link: http://www.wi-fi.org/files/FCC_Memorandum_on_UNII_Device_Operation_2010_07_27-M.pdf

As quick background, the FCC is the regulatory body in the US that is responsible for overseeing wireless usage, frequency allocation, and similar RF-related guidelines. In order to meet the FCC’s guidelines for wireless devices, transmission systems must be certified by the FCC as a system. In other words, from the radio to the antenna and everything in between, a “system” must be certified. Since wireless has become so prolific in the US, the FCC can’t possibly enforce every rule and regulation. For that reason, many end-users are pretty relaxed in their use of uncertified systems. However, some high priority issues warrant the FCC’s dedicated attention, and that is why the memorandum was drafted. To any readers out there with illegal/uncertified systems, listen up.

The issue at hand here is that of WLAN interference with Terminal Doppler Weather Radar (TDWR), which is used at airports to forecast weather patterns. TDWR operates in the 5.600 - 5.650 GHz (5600 to 5650 MHz) frequency band, which happens to sit right in the middle of the UNII 2e band (5.470 GHz - 5.725 GHz). Now you may be thinking that DFS takes care of the problem (I did), but the FCC seems to have found that even DFS-compliant systems are still causing problems.

From what I’ve gathered, the FCC has been experiencing issues with interference to TDWR for some time now, which is why they released new regulations in the Fall of 2009 stating that FCC DFS certification requires that wireless devices NOT operate in the 5600-5650 MHz frequency range at all. In other words, you can’t get a certification if you use this span of the spectrum. During certification testing, you must demonstrate that this span is unsupported. The source of this information is here:


First off, why didn’t I know that before now? For all intensive purposes, this cuts out four three of the eleven UNII-2e channels (120, 124, 128, and 132 -- Correction: Channel 132 (5660 MHz) was not affected by this...see comments). Given the number of 5 GHz channels, that’s no big deal really, but this takes us from twenty-four 20 MHz channels down to twenty twenty-one 20 MHz channels.

So, what else is going on here? For starters, the FCC doesn’t like the fact that high priority weather radar systems are experiencing interference. So, they’re tracking down interferers and forcing remedial action. Apparently, some interfering systems were illegal (too high Tx power, uncertified transmit systems, no permission for use, etc.), while others were perfectly within the regulations. WISPs with outdoor, line-of-sight, co- and adjacent-channel (to the TDWR equipment) transmissions were the interferers in this case, but the FCC’s response is a little more global. So…

1. The memorandum includes a list of TDWRs, their locations (long/lat), antenna height, and operating frequencies.

2. The FCC strongly encourages all wireless operators within 35 km (did we switch over to metric without my knowing?) of a TDWR to avoid use of frequencies within 30 MHz of the operating frequency of the nearby TDWRs.

For most of us, the impact of this memorandum isn’t severe, but it is important. If this applies to you and your WLAN, I’d encourage that you verify your setup and your system’s certification before the FCC comes knockin’. I suspect that fines come along with uncertified systems causing interference.

Tagged with: FCC, DFS, TDWR, UNII 2e

Blog Disclaimer: The opinions expressed within these blog posts are solely the author’s and do not reflect the opinions and beliefs of the Certitrek, CWNP or its affiliates.

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