In my last blog on 5 GHz spectrum, I discussed the recent FCC ruling that permitted outdoor access points to use the U-NII 1 band (5150-5250 MHz).

But the story doesn’t  stop there. As mentioned last time, there are significant technical challenges to using the 5 GHz band. It is not cleared spectrum. It contains incumbent uses that are important for national security and public safety. Therefore, it is imperative that Wi-Fi not create harmful interference to these incumbent systems. Cisco will not settle for less.

On the topic of interference, a particularly interesting component of the same  FCC ruling that opened the U-NII1 band for outdoor AP’s is that it also re-opened the Terminal Doppler Weather Radar (TDWR) band (channels 120, 124, 128) with new test requirements for DFS protection. Hold on, let’s backtrack a bit before diving into what this means:

What is TDWR?

In brief, Terminal Doppler Weather Radar (TDWR) “is a Doppler weather radar system used primarily for the detection of hazardous wind shear conditions, precipitation, and winds aloft on and near major airports situated in climates with great exposure to thunderstorms in the United States.” TDWR uses the frequency band from 5600-5650 MHz which is why wireless network equipment needs to be proven to “do no harm” to TDWR. If you’re curious for more information on TDWR, then please click here and/or here.

A Brief History

Many of you reading this will recall that the FCC closed the use of the TDWR band several years ago as the result of numerous reports of wireless equipment creating interference with TDWR. It’s important to note that “wireless equipment” was not only Wi-Fi.

The bottom line is that the flaws of a distinct minority resulted in the FCC being obligated to forbid wireless equipment operation in the TDWR band for everyone. This is completely understandable and defensible since the TDWR system largely contributes to the safe arrival and departure of commercial aircraft at airports across the country.

At this point you may be thinking “great history lesson Chris, but what exactly is changing and why should I care?”

What’s Changing

The FCC ruling is re-opening the Terminal Doppler Weather Radar (TDWR) band (channels 120, 124, 128) with new test requirements for DFS protection. The primary new requirement is that an access point (or any device acting as a master) must sense for radar signals at 100% of its emission bandwidth (the previous requirement was 80% of emissions bandwidth).

New test requirements mean a new test procedure (and certification) so that access points can prove themselves to detect and avoid radar (of course, this also means firmware modifications and recertification of existing access points). Although the new test procedure is still being defined it is anticipated to include additional radar profiles along with more stringent traffic loading requirements.

But what does this mean, especially in terms of 802.11ac?

When we think about this change, we need to remember that the three channels included in the TDWR band represent 60 MHz of bandwidth. So, re-opening TDWR translates into restoring three 11a channels. More importantly, it helps to enable two extra 11n channels (of 40 MHz each)–and even more importantly, it helps provide an extra 11ac channel of 80 MHz.


Consider this: Without the DFS channels there are only two 80 MHz channels for 11ac (i.e., the combination of channels 36/40/44/48 for one 80 MHz channel and the combination of channels 149/153/157/161 for another 80 MHz channel).

But with the DFS channels there are five 80 MHz channels for 11ac. Better yet, with the restoration of the TDWR channels there will be six 80 MHz channels for 11ac.

More channels results in lower co-channel interference (tax) from neighboring access-points and therefore helps provide a superior user experience. This is highlighted in the blog from my colleague Jim Florwick. Note that the 60 MHz made available from reopening the TDWR channels is the same as the entire usable spectrum at 2.4 GHz (three non-overlapping 20 MHz channels).

Why is this significant?

The advantage of having many channels should be obvious to anyone deploying in high density environments. It’s simple:

More channels = less probability of spectrum overlap.

Less spectrum overlap = better airtime efficiency.

Better airtime efficiency = higher network capacity.

In terms of additional capacity this means that we go from a total theoretical maximum capacity of 1820 Mbps when using only two 80 MHz channels (each channel theoretically capable of 910 Mbps) to 5460 Mbps when using six 80 MHz channels for an overall tripling of capacity.


Bottom Line

Adding more spectrum for broadband and Wi-Fi will lead to new ecosystems, new industries and new jobs, as well as help ensure technological leadership around the globe. Spectrum is not a commodity, and the use of shared spectrum is a privilege, not an entitlement.

Just as it was with achieving the outdoor use of U-NII 1, regaining the TDWR band is the result of many years of work in the industry, and has required open collaboration between many wireless stakeholders, especially network operators, network equipment vendors, and mobile device manufacturers.

I’d like to dedicate a special “shout out” to Mary Brown, from Cisco’s Government Affairs group as she continues to lead the effort in coordinating Cisco technical resources, while also working closely with industry stakeholders. As our lead corporate representative, Mary continues to be instrumental for Cisco acting as a “trusted advisor” to the FCC. Cisco dove into this initiative several years ago with the objective of seeing additional 5 GHz spectrum opened up and Mary is helping to fulfill that vision.

We at Cisco are committed to thought leadership in all aspects of the wireless industry. This FCC decision is just one of many efforts we have to create a more connected world. We look forward to sharing more exciting news about our projects and initiatives. Stay tuned!


Chris Spain

VP Product Management