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NTIA weighs in on FCC's 195 MHz spectrum announcement

By Matthew Gast in · HiveMind Blog · January 31, 2013

In the wake of the FCC’s announcement of 195 MHz of new spectrum, we’re all waiting with bated breath on the proposed rules. Here at Aerohive, we’re not alone, since many organizations are working through the implications of the proposed spectrum.

I’ve been waiting for the National Telecommunication and Information Agency (NTIA) to weigh in, since they act as the executive branch’s advisor on spectrum policy. NTIA doesn’t have regulatory authority, but they often work to coordinate the government agency response to FCC rules. Given that the frequency bands that the FCC announced are currently used by U.S. government agencies, NTIA would write the initial response, which was published Friday in a report

NTIA’s report is interesting reading. I think my favorite part was Appendix C, which lists all of the uses of the proposed new frequency bands: weather radar, various forms of navigation radar, unmanned aerial system (drone) links, and even electronic warfare. The report steps through the ways in which 802.11 works as a secondary user of a frequency band. Roughly speaking, 802.11 uses two methods to subordinate itself to a primary user. The primary method is classified as a sensing approach, in which an 802.11 device detects a primary user (such as radar) and backs off. Dynamic Frequency Selection (DFS) is a well-known example of this type of approach, and indeed, it features prominently in the NTIA report.

When the old TV broadcast spectrum was released, a second type of approach, called geolocationing, was developed. In a geolocation-type approach, a database is maintained of existing primary users and devices must be GPS-enabled and consult the database before beginning operation. For example, if a device were installed at 37.4 degrees north latitude, 122.0 degrees west longitude (Aerohive’s headquarters!), a geolocator would consult its database and from that determine if any channels needed to be blocked. Geolocation currently is not required for any existing 802.11 devices, but it may be required for devices deployed in the TV white space area.

The core of NTIA’s report is found on just four pages: 4-4 and 4-5 plus 5-4 and 5-5. These four pages have tables of the risk elements that NTIA has identified, along with proposed mitigations that will be submitted during the FCC rulemaking process.

For these radio systems, the NTIA summary is straightforward. The recommendations can be summed up as ensuring that the existing DFS regulations provide the protection that is intended. It is a good sign that so many of the recommendations are written around DFS rules because it means that basic framework is likely to be extensible. (Because the FCC has not yet released its proposed rules, many of NTIA’s suggestions may already be incorporated into the forthcoming FCC action.) Were it not, alternative approaches to spectrum sharing likely would have been suggested in the report.

The report’s recommendations include:

  • Study whether current DFS rules enable unlicensed devices to detect existing primary users. The guiding principle behind DFS is that when a primary user is on a given frequency, the secondary user (like 802.11) must give way. Before DFS can be effective, the detection algorithms must be robust. Studying algorithms to understand whether a band is in use is a key part of any FCC effort to open up new spectrum.
  • Change DFS rules to ensure that existing primary users do not suffer from significant interference. Although identified as a separate risk factor, it is part and parcel of modifying DFS rules. Aerohive, like all Wi-Fi equipment vendors, submits products for FCC certification before they can be sold. As part of these certification tests, we demonstrate that our equipment does not cause excessive interference. Part of making that demonstration possible is to develop good lab tests that replicate real-world scenarios, and if the secondary users cause serious harm, modify the regulations to ensure that they do not.
  • Some of the communication systems may suffer from hidden node problems, where an airborne system can receive a signal from a ground station as well as interference from ground-based devices on the same frequency some distance away. Wi-Fi networks are often deployed indoors, which should substantially mitigate the risk of hidden nodes resulting from indoor enterprise networks, so this recommendation is a consideration that likely looms larger for WISPs. One of the potential mitigation techniques for hidden nodes is geolocation, so that devices deployed near a ground station can avoid using the same frequency. Studying the extent to which hidden nodes are a problem is important, but its likely effects are largest for outdoor deployments.
  • The biggest concern raised in the report is that low-level interference from secondary users might cause harm to primary users. There’s not much that can be done other than study the problem, which is done as part of the detailed FCC rulemaking process (and which may already be underway at the FCC’s labs, but I have no particular insight into what the FCC is doing as part of creating their proposed rules).

Research into the effects of unlicensed devices on the existing primary users is a key component of opening up new frequencies. Whenever the FCC has opened up new spectrum in the past, a large part of that effort has been finalizing the testing rules for product vendors so that we do not interfere with primary users.

As with much in engineering, the answer to when we get the new spectrum is that it depends on the results of testing and whether the DFS rules need to be modified. Given that the report seems to accept the basic framework of DFS, I am hopeful that we will be getting more spectrum soon, and look forward to reading the FCC’s proposed new rules for myself.

Wi-Fi is beloved because it works, and it will work better with new spectrum – whenever it becomes available.

Read other spectrum-related blogs by Matthew:

Matthew Gast (@MatthewSGast)

Matthew Gast is Director of Advanced Technology at Aerohive Networks. He currently serves as chair of both the Wi-Fi Alliance's security task groups, was the first chair of the Wireless Network Management Marketing task group, and is the past chair of the IEEE 802.11 revision task group. Matthew is also the author of 802.11 Wireless Networks: The Definitive Guide (O'Reilly), which is now in its second edition and has been translated into six languages. His companion book, 802.11n book, 802.11n: A Survival Guide (O’Reilly), was recently published and provides information on how 802.11n works and what it means for the WLAN planning process. Most recently, Matthew completed 802.11ac: A Survival Guide (O'Reilly).


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