Friday, April 4, 2014

Impact of the FCC 5 GHz U-NII Report & Order on Wi-Fi Networks

Following the news release of the FCC's actions to change some technical rules for the 5 GHz U-NII bands, the official Report and Order was released on Tuesday.

I've read through the R&O, and here are the technical modifications that were approved:
  • U-NII 1 band (5.150 - 5.250 GHz) indoor operation restriction is removed. This allows use of the band for outdoor hotspots, WISPs, and bridge links. The growth of public hotspots will clearly benefit from this change.

  • U-NII 1 band (5.150 - 5.250 GHz) power level restrictions are changed. 
    • AP power levels at the Intentional Radiator may be 1W (previously 50mW) and the EIRP may be 4W using a 6dBi antenna (previously 200mW), and following the 1dB reduction rule in transmitter power for every 1dB of antenna gain above 6dBi. 
    • Client power levels at the IR may be 250mW and the EIRP may be 1W, following the 1:1 dB reduction rule for antenna gain above 6dBi.
    • WISPs may use up to 23dBi antennas on fixed point-to-point links without any corresponding reduction in transmitter power.

      These changes help to unify the U-NII 1 band with the U-NII 2A/2C and U-NII 3 bands so that larger contiguous swaths of spectrum can be combined using 802.11ac, which provides for 160 MHz channels comprised of two 80 MHz channels. In addition, by adopting power levels commensurate with U-NII 3, 80 MHz channels that are not adjacent to one another can easily be combined as well to form a 160 MHz channel. This ultimately provides greater flexibility in combing U-NII 1 with other U-NII 2A/2C/3 channels. The higher power limits also benefit WISPs for use on point-to-point links with higher gain antennas to achieve greater distances and throughput.
    In the [2013] NPRM, the Commission envisioned that harmonizing the power and use conditions across the lower 200 megahertz of U-NII spectrum (U-NII-1 and U-NII-2A) would likely permit the introduction of a wide-range of new broadband products capable of operating at higher data rates than is now possible.
    Globalstar MSS is the only user of this band in the U.S., using it for terrestrial gateway uplink transmissions to the Internet and phone networks (other spectrum bands are used for the satellite spot-beam transmissions, namely the Lower Big LEO and Upper Big LEO bands). Globalstar initially objected to the U-NII 1 outdoor use and higher power level changes. However, the NCTA analysis found little risk of interference if any one of these three conditions are met:
    1.) Outdoor APs do not radiate more than 125mW (21dBm) EIRP at elevation angles above 30 degrees
    2.) The device is used for a Point-to-Point link
    3.) The device operates indoors

    Devices that do not meet one of those three conditions are limited to 250mW conducted power. Additionally, before deploying an aggregate total of 1,000 APs or more outdoors in U-NII 1, companies must report to the FCC, which will facilitate corrective measures if harmful interference does occur. This is due to the fact that all U-NII 1 devices deployed outdoors in the U.S. will contribute to the noise level of the Globalstar MSS.

  • U-NII 3 band (5.725 - 5.825 GHz) is expanded up to 5.850 GHz, adding 25 MHz of bandwidth that now fall under the FCC Part 15.407 rules for U-NII (effectively consolidating FCC Part 15.247 into 15.407). This means that Wi-Fi channel 165 (5825 MHz) now falls under the U-NII 3 band rules instead of the ISM rules. Hopefully this will also mean more consistent support for channel 165 in vendor implementations (it's been spotty thus far). This additional 25 MHz provides for 1 additional 20 MHz channel (165) but no additional 40, 80, or 160 MHz channel capacity. You may want to revisit my post on 802.11ac Channel Planning.

    Other technical changes to the U-NII 3 band include:
    • PSD changes from 17dBm/MHz to 30dBm/500KHz.
    • No power reduction for antennas above 23dBi on fixed point-to-point systems. This should benefit outdoor WISPs and other point-to-point deployments. (Note - all non-PtP systems still require a 1dB reduction in power for every 1dB antenna gain over 6dBi).
  • The FCC also rejected ARRL proposed changes that would have required DFS operation from 5.65 - 5.925 GHz (including U-NII 3 and the proposed U-NII 4 band), citing no demonstrated need and that it would be overly burdensome.
Channel 165 is now part of UNII-3 (FCC Part 15.407), not ISM (FCC Part 15.247)
  • All devices (AP or client) operating in any U-NII band must be secured to prevent unauthorized software modification and to ensure it operates as approved to prevent harmful interference. The exact methods used to secure the software are left to the manufacturer, but must be documented in their application for equipment authorization to the FCC. The FCC is not setting specific technical security requirements since they are likely to change over time, but rather defining the capabilities that should be implemented by manufacturers. They do make note that more detailed security requirements may be necessary later as software-defined radio technology develops. They also declined to implement rules that would force manufacturers to render a device inoperable if unauthorized modifications were made, citing additional complexity and costs resulting in questionable benefits above the software security being mandated.

    All documented instances of harmful interference were found to be by devices certified for operation in the U-NII 3 band which had been manipulated through software controls to operate in the U-NII 2C band and interfered with TDWR (Terminal Doppler Weather Radar) in the 5.60 - 5.65 GHz sub-section.

    To quote from the R&O:
    "The primary operating condition for unlicensed devices is that the operator must accept whatever interference is received and must not cause harmful interference. Should harmful interference occur, the operator is required to immediately correct the interference problem or to cease operation."

    Other uses of the 5 GHz U-NII bands are show below:
5 GHz U-NII Bands with Primary and Secondary Allocations
    These FCC U-NII technical modifications are separate from another proposal currently under study by the FCC and NTIA that would add another 195 MHz of spectrum under U-NII rules in two new bands, U-NII 2B (5.350 - 5.470 GHz) and U-NII 4 (5.850 - 5.925 GHz). For further details read 'Wi-Fi May Get A Capacity Boost, Thanks to the FCC'.
  • DFS rules and compliance measurement procedures have been modified in the two existing U-NII 2 bands (U-NII 2A and U-NII 2C) to prevent harmful interference to TDWR and other radar systems. DFS is already required to be implemented if devices will operate in the U-NII 2 bands, but is being modified as follows:
    • Explicitly prohibits operators from using equipment without operational DFS in the U-NII 2 bands.
    • DFS must be turned on when operating devices in the U-NII 2 bands (it cannot be disabled).
    • Testing of DFS systems will be performed against a new radar waveform that more closely matches current TDWR systems.
    • Devices operating in U-NII 2 bands must now perform DFS radar sensing across 100% of the device emissions bandwidth (instead of 80% as specified in the 2006 DFS Compliance Measurement Procedures, Table 4). This will make DFS detection more stringent, but possibly also more prone to false-positives when radar is adjacent to the device's operating frequency range.
    • The DFS sensing threshold is modified. For devices operating below 200mW EIRP, the EIRP power spectral density (PSD) must now also be below 10mW/MHz in order to use the relaxed sensing threshold of -62 dBm. In practice this shouldn't cause any DFS change for most indoor Wi-Fi operation where AP power output is typically 100mW or less and EIRP is 200mW or less when using a 3dBi antenna. The minimum Wi-Fi channel width is 20 MHz, thus the 10mW/MHz PSD should be the peak limit, and larger channel widths will have even lower PSD. If operating APs at power levels above 100mW or with higher gain antennas above 3dBi then DFS sensing of radar at the lower threshold of -64dBm may come into play which could result in a slightly higher chance of detecting radar.
    • DFS devices no longer have to conform to a "Uniform Channel Spreading" requirement, which was intended to avoid dense clusters of devices operating on the same channel and might increase the risk of interference to radar systems. In fact, the FCC R&O acknowledges that the use of wide channels with an overall reduced number of usable channels can result in more effective spectrum use at a given location. Larger channels also spread the power of signals out uniformly over the frequency band in which the device is operating, rather than concentrated in a narrow bandwidth. This is not news to any Wi-Fi professional familiar with spread spectrum concepts :)

      By removing this requirement, they also acknowledge that dynamic or manual channel selection may be used. Previously, some manufacturers accomplished channel spreading by forcing dynamic channel selection. This should no longer be the case with such products, and manual selection of U-NII 2 channels should be available in all products moving forward.
    • The "Channel Loading" DFS compliance measurement test no longer requires the use of an MPEG video at 30fps, citing the need for greater flexibility to test and certify devices that are not designed or capable of streaming video. Instead, channel-loading tests will be performed with data types representative of the device under test. This should open up the use of U-NII 2 bands to more devices.
    • The TDWR frequency range of 5.60-5.65 GHz is now available for use by U-NII 2C devices again as long as they meet all new and modified rules! For Wi-Fi networks, this means we will gain access to channels 120-128 again. This adds back to the usable inventory: three 20 MHz channels, two 40 MHz channels, one 80 MHz channel, and one 160 MHz channel. This should be of significant benefit in enterprise environments, providing another 80 MHz channel (6 total now) with Wave-1 802.11ac equipment for channel re-use planning!

We will have to wait to see the effect these DFS rule changes have on the usability of the U-NII 2 bands in practice. Many users already shy away from using DFS channels due to the risk of Wi-Fi channel change and instability in the network. I fear this will heighten apprehension and further constrain use of these bands by network operators.

DFS Radar Detection (Sensing) Thresholds
  • Other DFS modifications were not implemented:
    • The FCC declined to adopt a geo-location database requirement to lookup TDWR in the 5.60 - 5.65 GHz band. They believe the updated DFS rules are sufficient.
    • The FCC declined to modify the out-of-band emissions limits for U-NII devices, as their field investigations have not found properly functioning equipment with the current limits to be a problem. Again, the majority of cases were from devices operating in frequency bands which they were not certified to begin with, or devices which had DFS disabled.
  • All of the new rules are subject to the following transition period before they take effect:
    • 12 months after the effective date of the R&O, applications for certification of 5 GHz devices must meet the new and modified rules.
    • Existing devices that do not meet the new and modified rules must cease to be manufactured, marketed or sold in the U.S. 2 years after the effective date of the R&O.
    • Existing devices that operate in U-NII 2 bands must always comply with DFS. If DFS is not implemented or is disabled, the devices may be confiscated.


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