By Rick Murphy
With important details from the upcoming Report and Order as posted in FCC-CIRC2004-1.
In October 2018, I first reported that the FCC was considering opening the 6 GHz band for unlicensed usage by way of Notice of Proposed Rulemaking (NPRM) 19-138, in Future-Fi, Chapter 2. In addition, I shared evidence that the 802.11ax Task Group was already embedding support for those upcoming channels into the new specification, which is still undergoing ratification. Here it is, just 18 months later, and we are expecting final closure on the matter by the end of this week.
The details of the upcoming FCC Report and Order concerning unlicensed usage of the 6 GHz band are written and have been posted. The FCC Open Meeting scheduled for Thursday; April 23 will decide whether this document becomes law. If it succeeds, as expected, the new spectrum addition for Wi-Fi will spark a huge change in capability and innovation. In this chapter of Future-Fi I will share details of the Report and Order specification, which is the target of Thursday’s vote.
Which do you want first? The good news? Or the even better news?
The Good News
Note: Quotations taken directly from FCC-CIRC2004-01
The good news is that the FCC will “authorize unlicensed standard power access points in the U-NII-5 and U-NII-7 bands through the use of an AFC (automatic frequency control/coordination) system. This will permit operations at the same power levels already permitted in the 5 GHz U-NII-1 and U-NII-3 bands (5.150-5.250 GHz and 5.725-5.850 GHz, respectively). Enabling synergistic use of both the 5 GHz and 6 GHz bands for promoting unlicensed broadband deployment.”
The even better news is that the FCC is “opening the entire 6 GHz band for unlicensed indoor low power access points. By authorizing use of the entire 6 GHz band for this type of use, we [FCC] provide opportunities for unlicensed operations to use up to 320-megahertz channels to expand capacity and performance capabilities.”
This announcement makes Wi-Fi a true superpower since the majority of enterprise deployments are contained indoors and a basic best practice for WLAN engineers is to limit the output power from each AP in order to optimize performance and lessen co-channel interference. The newfound ability to use an additional fifty-nine 20 MHz-wide channels at power levels of 18-30 dBm maximum EIRP is a game changer.
“The rules we adopt today are designed to optimize unlicensed access to the 6 GHz band while also protecting incumbent services so that they continue to thrive in the band.”
Although the standard power operations for U-NII-5 and U-NII-7 bands are also of interest to Wi-Fi engineers, I will address those new capabilities and restrictions in another episode of Future-Fi. For this chapter I’ll pint out the new rules and capabilities for Low Power APs in 6 GHz Unlicensed.
B. Low-Power Indoor Operations Across the Entire 6 GHz Band
“Based on the record before us, we open the entire 6 GHz band for unlicensed indoor operations without the need for AFC-controlled access. By doing so, we create new unlicensed use opportunities in these bands – including optimizing the potential for deployment of next generation Wi-Fi that makes use of 160 MHz channels – while protecting the various incumbent licensed services in the band, including fixed microwave services, various other fixed and mobile services, and fixed-satellite services.”
101. “Because there will be no AFC system to prevent interference to licensed services from occurring, the rules we adopt (include) three restrictions designed to prevent harmful interference. Devices are: (1) limited to indoor operation; (2) required to use a contention-based protocol; and (3) subject to low-power operation.”
102. “First, these low-power access points must operate only indoors. The signals transmitted by these unlicensed devices will be significantly attenuated when passing through the walls of buildings.”
103. “Second, we require that the indoor low-power devices, both access points and their associated client devices, employ a contention-based protocol. A contention-based protocol allows multiple users to share spectrum by providing a reasonable opportunity for the different users to transmit.”
“Wi-Fi devices share spectrum using a contention-based protocol. For IEEE’s 802.11, a “listen-before-talk” medium access scheme based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol functions as a contention-based algorithm to provide access to all traffic. Before initiating any packet delivery, a station listens to the wireless medium and if the medium is idle, the station may transmit; otherwise the station must wait until the current transmission is complete before transmitting. To ensure efficient and cooperative shared use of the spectrum, we require all unlicensed indoor low power operations use technology that includes a contention-based protocol.”
105. “Third, we limit the low-power indoor access points to lower power levels than the standard-power access points that operate under the control of an AFC. Consistent with the Commission’s approach for the existing U-NII bands, we specify both a maximum power spectral density and an absolute maximum transmit power, both in terms of EIRP. Specifically, we will allow a maximum radiated power spectral density of 5 dBm per 1 megahertz and an absolute maximum radiated channel power of 30 dBm for the maximum permitted 320-megahertz channel (or 27 dBm for a 160-megahertz channel).”
The Report and Order text further defines the provisions that will be required to enforce the low-power access points to remain indoors.
109. “Specifically, we adopt three equipment-related hardware requirements that are designed to keep these low-power access points indoors.
First, as suggested by Boeing, we will require that the access point devices cannot be weather resistant.
Second, we will require that the low-power access points have integrated antennas and prohibit the capability of connecting other antennas to the devices, which will prevent substituting higher gain directional antennas and make the device less capable or suitable for outdoor use as suggested by the Wi-Fi Alliance.
Third, consistent with the suggestions by Hewlett-Packard Enterprise and the Wi-Fi Alliance, we will prohibit these low-power access points from operating on battery power. Furthermore, we will require that the access points be marked as “for indoor use only” and include a label attached to the equipment stating that “FCC regulations restrict to indoor use only”, thereby informing consumers of the appropriate use.”
Client Device Restrictions
186. “As proposed in the Notice, we adopt a requirement that client devices operate either under the control of a standard-power access point or a low-power access point. The purpose of this requirement is to prevent client devices from transmitting outdoors at locations where they may cause interference to a microwave receiver or other incumbent.”
“When a client device is under the control of a low-power indoor access point, it should also be indoors and in close proximity to the access point, and therefore avoid presenting an interference risk to licensed services. However, we also adopt an exception to this general requirement to allow a client device to transmit brief messages (“probe requests”) to an access point when attempting to join its network as discussed below.”
188. “We recognize the utility of permitting probe requests to enable client devices to join an access point’s network. However, these probe requests have the potential to cause harmful interference to licensed operations. We will therefore only permit a client device to send a probe request to an access point after it has detected a transmission from the access point. The client device will be required to send the probe request on the same frequency as the access point’s transmission.”
“Under this exception, because the client device will have to detect an access point transmission, the client device will only transmit when it is close enough to an access point to be under its control and on a frequency on which the access point has permission to transmit. This will prevent harmful interference from occurring.”
105. “In addition, to ensure that client devices remain in close proximity to the indoor access points, we are limiting their PSD and maximum transmit power to 6 dB below the power permitted for the access points.”
113. “Accordingly, we conclude that the appropriate maximum power spectral density for low power indoor client devices in this band is 6 dB below the limit for access points (or -1 dBm/MHz based on the adopted PSD limit).”
To recap the upcoming Report and Order specifications for the use of low-power devices in unlicensed 6 GHz band:
The new rules will allow the entire 1.2 GHz of frequency space to be used by low-power, contention-based devices like Wi-Fi (DCF/HCF) without the need for AFC or DFS provided:
Access points intended for low-power usage;
- must not be weather-proof
- must use integrated antennas
- may not be battery powered
- must be labeled “for indoor use only”
Transmit output power is limited to the following maximum EIRPs using 6 dBi antennas;
- 30 dBm for a 320-megahertz channel
- 27 dBm for a 160-megahertz channel
- 24 dBm for an 80-megahertz channel
- 21 dBm for a 40-megahertz channel
- 18 dBm for a 20-megahertz channel
Client devices must operate under the control of low-power access points with the following exception;
- Client devices are allowed to transmit probe requests but only after they have detected a transmission from the access point
Client devices will also be limited to maximum EIRP of 6 dB less than the limits for low-power APs.
These requirements are a small price to pay and are completely in line with industry best practices for indoor Wi-Fi designs. Wi-Fi designers are accustomed to sharing frequency space and happy to operate in a manner that does not impede incumbent wireless operators. The benefit of having nearly sixty new, interference-free, 20 MHz channels is the golden ticket we’ve needed to spark futuristic communications capabilities and practices. Device manufacturers are ready to produce AP and client STAs that can utilize this frequency space. It remains to be seen which utility software vendors will be the first to update their diagnostic and design applications to provide the toolsets needed by WLAN engineers, in order to take advantage of these new resources.
The 802.11ax Task Group has already been hard at work crafting the protocol interactions that will allow soon-to-appear devices to transparently coexist and interoperate across the frequency divides of 2.4 GHz, 5 GHz, and 6 GHz.
The next step will be taken on Thursday, April 23 when, hopefully, the FCC approves this document and signs 6 GHz Unlicensed use into law.
Stay safe and optimistic.