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Is the 5 GHz U-NII-4 band useful for enterprise Wi-Fi?

David Coleman Director, Wireless Networking at the Office of the CTO Published 10 Feb 2022

I have been getting asked lately about the value of using the U-NII-4 band in 5 GHz for Wi-Fi. I must admit that I have an opinion that I think many people will find unpopular with or at least disagree with me. My opinion is simple, at this time, I do not see much value in using the new additional 5 GHz spectrum for Wi-Fi in the enterprise.

Let’s look at this historically….

In January 2013, the FCC proposed two new U-NII bands for unlicensed use. The first proposed U-NII-2 band was supposed to occupy the frequency space of 5.35 GHz–5.47 GHz and would have provided six 20 MHz channels. However, the FCC has decided that the U-NII-2B band will not be available for Wi-Fi use. Although the FCC has denied expansion of Wi-Fi into the U-NII-2B band, there was still the possibility for additional frequency expansion at the top end of the 5 GHz band. As shown in Figure 1, the U-NII-4 frequency band, 5.85 GHz–5.925 GHz, was reserved decades ago by U.S. and European regulatory bodies to allow Wireless Access in Vehicular Environments (WAVE) communications from vehicle to vehicle and vehicle to roadway. This is the realm of 802.11p, and the band was meant for dedicated short-range communications (DSRC). As we all know, the automobile industry has seen significant innovation toward self-driving automobiles and enhanced safety features, such as blind-side monitoring. These technologies are often referred to as an intelligent transportation system (ITS).

U-NII-4 band in 5 GHz

Figure 1: U-NII-4 band in 5 GHz

However, the automotive industry has had little usage of the U-NII-4 band. Thus, there was thinking by the FCC is that traditional Wi-Fi users could share this band in places where DSRC is not being used. In November of 2019, the FCC issued a Notice of Proposed Rulemaking (NPRM) to reconsider the 75 MHz U-NII-4 band rules. As shown in Figure 2, the NPRM proposed repurposing the lower 45 MHz of the U-NII-4 band to be used for Wi-Fi and unlicensed use. The upper 30 MHz of the U-NII-4 band would be reaffirmed for automotive ITS using a new LTE cellular technology called C-V2x. Cellular vehicle-to- everything (C-V2X) is an ITS technology meant to enable vehicles to communicate with each other and everything around them (example: traffic lights). The NPRM also proposed reconsidering whether the older ITS technology of DSRC should still be permitted.

The good news is that this has happened, and in September of 2021, The FCC issued guidelines on how Wi-Fi devices can be certified for use in the additional frequency space of 5.85 – 5.895.

 

U-NII-4 Channels

Figure 2 – U-NII-4 Channels

The FCC rules are very similar to the new low power indoor (LPI) rules for 6 GHz. An AP using this extra 5 GHz of frequency space can only be used indoors, must have an integrated antenna, cannot use a weatherized enclosure, and may not be battery powered. The APs also must be labeled that they are for indoor use.

As seen in Figure 2, this new 5 GHz spectrum offers the Wi-Fi potential of three additional 20 MHz channels, two additional 40 MHz channels, and one additional 80 MHz-wide channel in the United States.

Interestingly, this is happening just as we are now seeing the availability of Wi-Fi 6E enterprise indoor APs with 6 GHz capability. Although I am usually very excited when new license-free spectrum is made available for Wi-Fi, I must admit that the availability of U-NII-4 does not excite me at all.

I could give a multitude of reasons why I see no enterprise value of this tiny new chunk of the 5 GHz spectrum. AP bandpass filter issues, lack of availability in other regions of the world besides the US…. however, the main reason is simple:

99.9% of the existing 5 GHz client population does not support U-NII-4. In most cases, a hardware upgrade will be required. Even if a firmware upgrade is possible, most client vendors will not make updates available as their engineering resources are always focused on new devices. Nor will vendors get the devices re-certified by the FCC. These legacy 5 GHz clients are not going anywhere, meaning that any 5 GHz channel reuse plan that includes the U-NII-4 channels effectively creates a roaming and coverage “dead zone” for all of the existing client population.

In 2022, we will probably see some Wi-Fi 6E clients that support 6 GHz, 5 GHz, and the new few extra 5 GHz channels. And some AP vendors will indeed aggressively market the capability of U-NII-4 for Wi-Fi. So what? To use these new 5 GHz channels, once again, the legacy clients cannot be in the mix. I suppose you could have an isolated area that provides coverage in a controlled environment using capable APs and clients. But this is just not practical in the real world.

Adding an extra 80 MHz channel for 5 GHz may sound sexy, but let’s be honest, 80 MHz reuse patterns for 5 GHz in the enterprise are rare. This simply does not scale in the enterprise because there is not enough frequency space. What is bound to occur is co-channel interference and performance degradation due to a decrease in SNR when using 80 MHz in channels in a 5 GHz reuse plan. 80 MHz channels in 5 GHz have mostly been for consumer-grade access or rare corner cases in the enterprise. Instead, You will want to use 6 GHz as the new bandwidth superhighway, where 80 MHz channels are expected to thrive.

Don’t get me wrong. I must commend the FCC for opening this extra bit of 5 GHz spectrum for Wi-Fi usage. But for now, it is just not practical. Bottom line: U-NII-4 provides no immediate value for enterprise Wi-Fi.

The 1,200 megahertz of new 6 GHz spectrum available for Wi-Fi is our future. The availability of the 6 GHz frequency space for Wi-Fi communication is expected to bring hundreds of billions of dollars in economic value to the worldwide economy. Wi-Fi 6E is the beginning of a tremendous spectrum bonanza.

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