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


I was recently asked about the merits of using the 5 GHz U-NII-4 band for Wi-Fi. I must admit I have an opinion that I think many people will find unpopular or at least disagree with me. My opinion is simple, right now I don’t see much point in using the new additional 5 GHz spectrum for enterprise Wi-Fi.

Let’s look at this historically….

In January 2013, the FCC offered two new U-NII bands for license-free use. The first proposed U-NII-2 band was supposed to occupy the frequency space from 5.35 GHz to 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 the expansion of Wi-Fi into the U-NII-2B band, there were always the possibility of further frequency extension at the upper 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 US and European regulators to enable wireless access communications in vehicle-to-vehicle and vehicle-to-pavement (WAVE) environments. . This is the domain of 802.11p, and the group was intended to dedicated short range communications (DSRC). As we all know, the automotive industry has seen significant innovations in self-driving automobiles and enhanced safety features, such as blind side monitoring. These technologies are often referred to as intelligent transport system (HIS).

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

However, the automotive industry has made little use of the U-NII-4 band. Thus, the FCC believed that traditional Wi-Fi users could share this band in places where DSRC is not used. In November 2019, the FCC issued a Notice of Proposed Rulemaking (NPRM) to reconsider the rules for the 75 MHz U-NII-4 band. As shown in Figure 2, the NPRM proposed to reallocate 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 reasserted for automotive ITS using a new LTE cellular technology called C-V2x. Cellular vehicle for everything (C-V2X) is an ITS technology designed to allow vehicles to communicate with each other and with everything around them (example: traffic lights). The NPRM also proposed to reconsider whether DSRC’s old ITS technology should still be permitted.

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

Figure 2 – U-NII-4 Channels

The FCC rules are very similar to the new LPI (Low Power Indoor) rules for 6 GHz. An access point using this additional 5 GHz frequency space can only be used indoors, must have a built-in antenna, cannot use a weatherproof enclosure, and cannot be battery powered. Access points should also be labeled for indoor use.

As shown 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 channel in the United States.

Interestingly, this is happening just as we see the availability of indoor corporate Wi-Fi 6E access points with 6 GHz capacity. While I’m usually very excited when new unlicensed spectrum is made available for Wi-Fi, I have to admit that the availability of U-NII-4 doesn’t excite me at all.

I could give a multitude of reasons why I see no commercial value in this brand new piece of the 5GHz spectrum. AP bandpass filter issues, lack of availability in other parts of the world outside of 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 though a firmware upgrade is possible, most customer vendors will not make updates available as their engineering resources are always focused on new devices. Vendors also won’t have devices recertified by the FCC. These old 5GHz customers aren’t going anywhere, which means that any 5GHz channel reuse plan that includes U-NII-4 channels effectively creates a roaming and coverage “dead zone” for the entire existing customer population.

In 2022, we’ll likely see 6E Wi-Fi clients supporting 6GHz, 5GHz, and the new additional 5GHz channels. And some AP vendors will indeed aggressively market U-NII-4 capability for Wi-Fi. So what? To use these new 5 GHz channels, again, legacy clients cannot be in the mix. I guess you could have an isolated area that provides coverage in a controlled environment using capable access points and clients. But that’s just not practical in the real world.

Adding an extra 80 MHz channel for 5 GHz might sound sexy, but let’s be honest, 80 MHz for 5 GHz reuse models in the enterprise are rare. It just doesn’t scale in the business because there isn’t enough frequency space. What is unavoidable is co-channel interference and performance degradation due to decreased SNR when using 80 MHz in channels in a 5 GHz reuse plan. The 80 MHz to 5 GHz channels were primarily intended for consumer access or rare corporate emergencies. Instead, you’ll want to use 6 GHz as your new bandwidth highway, where 80 MHz channels should thrive.

Do not mistake yourself. I have to commend the FCC for opening up that extra 5 GHz spectrum for Wi-Fi use. But right now it’s just not practical. Conclusion: U-NII-4 offers 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 global economy. Wi-Fi 6E is the start of a tremendous windfall of spectrum.


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