LAA and WiFi

University of Chicago students conducted a study on and near the campus, looking at how LAA (Licensed Assisted Access) affects WiFi. Cellular carriers began using LAA technology in 2017. This technology allows a cellular carrier to snag unlicensed spectrum to create bigger data pipes than can be achieved with the traditional cellular spectrum. When cellular companies combine frequencies using LAA, they can theoretically create a data pipe as large as a gigabit while only using 20 MHz of licensed frequency. The extra bandwidth for this application comes mostly from the unlicensed 5 GHz band and can match the fastest speeds that can be delivered with home routers using 802.11AC.

There has always been an assumption that the cellular use of LAA technology would interfere to some extent with WiFi networks. But the students found a few examples where using LAA killed as much as 97% of local WiFi network signal strength. They found that when LAA kicked in that the performance on nearby WiFi networks always dropped.

This wasn’t supposed to happen. Back when the FCC approved the use of LAA, the cellular carriers all said that interference would be at a minimum because WiFi is mostly used indoors and LAA is used outdoors. But the study showed there can also be a big data drop for indoor WiFi routers if cellular users are in the vicinity. That means people on the street can interfere with the WiFi strength in a Starbucks (or your home).

The use of WiFi has also changed a lot since 2017, and during the pandemic, we have installed huge numbers of outdoor hotspots for students and the public. This new finding says that LAA usage could be killing outdoor broadband established for students to do homework. Students didn’t just use WiFi hotspots when they couldn’t attend school, but many relied on WiFi broadband in the evenings and weekends to do homework. Millions of people without home broadband also use public WiFi hotspots.

LAA usage kills WiFi usage for several reasons. WiFi is a listen-before-talk technology. This means that when a WiFi device wants to grab a connection to the router that the device gets in line with other WiFi devices and is not automatically connected immediately. LAA acts like all cellular traffic and immediately grabs bandwidth if it is available, This difference in the way of using spectrum gives LAA a priority to grab frequency first.

LAA connections also last longer. You may not realize it, but devices using WiFi devices don’t connect permanently. WiFi routers connect to devices in 4-millisecond bursts. In a home where there aren’t many devices trying to use a router, these bursts may seem continuous, but in a crowded place with a lot of WiFi users, devices have to pause between connections. LAA bursts are 10-milliseconds instead of 4-ms for WiFi. This means that LAA devices both connect immediately to unlicensed spectrum and also keep the connection longer than a WiFi device. It’s not hard for multiple LAA connections to completely swamp a WiFi network.

This is a perfect example of how hard it is to set wireless policy. The FCC solicited a lot of input when the idea of sharing unlicensed spectrum with cellular carriers was first raised. At the time, the technology being discussed was LTE-U, a precursor to LAA. The FCC heard from everybody in the industry, with the WiFi industry saying that cellular use could overwhelm WiFi networks and the cellular industry saying that concerns were overblown. The FCC always finds itself refereeing between competing concerns and has to pick winners in such arguments. The decision by the FCC to allow cellular carriers to use free public spectrum highlights another trend – the cellular companies, by and large, get what they want.

It will be interesting to see if the FCC does anything as a result of this study and other evidence that cellular companies have gone a lot further with LAA than promised. I won’t hold my breath. AT&T also announced this week that it is starting to test LAA using the unlicensed portion of the 6 GHz spectrum.

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