When the cellular industry first lobbied for the introduction of LTE in unlicensed spectrum, the outcry from the Wi-Fi world was loud and furious: Don’t mess with our spectrum.
The reasoning went something like this. Wi-Fi, an IEEE technology, communicates in a way that could be construed as more polite than your standard cellular ways. It pioneered the “listen-before-talk” (LBT) mode. Cellular, on the other hand, in the U.S. has “grown up,” so to speak, in licensed spectrum. They’ve managed to get along enough to co-exist in unlicensed spectrum, but the going was a bit rough, to put it mildly, at least in the early days.
For several years now, Wi-Fi and cellular have both used unlicensed spectrum — which by its very nature means it's open to a lot more users than licensed. Things have been mostly quiet on the co-existence front, but measurements recently presented by researchers at the University of Chicago show severe degradation to Wi-Fi devices in the presence of LAA, which stands for Licensed Assisted Access and is based on LTE technology. They presented their findings in the paper, “Hidden-nodes in coexisting LAA & Wi-Fi: a measurement study of real deployments.”
In an environment where Wi-Fi usage is high — such as outside a university bookstore, where an outdoor LAA base station is in close proximity to Wi-Fi, it can significantly affect Wi-Fi performance. "Wi-Fi throughput can degrade as much as 97% when coexisting with LAA whereas LAA throughput only degrades 35%,” the researchers wrote.
Monisha Ghosh, a research professor at the University of Chicago who just finished a stint as chief technology officer at the FCC, was part of the research team looking at the co-existence of Wi-Fi and LAA. As reported here, she and her students started collecting measurements around the Illinois campus and presented their findings in the paper.
“The conventional wisdom has been that LAA is deployed outside mostly. Wi-Fi is deployed inside mostly, so there isn’t a huge area where they will interfere with each other,” she told Fierce. “The measurements that we made actually show that that’s not necessarily the case, especially in university environments where there is a lot of outdoor Wi-Fi available even though the access points themselves are mounted indoors. You can be sitting outside in a common area and be connected to university Wi-Fi.”
The researchers found an area near the campus bookstore where an AT&T LAA base station was deployed and set up their own Wi-Fi access points inside the bookstore, on the same LAA channels.
They discovered a hidden-node problem between Wi-Fi access points and the LAA base station. They were actually quite surprised by how much the Wi-Fi performance deteriorated. “I really wasn’t expecting it to deteriorate that much,” Ghosh said.
It certainly isn’t the first research into LAA and Wi-Fi and the hidden-node problem has been well studied in the context of traditional overlapping Wi-Fi access points. However, the researchers said they believe the same rigorous study has not yet happened for hidden nodes between Wi-Fi and LAA, especially in the context of their paper. Their experiments led to the conclusion that in a hidden-node scenario with LAA, Wi-Fi shows a performance degradation that is more severe than that experienced by LAA; therefore, more advanced protocols are needed for co-existence between LAA and Wi-Fi.
Because deployments continually change, they’re pursuing ongoing work to take measurements. When they started the work, it was with older equipment that used 802.11ac consumer grade access points. Recently, the university has been upgrading the infrastructure and installed brand-new 802.11ax access points, and “we are redoing some of these measurements to see if the same conclusions hold up” with the newer set of access points, she said.
By and large, they see less degradation with the newer equipment, but “we do still see a fairly steep drop-off” in Wi-Fi throughput when LAA in the area is activated, she said.
“We continue to engage with standards bodies,” mostly IEEE-affiliated co-existence work groups, and other venues in the hope that with the next generation of systems, which in the U.S. will be in the 6 GHz band, that co-existence techniques will improve.
LTE-U, which came before LAA was ratified through 3GPP, can be traced back to 2013 when Qualcomm was a strong proponent of it. LTE-U came up through the LTE-U Forum that was established by Verizon, Qualcomm, Ericsson and other leading vendors at the time. Since then, NR-U has emerged as the 5G technology for unlicensed spectrum, and it’s being geared up to support the new 6 GHz band in the U.S.
Contacted by Fierce this week, Qualcomm said it’s worked with many others in the industry, including Wi-Fi technology companies, to do extensive testing on the coexistence between LAA/NR-U and Wi-Fi using industry-developed evaluation methodologies. After years of testing and evaluation, the wireless industry and FCC determined the use of LAA has “no adverse impact” on Wi-Fi performance. It’s worth noting that broadly speaking, Qualcomm plays in both Wi-Fi and cellular industries.
“From our initial review of the paper, the authors have chosen a single scenario where Wi-Fi performance appears to be impacted from the deployment of LAA nodes. However, there are many other scenarios where the opposite can be shown,” a Qualcomm spokesperson told Fierce via email.
“It is therefore important to evaluate a rich and representative enough set of scenarios to assess fairness in spectrum access. The current coexistence mechanisms were developed via consensus by all stakeholders, including cellular and Wi-Fi-based interests, with the confidence that fairness in the use of unlicensed spectrum is being achieved across fundamentally different technologies,” the Qualcomm spokesperson said. “As the relevant 3GPP technical report found, a majority of the scenarios studied showed that combinations of LAA and Wi-Fi did not ‘impact Wi-Fi more than another Wi-Fi network (offering the same traffic to the same users) in any of the measured performance metrics.’”
So far, so good. But the Wi-Fi Alliance said it's ready to jump in if necessary, saying it’s committed to ensuring the “best connected experience for billions of users who rely on Wi-Fi for education, work, healthcare and other critical necessities” in their daily lives.
“We continue to monitor how the introduction of LAA impacts Wi-Fi performance,” said Kevin Robinson, SVP of Marketing at Wi-Fi Alliance, in a statement. “As more real-world information emerges, if it becomes necessary, Wi-Fi Alliance is ready to engage constructively with industry stakeholders to ensure that new technologies in unlicensed spectrum do not disadvantage Wi-Fi operation.”