By accessing multiple networks simultaneously, the group found that competition decreased performance—reducing the amount of data transmitted, the speed of transmission, and the signal quality.
This competition was particularly detrimental to Wi-Fi. When LAA was also in active use, data transmitted by Wi-Fi users decreased up to 97%. Conversely, LAA data only exhibited a 35% decrease when Wi-FI was also in use.
Ghosh explained that the incompatibility between Wi-Fi and LAA owes in part to the different protocols each employs to deal with heavy internet traffic.
“If everybody starts using the spectrum at the same time, it creates interference and no one’s information gets through,” Ghosh said. “But Wi-Fi and cellular have developed very different mechanisms for dealing with this issue.”
Because Wi-Fi depends exclusively on unlicensed spectra, it uses a protocol tailored towards unpredictable demand. This protocol, called listen-before-talk, mimics the interactions of a group of polite party-goers. Participants listen and wait for a gap in the conversation to speak. If two people start talking at once, one politely backs off to let the other speak, then chimes in afterwards. Similarly, if multiple Wi-Fi users attempt to access the network at once, each is assigned a brief wait time, and randomness among these wait times reduces the probability of collision.
In contrast, cellular providers can predict demand based on cellular access, and so assign each user a specific transmission time. Thus, LAA users are more like speakers in a tightly scheduled colloquia than at an informal party.
This difference in protocols posed little problem when cellular providers were restricted to licensed spectra, but as they’ve moved to the unlicensed spectra, the channel access parameters LAA employs makes it difficult for Wi-Fi users to get equal access to the medium. In spite of the fact that LAA recently modified the tight scheduling used in cellular bands to implement listen-before-talk, it still operates with different parameters. One crucial difference is how long each system holds the medium once it gains access: LAA can transmit for up to 10 milliseconds, whereas Wi-Fi transmissions are only up to 4 milliseconds long.
Competition in the shared spectrum occurs not just between Wi-Fi and cellular providers, but also within each network type.
“In our experiments, we compare Wi-Fi/Wi-Fi coexistence with Wi-Fi/LAA coexistence,” Ghosh said. “Wi-Fi/Wi-Fi coexistence isn’t too bad because of the listen-before-talk procedure, so we used this as a standard of fairness. But Wi-Fi/LAA behaves worse, and we were surprised by how much worse.”
In future studies, she hopes to also examine how LAA networks supplied by different cellular providers compete with one another.
Additionally, Ghosh has advised regulatory agencies on how to better align network protocols based on her research.
“These changes have resulted in better coexistence and better sharing mechanisms. But there’s still a long way to go,” she said.
The researchers acknowledge the assistance of Kemal Badur, Assistant Vice President and Chief Technology Officer at UChicago, and Beth Niestat, Executive Director of UChicago GRAD, for loaning access points and allowing use of campus offices for installing the access points.
Funding: This work was funded by NSF grant CNS-Award # 1618920 "NeTS: Small: Collaborative Research: Can LTE & WiFi Live Happily Ever After?”