SAN JOSE, Calif.—In the process of defining tests to make sure a new LTE specification fairly shares unlicensed spectrum with Wi-Fi networks, engineers at Qualcomm uncovered some interesting data: Wi-Fi access points are all over the board when it comes to how they share spectrum with each other.
“We tried three brands of the best quality access points and found the sharing of the link varies from 10 to 90% -- even when using access points from the same vendor coexistence is all over the place.” said Mingxi Fan, a vice president of engineering in Qualcomm’s R&D group.
Ideally, devices should negotiate a 50/50 split of access time to spectrum, Fan said. The problem is vendors have a wide variety of parameters for how they configure their systems to dynamically negotiate spectrum sharing. Thus sometimes one access point has spectrum 90% of the time, and in another scenario it has it just 10%, he said.
Qualcomm’s tests used “the best enterprise [access points] instead of cheap consumer ones, but even then performance was everywhere,” Fan said.
In its tests, Qualcomm found request-to-send messages from two Wi-Fi access point sometimes collided so end devices could not hear and respond.
Hence the transmission link can be stalled for a long time, a Qualcomm representative said. One way to address the problem is to remove crosslinks between end device and multiple access points, the company said.
Qualcomm showed results of multiple tests in which Wi-Fi access points were inefficient or unfair in sharing spectrum. (Images: Qualcomm)
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The reports come amid a heated controversy over a coexistence test plan emerging at the Wi-Fi Alliance. All sides agree new LTE-U systems should not interfere with Wi-Fi access points any more that the Wi-Fi devices interfere with each other, but opposing sides debate exactly what the baseline for Wi-Fi interference should be.
“No one has ever looked closely at how [access points] impact each other, but now everything under a microscope because of the LTE-U discussions,” said Fanny Mlinarsky, a wireless specialist with testing company Octoscope who has been involved in the LTE-U process at the WFA.
The impact of the Wi-Fi detection threshold “is surprising to a lot of people…the industry needs to come to grips with what it is and how it’s used,” Mlinarsky said. “The first step is to understand how bad it is and we don’t know, we don’t have enough data yet…a lot of work needs to be done by the industry as a whole with peer review and a test methodology agreed upon,” she added.
The process will be complex due to variabilities between products, vendors and target markets. “An access point from HP can be different from one from Cisco…and they change from rev to rev,” she said.
The uneven interference patterns are a largely ignored fact of life, said one Qualcomm executive.
“Most people in their day-to-day use know this, they experience it all the time -- whether something needs to be done about it or not is not our focus,” said Dean Brenner, senior vice president of government affairs at Qualcomm who is currently pushing for changes in the WFA draft test for LTE-U.
A Wi-Fi Alliance representative was not available to respond to a request for information for this story.
— Rick Merritt, Silicon Valley Bureau Chief, EE Times