CableLabs Testing Again Points to LTE-U Coexistence Problem, but Lack of Standard May Be Bigger Issue

LTE-U continues to raise red flags with some Wi-Fi backers, as CableLabs said its tests showed Wi-Fi performance "suffered disproportionately in the presence of LTE-U." The testing, using a pair of off-the-shelf Wi-Fi access points and an LTE-U signal generator, found that Wi-Fi throughput was degraded 70 percent when LTE-U was at a 50 percent cycle, with the throughput declining as the duty cycle increased, Jennifer Andreoli-Fang, CableLabs principal architect, said in a blog post Friday.

But tests showing Wi-Fi interference or coexistence are inherently limited because there is no universally agreed-upon standard, emailed Public Knowledge Senior Vice President Harold Feld. "There is no contradiction between CableLabs saying 'we think interference is likely based on these tests' and LTE-U Forum saying 'well we think it is unlikely because of these tests,'" Feld said. "It all depends on what your assumptions are about how people will use the respective devices and what are the most likely set of operating environments."

CableLabs said in its test the LTE-U signal had similar effects on Wi-Fi latency. At both a 50 percent and 70 percent cycle, the LTE-U signal caused Wi-Fi latency well above 100 milliseconds, which is the usual cutoff for real-time communications, CableLabs said. When the LTE-U duty cycle hits 70 percent, CableLabs said, "latency reaches levels where even latency-tolerant applications, like web page loading, are likely to become irritating." And limiting LTE-U duty cycle to lower levels doesn't necessarily mean it won't interfere with Wi-Fi, CableLabs said. "The effect that LTE-U would have across a range of real-world circumstances is, frankly, unknown. It has not been researched -- by anyone." While LTE-U Forum put out a new version of its Coexistence Specification last week (see 1511040059), CableLabs said that "we don’t think it changes much for our purposes here."

One LTE-U supporter was quick to dismiss CableLabs' results. “The cable industry’s cable-only lab results are flawed and misleading, which is not surprising given the motivation of the cable industry to block new innovation and investment in unlicensed bands," CTIA Chief Technology Officer Tom Sawanobori said in a statement Friday. "Contrary to cable industry’s view, unlicensed spectrum is not their industry’s exclusive spectrum.”

"CableLabs is not off base," Feld said. "CableLabs is showing that there are circumstances under which interference is likely to occur." Feld, who has said it's possible to create an LTE-U version that coexists with Wi-Fi (see 1509080046), said now "what people need to understand is that there is no magic, agreed upon gold standard 'interference test' for a new technology. That's why you need a standards body, or a regulatory body like the FCC, to make the call. There are a lot of factors to weigh and balance against each other. This isn't like a breathalyzer test where you either are under the legal limit or over it."

Parties ranging from cable and Wi-Fi companies to Google and Microsoft have been pushing the FCC on creation of standards and tests to ensure Wi-Fi/LTE-U coexistence (see 1510280053). An NCTA spokesman said the CableLabs testing "reinforces our concern that these issues need to be resolved collaboratively through the private standards-setting process." FCC Chairman Tom Wheeler has indicated he's interested in standards bodies working out the technical details (see 1509080046). An agency spokeswoman said Friday the Office of Engineering and Technology "will evaluate and ask technical questions about any device that comes to the FCC for review."

When asked about setting such standards, LTE-U Forum co-founder Qualcomm in a statement said that “as members of both the Wi-Fi and LTE communities, we are constantly collaborating and working very closely with all key stakeholders, including CableLabs and others in the cable industry, to ensure that LTE-U and Wi-Fi will coexist very well. We are addressing technical issues, answering technical questions, easing technical concerns, and working together. In this case, the implementation that CableLabs used violates fundamental coexistence principles in the LTE-U specification. All testing conducted in accordance with the LTE-U specification shows that LTE-U is a good neighbor to Wi-Fi. For example, recent tests from well-regarded analyst firm Signals Research Group concluded that 'LTE-U does everything possible' to operate in clear channels, and 'when LTE-U must share a channel with Wi-Fi, it makes fair use of the channel.'"