“Typically, there’s a standards process – and these licensed and unlicensed bands are handled by entirely different bodies, and LTE-U skipped both of them. It’s not in 3GPP nor is it in IEEE. It’s just a proprietary technology that Qualcomm and Ericsson are just putting out there all of a sudden,” Calabrese said.
This isn’t to say that the U.S. wireless industry is in lock step on this technology, he added – Verizon and T-Mobile are both reliant on Ericsson as a primary infrastructure vendor, while Sprint and AT&T are not. (Nevertheless, AT&T has joined the pro-LTE Evolve Coalition and begun its own tests of the technology.)
By contrast, LTE-U’s opposition has generally remained united. Philip Berenbroick, a counsel for government affairs at advocacy group Public Knowledge, said that the technology might not cross legal boundaries, but it could easily break the unwritten rules.
“There’s a general etiquette in the unlicensed space that you don’t disrupt licensed or other unlicensed services,” he said. “We would like to see LTE-U comply with the etiquette that is pervasive in that band.”
A protocol called “Listen-Before-Talk” could be key to resolving the problem, Berenbroick argued. LBT, which makes transmitters ensure their channels are clear before sending their data, is referred to as a “politeness protocol” in the wireless industry. It provides an even-handed way of ensuring the best possible shared access to a given piece of spectrum, and is legally mandated in the European Union and Japan, though not in the U.S., China or South Korea. An alternative to LTE-U, called LAA, or license-assisted access, incorporates LBT as a safeguard against interference. (LAA is currently under consideration by 3GPP, one of the standards groups that LTE-U’s backers bypassed.)
“Because they don’t check for other users that are using the bandwidth first, [LTE-U transmitters] can have the effect of slowing or degrading other unlicensed traffic that’s out there,” Berenbroick told Network World. “We would like to see LTE-U observe Listen-Before-Talk.”
LTE-U, instead, uses a coexistence protocol called “duty cycling,” which simply creates pre-set on and off periods for an LTE transmitter – the lengths of which are controlled entirely by the carrier. That’s not good enough, according to Jennifer Andreoli-Fang, principal architect at CableLabs, a cable-industry-backed research group.
“There is no requirement to share fairly in time, to avoid interrupting Wi-Fi transmissions mid-stream, or to adapt to different levels of Wi-Fi usage and traffic,” she wrote in a September blog post.
The concerns over LTE-U are unlikely to die down soon; reports indicate that Verizon and T-Mobile may start using the technology as early as next year, and although FCC Chairman Tom Wheeler has been reluctant to get involved with the issue, he stated that the wireless industry risks further regulation if the rights of existing unlicensed users aren’t sufficiently protected.
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