The use of LTE over unlicensed spectrum has gained significant interest within the industry, with multiple wireless operators interested in boosting their network capacity and speed by aggregating their licensed LTE spectrum with unlicensed spectrum at 5 GHz.
Verizon Communications brought together a number of major wireless vendors last year to explore the technical aspects of LTE-U. The LTE-U Forum was formed by Verizon along with Alcatel-Lucent, Ericsson, Qualcomm and Samsung put together a series of technical documents on the technology and its workability with Wi-Fi. The documents available on the site include a coexistence study as well as minimum requirements for both user equipment and base stations for LTE-U.
Here are three things to keep in mind in discussions as discussions of LTE-U progress:
– Global requirements for LTE in unlicensed spectrum aren’t uniform. In most of the world, the framework for making LTE-U work is generally left up to the standards bodies and the industry. However, in Japan and Europe, there are regulatory requirements to adopt a “listen before talk” or “listen then talk” (LBT or LTT) scheme to minimize interference with Wi-Fi.
According to Qualcomm, “the deployments in countries such as U.S., Korea, China and India will use [LTE Release] 10/11/12 with coexistence features such as [carrier sensing adaptive transmission], and regions such as Europe and Japan that have LBT requirements will use [LTE Release 13] LAA – Licensed Assisted Access.”
Sanjeev Athalye, product manager for Qualcomm Research’s WWAN projects, said that the differences in the regulation of LTE-U are likely to impact deployment timelines.
– There are major concerns around co-existence and the potential impact of LTE-U on Wi-Fi at 5 GHz. CableLabs, among others, has tested the coexistence of Wi-Fi and LTE at 5 GHz and found reasons to be concerned about how well LTE-U will play with Wi-Fi even with fairness mechanisms in place. (For CableLabs’ take on LBT and duty cycling, check out these blog posts.)
Nokia, which is developing pre-standards LTE-U small cells for deployment by T-Mobile US, has also done testing that showed significant impact. In simulation, Nokia research found that “LTE system performance is slightly affected by coexistence whereas Wi-Fi is significantly impacted by LTE transmissions. In coexistence, the Wi-Fi channel is most often blocked by LTE interference, making the Wi-Fi nodes to stay on the listen mode more than 85% of the time. This reflects directly on the Wi-Fi user throughput, that decreases from 50% to ~100% depending on the scenario.”
The LTE Forum’s testing, however, concluded that with coexistence mechanisms in place, LTE-U’s impact on Wi-Fi can be minimized. The group also found that “in general, the observed gain of LTE-U was smaller in indoor environments relative to outdoor.”
“When LTE is simply deployed in unlicensed spectrum without any coexistence mechanism, it was observed that LTE can cause significant performance degradation on coexisting Wi-Fi,” the group acknowledged. “However, when LTE-U is deployed with reasonable coexistence mechanisms … it was shown that LTE-U behaves as a comparable or better neighbor to Wi-Fi compared to Wi-Fi as a neighbor while LTE-U significantly outperforms the replacing Wi-Fi deployment.”
Operators, Athalye said, “have told us, and we understand, they have no interest in being a sore thumb” for the Federal Communications Commission and the Wi-Fi community and that if Wi-Fi interference results in complaints from the Wi-Fi community, then the FCC is likely to clamp down on LTE-U.
Qualcomm is already responding to those concerns with implementing coexistence measures. As Athalye explained it, the multiple channels and large amount of spectrum available at 5 GHz means that “the base stations can scan the frequencies and look for bands or channels that are unused, or if they cannot find that, it will camp on a channel that has the lowest utilization.”
– Products are already in development that may appear this year, with deployments expected over the next two years.
Athalye noted that T-Mobile US and Verizon Wireless have both made statements about trials, and Ericsson has made statements about product offerings, with a timeline toward the end of 2015 in mind and enabling commercial launches in the first half of next year. (Qualcomm and Ericsson have already been demonstrating LTE-U for operators including Verizon Wireless, T-Mobile US and SK Telecom.) For listen-then-talk implementations with LAA, he added, the standard is expected to be completed in the first half of 2016 with reasonable expectation for products in the first half of 2017. Athalye expects that the “first incarnation will support 2x to 3x carrier aggregation,” reflected by speeds of 450 megabits per second up to 600 Mbps.
Athalye will be speaking on the topic of LTE-U at the LTE Innovation Summit in Del Mar, Calif. this week.
