Believe me, I have been asked that question hundreds of times. And I like it because it is an excellent start—a start that happened several years ago, in our labs, inside the heads of our engineers, while conceiving the idea of LTE-U. This is the very question we asked ourselves, and tasked ourselves with finding a definitive solution. We found it, not by accident, but through innovative design and our unwavering commitment to coexistence. We checked and rechecked it, tested it, confirmed it and now here we are, showing it to the world.
Coexistence for us is just not a check box or an afterthought. Instead, it’s at the core of our LTE-U system design. So much so that, forget about the notion of LTE-U harming Wi-Fi—it actually benefits it many cases. We also go above and beyond minimum regulatory requirements to ensure fair coexistence. Let me dive a little into the details.
LTE-U based on the geography can be deployed in two ways:
- In countries such as the U.S., Korea, China and India, based on Rel-10/11/12, with coexistence features.
- In regions such as Europe, Japan who have specific channel occupancy regulatory requirements called as Listen Before Talk (LBT), a new standard Rel 13 will be needed. LTE-U in Rel 13 is referred to as Licensed Assisted Access or “LAA.” (Strange acronym, agreed, but states exactly what it is!)
In both the cases, the sharing starts with dynamically looking for an unused 20 MHz channel and using it whenever available. With up to 500 MHz of spectrum available in the 5GHz band that LTE-U is targeting, there is a fairly good chance that it happens. If so, it’s all good. Actually, it’s all good even when there are no free channels. Only that Wi-Fi and LTE-U have to share the same channel. Oh! By the way, the unlicensed spectrum is used only when the extra capacity boosted is required, and released immediately when the boost is not needed anymore.
In both of the deployments (Re-10 or Rel-13), the sharing of the channel between LTE-U and Wi-Fi happens in terms of time. In the Rel-10 version LTE-U, estimating the number of active Wi-Fi access points (AP) in the vicinity, adaptively turns LTE ON and OFF in such a way as to proportionately share the channel with Wi-Fi. We aptly call this feature Carrier Sensitive Adaptive Transmission (CSAT), to be used for Rel-10 based LTE-U. CSAT can be better illustrated with the following chart:
In the example, there are four Wi-Fi APs and two LTE-U APs. So, LTE-U turns ON during 2/6th of the time, and OFF during 4/6th of the time, allowing Wi-Fi APs to use the channel as they would normally do. This cycle of ON and OFF continues, adapting to the changing network and loading scenario along the way. That explains the “Adaptive” part in the CSAT name. The timing scale for sharing is configurable and could be as little as 20 msecs to even 100s of msec. LTE-U also turns OFF for very short durations during its ON time (short gaps in the chart), to allow for latency-sensitive traffic such as VoWI-Fi.
The technique is similar for Rel-13 based LTE-U, except that and LBT requirements are pretty specific on how to sense the channel before transmission and how much time the channel can be occupied. Because of that the time scale is shorter and ON times are 1-10 msecs, as shown in this chart:
So now, getting to the point of going above and beyond, apart from the minimum power and transmission levels required for using the unlicensed spectrum which vary even within the 5GHz band (and LBT in some regions), LTE-U incorporates the coexistence features discussed above, and will fully adhere to 3GPP R13 LAA standards, and specifications for CSTA based LTE-U using R10/11/12. As a final check, we expect to have conformance testing done before commercialization, similar to what’s done with Wi-Fi today, but only more rigorous. Pretty good, huh?
What I just told you might be a lot of detail, and you might ask “but does it really work?” Absolutely!
Here is the video of it working in our over-the-air stress test chamber.
It was also shown at MWC 2015 live in full glory. And as usual for more details checkout www.qualcomm.com/lte-u. And for people who are pondering when will LTE-U be available? Check out our press release.
Before you go, if you happen to meet me and ask the same question, I won’t get mad. As you can tell. I love explaining this stuff over and over.