LTE TDD: Everything You Always Wanted to Know…and Weren’t Afraid to Ask
August 30, 2014 Leave a comment
- Given the difference in duplexing and regional agendas, it’s easy to see TDD and FDD versions of LTE as distinct; in reality, they’re part of a unified standard.
- It’s also tempting to see LTE TDD as disadvantaged by the history of WiMAX or operator interests in FDD spectrum; again, this isn’t a fair way of looking at the technology.
Back in June, I promised to spend some time this summer (and maybe even into the fall) talking about LTE TDD. That started with a discussion of how massive deployments in the People’s Republic could lead one to think of LTE TDD as a Chinese technology, a notion that runs counter to any interests in seeing LTE’s TDD variant benefit from global manufacturing and R&D scale. A look back at the development of LTE standards backed up the notion that, from its inception, TDD was envisioned as an integral part of LTE, not a narrow, regional use case.
So, where does this leave us as the summer slowly grinds to a halt?
One: If you’ve already read June and July’s posts, you probably could have saved a bunch of time if I’d just kicked things off with the 100 words up top. Two: Beyond the broad brushstrokes above, there are still a lot of questions we haven’t answered. That’s right; whether from colleagues, inquisitive readers, or other folks with a stake in the LTE TDD space, I’ve actually gotten a number of questions on the topic. I can’t promise to get to all of them, but I can get to some of my favorites.
- Isn’t LTE TDD a Migration from TD-SCDMA? With China Mobile driving an interest in both technologies, it’s easy to understand where this type of thinking comes from. Still, it’s not the case. Yes, China Mobile (with copious TDD spectrum assets) had an interest in moving towards LTE TDD, but that was, in part, because TD-SCDMA deployments were confined to one market and LTE TDD promised global scale. And, from a technical perspective, the differences are clear. LTE and CDMA leverage fundamentally different air interfaces. LTE was built as one technology addressing TDD and FDD, not part migrated from TDD versions of 3G, part migrated from FDD versions of 3G. Perhaps most importantly, the decision to develop LTE TDD with a single frame structure actually removed commonalities with TD-SCDMA vs. bringing them together.
- Okay, If It’s Not an Evolution from TD-SCDMA, Who Is Responsible for LTE TDD Standards Development? The short answer: TDD duplexing features within the LTE standard were developed by the same people that developed LTE standards more broadly. Recall, TDD and FDD modes were developed as part of a common effort; separating out the efforts doesn’t make sense. Straightforward, right? If you’re looking for a longer answer, you might just be a masochist. That said, you can get that answer by looking to submissions to 3GPP RAN working groups around LTE. Better yet, you can just look at the work commissioned by Ericsson and Qualcomm on that front earlier this year. From a regional standpoint, at least, you get a nice, balanced mix between the U.S., Europe and Asia. China over-indexes on LTE TDD compared with FDD, but even there trails the U.S. and Europe.
- How Similar Can LTE’s TDD and FDD Modes Be If Their Duplexing Is Different? Ah, that’s a good one. And, like the last question, there’s a short answer and a long answer. Returning to that nifty Ericsson and Qualcomm commissioned research, the short answer looks to be just over 80%. How’s that possible? Easy. Duplexing is just one part of the LTE standard. The frame structure across LTE’s TDD and FDD modes may be different, but the core networks are identical and everything else is largely the same: MAC layer, modulation types, basic MIMO implementations, control channels, etc.
- Didn’t WiMAX Attempt to Benefit from TDD’s Efficiencies but Fail? Yes. So what? One doesn’t imply the other. The WiMAX ecosystem focused its efforts on TDD because lots of operators with TDD spectrum assets were looking for a global, mobile broadband standard with which they would launch services. Given market demand, it was a smart move. That doesn’t mean the limited success of WiMAX had anything to do with TDD. No, WiMAX had its own issues. Beyond beginning life as a “nomadic” (not mobile) offer, WiMAX never managed to build a global ecosystem that could compete with mobile broadband technologies like HSPA+ (and, later, LTE). Again, the importance of creating LTE as a global technology is clear. If you’re looking to understand the interplay between global standards, TDD and success, WiFi might provide a better example: it’s TDD-based, but built as a global standard…and undeniably successful.
- Aren’t Mobile Operators Predisposed to FDD? Maybe. But, again, so what? Let’s begin with the “maybe” part. The premium FDD spectrum fetches over TDD spectrum would imply a preference for one vs. the other. This is only because FDD is what mobile operators know. Based on the symmetric nature of voice communications, it’s what they grew up with. It’s what they understand. They know how to plan their networks around FDD. They’re comfortable with it. Yet, if operators are scrambling to make use of every single piece of viable spectrum they can get their hands on (part of why LTE was designed to address so many different bands, duplexing schemes and channel bandwidths), then they won’t pass up on deploying TDD networks, especially if they’re already sitting on un-utilized TDD spectrum.
The “so what” at the beginning of the last answer was really just shorthand for capturing a larger message. Even if operators are predisposed towards FDD – and that’s not necessarily even the case – they are deploying LTE TDD networks, right alongside their FDD ones. That, of course, begs another question: “How are those deployments going?”
I’m glad you asked. If you really are interested in the answer, you’ll want to tune in next month when I take a look at a few for insights into the ways in which operators are looking at, and integrating, TDD into their LTE strategies.