Back in June (yep, that long ago), I began a set of posts looking at LTE TDD building off my impression that key LTE TDD misconceptions were still circulating: that it's a fundamentally distinct branch of LTE, and that it was developed in China. Exploring (debunking?) these ideas, we looked at the birth of LTE standards for insights into how TDD and FDD versions of LTE were related as part of a common development and standard. We looked at LTE TDD in the "real world" through the lens of service deployments and device development. We answered your burning questions. With the end of the year fast approaching, it only seems right that we close out this discussion with a view to the future.
Put another way, now that we know where LTE TDD came from and where it is today, we need to ask, "Where is the technology going?" It's safe to assume that the commercial momentum we discussed back in September will continue. As that momentum ramps, the scale should support continued ecosystem development. But, what about the technology itself? How can we expect to see TDD leveraged as a part of LTE going forward? And what does that future tells us about those misconceptions that kicked this series off?
- Supplemental Downlink. Back when AT&T acquired Qualcomm's FLO TV (700 MHz) spectrum, it called out plans for using this spectrum to boost the performance of its mobile broadband services. Overnight, the world got an introduction to "supplemental downlink." Essentially a carrier aggregation use case, this involves the use of unpaired spectrum to boost the downlink performance of a full-duplex system. That full-duplex system might be TDD. It could be FDD. In the latter case, we're talking about an integration of paired and unpaired spectrum – leveraging a channel of unpaired spectrum to boost an FDD system's downlink capacity. In fairness, this isn't necessarily about LTE TDD (since there's no duplexing in the unpaired band). It's not even an LTE specific technology; it's been discussed for HSPA in the past. What it is, however, is an example of LTE addressing diverse spectrum types (paired and unpaired) simultaneously.
- LTE-U / LTE-LAA. As soon as LTE was first deployed, the technology seemed to acquire a series of lettered follow-ons. You had LTE-A (AKA LTE-Advanced). You had vendors talking about LTE-B, an ill-conceived term which could mean the next LTE evolution beyond LTE-A, or simply shorthand for LTE Broadcast (more on that later). Against this backdrop you might think LTE-U was some far-off LTE upgrade or something altogether different. Think, "altogether different." With the U standing for "unlicensed," LTE-U is an application of LTE in unlicensed spectrum. Most recently, this has been positioned unlicensed spectrum LTE as a tool for boosting the capacity of LTE in licensed spectrum vs. any sort of standalone operation. This might mean LTE in unlicensed spectrum serving as a supplemental downlink for FDD systems. It might mean additional full-duplex communications in the unlicensed spectrum operating alongside FDD systems. It might supplement TDD systems either on the downlink or in a full-duplex mode. With unlicensed spectrum being un-paired, any use in a full-duplex mode represents LTE TDD. To the extent that carriers would expect to launch LTE-U alongside their licensed spectrum LTE networks (and, by most accounts, there is carrier interest), it's another example of side by side TDD and FDD deployment.
- LTE Broadcast. By now, we're all familiar with what LTE Broadcast (or eMBMS for the acronym fans out there) is and how it works. If you need a simplistic primer, check out my column from earlier this year. Like Supplemental Downlink, there's no strict relation to LTE TDD. eMBMS can operate over LTE TDD networks but it doesn't actually imply any sort of duplexing. And that's the beauty here; LTE Broadcast can leverage LTE TDD as well as LTE FDD networks. In part, LTE Broadcast delivered over a LTE TDD network speaks to TDD's ability to balance uplink and downlink capacity. In part, it speaks to one common set of services deployed across a common LTE standard – whether deployed in paired (FDD) or unpaired (TDD) spectrum.
So, is there a common theme to all of these technology evolutions?
You might not think so. After all, two of the three aren't even strictly related to LTE TDD. And that's actually part of the point. In one case, supplemental downlink, we have unpaired spectrum (usually the stomping ground of TDD technologies) being used to supplement FDD networks. In another, LTE-U, we have LTE's TDD functionality being used to tap unlicensed spectrum resources. In the final case, LTE Broadcast, we have a technology that can be implemented equally in LTE TDD or FDD networks. In other words, we have nearly every combination of duplexing schemes and spectrum types (paired vs unpaired, licensed vs. unlicensed) being deployed in tandem and in an integrated manner – something virtually impossible had LTE TDD and LTE FDD been developed separately.
Where the past and present speak to the TDD and FDD versions of LTE being part of a common technology foundation, it's nice to see future LTE technology evolutions tell a similar story.
Peter Jarich is the VP of Consumer and Infrastructure at Current Analysis. Follow him on Twitter: @pnjarich.