TDD vs FDD for LTE

, posted: 14-Nov-2011 12:47

New Zealand is debating how to best use the 700MHz spectrum that'll be freed up for other uses after analogue television broadcasts are switched off in favour of digital TV. Mobile operators are jockeying to get a fat slice of the "Digital Dividend" (silly name really), to use for Long Term Evolution (even sillier name) wireless data transmissions.

The going is slow, and one of the issues seems to be that the Ministry of Economic Development (MED) prefers frequency division duplex (FDD) configuration of the spectrum. This means providers use two bands, one for uploads and another for downloads.

Others propose the use of Time Division Duplex or TDD. This uses a single band that's used for both uploads and downloads with "time-slices" to obtain the duplex or transmission in both directions capacity.

The two technologies have their own sets of pros and cons, with TDD seen as the cheaper, easier to deploy technology (important for more competition) and FDD providing better performance.

I was asking Phil Ore at Nokia Siemens about this a while ago and  his crew kindly put together a presentation for me (see below) that explains the NSN view of TDD vs LTE. The slide below might explain why the MED says FDD is the way to go:


3GPP FDD and TDD band definitions

The 3GPP standards organisation has not defined a TDD band in the 700MHz spectrum, by the looks of it. According to NSN, initial LTE deployments are in eight FDD bands and two TDD ones. The former are in the US, Europe and Japan whereas the latter are in China, India and Russia.

As for the performance difference, NSN has this comparison:

TDD FTE peak rates comparison


Noting that these are peak rates and not actual throughput figures, it would seem FDD is the way to go if we want fast uploads as well as downloads. Some will argue that mobile is all about consuming data and asymmetry, and that uploads don't matter. As someone who frequently sends huge files, I wish that "we don't need no steenken uploads" argument would go away. Give me 40Mbps mobile uploads now!

Not that TDD is bad per se. I've tried out TDD LTE in Shanghai, courtesy of Huawei. The trial set up hit 30Mbps using 20MHz of 2.6GHz spectrum, which is pretty good especially since we were zooming along at 200kph on the Shanghai Maglev train. Yes, the Huawei engineer was only downloading. they didn't show the upload speeds.

Here's the NSN presentation in full via Issuu.

Other related posts:
Huawei TDD LTE demo aboard the Shanghai MagLev train
Apropos that new Telecom logo
Pipe Networks signs PPC-1 MoU: Sydney-Guam cable to go ahead

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