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TD-LTE: Fact vs. Fiction

There’s a lot of conflicting information going around on what TD-LTE actually means to operators and subscribers. Here’s a little bit of help in separating fact from fiction:

Fact or Fiction? TD-LTE will replace TD-SCDMA.

True. This won’t happen overnight, but there’s a reason that TD-LTE Operating Bands 34 and 39 are identical to TD-SCDMA bands being used today.

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Fact or Fiction? China will see the first large-scale commercial deployment of TD-LTE.

50/50:

The addition of the word “commercial” turned this into a trick question. China Mobile has deployed TD-LTE in several Chinese cities and plans to have a staggering 200,000 TD-LTE base stations running within a year, but this is technically a trial network. Commercial deployment can’t take place until the Chinese government straightens out the licensing; a look back at Chinese 3G licensing reminds us that the Chinese government kept us on our toes trying to anticipate the release of those licenses. 4G licensing is no exception; during the course of writing this short blog posting, MIIT announced plans to begin granting licenses in the second half of 2013, about a year earlier than most had thought.

In the meantime, Indian licenses are all in place, auctioned and paid for. While Indian subscribers are not especially demanding of next-generation wireless, there’s a business case for replacing planned WiMAX rollouts with TD-LTE.

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Fact or Fiction? TD-LTE lets operators dynamically re-allocate resources to UL & DL.

Partially True:

The 3GPP has defined seven different frame structures that define the resource split between uplink and downlink capabilities. In theory, it is possible to switch from one frame structure to another whenever there’s a change in uplink/downlink bandwidth demand. However, implementing dynamic changes to frame structures introduces a host of interference issues regarding nearby cells. This may be sorted out in the near future, but TD-LTE deployments might be a couple of years old before we see this feature.

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Fact or Fiction? On a per-Hz basis, TD-LTE provides better data rates and coverage than FDD because you can deploy MIMO beamforming with TD-LTE.

True AND False. While it is true that most of the first large-scale TD-LTE deployments will take advantage of MIMO beamforming, it’s not true that this is exclusive to TD-LTE. You can deploy MIMO beamforming with FDD-based LTE, it’s just not as efficient.

For MIMO beamforming to work, a transmitter needs to know the state of the MIMO radio channel. In FDD, this requires either a very good channel estimation algorithm or a mechanism in which a MIMO beamforming receiver can measure the channel state and feed that information back to the transmitter.

TD-LTE (or any TDD-based technology) doesn’t require this feedback loop. Since the uplink and downlink channels use the same bandwidth and frequency, and since they are separated by tiny slices of time (on the order of a couple of milliseconds), the transmitting element can measure the channel state at its own receiver and assume that the transmit channel is nearly identical. This characteristic of TDD is called “channel reciprocity”, and it is part of the reason that some network operators are getting ready to deploy high-antenna-count (8 x 2 in the reasonably near future) MIMO beamforming with their TD-LTE launches.

For a better understanding of TD-LTE and MIMO beamforming, please download our white paper entitled, TD-LTE and MIMO Beamforming—Principles and Test Challenges.
 
 
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