4:45 PM -- SoftBank Mobile Corp. $20.1 billion play for a 70 percent stake in Sprint Corp. (NYSE: S) is likely to result in another 4G acronym becoming common parlance on the U.S. wireless scene.

The new 4G flavor you'll need to know is called Long Term Evolution Time Division Duplex (LTE TDD). SoftBank has already deployed an early version of the technology in Japan, so here's a little primer on the fresh 4G.

What it is
The crucial way LTE TDD, which is also known as TD-LTE, differs from the frequency division duplex (FDD) version of LTE deployed today in the U.S. is in what spectrum the technology is deployed on and how the signal is transmitted. TDD technology uses a single channel and a timed signal to separate uploads and downloads. FDD systems have two channels of paired spectrum separated with a guard band for uploads and downloads.

Advocates of TDD technology say that the single channel makes the network more bandwidth-efficient. It also makes it a very suitable technology for data-only wireless devices, much like SoftBank has introduced with its TDD launch this year. (See Just What Is LTE TDD Anyway? and What We Mean When We Say '4G'.)

LTE TDD can be deployed on the 2.6GHz band in the U.S. Sprint's WiMax partner, Clearwire LLC (Nasdaq: CLWR), holds large reserves of this bandwidth across the nation. It has already been testing LTE TDD in the U.S., although launch plans are less clear now. (See Clearwire Targets 31 Cities for LTE TDD .)

Deployments
Aside from SoftBank, which has been called the largest LTE TDD deployment to date by IDC , many of the networks using the technology are pretty small beans so far. The technology has so far arrived in Hong Kong, India, Russia and Spain. (See Euronews: Russians Turn On LTE TDD With NSN , CMHK Deploys Dual-Mode LTE TDD With ZTE and NSN Takes LTE TDD to Spain.)

The scale of the technology could, however, drastically increase in 2013. China Mobile Ltd. (NYSE: CHL), the major backer of the technology, has started to hand out contracts to build out the networks in Chinese cities. (See ZTE Wins LTE TDD Deal in China.)

Deployments in a massive market like China could help to kick-start the wider LTE TDD device and infrastructure market around the world.

For more
Catch up on our SoftBank-Sprint coverage below:

— Dan Jones, Site Editor, Light Reading Mobile