If it gets more money, Clearwire LLC (Nasdaq: CLWR) could be one of the first operators in the world to deploy Long Term Evolution Time Division Duplex (LTE TDD), so we thought it was a good time to remind you what this 4G technology is and how it is different from today's Long Term Evolution (LTE) networks. (See Clearwire Goes It Alone With Faster 4G.) The basics
The key difference between LTE TDD, sometimes also known as TD-LTE, and the frequency division duplex (FDD) version more common today is in what spectrum the technology is deployed in 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 suggest that the single channel makes the network more bandwidth-efficient. In the real world, however, operators tend to deal with the spectrum they were dealt through acquisition or auction. Therefore, Verizon Wireless is running FDD LTE on its 700MHz holdings, whereas -- as AnandTech points out -- Clearwire has enough 2.5GHz spectrum to consider various TDD deployment tactics while continuing to run its existing WiMax network. (For some history on this, see McCaw's Mystery Millions and A Clearwire Timeline .)

The devices
IP-based TDD devices are ideal for fast data downloads; indeed, Clearwire claims up to 120 Mbit/s down in tests in Phoenix. In the future, this will likely mean digital cameras, video recorders and MP3 players using the technology, as well as handsets, tablets and PCs. For now, vendors like Huawei Technologies Co. Ltd. are introducing multi-modal TD-LTE data cards that can support a variety of 3G and 4G technologies. (See Huawei Readies LTE TDD Device.)

The players
Huawei and other Asian vendors like Samsung Electronics Co. Ltd. (Korea: SEC) and ZTE Corp. (Shenzhen: 000063; Hong Kong: 0763) loom large as LTE TDD device suppliers now, largely because the technology has been spearheaded by China Mobile Communications Corp. and is being tested on the mainland and growing in popularity as a next-generation option in India. Alcatel-Lucent (NYSE: ALU), Ericsson AB (Nasdaq: ERIC) and Nokia Networks have all participated in various trials as well. (See China Mobile Trials LTE TDD With NSN and Market Spotlight: LTE TDD.)

The future
If it raises the $600 million it needs to deploy a TDD LTE overlay in the U.S., Clearwire could be among the first operators to bring the technology online in the world. ABI Research currently tracks 28 commercial FD-LTE networks, but says that no TD-LTE network has yet gone live. (See No TD-LTE Network has Gone Live Yet.)

— Dan Jones, Site Editor, Light Reading Mobile