The speed at which data can be uploaded over a mobile broadband network will become increasingly important as users gain access to a wider range of wireless applications and devices in the coming years.

Executives from U.S. carriers such as Clearwire LLC (Nasdaq: CLWR) and Verizon Wireless have already tacitly acknowledged the importance of the uplink by talking about how "non-traditional" devices will have a growing role on their respective mobile WiMax and Long Term Evolution (LTE) networks. (See CES: Sprint's in the XOHM.)

For instance, Verizon Wireless CTO Dick Lynch is keen on talking up the idea that digital cameras will soon be able to send pictures over his firm's proto-4G networks. This particular application would certainly require a strong and fast uplink on the network. (See MWC 2009: Verizon Picks LTE Vendors.)

Users, meanwhile, are increasingly leaning on existing 3G network uplink capabilities to update their statuses on social networking applications like Facebook and Twitter Inc. Any regular mobile user will likely already know the occasional frustation of waiting to upload new comments, links, or photos over a less-than perfect 3G connection.

Generally, however, carrier and press discussion of advanced 3G networks and the evolution of fourth-generation networks tends to focus on the -- generally impressive -- downlink speeds offered over mobile broadband, rather than the uplink. (See WiMax & LTE Meet the Real World.)

So, while AT&T's maximum download speed for its upcoming 3G network upgrade is 7.2 Mbit/s with likely averages in the 3 Mbit/s to 4 Mbit/s range when the user is stationary, the uplink is considerably less. The carrier is promising uplink speeds of between 500 kbit/s and 1.2 Mbit/s via its High-Speed Packet Access (HSPA) technology.

Clearwire, meanwhile, has said that it can offer uploads at "up to 1 Mbit/s" on its new mobile WiMax networks. The Clear service is currently available in Atlanta, Las Vegas, and Portland, Ore.

The exact upload speeds that will be offered by Verizon's LTE network in 2010 and beyond are unclear at the moment. The operator has said that it doesn't want to talk speeds until it has much more testing data under its belt.

The way that Clearwire is deploying mobile WiMax compared to Verizon's LTE plans marks a big difference between two technologies that are not hugely dissimilar at heart. Clearwire is deploying its network in 2.5 GHz spectrum suitable for time division duplex (TDD) applications, while Verizon is using a frequency division duplex (FDD) for its 700 MHz deployment.

The key difference between the two duplexing schemes is that TDD uses a single channel and uses a timing mechanism to coordinate uplink and downlink transmissions. FDD, which has been a standard in voice-based cellular systems for years, uses two separate channels for up and downstream traffic.

TDD is generally touted as a more spectrally efficent system for wireless data, since it is easier to dynamically adjust the amount of bandwidth available on the uplink and downlink on the fly. There isn't a huge amount of open TDD spectrum available in the U.S. and beyond, however, so standards bodies have developed -- or are working on -- both FDD and TDD profiles for WiMax and LTE. (See WiMax Edges Closer to LTE.)

Verizon told Unstrung recently that its decision to use FDD for LTE was motivated by the suitable radio frequencies available for the proto-4G technology. "Our decision to use FDD for LTE was driven by spectrum availability in the US," says Bill Stone, executive director of network strategy in an email reply to questions.

"All of our existing spectrum (Cellular, PCS, 700 MHz, and AWS) is FDD spectrum. We do not have any TDD spectrum and there is limited availability of TDD spectrum in the US. FDD also is ideal for real time applications," he added.

So, at present, we can say that uplink speeds are improving for 3G and proto-4G network technologies. What is not yet clear, however, is how much of an effect the channel widths available to each carrier and the duplexing technologies used will have.

Both Clearwire and Verizon are starting with 10 MHz channels for their respective WiMax and LTE services. Clearwire has much more spectrum overall than Verizon's 20 MHz of 700 Mhz holdings, but proponents of that band suggest that is better for covering long distances than the 2.5 GHz that Clearwire is using. (See CTIA: Clearwire Talks Android & More.)

– Dan Jones, Site Editor, Unstrung