AT&T and T-Mobile USA both say they have completed network upgrades to HSPA 7.2, which supports peak maximum speeds of 7.2 megabits per second. The improved 3G speeds will help the carriers handle increased smartphone traffic from the iPhone and Google Nexus One, respectively.

AT&T came under fire in 2009 when iPhone users reported widespread network problems. AT&T later admitted that its network was running close to capacity.

T-Mobile has not come under similar scrutiny, but its network upgrade news coincided with the launch of Google’s high-powered Nexus One. With a 1 GHz processor, the Nexus One will be able to run high-bandwidth applications that could be a burden to network performance.

AT&T said it is also “dramatically” increasing the number of high-speed backhaul connections to cell sites as part of its effort to prep its network for a 2011 LTE deployment. Backhaul deployments are already under way in the six U.S. cities: Charlotte, Chicago, Dallas, Houston, Los Angeles and Miami. AT&T plans to begin LTE trials this year.

“Even as we look forward to LTE, 3G will be the predominant mobile broadband network technology worldwide for smartphones for the next few years,” said Johan Stankey, president and CEO, AT&T Operations, in a speech given at Citigroup’s 20th Annual Global Entertainment, Media & Telecommunications Conference. “AT&T’s strategy will deliver faster 3G speeds over the next two years, while also allowing us to build the foundation for the LTE future.”

T-Mobile says HSPA 7.2 is now enabled across T-Mobile’s entire 3G network, which reaches more than 200 million U.S. residents. The operator plans to be the first in the country to launch HSPA+, which has peak data rates of 56 Mbps on the downlink and 22 Mbps on the uplink. T-Mobile says HSPA+ will be deployed “across the bulk of its 3G footprint” this year.