Today T-Mobile announced it has doubled the speed of its 4G HSPA+ networks in an additional 41 cities, includingSalt Lake City, San Antonio, San Diego and Virginia Beach. Back at the CES 2010, the carrier promised it will double speeds for its 4G users from 21Mbps to 42 Mbps, saying that it expects to the faster service to be available to 150 million people by mid-year.

“HSPA+ is already available on several T-Mobile devices, although only newer devices will be able to take advantage of the full 42Mbps speeds. The technology also has battery life advantages over LTE and WiMax, both of which require power-hungry radios.”

A total of 97 cities now have access to the 42Mbps connection, which (using T-Mobile’s Rocket 3.0 broadband card) averages download peaking at 27Mbps on its faster 4G network. This is similar to what Verizon LTE customers are seeing. T-Mobile’s previous expansion was late last month, when it doubled 4G internet speeds to 55 markets, including Chicago, Dallas, Denver, Los Angeles and other cities.

Every major mobile carrier is venturing into 4G just like the competition, and yet another example is Sprint. The fourth largest carrier’s most recent venture in this field is the upcoming HTC Evo View 4G tablet – essentially a rebranded version of the Android-powered   HTC Flyer with a few additions, most prominently support for Sprint’s 4G coverage.

This week, Verizon also expanded its 4G coverage. Demand, and, consequently competition, is growing around faster internet networks, and earlier this week the company expanded its LTE network to include 19 additional cities, bringing the total number to 74.

Carriers seem to be taking very different approaches to expanding, whether it involves leveraging existing networks, launching new products, or buying out other bankrupt carriers. That’s what Charles Ergen’s Dish Network is doing. The company bid $1.375 billion for bankrupt telecom company Terrestar in order to obtain its 4G assets; a bid that comes after the acquisition of several other companies from the same reason.