T-Mobile USA's plans to move to Long Term Evolution (LTE) could be more challenging if it can't upgrade its existing mobile broadband users fast enough.

T-Mobile said this Thursday morning that it will start to launch faster LTE services in 2013. It is the last of the four largest U.S. operators to reveal plans to move to the speedy mobile data technology. (See T-Mobile Will Launch LTE in 2013 and Mapping LTE: AT&T & Verizon's 4G Footprints.)

A T-Mobile USA spokeswoman tells Light Reading Mobile in an email that the operator plans "to cover the vast majority of the top 50 markets and [have] nationwide coverage by the end of the program." The service provider isn't yet saying which U.S. cities will be launched first.

The carrier says that it plans to spend $4 billion in all on the network upgrade. "The $4 billion will be mostly spent in 2012 and 2013," the spokeswoman wrote.

This suggests a relatively rapid technical upgrade to LTE from High-Speed Packet Access (HSPA) 3G for T-Mobile. The company has already upgraded to 21Mbit/s and 42Mbit/s HSPA+ with suppliers Ericsson AB and Nokia Siemens Networks over the last few years. (See T-Mobile's HSPA+ Rivals Clearwire, US LTE Speeds.)

Ovum Ltd.'s principal analyst for infrastructure, Daryl Schoolar, expects that T-Mobile will stick with Ericsson and NSN and try to reuse as much infrastructure as possible.

"Both of them have been saying for years that they have been deploying equipment that can be easily moved over to LTE," he tells us.

"Hopefully for T-Mobile they can at least use the same baseband units," Schoolar continues. He suggests that that the operator will have to locate LTE "radioheads" at its cell sites for the upgrade. T-Mobile says it will install new equipment at 37,000 cell sites as part of its LTE evolution.

T-Mobile gets to undertake this upgrade because it got $3 billion and Advanced Wireless Service (AWS) spectrum (1700MHz/2100MHz) from AT&T Inc. after the failure of their planned $39 billion merger. The operator will, however, also eventually have to move existing 3G users off its AWS spectrum and refarm its 1900MHz spectrum for HSPA service. (See AT&T Drops Bid to Acquire T-Mobile.)

"That is a long process," states Ovum's Schoolar. "The biggest thing with any kind of re-farming is what do you do with the end user?"

He suggests that T-Mobile may have to do things like increase subsidies on handsets to encourage its value-orientated users to shift.

Whatever happens, T-Mobile's marketing department has its work cut out for it. The operator now has to explain why users should move to a new 4G LTE handset when -- like AT&T -- they have already been marketing HSPA+, which has traditionally been called 3G, as a 4G service.

— Dan Jones, Site Editor, Light Reading Mobile