For all the promise of LTE, it appears that global roaming on so-called “world phones” is one thing it might not be able to deliver.

The reason: spectrum fragmentation. The GSM Association is warning that the wide variety of frequencies used around the world for the mobile broadband technology could make it impractical and expensive to manufacture LTE devices that work across different continents.

The association predicts that there will be 38 different combinations of spectrum used in LTE deployments by 2015.

"Spectrum fragmentation has the potential to hinder global LTE roaming if device manufacturers are required to include support for many disparate frequencies in their devices," said Joss Gillet, author of the GSMA’s Wireless Intelligence report on spectrum fragmentation. "Given the backwards compatibility already required for either HSPA or EVDO connectivity, we are unlikely to see a 'world' device in a handset form-factor soon."

The fragmentation of spectrum used for LTE is fueled in part by the variety of frequencies supported by the 3GPP's standards for the mobile broadband technology. The standards body lists nearly 20 different bands for FDD-LTE and another nine bands for TD-LTE.

The flexibility is a boon for operators since it gives them more options for deploying LTE, but it could prove a headache for vendors that have to support an ever-increasing array of different frequency and technology combinations in equipment and devices.

GSMA expects that there will be more than 200 live LTE networks in more than 70 countries by 2015, up from 40 networks in 24 countries today.

The IMT-extension band from 2500 MHz to 2600 MHz is the most commonly used frequency for current LTE deployments, accounting for more than half of live networks this year. The 700 MHz and 800 MHz bands are expected to be used in about a quarter of LTE deployments forecast for 2015.

In the United States, Verizon Wireless and AT&T are using 700 MHz for LTE. Clearwire plans to add LTE to its WiMAX network in the 2.6 GHz band, and Sprint plans to deploy LTE in its 1.9 GHz spectrum.