While some U.S. carriers are moving ambitiously to deploy VoLTE -- T-Mobile is the obvious example here -- U.S. Cellular is taking a more measured approach and plans to begin to bring the technology to market next year.

And one major factor is that CDMA operators face a greater challenge with VoLTE than their GSM-centric counterparts, U.S. Cellular CEO Kenneth Meyers said.

"With respect to VoLTE, yes, it is a pretty well documented fact that the coverage on the voice side with CDMA is a wonderful product," Meyers said this morning during a conference call with analysts. "And when you have built your network with that product and then try to overlay voice over LTE in the same cell sites, there's an initial coverage -- what I'll call 'gap,' almost."

U.S. Cellular observed that "gap" when it began testing VoLTE last year, Meyers said. Engineers are working to overcome the problems by moving equipment gear to the top of towers, among other strategies.

"And they're using other tools so that when we do turn on VoLTE we are able to deliver to our customers the same and better services as they're getting now. So that's why we haven't been rushing to it; we've been very deliberate with it, proceeding with caution, like we have in every other technology change in the past."

U.S. Cellular sees VoLTE as a way to increase its roaming revenue from other carriers as the technology is expanded throughout the nation's mobile networks. The carrier has also said it plans to spread its deployment of VoLTE over time to ease the strain on its quarterly capex budget.

T-Mobile has been the most ambitious U.S. carrier with VoLTE, routing "well over half" of all its calls through the technology, company executives said recently. Verizon and AT&T are also in various stages of VoLTE rollouts, while Sprint has opted to focus on technologies and strategies such as carrier aggregation and beamforming.

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