With an end-to-end connection over China Mobile Ltd.'s LTE TDD network, ZTE Corp. has completed what is believed to be the world's first TDD-based voice-over-LTE (VoLTE) call.

It's a small but important milestone for the TDD flavor of 4G, which is being deployed in a select but important number of markets, including China, India and the U.S. (See Defining 4G: What the Heck Is LTE TDD?)

VoLTE, or 4G voice, is not only the most efficient means of carrying voice calls over an LTE network, but also offers operators a chance to get ahead of their OTT rivals by offering tiered quality of service (QoS).

It's also a clear sign that China Mobile is among the handful of 4G operators keen to capitalize on the efficiencies and improved customer experience offered by VoLTE. And as is often the case, operators in Asia/Pacific are leading the way: South Korea's main mobile operators, led by SK Telecom, already offer commercial VoLTE services, having launched in 2012. (See South Korea Hears First 4G Voice Service, LG U+ Offers 4G Voice With Ericsson and KT Picks Tekelec's PCRF.)

The call was made from a car in a field trial in Guangzhou, southern China, using a smartphone prototype built on a five-mode, 13-band Marvell Technology Group Ltd. chipset, according to Isaac Liang, ZTE's marketing director of TDD products. (See Marvell Intros Quad-Core LTE Chip.)

Liang said ZTE was planning further trials using Qualcomm Inc., Intel Corp. and Broadcom Corp. chips, and would release its first commercial device in the second half of 2013.

The vendor hasn't yet decided which chipset platform it will use. Although it worked with Marvell in the trial, Liang says its only confirmed chip partner is Qualcomm.

Lu Chang, director of product marketing at Marvell's mobile business group, says "all of the components are ready" to go ahead with VoLTE. "It's all about when the operators are ready to deploy."

He says the biggest attraction of VoLTE for operators is reduced network running costs.

"What the operator doesn't want to do, which they're doing now, is manage three networks, plus Wi-Fi, or four networks," he said. With VoLTE they can migrate to a new network and gradually retire the old one.

For 4G operators that haven't yet embraced VoLTE, the alternative ways of providing voice services to 4G users are Circuit-Switch Fallback (CSFB) or dual standby.

With CSFB, voice calls default to GSM or 3G, but this requires implementation of new interfaces in the legacy networks. China Mobile made a study of the investment required and decided it would be too costly, Liang said. Dual standby requires devices to support the legacy network and 4G services simultaneously, draining the resources of both.

VoLTE, which is enabled by the IMS platform in the core network, delivers higher spectral efficiency, lower latency and better energy consumption than either of the other options, enabling operators to support more users on its airwaves.

In addition, VoLTE has a call setup time of just two seconds, compared with eight to 10 seconds on 2G or 3G, and the ability for users open up video or other apps during the voice call.

VoLTE also allows operators the ability to offer three to five different levels of voice QoS, including high-definition voice, a potentially powerful tool in the contest with OTT players.

Liang says that while it is still early days, ZTE is in discussion with operators about "these kinds of issues" right now.

— Robert Clark, contributing editor, special to Light Reading