Voice over mobile broadband, in particular Long Term Evolution technology — which already is being deployed by some of the nation's largest cities for public-safety use — could one day replace land mobile radio (LMR) systems, thus freeing up additional spectrum the public-safety community wants for mobile broadband services.

However, the consensus thinking at the moment is that such an event is several years away, because public-safety LTE is on the same developmental track as commercial LTE, and the current version of the standard doesn't address voice. So, commercial voice over LTE — let alone critical voice features such as group calling and talk-around that public safety needs — will need some time to develop.

Case in point: Verizon Wireless is one of the most aggressive operators in the world in terms of migrating to LTE, with plans to roll out 20 to 30 markets next year. But while its network will be all-IP, analysts believe it could be another 10 years before Verizon offers true VoIP services. In the meantime, it will be relying on its circuit-switched networks to support voice services.

Likewise, European operators don't see the immediate need for voice over LTE either, said Paul Steinberg, chief technology officer with Motorola Solutions. "They are happy with using LTE for broadband data capacity for near term," he said.

One reason commercial operators are content with leaving voice traffic on their 2G and 3G networks is the advances in codec technology that have made voice traffic extremely efficient on their circuit-switched networks, said Peter Jarich, analyst with Current Analysis.

"I think you'll start to see operators play around with VoIP next year, but there's a huge temptation to use 2G and 3G networks for voice," Jarich said. "You have technologies like CDMA 1xRTT Advanced that supports the increase of voice capacity, and you've got various voice enhancements coming out of standards bodies. Basically, having all sorts of voice improvements on existing networks makes it more tempting to keep the traffic on there."

However, a group of major operators and equipment vendors late last year put their weight behind an initiative that would result in a common method for handling voice and SMS traffic over LTE. The initiative, called One Voice, will utilize IP Multimedia Subsystem (IMS) technology to incorporate voice over broadband. The IMS architecture, which on a basic level replaces an operator's back-end network architecture with an all-IP system, is designed to make it easy to deploy services like SMS and voice via a VoIP-type service. The group is hoping to leverage a technical profile of the 3GPP IMS specifications that will be required to deliver basic voice and SMS.

While IMS is the ultimate future of mobile networks, it still is immature, noted Phil Marshall, president of Tolaga Research.

"Vendors are desperate to sell IMS, and it hasn't taken hold in the marketplace," Marshall said. "So voice devices 2012 and beyond make more sense, but then you have to question why I'm putting on voice when I can just use [3G]."

The other alternative is a solution called VoLGA (voice over LTE via generic access) that breaks mobile voice and messaging apart from the IP-based LTE network and puts it on the existing 3GPP Generic Access Network (GAN). Development continues as the interim step to IMS, but it appears that most operators would rather move straight to IMS.

But public-safety voice-over-LTE will require significantly more voice elements than conversational voice services delivered over LTE, said Mario DeRango, vice president and chief architect with Motorola Solutions. Public safety requires critical voice services, he said.

Talk-around is particularly important to public safety, because that capability allows first responders to communicate with each other when their radios are unable to connect to the network. One example would be when a firefighter is in a structure that lacks an adequate in-building coverage system. Another would be when the network has been rendered inoperable by a natural or man-made disaster. In such situations, the ability to talk with other first responders on a peer-to-peer basis can mean the difference between life and death.

Consequently, members of the public-safety community, in collaboration with the Department of Justice, recently re-affirmed the requirement for talk-around through a draft of requirements crafted specifically for broadband public-safety networks. While the next iteration of the LTE standard — which is due later this year — will include VoIP capabilities, it won't provide the push-to-talk voice communications, i.e., one-to-many and talk-around, that are necessary in a mission-critical environment like the one in which first responders operate.

"If you look at push-to-talk over cellular, which is primarily one-to-one traffic, it was never focused on being efficient for a large group of densely located users, as often occurs at major incident scenes," DeRango said. He added that attempting a group-calling feature for early LTE networks would require the network to replicate a voice packet to every user, which is an inefficient way to transmit data over the network.

However, the 3GPP, the standards body in charge of both W-CDMA and LTE, is working on broadcast and multicast services, collectively known as MBMS (multimedia broadcast and multicast service). Using MBMS, multiple subscribers can receive the same data, which is sent only once on each downlink. Users receive the data by joining the multicast or broadcast group associated with the service. For public safety, the service would not only make group calling for voice communications feasible, but also would be beneficial for transmitting key pieces of data, such as video clips, DeRango said.

While it's clear that voice over LTE will take some time to develop for the public-safety world, some rather large vendors are jumping into the space. That increases the likelihood that vendors will spend the research and development dollars to develop solutions for public safety that aren't necessarily dependent on the evolution of LTE in the commercial market, Marshall said.

This week Motorola announced that it entered into an alliance with Ericsson to have the Swedish equipment manufacturer provide key LTE networking gear to Motorola for public-safety wireless broadband systems that are being deployed on 700 MHz spectrum. Motorola, which recently agreed to sell its commercial networks business to Nokia Siemens, has been speaking with multiple commercial LTE vendors about potential supplier arrangements for base stations.

In the meantime, could public safety one day see a multi-mode LTE/P25 handset? "It is something we look at a lot," Steinberg said. "There are pros and cons." Advantages would include ease of use and space efficiency on first responders' belts. But a technical hurdle exists in the radio frequency realm, as P25 networks emit at a higher power than cellular and LTE networks, and interference issues can arise when the dual radios are operating simultaneously in the 700 MHz band, he said.

For now, public-safety likely will have access to the same early devices available to operators like Verizon Wireless. In San Francisco, where public-safety agencies have entered into an agreement with Motorola to have the vendor giant build a 700 MHz LTE system as part of the Bay Area Regional Interoperable Communications System (BayRICS) plan, first responders will have access to devices such as wireless dongles, modems and eventually ruggedized PDAs and tablets alongside their public-safety radios.