Decision makers need to consider many layers when determining future role of mission-critical voice over LTE

First, a mission-critical LTE network needs to be deployed in the desired coverage area. In the U.S., FirstNet could begin being built in some areas as early as the end of 2017, while other jurisdictions may not get coverage until 2020 or beyond. If a public-safety agency doesn’t have the mission-critical LTE coverage it needs, switching from an LMR system should not be considered.

Second, MCPTT over LTE will be a new technology, so it’s definitely possible that there could be some bugs or other performance issues at the outset—problems that first responders cannot afford to deal with during an emergency.

With this in mind, many industry experts believe MCPTT over LTE will be used first for non-mission-critical voice initially, as LA-RICS has planned for its public-safety LTE network. It could be some time before public-safety users trust MCPTT over LTE for mission-critical operations, but it is too early to guess how long that might be.

When we make a switch to MCPTT over LTE, all public-safety agencies in the jurisdiction will migrate at the same time. Many believe adoption of MCPTT over LTE for mission-critical communications could vary within the public-safety ranks. For instance, police detectives in some cities already have stopped using LMR portables and depend solely on cellular devices. And many industry experts predict that police and EMS personnel will be inclined to adopt LTE at an incident scene than firefighters.

Some have argued that firefighters will be last because they traditionally have been more resistant to technological change, but there also are technical reasons that may be case. Many fire departments like to conduct their mission-critical voice communications in direct mode, even if the incident is within the coverage area of an LMR network.

In Release 12 and 13, MCPTT over LTE supports direct-mode communications, but the realities of physics could hamper its performance compared to LMR. LMR devices typically radiate signals at power levels that are at least four time greater that most LTE devices today. In addition, some LMR systems benefit from the greater signal-propagation characteristics that VHF and UHF frequencies have in comparison to 700 MHz spectrum.

Of course, that could change. LTE devices could be approved to operate at higher power levels, particularly if spectrum was made available—some have even suggested that some LMR channels be reallocated for this purpose—but current rules give LMR the edge in direct-mode range.

Because of the potential physics advantages, LMR will provide better coverage for mission-critical voice. As is often the case in wireless communications, it all depends on the circumstances. LTE networks have many more sites than LMR systems, so they can provide similar or better coverage, depending on the location and number of sites.

And sites are not just standalone towers; carriers locate sites on rooftops, water towers and many other locations. The numbers are staggering. A Verizon representative noted that the carrier once thought about nationwide coverage using 30,000 sites, but the number of sites had jumped to 300,000—and that statement was made in 2014, so the figure may have increased since then.  

One aspect that promises to particularly interesting during the next several years will be the issue of in-building coverage. With the proliferation of small cells and DAS systems that are designed to provide in-building LTE coverage, many wireless experts believe that it is likely that LTE coverage will be better in buildings than LMR coverage—especially for LMR systems that depend on outside signals penetrating inside a structure.