LTE Support for Mission Critical Voice for Public Safety


LTE or fourth-generation (4G) wireless broadband was designed and implemented primarily as a data over broadband technology. Voice in the form of Voice over IP, which is being designed to implement voice calls in the traditional cellular fashion of dialing a number and completing the call using the LTE network as transport, is being developed. The issue is whether LTE can and will support other types of voice services, specifically Push-To-Talk (PTT) voice and most importantly, PTT off-network, when units are out of coverage of the network or when they need short-range communications in buildings and in other areas where the network does not provide coverage.

The standards for LTE are largely controlled by the 3GPP, an organization made up of hundreds of commercial members including chipset companies, infrastructure vendors, network operators, handset companies, software developers, and others. In order to add mission critical voice requirements to the LTE standard, the Public Safety community must petition the 3GPP for its inclusion AND there must be a number of other members of 3GPP that concur. Once (if) this happens, the amendment to the standard is assigned to a future release of LTE and when that release is being worked on, the amendment will be considered.

In order for the amendment to the standard to be considered, all of the requirements must be defined and support must be garnered from members of the 3GPP. At present, there is no incentive for network operators that largely drive the direction of 3GPP, to embrace mission critical voice, especially the part of mission critical voice that is of paramount importance to Public Safety: The ability to communicate between devices without having to make use of a network. Commercial network operators are not inclined to agree to this type of voice communications because they won’t have control of their customers and the minutes of use cannot be billed to the customer.

Therefore, Public Safety will have a difficult time convincing the 3GPP to address the issue of mission critical voice. If a non-standard workaround can be and is developed, it would mean that the devices used by Public Safety would not be nearly as standard as the devices being envisioned today for data and video, thus the cost of these devices would be considerably higher.

However, voice over LTE will happen. It might take longer than many people believe, and it will certainly be implemented in stages. The first voice over LTE smartphones will be available on commercial networks by the end of this year, and the first PTT LTE devices will be tested. Initially, neither of these voice services will meet all of the voice requirements of the Public Safety community. The first PTT service will probably be PTT over LTE for non-mission critical voice communications that will be bridged to existing narrowband P25 voice systems in order to provide for interoperability between narrowband voice and LTE PTT services.

For those trying to plan upgrades to or expansion of their existing narrowband voice networks, it is possible that voice over LTE, both on and off-network, will eventually be developed to provide all of the voice requirements for mission critical on and off-network services. If there is funding for research and development available from the federal government, the time frame will most likely be shortened. In either case, it will take time to first build out the nationwide broadband network, then it will take time for Public Safety to learn how to incorporate data and video into their everyday incidents and then how to integrate voice over LTE into their systems over time.

It is imperative, therefore, that those making decisions about the existing narrowband voice spectrum now available for Public Safety realize that this process could take many years. The commercial community is deploying its forth generation of wireless technology in thirty years while Public Safety is deploying its second generation of voice (P25) in fifty years. One advantage to Public Safety moving to a commercial standard for the first time is that it can piggyback on the standards developed for the commercial community and follow rather than lead the commercial networks with new and upgraded technologies, features, and functions. When it comes to voice over LTE, some of the requirements may not be relevant to the commercial network operators and vendors, thus the development of this functionality may take longer than functions driven by the commercial network operators.

LTE will be able to provide some of the voice capabilities needed by the Public Safety community. The questions remaining are how long will it take to implement the rest of these requirements, how much will it cost, and whether it is better in the near future to integrate voice and data services in the back-end network but not necessarily over the airlink. There is a lot of work to be done to transition from traditional narrowband voice to voice over LTE, and at this point no one knows how long it will take or even if all of Public Safety’s requirements can be fully met on a broadband network.

What Is Mission Critical Voice?

At the moment, there is no “official” definition for mission critical voice; both SAFECOM and NPTSC are in the process of flushing out a number of definitions. But the two below, one developed by Motorola and one by me, are pretty close to what is necessary to provide true mission critical voice communications that are Public Safety grade.

From Motorola

“Mission critical voice communication reflects the harsh realities on the emergency management scene: when every other commercial system is down, you expect mission critical voice to be there. The exacting standards for mission critical networks and devices disqualify many nascent technologies and devices in favor of proven, reliable standards. In a mission critical environment, all aspects of a device or technology must achieve interoperability, reliability, coverage, capacity, control and instant, real-time communications.”

“In an emergency, mission critical voice remains the single most powerful tool public safety and government agencies have at their disposal to ensure the safety and well-being of first responders and the citizens they protect,” says Thomas Quirke, director of Solutions Marketing at Motorola.

And the one I have developed is as follows:

1)   Mission critical communications

  1. Communications between two or more parties that must be conveyed no matter what the circumstances and it must be delivered in a timely (immediate) manner with clarity.
  2. These communications must be 99.999% reliable regardless of the conditions.
  3. This applies primarily to voice services but should also include data and video services during emergency situations
  4. An emergency is any event or incident where life and/or property is in danger, and includes the safety of the responders.

Mission Critical Voice Today

Today, mission critical voice is provided to the Public Safety community using narrowband voice channels in one of eight different portions of the spectrum. These channels are dedicated to Public Safety and the larger an agency is, the more area it has to cover, or the denser the population in its area of responsibility, the more channels it will have licensed to a given agency, city, county, or region. For a more complete explanation of the way in which mission critical voice communications is used today, please refer to Appendix A.

There are those who are working toward the goal of having the Public Safety broadband nationwide network replace today’s mission critical narrowband voice networks. There are many who believe that this will be possible to accomplish in only a few years, while others doubt it can be accomplished within the next decade, and some who believe that voice over broadband won’t be able to replace the need for narrowband voice channels beyond a decade, if ever.

The first thing that is required in order to determine whether broadband networks will be able to replace narrowband voice systems for mission critical voice is a common set of system requirements based on the needs of the Public Safety community. Many of those who are proposing the use of broadband for all of the Public Safety community’s needs for voice services do not fully understand the vast differences between Public Safety voice and voice over an existing cellular system and how different the requirements are for each. Working toward the goal of voice over broadband is fine but only with the understanding of exactly what is required by the Public Safety community and then developing hardware and software that may be able to meet all of these requirements at some point in time.

Mission Critical Voice Over LTE Impediments

The following is a partial list of the impediments that will present challenges to the development of mission critical voice over LTE including off-network, one-to-many communications:

1)     LTE is the fourth-generation standard for wireless broadband and voice over IP (VoIP) will be added for typical cellular phone calls.

  1. Neither the existing release nor future releases address the issue of mission critical voice over LTE or the issue of off-network, one-to-many direct voice communications.

2)     LTE, while a worldwide standard for broadband services, will not be using a common portion of the spectrum.

  1. Today, LTE is being deployed in more than fourteen different portions of the spectrum worldwide with many different off-sets between transmit and receive frequencies and as a time-division-duplex (TDD) technology using the same transmit and receive portions of the spectrum. This makes it even more difficult to develop a standard for off-network communications.

3)     Today’s LTE devices are controlled by the network and NOT only at the device level.

  1. If off-network communications is required, then additional intelligence will have to be built into the devices in order for them to be able to select an LTE voice channel manually or in some other fashion

4)     LTE commercial devices have an output power of 200mw as opposed to handheld LMR radios with power levels of 5 watts or more.

  1. Therefore, the difference in in-building communications will be substantial unless LTE devices are designed with increased power levels for off-network communications.
  2. However, the power levels will have to be different for on-network and off-network voice since higher-powered devices (with the current LTE specifications) will simply shut down a standard LTE network.

5)     How many voice channels can be supported by LTE both on and off-network?

  1. If LTE is to be used for voice dispatch, it will have to provide multiple, separate one-to-many on-network voice channels in order to replicate existing Public Safety narrowband voice systems.
    1. Many cities and counties make use of multiple dispatch channels as well as city or countywide channels (see appendix A). The number of channels required in major metro areas or large counties could run as high as fifteen to twenty separate dispatch channels. When using off-network voice communications, if it is possible to accomplish this, the number of off-network channels will have to be substantial.
  2. In a major incident such as a wild land fire, upwards of seventy discrete one-to-many, off-network voice channels will need to be deployed during the incident.

6)     How many voice channels will be available in a given metro area for one-to-many voice communications?

7)     What impact will adding PTT and voice communications have on the network’s ability to handle data and video at an incident?

  1. Most daily incidents will take place in a confined area within a jurisdiction and will usually be covered by only a single cell sector. Therefore, the total bandwidth available for the incident will have to be shared between voice and data/video services. Using Voice over IP, voice must have priority on the system.
    1. Further, if thirty or forty PTT users are registered within a single cell sector, the amount of data/video capability at the incident will be limited because of the requirement for voice within the same cell sector.

8)     What additional infrastructure will need to be added to the Public Safety LTE network in order to implement one-to-many voice services?

  1. eMBMS (Enhanced Multimedia Broadcast Services)?
  2. Other?

9)     Since LTE’s transport layer is based on UDP (User Datagram Protocol) rather than TCP/IP, this means that packets that are not delivered or that are delivered with mistakes in them are not re-sent and are basically thrown away.

  1. How will LTE’s Quality of Service (QoS) and Priority features ensure that
    1. Voice packets have priority over data packets
    2. Voice packets arrive in the proper sequence and with no errors or
    3. The user devices are capable of error correction to ensure intelligible voice being received?

10)  How will voice traffic, specifically PTT voice, be routed when a user is out of his/her prime area of operation?

The above list is only a partial list of the issues that must be dealt with and solved prior to LTE being able to support what is defined as one-to-many voice services for Public Safety.


IF LTE broadband can meet both the voice and the data requirements of the first responder community, a single device could be deployed that would provide not only data/video interoperability, but voice interoperability as well. This would be an ideal situation and one that is worth pursing. However, existing narrowband spectrum should not be reallocated for other uses until such time as LTE broadband can and does meet all of the requirements for Public Safety mission critical voice as well as data and video services.

Organizations such as NPSTC are working on defining the specific requirements for voice over LTE for Public Safety and there are those both within the federal government and within the vendor community that believe Public Safety will soon be able to use a single device for both mission critical voice and data on the common LTE broadband system. Others believe that some form of PTT over LTE, perhaps P25 digital PTT, will be able to be used over LTE but in conjunction with existing narrowband P25 voice systems.

Whatever the case proves to be, it is imperative that Public Safety not lose any of its meager spectrum holding for narrowband voice until voice over LTE is proven and readily available. As important as data and video are as new services to the Public Safety community, in the end, voice communications are the most critical form of Public Safety communications and it must remain mission critical, easy to use, and available on a moment’s notice. If it is not, Public Safety agencies will not be able to perform their tasks efficiently and will, in fact, place themselves and the public in harm’s way.

The most sensible approach for the short term is to work with vendors to develop dual-mode devices capable of LTE data/video services as well as narrowband P25 capabilities on both the 700 MHz and 800-MHz bands currently assigned to and licensed by Public Safety agencies. Such devices should become readily available within the next few years, and while they will still cost more than commercial wireless devices such as smartphones, these devices can and will provide both broadband data/video and voice over P25 using the narrowband channel capabilities. There will still be issues regarding voice interoperability with systems on other portions of the spectrum, but for the next few years there are workarounds that can be implemented to minimize, but not eliminate, the issue of voice interoperability.

LTE may be able to meet all of Public Safety’s requirements for fully interoperable voice and data services but that has not been proven yet and I believe if it happens at all it is many years in the future. In the meantime, LTE for data and video should be deployed as quickly as possible, and workaround solutions for interoperable voice should continue to be implemented on a local, area, regional, and statewide basis. To stop the drive for voice interoperability on the existing narrowband voice channels while waiting to determine if LTE can, in fact, be effectively used for both voice and data mission critical applications would be dangerous not only to the first responders but to the people they serve.

Andrew M. Seybold

Appendix A:

Mission Critical Voice Use

Voice channel requirements for Public Safety include the following:

1)     One or more one-to-many, on-network voice channels for dispatch.

  1. Dispatch channels must be capable of one-to-many communications since it is imperative that not only the units required for the dispatch are notified but that those in proximity to the incident be made aware of the incident and that the field commanders be aware of the dispatch and the incident as well.
  2. After the dispatch is made, either the units responding are directed to use a different one-to-many, on-network channel for unit coordination or they remain on the primary dispatch channel until the first unit(s) on the scene report the status of the incident.
  3. Usually fire and EMS personnel are dispatched on one set of dispatch channels while police agencies are dispatched on a different set of radio channels.
  4. In large populace and metropolitan areas, there need to be multiple dispatch channels. Normally they are assigned by geographic area within the agency’s jurisdiction.
  5. In addition to the dispatch channels, each agency normally has one or more jurisdiction-wide command and control channel for incident coordination.

2)     On-scene communications

  1. The first unit arriving on the scene of an incident will give both the dispatcher and the other units responding to an incident a report of the situation as visually observed upon arrival.
    1. This message needs to be heard by ALL of those responding, the dispatcher, and those within the area and/or line officers responsible for manpower and equipment allocations.
    2. In both police and fire incidents, if the incident involves additional responders, the incident is normally assigned to an off-network channel to free up the dispatch channel and to provide on-scene communications for those at the incident. Again, fire and EMS will normally operate on one or more off-network channels while police will operate on others.
    3. As the incident grows, additional off-network channels are assigned so each group at the incident has its own working channel. For example, in a hostage situation, the SWAT team may be assigned one discrete channel, uniformed officers on the scene another, detectives yet another, and fire and EMS support yet additional channels.
    4. The larger an incident becomes, the more off-network channels need to be assigned.

3)     Off-network communications on different channels are a necessity for Public Safety. Oftentimes there are multiple incidents in progress at the same time, separated by some distance, and even more off-network channels are required in the day-to-day operation of Public Safety agencies.

Narrowband Voice Channels Currently Licensed to Public Safety Agencies:


Frequency Band Type of Radio Channels Band Shared with other Users?
30-50 MHz shared spectrum
(6.3 MHz of spectrum)
Narrowband voice channels Yes, business, utilities, government others
150-170 MHz shared spectrum
(3.6 MHz of spectrum)
Narrowband voice channels Yes, business, paging, utilities, other
220 MHz channels (only one area of U.S. near Canada) Narrowband voice channels No
450-470 MHz shared spectrum
(3.7 MHz of spectrum)
Narrowband voice channels Yes, business, alarm, utilities, paging, local government, others
470-512 MHz (shared TV channels certain areas only) Narrowband voice channels Shared with TV station and business radio/wireless mikes
700 MHz narrowband
(12 MHz of spectrum)
Narrowband voice/data No—contiguous spectrum
700 MHz broadband
(10 MHz of spectrum)
Broadband data No—contiguous spectrum
800 MHz narrowband
(9.5 MHz of spectrum)
Narrowband voice/data N0 AFTER rebanding is completed
4.9 GHz broadband
(50 MHz of spectrum)
Low-power data Suited only for local use and does not penetrate buildings

Note: Cleveland, Buffalo, and Detroit are using shared NTIA channels in the 421-430-MHz band
Note: There are a few 220-MHz systems in use in other areas such as Long Beach, CA.

As the chart above illustrates, today’s Public Safety mission critical voice channels are spread across seven vastly different portions of the spectrum. It should be noted that except for the 700 and 800-MHz voice allocations, all of the other portions of spectrum allocated to Public Safety are shared with other services. It should also be noted that a radio system operating on the 30-MHz band, 150 MHz, 450 MHz, or 800 MHz will have different coverage capabilities on each; the higher in the spectrum that you operate a system, the more infrastructure is required to cover the same given geography.

Different types of narrowband voice services and technologies available and in use in the various segments of the spectrum are as follows:

Possible Combinations of Systems that Need to Be Interoperable 


Frequency Band Analog FM P25 Digital Base to Mobile Repeater Simulcast Trunked
30-50 MHz X X X X
150-174 MHz X X X X X X
220 MHz X X X X
450-470 MHz X X X X X X
470-512 MHz X X X X X X
700 MHz NB X X X X X X
800 MHz NB X X X X X X

One-to-many voice communications, available on multiple on and off-network channels, is a prime requirement for Public Safety communications but is not a requirement for commercial wireless networks.



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