Public Safety Advocate: MCPTT over LTE and Direct-Mode

[Not quite] Mission Critical PTT over LTE is being tested and put into service for live network beta testing. Two things are missing from these trials and tests: a mission-critical or public safety-grade network for the Mission Critical Push-To-Talk application to run over and perhaps more importantly, the way forward to provide direct-mode communications. The UK’s LTE system for public safety to go live in 2020 and replace its existing Tetra system is probably, at the moment, the most robust of the existing LTE networks available.

Recently in France, the European Telecommunications Standards Institute (ETSI) held MCPTT testing sessions and a number of vendors participated to see how well they performed and if they met the 3GPP Release of LTE version 13, which includes specifications for on-network push-to-talk services but does not yet address the issue of direct-mode on and off-network communications. In many instances, direct-mode PTT is as important and in some instances, more important than network-based PTT services. While I recently wrote an Advocate about PTT over LTE and over Land Mobile Radio (LMR) that detailed some of this information, there are a number of things happening in Europe and especially the United Kingdom that could have far reaching effects on how and when PTT services are actually deployed over LTE on and off-network systems.

Gordon Shipley, Programme Director for the Emergency Services Mobile Communications Program (ESMCP), spoke at the recent Public Safety Communications Research (PSCR) meetings and again was quoted in a recent article by Ken Rehbehn. Both times he discussed providing direct-mode services making use of Direct Mode Tetra using a bolt-on remote speaker microphone with a second direct-mode radio as one possible approach to direct-mode.

In the United States, direct-mode could be P25 or analog on any of the VHF, UHF, or 700/800-MHz channels. The main issue, as I see it, still has to do with those working on the solutions truly understanding the needs of the public safety community. There is a huge difference between the range of a broadband phone with a ¼-watt (250 mW) output and a series of built-in antennas and today’s 5-watt land mobile radio handheld with an external antenna. Tetra handhelds come in various power classes from 1 watt up to and including 3 watts and again, they have external antennas.

The tough part about explaining the differences in range between an LTE broadband device and an LMR handheld radio is that there are a number of complex discussion points. Land mobile radios are using either analog or digital voice capabilities transmitted on an assigned radio channel. The LTE broadband device is converting voice to data bits and sending it as voice over IP (VoIP) or Voice over LTE. It is packetized on one end and the packets are put back together on the other end. While you might think P25 and Tetra are really the same at LTE VoIP, there are differences. Suffice it to say that LTE devices are using far less power, the antennas are usually built into the case of the device with a few exceptions such as the Sonim phones, and the broadband phone has many different portions of the radio spectrum built into it. The antennas, filters, and duplexers are normally used for multiple portions of the spectrum meaning that these elements are not designed or optimized for a specific portion of the spectrum as are most LMR radio components.

It is still amazing that you can use a broadband phone, talk to a cell site a few miles away, and the voice and data calls are not garbled. However, if the broadband phones were capable of direct-mode, in most instances you would only be able to hold a conversation between the two devices for a few hundred feet and certainly not from the street to a sub-basement as you can with a handheld LMR radio.

When I reached out to Mr. Shipley in the UK, his response regarding this issue was provided in an email to me which reads:


Whilst ProSe was approved in Mar 15 by 3GPP as part of R13, it is not yet widely available commercially. The range and performance of ProSe is therefore unclear. In the meantime, ESMCP is intending to procure a Device to Device (D2D) service based on TETRA Direct Mode of Operation (DMO) for those who require it. This will require TETRA spectrum but not the TETRA network. The D2D service can be housed in a Remote Speaker Mike (RSM), which would also control the ESN device.

To ensure interoperability, our TETRA network, Airwave, will continue to operate in parallel as users transition onto ESN.


Several important points were covered in the email. First of course are the unknowns regarding LTE ProSe for LTE direct-mode (see below). The other is the last comment that the TETRA network will continue in operation in parallel with the LTE network as users transition into ESN. The TETRA system is a nationwide system run by Airwave (now owned by Motorola). Unlike in the United States where local jurisdictions have their own FCC licenses and radio systems, the Airwave Tetra system covers the entire country and it will be shut down once the LTE network is deemed public safety-grade. The effort to find a solution for direct-mode is only one of the issues being worked on with this system.

Of interest to me is the fact that both at the PSCR event in Dallas and in his email to me, Mr. Shipley referenced a speaker-microphone that would house the Tetra radio portion of the device and still control the PTT of the LTE device. Looking back at previous endeavors along this line, I am reminded that in 1996 Motorola’s LMR division designed a speaker microphone that controlled the LMR handheld and had a GSM phone built into it. This device might have been a game changer, especially in 1996, but the Motorola cellular division blocked the sale of the product because it might compete with its own cellular phones. I have often wondered what the world of public safety communications would look like had this product been permitted to be sold.

It is interesting that in the UK they recognize that the 3GPP specification for ProSe is not really ready for prime time, and the issue is whether it ever will be. The first blush of the PreSe (direct-mode) specifications call for the ability to have a device between two users to relay the voice or data in order to increase the range. In the world of public safety, it is not a valid assumption that there would be a unit mid-way between two units that need to talk to each other nor it is clear how that session would be set up.

FirstNet has one advantage over most other LTE systems since by law user devices are permitted to transmit at a power level much higher than the typical ¼-watt broadband devices. Since the FCC has categorized band 14 (FirstNet spectrum) as part 90 spectrum (same as all public safety spectrum), it permits devices up to power levels of 1.25 watts (+31 dB), which will help with the range but probably will still not meet the needs of the public safety community.

The biggest issue in combining LTE and Tetra or LTE and P25 or analog into a common device or with one or the other being built into the speaker-mic is the battery life. Below you will find a suggested test of range using simple Family Radio Service (FRS) and General Mobile Radio Service devices (GMRS) . The handhelds offer both services (GMRS users must obtain an FCC license), they have been designed to provide the ½ watt permitted on the FRS channels and 1 or 2 watts (as opposed to 5 watts permitted on GMRS channels) because of the battery life of the devices. Increasing the transmit power to 5 watts would drain the battery much quicker and a radio that today works for eight or more hours would barely last three hours on the same size battery with the power increased to the legal 5 watts.

Try an Experiment

If you have any Family Radio Service (FRS) and GMRS radios available (they have to be capable of both FRS and GMRS), try this. If you don’t, go to Costco, Sam’s Club, or WalMart and spend $30 for a pair of them. If you want to use the GMRS channels fill in the FCC license application and submit it since for this experiment you will need access to both FRS and GMRS channels. Most of these radios operate on FRS with ½ watt and on the GMRS channels (license required) at 1 to 3 watts due to the battery limitation of the radio. To really test the differences, you will need both an FRS radio and a true 5-watt GMRS type approved radio.

Set up a course, say open fields or roads, over which you can see a substantial distance. Now tune to an FRS channel (8 through 14) and start talking as you walk away from each other. Once you get to the point where you can no longer understand the other person, switch to a GMRS channel, say, channel 2 or 3 and continue your conversation as you move further away. There will be a difference in range since the FRS radio is limited to ½ watt (500 MW, still twice as much as a broadband phone) but the GMRS channels have more than twice the transmitter power. Try this same test from outside a building to inside, from inside to inside, and you will understand the difference between ½ a watt and 2 and then 5 watts using a handheld radio.


As I mentioned in my Public Safety Advocate a few weeks ago, direct mode is used in a number of different situations. Broadband network-based PTT devices are used only when devices are within network coverage. When they are not in coverage the devices won’t communicate. Period. This is not a situation a first responder can ever be faced with! With direct-mode, even if out of range of the network, the chances of others at their incident hearing them talking in direct-mode will increase the opportunity of being heard by someone.

Direct-mode is sometimes called simplex, talk-around, and if you are an IT type, peer-to-peer communications. The difference is that direct-mode must function:

  • One to one-unit communications
  • One to many-unit (groups)
  • Even while in network coverage
  • When some units are in network coverage and some units are not
  • When every unit is out of network range
  • One of the most vital uses for direct-mode is to be able to talk from street level deep into a building or parking structure (usually where there is no network coverage)
  • The final and perhaps most important use of direct mode is to be able to continue to communicate when the network does not work at all
  • Many LMR systems permit high power transmitters located at a high site to communicate with a handheld 20 or 30 miles away (depending on the spectrum in use and the terrain)
    • Many times, when a public safety network has failed, dispatches are still received in the field because of direct-mode operation
    • This means some type of network to device service must be maintained even if the network core is offline (this is a tough but hopefully solvable problem). However, because LTE is based on a cellular-type architecture, the size of the cells and the areas they cover can be made larger or smaller on demand. Unless there is a failure that impacts the entire network segment (cut fiber cable for example), the network should remain operational

It is important to understand when and where direct-mode is required. A recent example might be the wildfires burning in the west. As each of these fires grows in size, those fighting the blaze are assigned to different areas for different tasks. In many cases there is no cellular or LMR coverage in these remote areas but the devices carried by fire fighters support direct-mode and are used to keep track of those on the fire line. Normally this is accomplished by locating the division commander at a spot where he can talk to the members of his group and still be able to communicate back to Incident Command on the command channel. During these fires, recently the need for direct-mode channels required a reuse of some of the channels in different areas. This reuse was put into place to minimize any interference to others on the same channel operating elsewhere. It is not clear how different radio channels for simplex will be translated on LTE direct-mode if it ever is a viable option to direct-mode LMR.

But direct-mode is not only about wildfires, it is used daily by law enforcement, fire, and ems personnel. Swat teams use direct-mode to contain their conversation to a local area, fire departments use it to communicate inside buildings where the network does not cover, and there are many other times it does not make sense to be tethered to a network when operating on a local basis. If you ask most public safety personnel they will tell you that direct-mode is a vital part of their communications capability and they would not be able to function efficiently or safely if it was not available to them.

Combination Devices

Just as the vendor companies in the UK and elsewhere are working on combination LTE/Tetra devices, the same type of activity is also going on in the United States. There are a number of ways to accomplish the direct-mode requirement. One is, as Mr. Shipley stated in his PSCR presentation and his email, to use the speaker-microphone to house the non-LTE device, but then you have to increase the size of the battery on the main device to be able to power it. In reality, the main issue when it comes to designing combination devices is the battery life. A device must remain operational for more than a single eight-hour shift.

Other options I have seen or heard about include a Bluetooth-enabled speaker-mic controlling both an LTE and an LMR device, or an LTE device with Bluetooth and/or WiFi capability that can also be used as a control platform for the belt-worn LMR radio. There are probably companies working on other combinations of LTE and LMR devices and I certainly encourage that work for two reasons. First, the demand for direct-mode is not going to go away, and the second, the ability to have LMR systems running in parallel with the FirstNet system gives the first responder community more choices, better choices, and yet another level of failsafe capabilities.

Recently I attended a state planning meeting to review the state plan presented by FirstNet and AT&T. The session was led by the state with representatives from FirstNet and AT&T presenting. Over and over again during this session the point was made that FirstNet/LTE is not designed or planned to replace LMR systems but to augment their capabilities. LTE push-to-talk is a great way to implement better interoperability at the scene of an incident involving multiple agencies and AT&T is providing a number of different ways in which an LMR network can be integrated into the LTE system.


Not only is LMR not going away anytime soon, the need for direct-mode is not going away even if and when the FirstNet system becomes the primary communications system for public safety. Direct-mode is necessary and must be provided. By the way, when Congress passed the law creating FirstNet it also required the eleven cities and surrounding areas to give up the T-Band channels (shared TV channels) they rely on so heavily. While FirstNet is being rolled out it is vitally important that T-band users find a way to extend the timeframe of their use of the T-band channels. I have proposed a five-year extension of the 2022 deadline for moving off the T-band, which should be enough time to see if FirstNet really is capable of becoming the one and only communications system needed by the public safety community. I, for one, am doubtful any network should become the one and only network for public safety—especially when it comes to direct-mode.

Andrew M Seybold
©Andrew Seybold, Inc.

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Andrew Seybold, Inc. is the mobile wireless industry’s trusted resource for strategic consulting, research, and analysis. Your source for clarity, insights, predictions, and actionable knowledge that will guide you into the future. Our partners bring more than 180 years of wireless industry experience across the entire spectrum to every assignment.

About the Author

Andrew Seybold
Andrew Seybold, Inc. is the mobile wireless industry’s trusted resource for strategic consulting, research, and analysis. Your source for clarity, insights, predictions, and actionable knowledge that will guide you into the future. Our partners bring more than 180 years of wireless industry experience across the entire spectrum to every assignment.

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