Public Safety Advocate: Waiting on Mission Critical Push-To-Talk (Data, Video)

According to Motorola, its Kodiak Networks has developed the first Push-To-Talk (PTT) application for on-network use. On-network means the network hosts the application as opposed to an over-the-top application such as ESChat or Orion Labs, the other FirstNet-approved options. Motorola’s claim is that the Kodiak PTT solution as deployed in both the AT&T and Verizon networks (though not compatible across the two, I am told) is close to meeting the Mission Critical Push-To-Talk specification released by the 3GPP standards organization.

All that is needed, claims Motorola, is for companies to start producing Proximity Services (ProSe) chipsets, which Motorola claims remains the greatest hurdle to being fully MCPTT-compliant. I am using Motorola as an example here since it recently stated its PTT solution for FirstNet (Built with AT&T) works and is embedded in the FirstNet network. ESChat and Orion Labs are also certified on FirstNet. I find Motorola’s comments about ProSe to be peculiar coming from one of the premier Land Mobile Radio (LMR) companies in the world that for many years has built mobile and handheld radios that provide push-to-talk on public safety and business radio networks and further provides the capability for off-network push-to-talk.

Off-network push-to-talk, which I have written about over the years, is critical to the ability of those in the field to communicate with others even when a network is not available—either because they are out of network range or because they are deep inside a structure and cannot reach the network that may be available on the street. Simplex, talk-around, or whatever you want to call it is vital to the operation of the public safety community. It is used every day by police, fire, and EMS personnel. Simplex provides one-to-one and one-to-many communications to any unit in range of the transmitting unit.

This is, perhaps, the most important point: With simplex, the transmission from the sending radio can be heard by any and all units, or specifically-chosen units, within range of the transmitting unit. So my question is whether Proximity Services is simply a checkbox on a set of specifications for mission-critical push-to-talk or is it meant to be a serious contender for off-network communications? If Motorola or others need to be able to say they are 100-percent compatible with the 3GPP Mission Critical Standard, must they include ProSe in their system even though they fully realize there are severe limitations with ProSe and that the likelihood of ProSe replacing LMR off-network communications using today’s LTE technology is slim to none?

Later in this article I will look at mixed LTE and 5G systems and perhaps those planning and implementing these networks can address this issue as well. However, again, once you are off the network, whether LTE, 5G, or WiFi, and need to communicate with others on the scene, you must have a robust system that will communicate over reasonable distances and into buildings, sub-basements, ravines, in mountainous and other difficult-to-cover areas, and in short, anywhere the network is not able to communicate.


What are the issues with ProSe over LTE? First and foremost are the transmit power levels currently being deployed over LTE. Today, a device on LTE is limited to 1/4 watt (250 milliwatts). Further, most (but not all) FirstNet-approved devices are smartphones with the antennas built into the phone. Even with the best-engineered antennas inside a smartphone, the total permissible effective radiated power permitted from a smartphone is 30 dBm or 1 watt, which is really stretching the performance of a smartphone. Normally, the LTE network controls the output power of the smartphone, setting the power to the lowest level possible for communications to a given cell site. The exception to this power restriction is that in Band 14, and only Band 14, a higher power can be transmitted by a device. The power limit in this case is 1.25 watts or 1 ¼ watts. Thus it may be possible to build smartphones and tablets that use the higher power on Band 14, but along with this increased transmit power comes the issue of battery size and capacity. 

And there are also other factors at work here. All LTE devices transmit in the 700-MHz band in the low end and receive in the high end of the band. This means, for example, the transmitter would transmit on 758 MHz to the cell site and the cell site would transmit back out to the device at 788 MHz. This is a 30-MHz separation between the transmitter and receiver. In order for ProSe to work, the smartphone transmitter has to be enabled to transmit at, for example, 758 MHz and also at 788 MHz, or the receiver could be designed to receive on both the 788 and 758 channels. With today’s technology, this should be no problem, correct? Not exactly. Between the transmitter, receiver, and antenna there are filters and duplexers designed to limit the amount of interference between the transmitter and receiver. 

Of course, this can all be “fixed” by the smart engineers who for years have been putting more radio bands, filters, and duplexers into smartphones than we ever thought possible. So, ProSe devices can and probably will be built, but what then? ProSe was not designed for public safety push-to-talk. If it had been, attention would have been paid to the issue of distances that need to be covered. Instead, it appears as though the intent was to enable network customers to share data and pictures when they are in close proximity to each other. Yet we are being told by some that ProSe will be the final piece needed to enable FirstNet PTT to replace LMR PTT. At this point in time at least, this is a mis-statement.

We will see what happens when we have a ProSe device and can run it through its paces. We would take two of any company’s type-accepted handheld radios on 800 MHz or 700 MHz and pit them head-to-head with two ProSe devices. Some will say this is not a fair comparison. They might say the 700-MHz LMR radio has a higher power output of 1 to 2.5 watts according to the Motorola APX 8000 Specification, while other models provide more power on VHF and UHF. The unit has an external antenna and a battery that will provide for operation for more than a full shift. So here are the tests I would run if I had two of each radio:

  • Two users. One stationary, one moving away from the first one on a level street in an urban area. How far away can they continue to communicate?
    • ProSe
    • APX
  • Two users. One stationary on the street, the second entering a building, taking an elevator to the top floor, and moving to the back of the structure.
    • ProSe
    • APX
  • Two users. One stationary on the street, the second in a building and descending to the parking garage in the basement of the structure.
    • ProSe
    • APX
  • Group Communications. Multiple users with multiple devices, surround a city block that is built up. Invoke one-to-many group mode.
    • ProSe
    • APX

Anyone want to bet who wins this set of tests? Back to those who may feel this would not be a fair test. I am not worried about being fair here, I am making the point that off-network operation should not be sacrificed so a standard can be met and the spec sheet can declare “Fully-MCPTT compliant.” If you want to run the test with Band 14 radios, let’s go ahead and do so at the full 1.25 watts. Perhaps those results might favor ProSe, but I doubt it.

Now, simply because a company has a ProSe-capable device does not mean the network is ready for MCPTT services. Mission Critical, or Public Safety Grade as I prefer to call it, requires hardening of the network. This is being addressed by FirstNet (Built with AT&T) and in some areas, it has met most of the criteria while many LMR systems do not yet meet all Public Safety Grade recommendations. However, let’s not forget about another aspect of push-to-talk that is not being addressed and can only be addressed from the network side. This requirement, as described in a specification for LTE, is called MBMS or Multi-Cast Service. Multi-cast capability is essential if voice dispatch over LTE is to become a reality, or if FirstNet is to operate in “zones” as many cities operate their LMR radio systems. Until we have MBMS one-to-many, PTT must be via groups rather than zones, and there is a difference. 

Getting back to 5G, I have heard from non-public safety experts that PTT over 5G will be amazing and will solve the ProSe issue. No, it won’t. Even with 5G there will be uncovered areas, areas too deep into buildings, or buildings not covered with Distributed Antenna Systems (DAS). Off-network PTT will continue to be a critically needed capability. Even with 5G, until I see a different approach to ProSe, LMR will continue to be the only system to provide off-network PTT for our public safety personnel who need it, use it, and will not do without it.

Winding Down

First, I would like to thank Sierra Wireless, FirstNet (Built with AT&T), and Sonim Technologies for sponsoring our up-coming Phoenix area (and friends) Spring Get-Together cocktail party and BBQ. We are looking forward to a fun social time without the pressure of business. Next, I saw, as perhaps you did too, the reunion of four men we called the “four horsemen” when we were working with Congress, the Executive Branch, and with (or sometimes against) the FCC. The four, Chief Johnson, Chief Dowd, Chief Moore, and Dick Mirgon, were instrumental in convincing Congressional leaders of the need for public safety access to the D Block (Band 14), bringing their energy and influence to our ongoing efforts. 

Chief Moore (Ret), Chief JJ. Johnson (Ret), Dick Mirgon, Deputy Chief Dowd (Ret)

The first time any of us heard about this idea was when Morgan O’Brien, at his keynote at IWCE, put forth the idea of having a nationwide broadband network for public safety. Chief Harlin McEwen (Ret.), who I consider to be the father of FirstNet, picked up the ball and ran with it, then the four horsemen joined in and moved us along. As my co-author and I write our book about this adventure, it is amazing how many people pitched in, helped, contacted their representatives, wrote articles and letters, and more. It is, honestly, a huge undertaking and we hope we will have named everyone who should be included. While a few started the process, it took the efforts of dozens to carry it through to completion.

Today, FirstNet is real, it works, and gains momentum with each month. The winning bidder for this project, AT&T, is ahead of schedule and, I believe, is on course to continue at this rate. As a result, we have more FirstNet now than we ever expected and there is more to come as additional agencies sign on.

Andrew M. Seybold
©2019, Andrew Seybold, Inc.


6 Comments on "Public Safety Advocate: Waiting on Mission Critical Push-To-Talk (Data, Video)"

  1. Randy Kaminsky | April 25, 2019 at 12:48 pm | Reply

    There is one way that ProSe can be integrated with LTR but it is complicated. Example-ProSe in use at large disaster scene where existing LTE network has been compromised. Mobile routers have already formed a mesh network with lack of site signal. APX or similarly designed portables that lose minimal mesh signal at scene would auto-revert to a preassigned (by programming) simplex (or half duplex) LTR channel connected to a JPS ACU or similar in a communications vehicle that would revert back to the LTE talkgroup. Some digital delay issues in play but overall it could work. The same APX mentioned could be a ‘repeater’ for LTE only devices. Feels like I’ve taken the Car, Train, Bus, and Bicycle to get around the block but it could bridge a way around an area with heavily damaged infrastructure. Your thoughts Andy?

  2. Andy,
    I mean this in the most respectful way, but I believe your list of proposed “ProSe” tests is not completely realistic. Take, for example, your first test utilizing two radios “on the street” trying to talk to each other on simplex – what would be the result (e.g., distance covered) if one (or both) were positioned (moving or stationary) behind an obstruction such as a large tour bus, for example? Or for the second two tests, take the same issue of “Urban Clutter” or such losses as elevator car attenuation. Or if one of the radios is being operated from inside a mobile command post vehicle? I know it’s difficult to discuss this conceptually in terms of dB of loss or margin, but the operational equivalent of that is one issue that’s in play here. What impact would interference from a nearby cellular system have?
    Also, what about the model in which more than one “ProSe” talkgroup is in use at an incident?
    Certainly, equipment purveyors would be reluctant to support demonstrations in the real world environment, yet for us First Responders and sympathizers, it is reality.(Just my thoughts!)

  3. Randy agreed, what I did not discuss and probably should have is the fact the ProSe also includes a way to use a device between devices to increase its coverage. The problem I have wiht that is the middle device might move out of the path and therefore cannot be counted on for extended periods of times. Mesh is a great way to do things but is it worth a trouble when LMR simplex is so good in most cases?

  4. Dennis, you make a number of valid points and I was certainly not suggesting that my tests were the best way to measure the differences, although my point was than distance and penetration into buildings are important uses for LMR simplex. If I were going to write up a real test document it would be a lot more concise than what I laid out, thanks for the comments as always.

  5. Andy, thanks for all the time you spend keeping us informed on the progress of FirstNet.
    My reading of the 3GPP ProSe standard and subsequent research for a paper I presented indicates ProSe will only operate in the presence of a working E-NodeB to mitigate the SC-FDMA UL channels rather than use the LTE OFDMA DL modulation. If such is the case, it does not bode well in a disaster when the E-NodeB is cut off from the carrier’s EPC. Furthermore, the embedded MCPPT function is contained in the carrier’s IMS which would also be unreachable by the E-NodeB on a backhaul failure. What say?

  6. Richard, first, thanks for the comments. I do my best to keep information flowing. And after I read your response to this column I will go back and revisit ProSe, if what you saw is correct, and I don’t doubt it is, then neither ProSe or Network centric PTT will work is out of range of a cell site or if the cell site is down. Thanks, will follow up in the next week or so.

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