From: invalid@see.sig.telecom-digest.org   
      
   On 4/1/2022 8:28 PM, Telecom Digest Moderator wrote:   
   > ...   
   > One of the things that happens when I take a vacation from the Digest   
   > is that I come to work with "nw eyes" - I notice things that weren't   
   > grabbing my attention before, and I've just realized that I don't know   
   > as much about radio and Cellular technology as I had thought I did.   
   >   
   > Ergo, I'll ask you to give us more detail about the underlying   
   > technology behind FIrstNet(R), and to explain some of the acronyms   
   > that have been mentioned. I hate to do it, but I'll (respectfully)   
   > request an "Executive Overview" that gives a layman's view of the   
   > possibilities and problems.   
      
   Happy to oblige.   
      
   >> FirstNet is a "broadband" public safety network intended to complement   
   >> the "narrowband" voice walkie-talkie systems that first responders   
   >> (police, fire, EMS) typically carry.   
   >   
   > OK, here's my first double-take: my only experience with two-way radio   
   > technology, outside Amateur radio, was fixing the radios in the snow   
   > plows and staff cars used by the Commonwealth of Massachusetts, back   
   > in 1972. At that time, "Narrowband" was what we called FM two-way   
   > radios that were set for plus-and-minus 5KHz deviation. However, I've   
   > been told that "Narrowband" now refers to radio transmitters using   
   > plus-and-minus 2.5KHz deviation, and "Wideband" is the older +/- 5KHz   
   > system.   
   >   
   > Pleae tell us if I'm right, and what that change did to increase the   
   > available bandwidth in the bands used by First Responders, and why   
   > FirstNet is considered "Wideband."   
      
   "Broadband" in the modern sense means that it supports more than   
   voice, and supports high-bandwidth applications like video. Think   
   high-speed Internet access, in this case via smartphones. FirstNet is   
   a cellular network, just one with special features for first   
   responders.   
      
   >> It's basically a 700 MHz LTE network, where the 700 MHz band has   
   >> good indoor and cluttered-area coverage. The idea is that AT&T gets   
   >> to use the spectrum for commercial (cellular) customers, but   
   >> reserves and prioritizes its use for first responders when they need   
   >> it. FirstNet's customers, the first responders, pay for the service,   
   >> which allows them to download images and video, which could help   
   >> them in their front line work.   
   >   
   > IIRC, 700 MHz was the range used for "trunked" two-way push-to-talk   
   > systems: I thought it was still being used for that. Correct me if I'm   
   > wrong, though: didn't T-Mobile have it's "Push-To_Talk" service in   
   > that band as well?   
      
   No. 700 MHz was TV channels 51-69, repurposed for mobile use around 15   
   years ago via the DTV transition. Trunked two-way uses frequencies in   
   the 800 MHz range (near but not the same ones as cellular) and on the   
   older UHF band (450-470 MHz). - When the second 700 MHz auction - was   
   being planned, there was a proposal on the table to reserve some for   
   first responders, and that ended up in FirstNet.   
      
   > And, Ghod forgive me, I have to ask what "LTE" means in this   
   > context. Trunked radio systems are now decades old, so if that's what   
   > AT&T is calling "Long Term Evolution," well, I want my tax money back.   
      
   LTE is the name of the "4G" air interface, which has been the standard   
   for at least a decade. Since the 3G networks are being shut down, your   
   mobile phone either has LTE or it won't work. "5G" has an air   
   interface called "NR", for New Radio, but that is really just a set of   
   evolutionary changes to LTE to support higher frequencies, wider   
   channels, multi-band aggregation, and similar tweaks. To make a long   
   story short, LTE uses full-channel OFDM in the downlink (base to   
   mobile) while the uplink divides the channel into narrower   
   sub-channels called Resource Blocks and thus allows multiple mobile   
   units to transmit at once. OFDM means that there are multiple   
   narrowband carriers, 15 kHz apart in LTE (can be more in NR), across   
   which the payload is divided.  It's a basket of clever ideas (and   
   maybe some weird ones) thrown together by committee.   
      
   >> Not all first responders buy into this; real-world police in many   
   >> places, for instance, carry ordinary smartphones, which generally work   
   >> fine. But in some places where cell coverage is spotty, FirstNet gives   
   >> AT&T an incentive to build out, and it gives local governments an   
   >> incentive to permit the necessary towers to be built. Whether that's   
   >> good or bad is a matter of perspective...   
   >   
   > Let's pull over into the learning lane for a moment, and I'll ask a   
   > few questions I hope will clarify what is going on.   
   >   
   > 1. Is FirstNet(R) a service that uses single-channel radios, like the   
   >     ones that Police used to have for their exclusive use, or is it for   
   >     "trunked" radios like the ones taxicabs, courier services, and   
   >     delivery trucks use now? Some Police and Fire departments have   
   >     switched to "trunked" systems, because some municipalities have   
   >     cobined all their services into a single "trunked" system in an   
   >     effort to save money.   
      
   It uses a conventional cellular phone, so the channels are wide and   
   allocated using LTE's methods. Trunked systems are voice systems with   
   a lot of narrowband voice channels assigned on demand. Totally   
   different.   
      
   > 2. If FirstNet is a "Wideband" service that allows First Responders to   
   >     "download images and video," how can it be shared with older   
   >     "narrowband" push-to-talk users? Are there multiple systems with   
   >     different capabilities sharing the FirstNet band(s)?   
      
   It is totally separate from old PTT. It may synthesize PTT but that's   
   an "app". The network is full duplex with paired (frequency division   
   duplex) channels.   
      
   > 3. Unless I misunderstand the FirstNet PR, the system is equipped to   
   >     allow First Responders to interrupt existing "other" users when   
   >     First Responders make a call. Is that correct? Is there any public   
   >     info you can point us to?   
      
   LTE supports a lot of simultaneous users per cell, and divides   
   capacity among them. FirstNet prioritizes first responders, so if   
   there is a shortage of capacity on a cell, first responders get theirs   
   first, and ordinary cellular users either get less capacity or in   
   extreme cases may get thrown off. Basically the license is divided   
   into a cellular channel and a first responder channel, and the first   
   responder channel's share of capacity is available to cellular users   
   when first responders don't need it.   
      
   --   
      
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