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Old 12-08-2017, 10:00 AM
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Thanks for the clarifications, that helps.

Originally Posted by extan View Post
Thank you all, very much, for your replies.

Apologies for my unclear description. When I speak of "PTT" I'm referring to push-to-talk (half-duplex) cellphones. I believe Nextel was first to market many years ago, but now ATT and Verizon offer it. It's my understanding that it's packet-switched, running over the data network. Last year the march used "AT&T Kyocera DuraXE 4G LTE E-PTT" and "AT&T Sonim XP5 4G LTE E-PTT". They were a complete failure, I assume primarily because the cell "towers" were completely overwhelmed by the extraordinarily high concentrations of cellphones. (I'm interested in any feedback on this assumption!)

Understandable failure for a few reasons…

Cell sites only have a specific amount of capacity, and once it's full, calls get rejected. On a normal day, it's not an issue, cell sites are designed for "every day". What they often are not designed for is large events, like a 20,000 person march. Considering just about everyone has a cell phone, and AT&T is one of the more popular carriers, you can imagine what happens when a few thousand people are all trying to talk, upload photos, post on facebook, etc. all at once. Without any sort of prioritization, your calls get lost in the mix.

Cellular companies do have the ability to bring in portable cell sites to solve these sorts of issues, but getting them to do it is hard. Big sporting events, disasters, etc, it can happen, but you'll have a hard time even getting to the people that could make this happen.

The other issue is cellular PTT has an inherent delay. The call setup time can be long, and someone who's not accustomed to using it will often key the device and start talking. The other end misses most of the transmission since the phones/network are still trying to set up the path.

Cellular PTT is good for some applications, but not this.

Originally Posted by extan View Post
We understand it's a very challenging environment for rf, but the march will go on, and we feel like we have to do the best we can with what we can get. That's what I'm hoping you guys can help us optimize.
This is the application where two way radio shines. This is one of the reasons public safety uses their own radio systems and doesn't rely on consumer cellular for their critical voice communications. Good news is that it's easy to rent what you need from a radio shop. This is what they do. They'll set everything up, you show up and use it, and at the end of the day they take it away.

Originally Posted by extan View Post
I think we're talking about several dozen (maybe up to 60) handsets. These are non-technical volunteers divided into functional groups (eg medical, marshals, legal, stage/speakers, etc) that would like to be in close contact with others in the same group. Each group has a "head", and all the heads would constitute another group, facilitating inter-group communications. (I don't think it's practical to consider obtaining licenses, at least for this march, which is only about 6 weeks away.)
60 radios won't be an issue for a decent radio shop.

As for "non-technical" volunteers, that's the beauty of a two way radio. They are simple to use. Just make sure it's turned on and you are on the correct channel.

It would be difficult to get a license that fast. While there are ways to do it, it takes some work. But, there's no reason for you to go that route. Since you don't already own the radios, you'll have to rent. The shop that rents you the radios will have the license that covers their use.

Originally Posted by extan View Post
So it seems we are left with VHF/UHF with 2-way radios?
That would be the logical solution. Cellular PTT isn't going to work well, as you've discovered. You don't want to rely on amateur radio operators for this as they have to be standing their with the radio, and that adds a layer to your coordination you don't need. While they would have the equipment and skills, it adds a layer you don't need. You want 60 or so volunteers with their own radios to communicate quickly and efficiently. Having to double up each volunteer with a "shadow" radio operator isn't the way to do this.

Originally Posted by extan View Post
I'm thinking if we could station a repeater on each leg of the route, ideally over the heads of the marchers, ideally within line of sight of the neighboring repeaters, it would offer significant benefit. Thoughts?
Perhaps we could park repeater-equipped cars along the route? Perhaps we could mount fairly tall antenna masts on the cars? Or would it be much better to try to find friends with offices directly overlooking the route?
I think you are overthinking this. Talk to a radio shop and tell them that you need 60 rental radios and coverage over your area. Let them deal with the engineering side. Chances are they'll have repeaters in the area that will cover what you need. If they need to add additional repeaters, they'll handle that. Ideally the radio shop would have a trunked radio system that would provide everything you need. You'd just show up, grab a radio and go.

Originally Posted by extan View Post
It's not obvious to me what the primary sources of interference are. Is it all of the tightly-packed bodies? Is it rf noise from all the cellphones? Is it the buildings? Is it ambient noise from all the craziness of midtown?
Not a complete list:
High RF noise floor.
Interference from other users.
Meat-bag bodies do absorb RF energy really well.
User error.
Reflections off buildings (multipath, etc)

Originally Posted by extan View Post
We've had both the CP200 and XPR families recommended to us by rental houses...
Let the radio shop decide that for you. You'll be a radio user, no need to engineer a system. You'll be paying someone to do that.
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