Simulcast - Problem Question

[Tom_G]

CT isn't running a statewide simulcast - they are running a statewide Smartzone system. That's not the same.

rdale: Yes, I went back an verified that.
Although they do incorporate Simulcasting within the system, each prime site has its own set of frequencies.
And the way I understand it, each of the prime sites is simulcasted.
From reading the GRE document, it almost sounds more like a multicast system.

Now I need to find some info on Smartzone..... Thanks,

 

[dpm3]

The issue with the GRE's, the way I understand it, is they can't handle a simulcast signal due to the in and out of phase signals generated from more than one tower as well as a Motorola radio can. Generally with a Simulcast system you're always going to be receiving more than one tower in the sub-system. Because of things like buildings, etc the signals bounce around changing the point in time they reach the receiver (called being "out of phase"). And this, as said, is why Simulcast is reliant on precise timing at all sites. When you introduce a yagi in to the equation you're focusing your area of reception to a narrow area which in turn reduces the signals from other sites (outside the beamwidth) therefore reducing the out of phase signals you would be receiving.

Then am I to take it that no matter how proximate you may be to any single transmitter "site" the "out of phase" phenomenon is going to interfere with your listening to the system? It has occurred to me that those of us on the "fringes" (as it were) of a system are more likely to encounter seriously downgraded reception than those close to one or another of the transmitter sites. Excuse my naivete (or downright ignorance) but this seems to make sense to me. If I'm wrong it would seem then that monitoring a simulcast system would be virtually impossible without professional grade, properly programmed Motorola equipment.

 

[jim202]

After reading all the posts, there are a few things that haven't come to the surface on
discussing the differences between the SIMULCAST and regular multi site trunking.
First, there is the issue of the 3 db capture range of a signal strength. If one signal
is stronger than a second signal by 3 db on the same frequency, the stronger signal
will win and capture the receiver. Doesn't matter if it is SIMULCAST or regular
trunking. If you get into the non capture zone between two transmitters, on a regular
trunking system or a non SIMULCAST system, you may or may not be able to hear
a usable signal.

When a SIMULCAST system is used, all the transmitters are on the exact same frequency
and the audio sent over each transmitter is delayed by some fixed number of a fraction
of a second. Normally this is set up for the most distant transmitter with no delay and as
you get closer to the origination point of the audio, more delay is put into the audio path.
you need to have a fixed audio path to make this work. renting T1 circuits from the phone
company won't hack it as they have a tendency to change circuits around as they do
service work on the circuits. When they change a circuit routing, this causes the delay
to change and messes up the audio delay the system was set up with.

Another problem with SIMULCAST, is that there is a limit in the max distance that can
be adjusted for between the transmitter sites. This again has to do with the delays that
are added into the audio path so the transmitters will function correctly. If the delay
gets beyond the critical point, the receivers in the field will not be able to decode the
digital signal.

A normal SMARTZONE trunking system is cheaper to install, cheaper to maintain and
doesn't have the timing issues to contend with like a SIMULCAST system. There are
2 reasons to use a SIMULCAST system. The first is a lack of channels. The second
is so the vendor can make more money. Your much better off to go with a normal
SMARTZONE trunking system. The equipment is lower in cost, the radios in the field
play much better with it, you don't need a tech on the road all the time, it will cover a
greater distance in miles between the sites, and I think in the end it sounds much
clearer.

I am sure that my comments will bring the rath of the sales teams out in force. It
may also stir up a number of system managers, but the bottom line, unless there
is no other option, stay away from SIMULCAST. It will only cause problem in the
long run. Plus your forced into installing your own microwave system to carry
all the radio system traffic. This tends to swell the cost of a system in a big
hurry. Remember we are talking Public Safety systems here so your going to
need a hot standby microwave radio system. A complete loop is the best way,
but not always doable. Last time I looked, a 6 GHz, hot standby, with antenna,
waveguide, installation, license and a tower heavy enough would run you in
the order of $100 k per end of each microwave path. If you have just 3 paths
to make a loop from 3 locations, (don't forget this includes the dispatch) that is
only 2 remote tower sites, your talking about $600 K just to get it off the ground.

Jim

Hi Guys,

I noticed alot of the new systems are switching to P25 Simulcast systems, and this creates issues for people who may want to monitor the system. I am guessing that this is because the transmiters are overlapping. It has been pointed out in many threads that the only way of solving this problem is to point a directional antenna a one of the transmitters.

My question is this: How are the Motorola radios configured so that this not an issue for them? Can these settings be duplicated on the GRE? Perhaps a firmware udate would allow for these changes to be made???

Thanks,

Joe

 

[kevins669]

A normal SMARTZONE trunking system is cheaper to install, cheaper to maintain and
doesn't have the timing issues to contend with like a SIMULCAST system. There are
2 reasons to use a SIMULCAST system. The first is a lack of channels. The second
is so the vendor can make more money. Your much better off to go with a normal
SMARTZONE trunking system. The equipment is lower in cost, the radios in the field
play much better with it, you don't need a tech on the road all the time, it will cover a
greater distance in miles between the sites, and I think in the end it sounds much
clearer.

Jim

And it makes it even worse when there appears to be a mismanagement of PS frequency allocation and system usage. Which, to me, seems to be the case in New Orleans.

-- Kevin

 

[rdale]

Then am I to take it that no matter how proximate you may be to any single transmitter "site" the "out of phase" phenomenon is going to interfere with your listening to the system?

That's fair. Unless you are very close to a tower, or distant from all of the (i.e. out of the county assuming antennas aren't pointed inwards) you will not get reliable comms on a digital simulcast CQPSK.

 

[Tom_G]

And to make it worse.....

And to make matters worse, here in Connecticut the State Police run a Smartzone system in which each of the prime sites has its own set of frequencies.
And each of the prime sites has its own group of simulcast sites....

Trying to get a scanner setup to monitor this is not easy.....

 

[seamusg]

And to make matters worse, here in Connecticut the State Police run a Smartzone system in which each of the prime sites has its own set of frequencies.
And each of the prime sites has its own group of simulcast sites....

Trying to get a scanner setup to monitor this is not easy.....

Try the Michigan MPSCS system, it's a Statewide P25 system and the larger areas have simulcast subsystems (ie. Detroit, Macomb, St. Clair, Downriver etc.).

 

[rdale]

seamusg - what he's saying is that every CT group is a simulcast system, so it's worse than MI.

 

[rhodester]

My county is going to a simulcast digital system very soon there is a tower about 8-10 miles from me.Will i have much of a shot at monitoring this system?

 

[seamusg]

My county is going to a simulcast digital system very soon there is a tower about 8-10 miles from me.Will i have much of a shot at monitoring this system?

The best shot at monitoring a simulcast is with a directional antenna pointed at one tower.

 

[RoninJoliet]

Here in the Chicago area the Digital Starcom Statewide 700/800 system has taken a toll with the multipath problems on both GRE500 and Uniden396/996...In certain spots here at my home location signals seem fine but they also change daily it seems with wind and weather...I still have the old 1.11.03 firmware in my Uniden396, it works well near these towers and awful in the car mobile...The Uniden396 worked great in "central ILL on Starcom with the stand alone towers....The GRE500 also suffers from missed drop-out calls on these "echoing" towers....A scanner friend of mine has tried the NEW firmware changes from Uniden and still cannot get it rite....