Simulcast - Problem Question

[kb9tfh]

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

 

[davidbond21]

Well, if you understand the difference between analog and digital well enough, you're halfway there. Basically with a digital signal you either receive it or you don't, no staticky half voice, because the digital radio(or scanner) only sees a 1 or 0 bit, nothing in between.

The other half is timing. All the repeater sites are precisely timed to many zeros past the decimal point of a second. When you are different distances from all the simulcast sites, the signals all reach you at slightly different times, but, with the high dollar radios, as long as those signals come within a certain frame of time of one another(meaning they don't step on one another too much), the radio can understand whether the data bit is a one or a zero. This is expensive to do though, requiring sophisticated technology, hence the price difference, and quality difference between a scanner and a motorola radio.

This is much simplified and I'm sure others on here will either correct me or provide links to better explanations.

 

[fmon]

In addition to David's note, read this from GRE support desk.

 

[rdale]

That's multisite though - he's asking about simulcast.

 

[Tom_G]

I'll Second that !!!

Here in Connecticut, the State Police are running a large Simulcast system.
I can hear several of the control channels across the state, but what I don't understand is why I hear certain troops on multiple sites especially where I wouldn't expect to.

I have tried listening to just one tower/control channel, but I don't believe I am hearing everything there. Not sure though....

There is a lot I don't know.... But I would love to find out...
Anybody have more scoop on this ???????

 

[detroit780]

Control Channel

Tom,

The state of Michigan also has a statewide P25 system. In my county we have just one tower. We have other towers in other counties that surround us. So as the radio moves from one location to another it can and will change tower affiliations. When in the center of the county the radio will affiliate with the county tower. When on the edges of the county it will affiliate to another tower. Each tower that has affiliations of say My County Police 9-1-1 will pass those conversations. So even if my county had a vehicle 5 counties away on a prisoner exchange as long as his radio was tuned to my county 9-1-1 all that traffice would pass through that tower. The radio handles this on it's own.

I hope this helps.

Les

Here in Connecticut, the State Police are running a large Simulcast system.
I can hear several of the control channels across the state, but what I don't understand is why I hear certain troops on multiple sites especially where I wouldn't expect to.

I have tried listening to just one tower/control channel, but I don't believe I am hearing everything there. Not sure though....

There is a lot I don't know.... But I would love to find out...
Anybody have more scoop on this ???????

 

[texasemt13]

This applies to repreated and simulcast though...

Imagine a different (but not much) scenario. Say a central light switch turns on 2 lights on either side of town. You happen to be standing right next to one and about 2 miles (but still in sight) of the other light pole. As the metaphorical "mayor" (or transmitter/dispatcher in the sense of electromagnetic waves), throughs the "switch" (keys up the mic), a signal is sent to both sites. Now if your standing closer to one light pole you will see it come on first, then the second (this is because the light travels at the same speed and just has to travel less distance from the first pole to your eye). In the sense of EM waves, imagine they are simulcasted (turned on), at the exact same moment. Now being closer to one, you will still receive the signal from the closer one first, the radio, receiving both at different times would have a problem decoding multiple signals.

How do the radios deal with this? They can (depending on the manuf.) like a cell phone, do a "roam" function allowing them to find the signal that would be better. Most radios allow users to set a threshhold level stating when it should start to search for a better source, others will automatically roll to another closer tower at a certain signal strength. I don't think this could be fixed by firmware...

 

[rdale]

How do the radios deal with this? They can (depending on the manuf.) like a cell phone, do a "roam" function allowing them to find the signal that would be better.

No. Completely wrong. All towers are sending the exact same signal on the exact same frequency. What you are saying would be true about non-simulcast systems. This thread was asked about simulcast systems, and that does not (cannot thanks to a little law called "physics" ) happen with any radio or scanner.

 

[fmon]

That's multisite though - he's asking about simulcast.

Apparently you didn't read the GRE page.

To understand how MultiSite works, it is important to make the distinction between "simulcast systems" and "networked" systems. Both types of systems have multiple sites, but they are quite different in how they operate.

A simulcast system utilizes two or more sites, up to 15 depending on the technology. Each site is exactly the same in terms of number of transmitters and the frequencies that are used. When a channel is assigned to a voice call by the trunking system controller, the assigned frequency transmits the call from every site in the system simultaneously, providing simultaneous multi-site coverage to the service area.

 

[Tom_G]

Interesting....
So in theory, I should be able to monitor 1 single site (control channel) and should hear all traffic. True ?

 

[rdale]

Interesting....
So in theory, I should be able to monitor 1 single site (control channel) and should hear all traffic. True ?

It's not theory It's the way a simulcast system is set up.

Apparently you didn't read the GRE page.

I did. It's not talking in ANY fashion about listening to simulcast systems. It's telling you how to use Multisite settings on non-simulcast systems.

 

[seamusg]

Interesting....
So in theory, I should be able to monitor 1 single site (control channel) and should hear all traffic. True ?

True since a simulcast broadcasts the same signal from all sites on the same freq. at the same time.

 

[texasemt13]

No. Completely wrong. All towers are sending the exact same signal on the exact same frequency. What you are saying would be true about non-simulcast systems. This thread was asked about simulcast systems, and that does not (cannot thanks to a little law called "physics" ) happen with any radio or scanner.

Where did I suggest that they were searching for different frequencies? Nowhere, my point was that they are trying to find a stronger signal from a closer tower.

 

[texasemt13]

No. Completely wrong. All towers are sending the exact same signal on the exact same frequency. What you are saying would be true about non-simulcast systems. This thread was asked about simulcast systems, and that does not (cannot thanks to a little law called "physics" ) happen with any radio or scanner.

And physics isn't a "law." There are "laws" of physics, but physics itself is based on the manifestation of 4 known fundamental forces. If you'd like I can get more technical...

 

[fmon]

Tom,

The OP mentioned "P25 Simulcast" systems and my post only quoted part of the GRE note that may or not have dealt with his issue. For other usage, read it's totality in the GRE link of post #3.

 

[rdale]

Nowhere, my point was that they are trying to find a stronger signal from a closer tower.

That's not physically possible... Let's take a sample:

I am closer to tower 2 than tower 1 on a simulcast system. Both have a control channel of 866.500MHz. The signals are EXACTLY the same. It is physically impossible for a radio to say "ignore the 866.500MHz signal you are getting from tower 1, only get it from tower 2."

davidbond explained it perfectly. It is not the same as how cellphones or radios on a statewide system jump from tower to tower.

 

[rdale]

Here in Connecticut, the State Police are running a large Simulcast system.

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

 

[texasemt13]

It is physically impossible for a radio to say "ignore the 866.500MHz signal you are getting from tower 1, only get it from tower 2."

From the link in post #3: "Roam" is used when you want the radio to automatically find and use the best available site as you move around in the service area of the system.

I understand the above post is for a MultiSite but the technology is possible. I might be mistaken about being able to decode the same frequency, as you suggest, but the technology is available to "roam."

 

[rdale]

Yes, multisite is a great feature. It lets you add all the DIFFERENT control channels, and goes through the frequencies and selects the strongest. So as you move around, it picks the best. It specifically says that it does not get used with simulcast systems.

That cannot happen on a simulcast system, which is what the original post is about

 

[WayneH]

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.

And Rob is right, the original post has nothing to do with MultiSite trunking (which the link was also about).