Simulcast Overlap

[70cutlass442]

We are going to try a two-site simulcast system for a local school district. Geographically, the schools are all withing 1 mile of each other, but the steel and concrete causes issues with their single site conventional system.

We will be using NXDN equipment with GPS and NTP timing and a fiber LAN for networking.

I am concerned about the extensive overlap that this system will generate.

Can anyone shed some light on whether or not this system may even work? If we have issues, which I am anticipating, We have other options available. But if this can work for them, this will make their use of the system much simpler.

 

[wtp]

some schools down here have gone to an onsite repeater system.
most of the school walls, windows and even the roof is energy efficient, or in other words, Aluminium Faraday cage.
so handhelds would not go from one end to the other. a repeater helped.
i figure you have read all the problems with a simulcast system on here.
so can you shed some more light on what the problem is.

 

[70cutlass442]

some schools down here have gone to an onsite repeater system.
most of the school walls, windows and even the roof is energy efficient, or in another word, Aluminium Faraday cage.
so handhelds would not go from one end to the other. a repeater helped.
i figure you have read all the problems with a simulcast system on here.
so can you shed some more light on what the problem is.

District staff identified the "hardened" areas of several buildings as being the trouble areas. These are areas that students would congregate during inclement weather.

The antenna for RPTR 1 is on a tower that appears to cover most of the schools with little issue. RPTR 2 will be at a school the furthest away (1.1) miles. This second location is where they describe having the most issues.

My plan B in this would be to leave the RX antenna on the roof of RPTR 2, but to place the TX antenna in the basement near the problematic area. That would reduce the overlap substantially and focus the signal where needed.

Plan C, which is what I initially purposed, and they are not opposed to, is to RX at several sites on the same frequency and utilize the voting feature of the system, but to TX on different frequencies. The user would have to manually select the "wide area" system if we did this. I fear that introducing the idea of changing channels could complicate things, especially during times of incidents of high acuity.

Vote scan was considered with Plan C however, most radios will only allow conventional scan, OR vote scan. This means they would not be able to monitor a channel reserved for their respective school while also monitoring a multi-site conventional system while vote scanning.

 

[mmckenna]

Have you looked at IP linking between sites?
Put in some small sites at each campus on their own frequencies pairs. Link all the repeaters together over IP so it looks like one big system. Each campus just select their own site.

 

[70cutlass442]

Have you looked at IP linking between sites?
Put in some small sites at each campus on their own frequencies pairs. Link all the repeaters together over IP so it looks like one big system. Each campus just select their own site.

That is exactly what we will end up doing if the simulcast does not work the way we want. These repeaters will allow for a common input frequency so we can make use of voting. The desire to simulcast if rooted in the need to not change channels when staff migrates to another site. That rarely happens but is necessary for off-site reunification area that could be a mile or so away from the school.

 

[mmckenna]

I've got a couple of NXR-1800's sitting at work that are linked together. 3 repeaters, 3 different pairs, all analog. So far, it's looking good.
I believe you can set them up with their own PL tone at each site, and a second tone that links all the repeaters.

I'd like to run these on NXDN, but the end users are still running some analog gear.

 

[70cutlass442]

I've got a couple of NXR-1800's sitting at work that are linked together. 3 repeaters, 3 different pairs, all analog. So far, it's looking good.
I believe you can set them up with their own PL tone at each site, and a second tone that links all the repeaters.

I'd like to run these on NXDN, but the end users are still running some analog gear.

My preference would be to run these in analog for the same reason. I am not sure Icom gear will let us do that.

I will try the Simulcast method first, but I think you are right, I will likely end up with a common input (for voting) and separate outputs.

 

[lenk911]

We are going to try a two-site simulcast system for a local school district. Geographically, the schools are all withing 1 mile of each other, but the steel and concrete causes issues with their single site conventional system.

We will be using NXDN equipment with GPS and NTP timing and a fiber LAN for networking.

Simulcast transmitter spacing is a function of the transmission rate of the symbols or the frequency of the analog signal and the laws of physics. Your one mile spacing is normally not a problem. There is an upper mileage limit depending on the quality of modulation of the transmission such as from a class C FM transmitter or a linear modulated transmitter.

I have simulcasted DMR with high quality Tait linear transmitters interconnected with fiber controlled by a console. Each of the 14 sites has one simulcast station and 2-4 local DMR stations. I would be concerned with the quality of the NXDN transmitters. Watch your roof top antenna gain--no more than 3 dbd or the cone of silence will affect you. 20 Watts ERP should be sufficient to cover the largest high school properly engineered with roof top antennas.

 

[70cutlass442]

Simulcast transmitter spacing is a function of the transmission rate of the symbols or the frequency of the analog signal and the laws of physics. Your one mile spacing is normally not a problem. There is an upper mileage limit depending on the quality of modulation of the transmission such as from a class C FM transmitter or a linear modulated transmitter.

I have simulcasted DMR with high quality Tait linear transmitters interconnected with fiber controlled by a console. Each of the 14 sites has one simulcast station and 2-4 local DMR stations. I would be concerned with the quality of the NXDN transmitters. Watch your roof top antenna gain--no more than 3 dbd or the cone of silence will affect you. 20 Watts ERP should be sufficient to cover the largest high school properly engineered with roof top antennas.

I did a test on the bench. Very non-real world. I could not create a scenario in which the receiving radio was distorted or no-audio at all. This ranged from having a RX radio within 1' of two dummy loads that were transmitting at the same time, to walking around a 1 acre lot.

What conditions create dead zones in a simulcast network?

 

[mrsvensven]

I want to make sure I am understanding this right. You say that you are putting one repeater in each school. Are you connecting these repeaters together so that they both key up and transmit the same signal at the same time, regardless of which repeater (or repeaters) is receiving the incoming signal?

Simulcast distortion is caused by signals being received at different times- on analog, if you are right next to one repeater and also receiving a signal from a second repeater 15+ miles away, that second signal will arrive at your radio 80 microseconds later and the two signals will be out of sync enough to cause the distortion. This isn't applicable in your scenario where the repeaters are within 15 miles of each other. The minimum interference-causing distance goes down as low as 8 miles on a P25 phase II system. It probably varies based on the manufacturer and quality of your equipment, but either way 1 mile should not be an issue.

 

[N4DES]

Is there an operational reason why simulcast is a must, and the same local traffic needs to be heard at both locations? Not trying to compare, but I have never seen this and around here and schools all operate in a silo. The only school district division that operates on simulcast is the school police department and they are on the County system using AES-256.

 

[70cutlass442]

I want to make sure I am understanding this right. You say that you are putting one repeater in each school. Are you connecting these repeaters together so that they both key up and transmit the same signal at the same time, regardless of which repeater (or repeaters) is receiving the incoming signal?

Simulcast distortion is caused by signals being received at different times- on analog, if you are right next to one repeater and also receiving a signal from a second repeater 15+ miles away, that second signal will arrive at your radio 80 microseconds later and the two signals will be out of sync enough to cause the distortion. This isn't applicable in your scenario where the repeaters are within 15 miles of each other. The minimum interference-causing distance goes down as low as 8 miles on a P25 phase II system. It probably varies based on the manufacturer and quality of your equipment, but either way 1 mile should not be an issue.

This is a NXDN system that will be linked via private fiber and uses GNSS and NTP for time referencing. The repeaters vote the strongest incoming signal as part of this.

SC is not my first choice for this project, but the users need to have a very seamless experience on a shared channel and changing channels regularly is not preferred. Subscriber vote scan is also not an option because they also need to scan other conventional channels and I have not found a way to scan do vote scan and conventional scan simultaneously.

Thanks for the help!

 

[70cutlass442]

Is there an operational reason why simulcast is a must, and the same local traffic needs to be heard at both locations? Not trying to compare, but I have never seen this and around here and schools all operate in a silo. The only school district division that operates on simulcast is the school police department and they are on the County system using AES-256.

The desire for SC has to do with the want to monitor a conventional channel in a scan list. I have done vote scan systems that work very well which would allow us to use different output frequencies, but the radio in the system will not allow for monitoring of a conventional channel list as well as a vote scan list. The next option to meet their goals would be simulcast.

 

[N4DES]

The desire for SC has to do with the want to monitor a conventional channel in a scan list. I have done vote scan systems that work very well which would allow us to use different output frequencies, but the radio in the system will not allow for monitoring of a conventional channel list as well as a vote scan list. The next option to meet their goals would be simulcast.

OK so exactly "who" has a need to monitor the traffic from a distant location? If it's routine traffic just set up a base station at each location with a yagi pointed at the adjacent school and have a secretary or staff member monitor it. This looks more and more with a solution looking for a problem.

 

[70cutlass442]

OK so exactly "who" has a need to monitor the traffic from a distant location? If it's routine traffic just set up a base station at each location with a yagi pointed at the adjacent school and have a secretary or staff member monitor it. This looks more and more with a solution looking for a problem.

They want to have a channel to broadcast announcements to all radios in the district. They have this now but with a single NXDN repeater that does not penetrate into several of the schools on the other side of the village (even though they are a mile from the tower).

I am not sure as to their protocols on how they use the channel however, they want to use it the same way they have been but with better inbuilding coverage into these two outlaying schools.

 

[N4DES]

Set up control stations in the hardened locations around the school on this "announcement channel" where they have bad coverage and call it a day. I'm assuming this is UHF, if so, a BDA is another option. ProtectLink UHF BDA, Class A/B

"District staff identified the "hardened" areas of several buildings as being the trouble areas. These are areas that students would congregate during inclement weather."

 

[70cutlass442]

I want to make sure I am understanding this right. You say that you are putting one repeater in each school. Are you connecting these repeaters together so that they both key up and transmit the same signal at the same time, regardless of which repeater (or repeaters) is receiving the incoming signal?

Simulcast distortion is caused by signals being received at different times- on analog, if you are right next to one repeater and also receiving a signal from a second repeater 15+ miles away, that second signal will arrive at your radio 80 microseconds later and the two signals will be out of sync enough to cause the distortion. This isn't applicable in your scenario where the repeaters are within 15 miles of each other. The minimum interference-causing distance goes down as low as 8 miles on a P25 phase II system. It probably varies based on the manufacturer and quality of your equipment, but either way 1 mile should not be an issue.

I ran some numbers based on the two TX locations and the buildings we need to cover. The most delay spread we will have is 6 microseconds. The least is .05 microseconds. even if we introduce additional transmitters, those numbers dont change based on their locations.

 

[kv5e]

Down tilt on the antennas helps in cases like this to mitigate inter symbol interference.

 

[lenk911]

In reading above I am getting concerned for the system you are designing. When a committee designs a horse you can end up with a camel! A word of advice from an old engineer.

The saving grace is there is not a decent school in this country that completely teaches radio communications engineering. You learn the basics in college. Much more at the knee old masters. The rest by getting certificates from old Murphy. You may think you know a lot and jump into the system engineering without fear but old Murphy will humiliate you and may cost you money. But by the time you graduate though...if you do, you will will have a PHD in radio engineering. You will have mastered that A+B usually equals C but sometimes it doesn't! Know when, be wise--be careful!

 

[speedway_navigator]

To recap you need a dedicated talk group / channel in each of the 2 schools with full indoor coverage and external up to a mile, then each user needs access to the separate all call channel which both sites need to hear indoors and out.
Would the all call traffic have priority over the local one?