Possible DUMB question regarding P25 Simulcast

bobruzzo

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I am sitting here staring at the SDS200 screen watching the RSSI and D-ERROR numbers. There are 4 sites in my state (RI) and each site has between 5 and 9 (-/+) towers. I dont know much about simulcast P25 systems but we'll take one of the sites in my area as an example. The PROVIDENCE CITY site has 5 towers in various locations around the city. The output power is a bit lower than other sites because its mainly for downtown reception. I am only 6 miles from Providence. Now say out of the 5 towers, there is ONE tower in the group (or could be 2, 3?) that might be a source of simulcast distortion. I was trying to tally the list of these 5 freqs looking at scanner and when radio stops on a transmission, see what the freq is, write it down and note whether is had good/bad audio. What I am trying to say is, is it possible to DELETE from the programming in radio or software a TOWER in a site? If the 5 towers in the PROV site are all transmitting at the same time, each towers transmitting the SAME THING, why couldnt I DELETE all but the CLOSEST tower? In this particular case the closest tower to me is 4 miles away. Would this somehow CUT DOWN on simulcast distortion from the other towers in the same site further away? The variables for troubleshooting P25 simulcast distortion are WAY too many. Shouldnt I be able to recieve the transmissions from a 5 tower site, but only recieve from 1 tower? Like I said I dont understand how this works. I've been fighting this distortion since I first got my BCD996P2 back in March. I got the SDS200 thinking it would solve the simulcast issues. It didnt completely but did help on 2 of the 4 sites in my state. The other 2 sites still have occasional crappy audio. I did a lot of testing with antennas and the only one that really made a big difference, cutting way down my D error and RSSI numbers is a 6 element yagi. I can receive 2 (NORTH & SOUTH) sites pretty much with NO d-error and a low -45 to 60 RSSI. But the other 2 sites still have the occasional distortion. RI is a small state and I am right smack in the middle on a small hill. I can receive all sites in the system with both a vertical and the yagi but the yagi performs hands down the best. Filters make no difference. Coax is 50' LMR400 yagi up about 20 feet. The yagi is pointing a bit southeast. Seems to work best in this configuration. Its almost impossible to troubleshoot because there are too many factors involved. So I have been taking a step at a time. Sorry for my usual rambling but I just wanted to give details.
 

sallen07

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What I am trying to say is, is it possible to DELETE from the programming in radio or software a TOWER in a site? If the 5 towers in the PROV site are all transmitting at the same time, each towers transmitting the SAME THING, why couldnt I DELETE all but the CLOSEST tower?
No.

I think you're missing the fundamental principle that makes P25 LSM systems hard to monitor. EVERY tower in a cell transmits the SAME information on the SAME frequency at the SAME time. When you watch your scanner and see it switch to a voice channel, EVERY tower is transmitting on that voice channel. It's not like tower #1 uses voice channel #1, tower #2 uses voice channel #2, etc. All of the voice channels are used by ALL of the towers, at the same time. So there is no way to program your scanner to only "hear" one tower.

The advantage of the SDS scanners is that the software can analyze the combined signals being received and "focus" on the strongest one, so you get much better results (but still not perfect).

Yes, Rhode Island is small (actually smaller than Monroe County) but I have a feeling your poor audio on some of the sites is that you are too far away. 800 MHz systems are designed to only be picked up in their service area, and not beyond. The fact that there are four sites in Rhode Island highlights the fact that each one was only engineered to cover (approximately) 1/4 of the state.

Another thing you might want to do is listen to each cell separately and see if there is one that carries all (or at least most) of the traffic you are most interested. I think you'll get better results if you are monitorig just one cell instead of trying to monitor all four.
 

bobruzzo

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No.

I think you're missing the fundamental principle that makes P25 LSM systems hard to monitor. EVERY tower in a cell transmits the SAME information on the SAME frequency at the SAME time. When you watch your scanner and see it switch to a voice channel, EVERY tower is transmitting on that voice channel. It's not like tower #1 uses voice channel #1, tower #2 uses voice channel #2, etc. All of the voice channels are used by ALL of the towers, at the same time. So there is no way to program your scanner to only "hear" one tower.

The advantage of the SDS scanners is that the software can analyze the combined signals being received and "focus" on the strongest one, so you get much better results (but still not perfect).

Yes, Rhode Island is small (actually smaller than Monroe County) but I have a feeling your poor audio on some of the sites is that you are too far away. 800 MHz systems are designed to only be picked up in their service area, and not beyond. The fact that there are four sites in Rhode Island highlights the fact that each one was only engineered to cover (approximately) 1/4 of the state.

Another thing you might want to do is listen to each cell separately and see if there is one that carries all (or at least most) of the traffic you are most interested. I think you'll get better results if you are monitorig just one cell instead of trying to monitor all four.
OK I am beginning to see your point a bit clearly now. The way the state has the towers and sites positioned is VERY confusing because there is some overlap of the 3 (NORTH, SOUTH, & EAST) zones but you wont hear any of the PROV transmissions on those 3. Only on those towers surrounding the city. And seeing that I am only about 6 miles from Providence, I assumed reception would be good. I get near perfect reception on NORTH and SOUTH. EAST is a little flaky at times, but it is 16 miles away. To the east. So maybe with a high gain bigger yagi pointing towards the city using a different radio would still allow me to hear that zone. The way the state arranged the sites, and I guess thats the way they had to do it, there are towers for the North zone in the middle of state, there is a tower for the south zone right up the road from me and I am in center of state. So they seem scattered about. So since the closest tower for Prov is 4 miles to my north, maybe a bigger yagi pointing towards it will help. Well I can either just monitor the 2 strong sites and not bother with the other 2 or just deal with the occasional distortion. I am kinda disappointed because I thought the sds200 would solve the problem. I think it would be good if I could monitor one zone but the overlap is not very much so I'd miss alot of good traffic.
 

jonwienke

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We've discussed this before. Your problem isn't simulcast distortion, it's weak signal. Once your RSSI gets below about -110dBm, your decode error is going to start going up. By the time it gets down to around -120dBm, you're going to have dropouts and other audio artifacts; that's getting close to the limits of the receiver.

Most simulcast sites are designed to leak the least possible signal out of the intended coverage area. Limiting signal to a specific area allows the frequencies to be reused more often. You're trying to do something the designers of the system are intentionally trying to prevent.

You probably need to go with multiple antennas; a yagi or similar directional antenna for each distant site, plus an omnidirectional antenna (probably a discone) for the nearby sites with strong signal and miscellaneous local scanning. You can combine the yagis with a splitter if you use equal coax lengths between the yagis and the splitter and mount them on the same pole to avoid phasing issues. The omni antenna should probably have its own coax and go to a separate radio.
 

bobruzzo

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We've discussed this before. Your problem isn't simulcast distortion, it's weak signal. Once your RSSI gets below about -110dBm, your decode error is going to start going up. By the time it gets down to around -120dBm, you're going to have dropouts and other audio artifacts; that's getting close to the limits of the receiver.

Most simulcast sites are designed to leak the least possible signal out of the intended coverage area. Limiting signal to a specific area allows the frequencies to be reused more often. You're trying to do something the designers of the system are intentionally trying to prevent.

You probably need to go with multiple antennas; a yagi or similar directional antenna for each distant site, plus an omnidirectional antenna (probably a discone) for the nearby sites with strong signal and miscellaneous local scanning. You can combine the yagis with a splitter if you use equal coax lengths between the yagis and the splitter and mount them on the same pole to avoid phasing issues. The omni antenna should probably have its own coax and go to a separate radio.
Yeah I keep experimenting but it gets to the point where its useless. the highest I have seen the RSSI go is about -118. I think what also makes it hard is there arent hardly any other people in my area that I know of on RR so we could compare results. I am curious to know what the situation is like a few miles east of me. I agree that it could be weak signals but I didnt think being only a few miles from the city, like 6 to 8 miles at the most would still be weak! One of the PROV towers is 4 miles away from me! I would like to see about getting a splitter/combiner to use 2 antennas together. I can point a yagi towards the city and the yagi I am using now facing east can stay cause it receives everything else pretty well. I ordered an 8 element yagi the other day and some new LMR400. I just need to find the right device to connect the 2 yagis to one radio. I have seen the STRIDSBERG couplers which are very good but I am not sure they will do what I need. I have room on my mast for another yagi but I need to get the right equipment and set it up right. So thats my next objective.
 

hiegtx

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Yeah I keep experimenting but it gets to the point where its useless. the highest I have seen the RSSI go is about -118. I think what also makes it hard is there arent hardly any other people in my area that I know of on RR so we could compare results. I am curious to know what the situation is like a few miles east of me. I agree that it could be weak signals but I didnt think being only a few miles from the city, like 6 to 8 miles at the most would still be weak! One of the PROV towers is 4 miles away from me! I would like to see about getting a splitter/combiner to use 2 antennas together. I can point a yagi towards the city and the yagi I am using now facing east can stay cause it receives everything else pretty well. I ordered an 8 element yagi the other day and some new LMR400. I just need to find the right device to connect the 2 yagis to one radio. I have seen the STRIDSBERG couplers which are very good but I am not sure they will do what I need. I have room on my mast for another yagi but I need to get the right equipment and set it up right. So thats my next objective.
There is one local city, using simulcast, that is about 5 miles from me (to their city limits; the actual transmit sites are farther). But when they switched from their old Motorola Type II to P25 Phase II Simulcast, I lost the ability to hear them from home. As part of their conversion, they reduced the transmit power levels. They also remounted the antennas lover on the site towers, and adjusted them to concentrate the signal in their jurisdictional area. If I go over into that city, I can monitor them with no issues. But on the way back home, once I get much over two miles from their city limits, I can no longer get enough signal for consistent reception.

Possibly, if I mounted a directional antenna as high as possible, I might hear them. But I don't want to dedicate one scanner to mostly that one city, since the direction is away from most of the cities and agencies I normally scan. It's not worth it, since their PD is now fully encrypted. Not sure about fire, but I very rarely hear their FD, even when in that specific city. There is another city, just past this one, that I also lost when they switched. But since they are even farther away, and PD & FD are fully encrypted, there's no point trying to scan the second city at all.
 

Spitfire8520

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Yeah I keep experimenting but it gets to the point where its useless. the highest I have seen the RSSI go is about -118. I think what also makes it hard is there arent hardly any other people in my area that I know of on RR so we could compare results. I am curious to know what the situation is like a few miles east of me. I agree that it could be weak signals but I didnt think being only a few miles from the city, like 6 to 8 miles at the most would still be weak! One of the PROV towers is 4 miles away from me! I would like to see about getting a splitter/combiner to use 2 antennas together. I can point a yagi towards the city and the yagi I am using now facing east can stay cause it receives everything else pretty well. I ordered an 8 element yagi the other day and some new LMR400. I just need to find the right device to connect the 2 yagis to one radio. I have seen the STRIDSBERG couplers which are very good but I am not sure they will do what I need. I have room on my mast for another yagi but I need to get the right equipment and set it up right. So thats my next objective.
Something to keep in mind is that radio reception depends on more than just simple distances. A lot of times line-of-sight is considered the best if you want to receive a signal. If you have geography like a significant hill between you and Providence, then you might be getting a weak signal as a result and all radios will perform badly with a weak signal. In cases like that, then pointing your yagi right at Providence or maybe some direction that reflects the signals from Providence to your location might help.

The advantage of the SDS scanners is that the software can analyze the combined signals being received and "focus" on the strongest one, so you get much better results (but still not perfect).
This explanation is not correct for why the SDS works with simulcast sites. The SDS reads the signal properly with the correct demodulation method. Simulcast uses what could be simplified as a phase modulated signal, but most scanners other than the SDS attempts to demodulate it as an frequency modulated (FM) signal which is why they fall short of being able to demodulate the signal correctly. This would be like trying to decode an FM signal using amplitude modulation (AM) mode on a radio.

A real world analogy would be translating between languages, but using the wrong language. If one takes a written sentence in Spanish and tries to translate it as if it were Italian, they might be able to get the general idea of what it means, but the translation would probably not be great.
 

bobruzzo

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Something to keep in mind is that radio reception depends on more than just simple distances. A lot of times line-of-sight is considered the best if you want to receive a signal. If you have geography like a significant hill between your and Providence, then you might be getting a weak signal as a result and all radios will perform badly with a weak signal. In cases like that, then pointing your yagi right at Providence or maybe some direction that reflects the signals from Providence to your location might help.



This explanation is not correct for why the SDS works with simulcast sites. The SDS reads the signal properly with the correct demodulation method. Simulcast uses what could be simplified as a phase modulated signal, but most scanners other than the SDS attempts to demodulate it as an frequency modulated (FM) signal which is why they fall short of being able to demodulate the signal correctly. This would be like trying to decode an FM signal using amplitude modulation (AM) mode on a radio.

A real world analogy would be translating between languages, but using the wrong language. If one takes a written sentence in Spanish and tries to translate it as if it were Italian, they might be able to get the general idea of what it means, but the translation would probably not be great.
Interesting. Yep....I am willing to bet after reading all of this that the culprit is the topography here. There is a ridge about 100' higher than my location to my east and slopes downward a little towards Providence. I have absolutely no issues scanning conventional....I pick up stuff far away like 50 miles from here. I am going to have to find out how to connect 2 yagi's to 1 radio. I have an 8 element yagi coming soon but I need to learn about hooking them both to scanner. I will point the 8 element yagi towards Prov and the 6 element I will leave pointing east cause it works fine with the other zones. In attached pic, my house has red line under it...you can see how land rapidly rises to my east......picture not too clear but you can make out houses on top of that hill.
 

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sallen07

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This explanation is not correct for why the SDS works with simulcast sites. The SDS reads the signal properly with the correct demodulation method. Simulcast uses what could be simplified as a phase modulated signal, but most scanners other than the SDS attempts to demodulate it as an frequency modulated (FM) signal which is why they fall short of being able to demodulate the signal correctly. This would be like trying to decode an FM signal using amplitude modulation (AM) mode on a radio.
My sentence wasn't meant to be a detailed technical explanation of why an SDR (which is what the SDS scanners are) does better on simulcast than a traditional scanner. Hence the quotes around "focus on". :)
 

Whiskey3JMC

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Bottom line is & as I have said here before, the system engineers do NOT design these systems with the hobbyist in mind. Part of the fun and frustration at times is experimentation.
 

jonwienke

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the highest I have seen the RSSI go is about -118.
That's not high, that's low, because it's a negative number. The minus sign means it's that many dB below the reference level (1 millivolt). Think of it as being underground--the bigger the negative number, the farther you are below the surface.
 

ofd8001

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Back when our community migrated to a simulcast trunked system, it was designed for coverage to be 3 miles past the county line.

I suspect Rhode Island may have some form of a linked system, where a unit from one area (or site) was to roam into another site's area, it would affiliate with the "new" site and link back to the original site. That helps for comms to continue and cover a much larger area. Great for users of the system, but adds challenges to monitoring systems.
 

bobruzzo

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you don't want to receive more towers.
you want to receive one clearly.
you might want to turn the antenna to do that. VERY slowly, and it might not be pointing at the tower you want to hear. don't get hung up on "it has to be that way". even go as far as turning it all the way around.
View attachment 89932
Thats what I did. I pointed my yagi to the east very slightly south of east and reception improved greatly. I am receiving the north/south/Prov zones off side lobes and the east zone which is the furthest away (16 miles) in direct path of yagi.
 

bobruzzo

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That's not high, that's low, because it's a negative number. The minus sign means it's that many dB below the reference level (1 millivolt). Think of it as being underground--the bigger the negative number, the farther you are below the surface.
OK averaging it out the RSSI normally is hovering from -50 to -60
 

bobruzzo

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Bottom line is & as I have said here before, the system engineers do NOT design these systems with the hobbyist in mind. Part of the fun and frustration at times is experimentation.
Thats what I plan to do. I ordered a larger yagi and more LMR400. I am going to set it up where I can experiment turning it and checking results. Right now the current yagi is very inconvenient to play with due to its annoying temporary location. But it is the only option I have at the time. So I am waiting till cooler weather gets here and I can do more experimenting.
 
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