Understanding the complexity of P25 Simulcast systems.

[an39511]

I am posting this in the Uniden forum because I am using a BCD536HP to observe with.

I live in an area with several simulcast P25 systems and for several weeks been observing receiving reception. I had been using a RadioShack PRO-106 and now have the BCD536HP. The 536 has a nice feature that you can enable that show you the ERRor rate on decoding the digital audio so that is the only tool I am using at this point.

I have already read much on the subject of multipath and how it causes issues in the transmit signal and digital audio decoding. I also understand that each system can be deployed differently. I understand the location and weather has a lot to do with how much interference the receiving radio is subjected to.

To simplify all this I am observing one talk group in a P25 simulcast system.

Order of events;

Base calls mobile. The ERR rate is zero(0) for the entire transmission
Mobile replies to base. ERR rate is zero for the entire transmission
Base responds to the mobile. ERR rate ranges from 8-15
Mobile replies. ERR rate is zero.
Base responds to mobile. ERR rate is zero.
Mobile replies. ERR rate ranges from 30-50.
All this back an forth happens in less than 40 seconds.

What I am trying to understand is why does a transmission from a fixed location have a high error decode rate one minute and ten seconds later have no decode errors? I can understand this happening with mobiles but not a base. This scenario can also happen with mobiles but I choose to stick with fixed locations for simplicity.

To add to all of this I noticed during long transmissions either from a base or a mobile, is if the ERR decode rate begins at zero, it always stays at zero or very near it during the entire transmission and if the transmission begins with higher decode ERRs then it stays in the higher range and never drop down to zero.

The only theory I have is that each transmitting tower is changing dynamically each time a radio calls for transmission. Everything I have read so far is in a simulcast system each tower in a simulcast system broadcasts the same radio traffic on the same frequency. I am wondering it this is true in all cases. I am beginning to think that there is some algorithm that signifies if all or just some of the towers broadcast, or possibly altering their power output based on the location of the mobiles.

Is there anyone out there that works on these types of systems that can explain what I am observing?

 

[UPMan]

You are always receiving a signal that is originating from a fixed location. You are not receiving a direct signal from either mobile or base units, you are receiving from the fixed-location repeater towers.

 

[an39511]

You are always receiving a signal that is originating from a fixed location. You are not receiving a direct signal from either mobile or base units, you are receiving from the fixed-location repeater towers.

I understand that I am only receiving signals from the towers but I am wondering why the decode errors vary from one transmission to the next. It isn't the scanner because other brands suffer the same way it is just the Uniden has that feature I can see more of what is going on.

 

[UPMan]

Atmospherics (wind, pressure systems, storms), vehicles in your area reflecting the signal differently, in the Spring, the incremental growth of leaves on trees

 

[eorange]

All this back an forth happens in less than 40 seconds.

I have observed and wondered this myself with my 396XT. What could possibly be causing this high rate of change in < 1 minute?

 

[an39511]

Atmospherics (wind, pressure systems, storms), vehicles in your area reflecting the signal differently, in the Spring, the incremental growth of leaves on trees

Again, I understand that too but there is something else going on. Because there are so many variables finding all the answers is not possible but in my original observation demonstrates that if a transmission starts out with a ERR rate of 0 or near it, it remains that way for the duration of that transmission even if it goes on for a very long time, sometimes more than 60 seconds. If that station breaks, even for a second or two when that same station keys up again the ERR rate may range between 30-50 and stay there for as long as they have the mic keyed. If it was atmospheric I would expect that ERR rate to fluctuate more. Example; Transmission starts with a ERR at 0 but during the course of that specific transmission rise way up and back down again but I have yet to see that happen. If the transmission starts out good it remains that way.

 

[an39511]

Just to clarify, this not a fault with the BCD536HP but I am pretty sure it is a phenomenon of the P25 simulcast system.

 

[troymail]

...and the expensive subscriber radios (Motorola, Harris, etc.) were designed to deal with these signal variations and are engineered to process the signals better.

The (relatively) lower costs scanners are simply doing their best given the amount of dollars put into solving it in a scanner.

Also, I'm not really sure it makes a difference while inside the same "site" (with multiple towers), but keep in mind that the subscriber radios are activity "talking" to the system - scanners are receive only.

 

[jcardani]

That's correct, it's the multi path from multiple towers that's causing the fluctuation you see in signal strength. This causes the distortion in audio (garbled, choppiness, missed calls, etc.). Basically the scanner is not seeing the correct CQPSK waveform and therefore not detecting the correct transition peaks during symbol recovery. This is most likely due to the fact the DSP is using baseband discriminator instead of the scanner's IF, which is needed for proper CQPSK demodulation. See:

http://forums.radioreference.com/di.../283705-lsm-why-scanners-suck-real-story.html

Just to clarify, this not a fault with the BCD536HP but I am pretty sure it is a phenomenon of the P25 simulcast system.

 

[jcardani]

Furthermore,

Correct CQPSK demodulation has been proven to work with the OP25 project under GNU Radio, which is open and free software. Basically instead of using C4FM demodulation and the baseband audio discriminator, the radio's IF is used and the CQPSK demodulator performs the IQ translation. The demodulator basically uses a Costs loop to recover the carrier and the Gardner algorithm for bit timing. Max (KA1RBI) is credited for putting a lot of hard work updating the CQPSK demodulator.

 

[policefreak]

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To the OP, was each transmission on the same frequency? That could make a big difference.

 

[an39511]

Wirelessly posted (Mozilla/5.0 (Linux; U; Android 4.1.2; en-us; LG-LS720 Build/JZO54K) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30)

To the OP, was each transmission on the same frequency? That could make a big difference.

I do not know if they were. I agree that may be another piece to the puzzle. Does anyone know how to get the frequency to display on a BCD536HP?

 

[troymail]

I do not know if they were. I agree that may be another piece to the puzzle. Does anyone know how to get the frequency to display on a BCD536HP?

This is an excellent point! - even on analog systems, you can tell some system frequencies are better/worse than others.

 

[WA0CBW]

In a multi-channel simulcast trunk system each exchange of transmissions can be on a different frequency. The transmit combiners can handle only from 5 to maybe 10 frequencies. So there may be more than one transmit antenna mounted at different locations on a tower. Due to the physical location of the transmitters with respect to the combiners the output power will be different from one frequency to the next. Usually the transmitters are adjusted to maintain the output power within 5 to 10 watts of each other. All of these factors add up to a constant varying of signal strength and phasing. So what you observed from one transmission to the next is not unusual.

 

[an39511]

That's correct, it's the multi path from multiple towers that's causing the fluctuation you see in signal strength. This causes the distortion in audio (garbled, choppiness, missed calls, etc.). Basically the scanner is not seeing the correct CQPSK waveform and therefore not detecting the correct transition peaks during symbol recovery. This is most likely due to the fact the DSP is using baseband discriminator instead of the scanner's IF, which is needed for proper CQPSK demodulation. See:

http://forums.radioreference.com/di.../283705-lsm-why-scanners-suck-real-story.html

I wish to point out that during the time of my observation the signal bars were not fluctuating. I know that the signal bars a very subjective way to go about it but that is all I have at this point. I do understand about how the signals from multiple paths can cancel each other out but I was not detecting that situation at the time.

 

[an39511]

In a multi-channel simulcast trunk system each exchange of transmissions can be on a different frequency. The transmit combiners can handle only from 5 to maybe 10 frequencies. So there may be more than one transmit antenna mounted at different locations on a tower. Due to the physical location of the transmitters with respect to the combiners the output power will be different from one frequency to the next. Usually the transmitters are adjusted to maintain the output power within 5 to 10 watts of each other. All of these factors add up to a constant varying of signal strength and phasing. So what you observed from one transmission to the next is not unusual.

Good information, thank you.

 

[troymail]

I wish to point out that during the time of my observation the signal bars were not fluctuating. I know that the signal bars a very subjective way to go about it but that is all I have at this point. I do understand about how the signals from multiple paths can cancel each other out but I was not detecting that situation at the time.

From what I've seen on the 536 and even some older radios (including GRE), I don't think I've really trust the "bars". I can get really good reception from some systems with 1-2 bars while others show 5 solid bars and I can't hear much of anything. I'm not even going to try and understand that....

Also - FWIW - my guess is that of you're receiving really strong signals (particularly from two or more different sites at the same time), that really causes the scanners to be "confused". I think the reason I receive some distant systems/sites well is because I'm receiving from the closest tower/transmitter and the others are farther away than that one.

 

[jcardani]

I wish to point out that during the time of my observation the signal bars were not fluctuating. I know that the signal bars a very subjective way to go about it but that is all I have at this point. I do understand about how the signals from multiple paths can cancel each other out but I was not detecting that situation at the time.

I guess I read your post too quickly, please ignore my replies. My apologies.

 

[an39511]

From what I've seen on the 536 and even some older radios (including GRE), I don't think I've really trust the "bars". I can get really good reception from some systems with 1-2 bars while others show 5 solid bars and I can't hear much of anything. I'm not even going to try and understand that....

Yes the signal bars are not very good for determining the strength of signal but one thing I have noticed when "that type" of interference occurs I have noticed rapid fluctuation in the bars. From 5 bars to 1 bar in a fraction of a second. Whenever I see that I know I am going to get a bad decode, for obvious reasons. But that was not happening at that time either.

I understand there are many reasons that cause interference and I am trying to separate the problems to get a better picture of what is happening.

 

[Josh]

In my experiences with trunking and scanners, the issue I've found which supports your error rate going all over the place is adjacent channel interference.

Scanners have a hard time blocking out adjacent channels and depending on how your system is set up there may be repeaters on the same tower transmitting occassionally at the same time. For instance here in Detroit, I can see the tower, it's about a mile away. It has frequencies of 851.675 and 851.7375 in service. Needless to say that I can hear one or the other fine, but combine the two and the BER goes through the roof to the point you cannot decode anything, but still have "full signal bars".

Motorola and other professional radios have ultra narrow filters to keep adjacent channels from interfering.