Understanding the complexity of P25 Simulcast systems.

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an39511

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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.
OK, I can understand that from using analog scanners and high power paging towers near the frequency I am trying to receive. But what is happening is if the transmission starts out good it remains good. Here in Cleveland we have some long winded dispatchers that go on-and-on. I would think that at some point in their transmission, sometimes more than a minute, I would expect that another interfering frequency, perhaps on the same tower would kick on and cause my reception/decode to deteriorate in the middle but that isn't happening to me. Either the transmission starts out good and remains that way, or it starts bad and remains that way.
 

whsbuss

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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.
Those signal bars fluctuation might be from multi path, where the signals out-of-phase cancel each other and decrease the original received signal.
 

an39511

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Those signal bars fluctuation might be from multi path, where the signals out-of-phase cancel each other and decrease the original received signal.
Correct! I know it happens, I have seen it happen many times, but that is not what was happening in my original observation but it is one of several that causes problems. I think once I can see the actual frequency during the transmission I will know more. Thanks for your contribution.
 

policefreak

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an39511 said:
Those signal bars fluctuation might be from multi path, where the signals out-of-phase cancel each other and decrease the original received signal.
Correct! I know it happens, I have seen it happen many times, but that is not what was happening in my original observation but it is one of several that causes problems. I think once I can see the actual frequency during the transmission I will know more. Thanks for your contribution.
What you might want to do is have your PRO-106 and your 536 next to each other so that while your 536 is operating you can look at the pro 106 for the frequency. With these systems, even on the same band one frequency can propagate totally different than another.
 

an39511

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What you might want to do is have your PRO-106 and your 536 next to each other so that while your 536 is operating you can look at the pro 106 for the frequency. With these systems, even on the same band one frequency can propagate totally different than another.
Good Advise! Thank You!

I did this and it worked well. I took a video so I could keep track of the frequencies. What I found there is one frequency that seems to produce more errors than the others. This looks like it explains why there seems to be decode problems from one transmission to another.
 

ofd8001

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I don't know if this is a "wive's tale" but from what I've been told, with digital you either got it or you don't. Thus the audio signal should be just as good at 2 bars as it would be 5 bars. I listen to a conventional digital system and get quite clear audio even with no bars showing on occasion.

This might be in left field, some of the issue could be how loud the person is talking into the radio, how far the microphone is from their mouth and whether any background noise is present. For example someone calling the dispatcher may speak loudly at first because sub-consciously they are trying to get someone's attention. Their follow-up transmissions could be a little softer.
 

policefreak

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The meter is not determined by volume, but the amount of RF going into the front of the receiver. And you will see, even when nobody is talking, you can have full bars. That is also to say that while "you either have it or you don't," fluctuating signal strength can be indicative of multipath interference. On some of the old analog simulcast systems or simulcast conventional frequencies, this may sound like bacon frying (making you hungry) or a slight crackling. Throw in the timing component of a digital processor for P25 audio, and the fact you either got it or you don't, and the problem we all have with simulcast.
 

an39511

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I want to follow up with everyone and let you know what I found out. The system I am monitoring has a total of 30 frequencies assigned with 4 of those control channels. Out of the 26 remaining there is only one frequency that is showing high error rates. So what I thought was random, isn't. I logged 130 calls and out of those 7 landed on that specific frequency causing decode problems.

I will perform this test again at a future date to see if this type of problem changes / moves to other channels. I guess if this type of interference happened on a control channel the entire system may not be able to be monitored.

Anyway, this may be something you want to observe to see if this is happening to you.
 

ofd8001

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Of those four control channels, most likely one is the primary with the three others alternates. My understanding is that only one control channel is in use as a control channel at any given time. The others are "just in case" and can be devoted to voice channels.

If one frequency happens to be the "culprit" possibly there is some type of interference going on some kind of way. That interference could be another source of RF or possibly some electronic equipment near your scanner emitting RF. Also, one cannot rule out the possibility of a system transmitter issue (as in not putting our full power).
 

WA0CBW

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It is very possible that one (or more) channels have a problem. In a simulcast system where all sites transmit the same thing it can be very difficult (read that time consuming) to determine if a particular frequency is working properly. There are several adjustments that must be right on. Interestingly a scanner makes a good troubleshooting receiver because errors in either frequency or phase show up on a scanner whereas a commercial receiver is still able to recover the audio without error. Sites using more than 10 channels can take a while to check each frequency for the proper adjustments and it has to be done for each site.
 

pratzert

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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.
Thanks for that info.

Why do you think scanner Mfg's don't use the IF to help the simulcast reception if it works ?

Is it more expensive to do it that way or too much engineering to go down that path instead of what they are using now ?
 

whsbuss

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Thanks for that info.

Why do you think scanner Mfg's don't use the IF to help the simulcast reception if it works ?

Is it more expensive to do it that way or too much engineering to go down that path instead of what they are using now ?
I think its because scanners are the swiss-army knife devices to receive all types of freq's/services, etc. Analog and digital are different animals when it come to decoding audio.
 

an39511

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I have done some more research by connecting up my FUNcube and HDSDR and tuning to the frequency (854.2375) that has decode issues. What I am seeing is this https://dl.dropboxusercontent.com/u/11263151/854.2375.jpg
It looks like the transmission is not on that frequency. How can this be? And can this cause the problem?
Is there any way to correct it? Or is the control channel telling the scanner to tune to that exact frequency.

I have checked HDSDR and it appears to be calibrated correctly.
 

an39511

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Something must be wrong with HDSDR because when I tune the scanner to the frequency that HDSDR shows the scanner won't recognize it as a digital channel and won't decode but only hear the digital noise.
 
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