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P25 simulcast help

Posted: Wed Feb 04, 2009 5:48 pm
by walong
Hi all.

After searching the forum, many questions have been answered, and I have gained a whole 'lotta knowledge. But I still need some advice/help. I have experience with Moto, but just analog stuff. So here I go.

My problem: Local PS is on a regional 700/800 Moto trunked P25 system (1a7). My particular county's site is actually a five tower Motorola P25 simulcast system. The simulcast is near impossible to rx using the GRE (other "regular" sites are perfect).

What I'm thinking: After pouring over white papers on P25 theory, etc, and looking over the data structure of a P25 transmission, the best I could come up with is this as a poor-mans solution.

Any P25 800 XTS, setting up as conventional rx, and using the TG #s to "filter" what comes through. I figure 11 voice channels, not too many TGs on system to "filter" through. Kind of similar to scanning for a particular CTCSS tone over many conventional channels, just using the TG info instead. Should be better than just randomly scanning the vc's.

My questions:
1. Is this possible? Is there a way to unmute audio based on TG info in conventional mode?
2. After bringing scan hang time to 0, will the radio scan and check for decode at a fast enough rate to pull this off?
3. Im thinking channel 1's scan list would have the systems voice channels in it's list, but just looking for specific TG info, repeat for ch 2, ch 3 , etc. Am I on the right track?
4. Will I gain anything as far as the simulcast is concerned? Will I be able to adjust for the systems' simulcast?
5. IF this is possible, any specific fw, etc I should look for? I assume an older 3600 radio would still work, as I wouldn't be trying to follow the 9600 trunk. (yes, I know the baud rate deal, just keeping it simple)
6. And last, any pitfalls to watch for would be appreciated. And if it is feasable, any particular flashes I should look for that would be conducive accomplishing this.

Just trying to get ANYTHING to hear this simulcast before moving on to "other" options. Not mission critical, just continuing my hobby.


Re: P25 simulcast help

Posted: Mon Feb 09, 2009 3:44 pm
by walong
Ok, maybe I approached this wrong.

First, hello to all. This is a great site, rich with information. I have been refering to this site for years, but just now registered on the forum, and discovered the additional topic-specific forums. Many thanks to the volunteers who keep this site running and provide such a wealth of info.

I did post this in the wrong sub-forum. My apologies.

Let me filter my question as so as not to appear as the "typical" lurker/troll, as I have searched and read the related topics in the (proper) Mot P25 forum:

?- Specifically, can adjustments be made to better recieve what I belive to be LSM in convetional modes, even though I am monitoring a P25 sys? Or are those adjustements only available through the trunking menu?

Thank you ,

Re: P25 simulcast help

Posted: Tue Feb 10, 2009 1:46 pm
by motorola_otaku
Well, part of your question has already been answered. See here.

Regarding the GRE and poor decode of CQPSK simulcast, I think there was thread in the Illinois subforum of Radioreference where some guys up there were having problems decoding a simulcast system and found solutions either in the form of directional antennas pointed at one simulcast tower or trading their GREs for Bearcats. I know I read it, but I can't find it in a search atm.

Re: P25 simulcast help

Posted: Tue Feb 10, 2009 3:01 pm
by Wowbagger
That you cannot decode the LSM format is odd, because at symbol time there is no difference between LSM and C4FM. The only difference is that the LSM signal will reach the symbol deviation value (+/- 600 or +/- 1800 Hz) earlier in the symbol time than C4FM, and will leave that value later in symbol time than C4FM. That's the "magic" in LSM: In the face of multiple systems being received, the longer dwell at symbol deviation means you can tolerate a larger flight-time difference between the sites without interference (at the expense of either needing a linear RF power amp or having a larger occupied bandwidth).

Any data slicer that can do C4FM should be able to do LSM *if it was done right*.

Whether the data slicer in question was done right is an open question.

Re: P25 simulcast help

Posted: Wed Aug 04, 2010 1:47 pm
by MattSR
Hi guys, for interest, I came across some good information on demodulating LSM here - the author of that page has cracked the LSM problem wide open for all to see :)

Re: P25 simulcast help

Posted: Wed Aug 04, 2010 2:48 pm
by Bill_G
Interesting. Thank you.