P25 LSM (Linear Simulcast Modulation) Difficulties

[MattSR]

I haven't found a refernce to back up the C4FM/CQPSK point I made, but I did test your new python code. It seems to work very well - the following screenshots were grabbed with a Ubuntu VMware guest, running OP25 and GNUradio off a soundcard.

I was finally able to capture the P25 AlgID for ADP (RC4 based) encryption, something I never got a response to on another particular Motorola forum.

I'm hoping that some code that can be used to create a software based P25 repeater comes out of this effort, I think that some Encryption/Decryption GNUradio blocks, and Tx/Rx blocks would make for a powerful platform. Project54 even managed to do full transmit and receive using nothing more than a soundcard, so the potential for OP25 is limitless.

Thanks,
Matt
VK2TVK

 

[KA1RBI]

Thanks Matt, that's great feedback!

Noticed a couple things from your screen prints. First, you had the audio gain control set at maximum. This suggests that we might want to increase the upper limit a bit. Second, the "demodulated symbols" plot shows a fair bit of noise. There's a P25 station close by here that is very strong and provides utlra-clean copy; however, yours shows reduced noise margins, some of the symbols are getting uncomfortably close to a slicer decision point. The plot overall shows either a relatively weak signal or else we still have further work to do in the demodulator.

Can you please do two things
1) tell more about the signal (strength, type of transmitter, type of equipment, etc...
2) send me a screenshot of the Datascope window

Best Regards

Max

 

[scanthis]

Simulcast APCO 25 systems!!!! Aaaaaargh!

I live with a large simulcast system, and with a handheld, reception depends on being very close to one tower, relative to the others. If you are somewhat between towers, its rough.

At home matters are much better. I have a dedicated yagi / scanner combination just for this system. It is pointed so it attenuates all but my primary tower as much as possible, per the field pattern of the antenna, and it works pretty well. To really attenuate the 2nd and 3rd closest towers, I end up pointing it about 11 degrees off of my primary tower oddly enough. Pointed RIGHT AT the primary tower, one of the other towers starts having an impact, as that angle on the field pattern starts allowing it to be less attenuated it starts doing bad things to reception. Keep in mind, being 11 degrees off meant that I went from a gain of 11db to 8db, but the attenuation of the number 2 tower went from -20db to -50db, so the ratio was MUCH better this way. The field pattern of a high gain yagi is a pretty funny looking thing, with sharp dips in places. So really, in this case you are using a yagi in a way that was never really intended, as an ATTENUATOR, more than as a device to pick up a weak signal from one location, as it was intended to be used. This took many hours of tinkering to figure out, but in the end it makes sense.

My point is, pointing a yagi right at your nearest tower may well not be where it needs to be pointed. A good ratio of signal strength is the key, and that comes from plotting out nearby towers and aiming the antenna based on its field pattern to achieve the best ratio. Properly pointed, it works like a charm. Not 100%, but with the new PSR-600 / Pro-197 with updated dsp, its probably about 95% reception, and that is about as good or better than the Police portables get, judging by the number of times they have to ask for a repeat because of a break-up or someone "going digital".

Now, as these systems become commonplace, I hope that scanner manufacturers start including the guts to make these things work like police radios do. However, I suspect that will mean a BIG jump in price. I have heard a Phoenix PD portable is almost two grand, but since the scanner doesn't have to transmit, maybe it can be done for less than half of that. People like myself would come up with the money if it worked well.

Phoenix, I have the same problem here (Sioux City IA)...it's frustrating as hell. Have you heard of any scanner manufacturers doing anything about this yet???? I would gladly pay big bucks to have a handheld scanner that would reliably decode simulcast APCO 25 systems without having to be tethered to a yagi!