Orlando P25

[Patch42]

Like many others, I'm having great difficulty hearing most of the Orlando FD transmissions since they and the PD moved to the new P25 system. My PSR-500 tracks them and attempts to decode the FD transmissions, but probably 90% of the them come through so garbled they're illegible.

The prevailing theory seems to involve simultaneous broadcast from multiple sites resulting in some serious multi-path reception that really messes with the radio's ability to decode the digital. I'm wondering if this is really the case since there is only one site listed in the database here. Is there another site that hasn't been fully nailed down as far as frequencies? Are we getting multi-path reception due to the signal bouncing off buildings rather than being from multiple sites? Are there multiple sites using the same control channel frequencies? Or is there something else causing the poor digital decoding?

Just to be clear, I'm talking strictly about the Fire Department transmissions, which are digital but are not supposed to be encrypted. I know the Police Department has gone fully encrypted. I'm not talking about those transmissions. I have those groups programmed and permanently locked out.

 

[WX4JCW]

The Same Frequencies are used on 5 Towers in a 9 Mile radius
http://www.radioreference.com/apps/db/?cs=WNXS401

1 9300 AIRPORT BLVD (OIA)
2 5959 PERSHING AVE
3 100 S HUGHEY AVE (OPD HQ)
4 5100 L B MCCLEOD RD
5 CLARCONA RD AT PINE HILLS RD

All 5 Towers Use the Same Frequencies, Hence the Theory of Simulcast Distortion as the tower sites are extremely Close Together.

I was Able to Listen to Patrol East Standing next to an OPD officer (Didn't ask him anything as he might have thrown me to the ground and arrested me) But the Audio Sounded fine, Clear and Understandable.

So i am going to RADCOM (Robinson & Bumby) and get a 3 Element Small Yagi and see if that helps

FYI The Base Radios at the Fire Stations use Yagi's (11's is Pointed to OPD HQ)

 

[Patch42]

Okay, that explains a lot. I'll have to think about the Yagi. I might be able to zero in on just the airport site, depending on how narrow the angle of reception on the antenna is.

 

[dgoodson]

Why are the PD radios apparently not affected by this? Seems like the physics of the phenomenon theorized would affect any radio "out there"

 

[WX4JCW]

I read the RR wiki on this and the Actual Radios have better reception of course, but their DSP is way better in a $5000 portable than a $500 scanner, OPD has no mobiles, the use only portables, been that way since they were on 460 MHZ, OFD has had different issues having to do with the DSP, but apparantly the radios are not affected

 

[Patch42]

From what I've read, it's also possible for the radio technicians to adjust parameters on the transmitters to minimize the problem with the radios being used. We're stuck with trying to adjust our radios to accommodate the transmitters. And, in my case at least, working effectively blind since I have no clue what most of the radio parameters do.

 

[WX4JCW]

I think Orlando is the Worst Example of this Issue, I don't know how it will play out when the county migrates, but hopefully the scanner mfr's will be able to work on a solution.

 

[Patch42]

If Orlando alone is using five transmitter sites, one has to assume it would require at least a dozen to cover the rest of the county. That is likely to be quite a challenge for the scanner makers.

 

[Patch42]

You know, the more I think about this and look at the numbers, the less I'm convinced this multipath theory is what's at work here. Guesstimating from the locations on the map linked in the first reply, it looks to be about 15 miles between the two most distant transmitters. If my math is correct, that comes out to a worst case of about 22ns difference in the arrival time of signals from those towers. It's hard to believe that would make a significant difference. Assuming a worst case of transmission from the central office we would only double that figure to 44ns.

Add in the fact that some transmissions come through just fine, and I think the inescapable conclusion is if there is an issue with signals arriving from the various towers at different times, it has to be due to something other than simple distance from the tower. If that was the only issue, ALL signals should be equally distorted at any given location.

I'm wondering if the signal going from the central office to the towers is using some kind of packet switching network that is introducing additional delays.

EDIT: I checked my math again and found I used the wrong number for the speed of light. (I should have known not to do this before I had my first cup of coffee.) I believe the correct number should be about 81us for the signal to travel 15 miles. That should be the worst case difference in signal arrival times, assuming the signals are perfectly in phase when they leave the transmitter tower. I originally thought worst case delay getting to the tower was double that, but I've since realized the signal may be traveling by wire, meaning it would be virtually impossible to know the delay getting to the various towers without actually measuring it. For the moment I'll assume double is close, meaning we'd be looking at a 162us difference in signal arrival times.

According to the only source I could find during a short search, P25 digital is 4800 baud. That would give each bit a duration of 208us. I guess 162us is getting close enough that it might cause significant confusion in decoding.

 

[WX4JCW]

I did a range circle in streets and trips, centering on opd hq, all the sites fit within a 9 mile radius

 

[Patch42]

I was trying to look at the worst case, where, for instance, you are right next to the airport site. The furthest site appears to be about 15 miles from the airport.

If the most distant site is 9 miles from OPD HQ, then the difference in delay getting to the sites is probably a bit less than I estimated. It still looks like the timing differences could be causing the issues described, though why some transmissions are decoded properly is a real mystery. It seems like if any of them have a problem, they should all have a problem if the source of the problem is the multisite propagation time difference.

 

[WX4JCW]

I have Listened to Many P25 Systems and Orlando is the Only one i have heard so far that has issues like this, It is miserable being at home trying to listen to FD, I listen to OCFR Mainly but I am in OFD 6's First Due so I like to know whats UP