Decode threshold while roaming

dispatchgeek

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I use my P25RX as a mobile scanner on our statewide P25P1 system. My receiver lives in my center console, hooked to an 800Mhz 1/4 wave mobile antenna on the roof and send audio to a bluetooth visor speaker.

I roam several different sites that span my commute to work. I've found that the P25RX tends to hang on to marginal sites with high error rates much more readily than my SDS100 under the same conditions. This results in choppy audio due to a high error rate on the P25RX. Is there a way to set a decode threshold so the P25RX prefers sites with a better signal? I would rather roam to a closer site to hopefully catch that radio traffic with a better decode.

Any ideas or options to try?
 

FreqNout

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Interesting topic. This type of mobile switch over would be like a hand off in a cellular network. I have a decade of experience in cellular. Hand off thresholds are a science and an art to fine tune.
Preventing ping ponging in fast fade or poor coverage as you drive through your strongest system will be the biggest challenge. Harder to do with a single rx. It can be done. It will require a lot of thought in how best to implement a feature.
 

btt

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I'm currently working on fine tuning for mobile operation with the P25RX-II. In the new firmware, I've added a calculation for EVM (error vector magnitude). This is a superior way to measure signal quality. It is nearly instantaneous and a very accurate predictor of signal quality (independent of the BER or signal level). After I get this all figured out, I will port this to the P25RX.
 

dispatchgeek

Complex combination of diodes, marbles and valves.
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Maybe a bit more simple than you are thinking FreqNout. I have my roam set up to move off a site after 2 seconds of inactivity so I can catch regional locked talkgroups. In a more traditional scanner, if the control channel decode rate doesn’t meet a set BER, it simply moves on to the next site in the roam list. On my Radio Shack scanners, I could set it low enough to lock on to sites with poor decode rates.
 

dispatchgeek

Complex combination of diodes, marbles and valves.
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Thanks Todd! I’m certainly not in a rush, as I am super satisfied with my current setup. I do travel through some coverage challenged areas which tend to exhibit choppy audio and garble. (I have a site in the works to fix that area!)
 
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goldmyne99

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Probably a future design idea here...Any thoughts on a single receiver with 2 antenna spaced diversity to improve mobile fading performance?
 
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FreqNout

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Maybe a bit more simple than you are thinking FreqNout. I have my roam set up to move off a site after 2 seconds of inactivity so I can catch regional locked talkgroups.
Understood. I am not saying simple won't work. I believe using multiple thresholds and timers would make for a better performing switch over. I would also suggest the idea of a neighbor list for each roaming network. Meaning limit the roaming choices for the rx to search to the only those that are likely to be adjacent to your current system on your drive. No reason to waste time looking and switching to a system that is sure to be weak.
 

btt

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Probably a future design idea here...Any thoughts on a single receiver with 2 antenna spaced diversity to improve mobile fading performance?
It didn't make it into the P25RX-II design, but that is something to consider. I assume you are thinking 2 SMA connectors and a switch to select the better of the two ports (EVM would be the perfect calculation to decide which port). Great idea. This would also let you switch between two antennas for different bands as well. Yes, sounds like a future product once I can afford to pursue it.
 

goldmyne99

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you are thinking 2 SMA connectors and a switch to select the better of the two ports
Yes, RX Spaced Diversity antennas as you describe. Comparing ports - selecting the best and/or adding them for best decode, etc.

I know a lot of ideas brought up on this forum are easy for us to suggest and will never make it to production. Especially, when we do not have to write the code and build the hardware. But, it is fun to provide some ideas and see how the product comes to life. Todd, Thanks for engaging in the discussion of many ideas.
 
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FreqNout

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RX Spaced Diversity antennas as you describe.
No doubt. Diversity rx in a p25rx rig would be great in a mobile setup to improve performance. Has a 'scanner' with rx diversity ever been on the market?


I've found that the P25RX tends to hang on to marginal sites with high error rates much more readily than my SDS100 under the same conditions.
Speaking of rx diversity. Two rigs on different antennas are also rx diversity. Comparing the p25rx rig and the sds100 rig on different antennas may not be a fair test as they may not have the exact same rx sig. For a true compare they probably should be on the same antenna.
 
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dispatchgeek

Complex combination of diodes, marbles and valves.
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This was testing conducted over about a month of using either the SDS100 or the P25RX mated up with my mobile 800mhz 1/4 wave. Not a super scientific test by any means, but did provide some comparison data.

I am not aware of any P25 radios, commercial or scanner with diversity antennas. This does provide an interesting take on things.

Some of the areas I am referring to are known challenges to any site in the system. The deputies don’t call it “Radio Free Galesburg” for nothing!

No doubt. Diversity rx in a p25rx rig would be great in a mobile setup to improve performance. Has a 'scanner' with rx diversity ever been on the market?



Speaking of rx diversity. Two rigs on different antennas are also rx diversity. Comparing the p25rx rig and the sds100 rig on different antennas may not be a fair test as they may not have the exact same rx sig. For a true compare they probably should be on the same antenna.
 
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