Loose (Open) Squelch Improves LSM (Simulcast) Reception

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UPMan

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In recent studies, we've determined that the squelch setting can dramatically affect P25 decode quality, especially on Simulcast (LSM) sites. For more background on Simulcast issues, see the wiki article with the caveat that it is not fully correct on the root cause of the problem. Here is a quick bullet-point summary that more correctly describes the root cause:
  • Any receiver dropout during digital reception creates data loss that results in bad decoding.
  • Because multiple towers are transmitting the same signal, simulcast systems can have many areas that have “nulls” where the same signal from different towers arrives at slightly different times, causing the signal strength to drop and/or vary suddenly.
  • Even atmospheric changes or someone opening/closing a door can affect where the nulls are, so even optimizing antenna location cannot prevent dropouts 100%. Moving a receivers antenna even a few inches can sometimes move it out of a null zone. Vehicle and fixed-location radios typically avoid the problem by use of a diversity antenna system. Also, system engineers try to design the system so that predicted null areas occur in areas that system users are not likely to be, such as over water or unpopulated areas. Even then, nulls cannot be 100% eliminated in intended service areas. Handheld radios w/single antennas and scanners are both subject to a higher level of signal amplitude changes.
  • When the signal strength drops below the squelch threshold point, the scanner’s squelch action immediately stops the receiver from providing a signal to demodulate/decode, even if the drop is very brief. This results in data loss and choppy or no decoding.
We are studying to determine what things we can do in the scanners themselves to mitigate these drops (no guarantee that we can do anything for an existing model, but we are looking). For now, though, we strongly recommend that you set your squelch level as low as you can without having conventional channels w/no tone programmed break squelch w/no signal present. If you are only scanning trunked systems, for example, you could effectively set squelch to 0 (always open) with a couple of caveats:
  • You must have End Code detect enabled. Otherwise, when an analog comm ends on a trunked system, the scanner will remain on the voice channel and you will either hear open squelch or the next comm assigned to the voice channel.
  • It will take longer for the scanner to acquire the trunked control channel (normally, it only looks on those programmed frequencies that have a signal strong enough to break squelch). You can mitigate this delay by only programming control channels into your scanner.
If you have any conventional channels programmed with no CTCSS/DCS, then you must set the squelch to threshold: typically 3 or 4. Even if all your conventional channels have CTCSS/DCS, you might want to keep the squelch at threshold, as it will take much longer to scan conventional channels with squelch fully open. The scanner will have to evaluate every channel for the presence of CTCSS/DCS before moving to the next channel. Typically, this check is only made for those channels with a signal strong enough to break squelch.
 
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UPMan

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I will add that I do not know whether the squelch setting will result in performance changes for GRE scanners. Probably worth the experiment. Here is a sample setup for checking (requires two of the same model).

Connect both scanners to the same antenna, preferably through a multicoupler, but it is probably possible to do this with direct connections. The goal is really just to ensure that both scanners are getting exactly the same signal.

Tune both scanners to the same digital channel and hold on the channel.

Fully open squelch on one scanner. Set the other scanner at/near the squelch threshold.

Compare results between the scanners.
 

Thunderknight

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Vehicle and fixed-location radios typically avoid the problem by use of a diversity antenna system.
You say typically. Can you elaborate on that? I've never seen a P25 LMR mobile with diversity receive. Motorola, Harris etc. P25 radios are single antenna.
The only place I've seen it used in LMR-like (not talking about cellular like systems) settings is dedicated mobile data systems.

A lot of avoiding the problem in a simulcast system is minimizing delay spread to below the threshold for the receiver to decode the signal at the designed DAQ. I'd guess that scanners have less tolerance (lower required spread) than purpose built P25 subscribers (which I think usually have a max delay spread of 40-70µs for DAQ 3.4, depending on modulation, with LSM having more tolerance for higher delay spreads).
 

ofd8001

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I'm encouraged that Uniden is continuing engineering efforts to deal with this situation.

Thanks for persevering!
 

Voyager

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You say typically. Can you elaborate on that? I've never seen a P25 LMR mobile with diversity receive. Motorola, Harris etc. P25 radios are single antenna.
The only place I've seen it used in LMR-like (not talking about cellular like systems) settings is dedicated mobile data systems.
That was one of two points I was going to make as well. I've never seen that used on Portable, Mobile, Base, or Station installations.

The other point was that I've never found a squelch setting other than '2' to be required. At times, I've run it down to '1', but my normal setting is '2'.

I've also never found simulcast issues on my local simulcast systems.

Joe M.
 

UPMan

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Sorry, I overgeneralized the use of diversity antennas. It is indeed used for many other multipath-sensitive technologies (wifi most commonly...the reason for the two antennas on most wifi routers).
 

mharris

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Let me just say it's great to see that Uniden is conducting research on this problem. With research comes answers, and it's pretty safe to say we will see improvements in future models. At this point I don't even try to decode simulcast unless I'm within a certain geographical area where I know the decode will be ok, if not I don't even try.
 

SquierStrat

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I'm encouraged that Uniden is continuing engineering efforts to deal with this situation.

Thanks for persevering!
Agreed! i am VERY pleased to hear this. Even though the last firmware update did amazing things for the ORION system in douglas county nebraska. With adjust mode set to 15, and adjust level set to 50, It is by far the best ive heard out of this scanner yet! Cant wait to get back down there to try this squelch "trick"
 

KevinC

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Even atmospheric changes or someone opening/closing a door can affect where the nulls are, so even optimizing antenna location cannot prevent dropouts 100%. Moving a receivers antenna even a few inches can sometimes move it out of a null zone. Vehicle and fixed-location radios typically avoid the problem by use of a diversity antenna system. Also, system engineers try to design the system so that predicted null areas occur in areas that system users are not likely to be, such as over water or unpopulated areas. Even then, nulls cannot be 100% eliminated in intended service areas. Handheld radios w/single antennas and scanners are both subject to a higher level of signal amplitude changes.
And not to beat you up even more, but nulls on a Motorola P25 LSM system won't be predictable. A little "jitter" is introduced so the nulls are constantly moving.

If this wasn't implemented nulls would invariably be at either the police station or the chiefs house. :D
 

torontokris

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also to add be sure to update to the latest version of the 396XT & 996XT as the newest firmware is specifically for improving LSM systems.
 

alphaacres

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I would also make sure that talkgroup audio type is set to digital the default is auto this seems to help sometimes too.
 

AE7Q

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Diversity reception

Sorry, I overgeneralized the use of diversity antennas. It is indeed used for many other multipath-sensitive technologies (wifi most commonly...the reason for the two antennas on most wifi routers).
It was a feature in my first cellphone, a 5W trunk-mounted unit from EF Johnson. I installed it with two antennas on my car back in 1984, and the benefits were astounding: Back then, there were few cell sites in Los Angeles, and most users lost their connection every time when driving in hilly areas (eg, Sepulveda Pass), whereas I could always hold the connection.

I recall a claim somewhere that diversity reception could provide effectively a 12dB gain in some situations, and from my experience with that cellphone, I'd believe it.

I currently have an Icom IC-2820H amateur radio transceiver with a diversity reception feature, and I've installed it in my current car, also with two antennas. I haven't seen the same benefit, but I really haven't tested it in problematic areas. This week I'm making a long drive, and I'll try to remember to test it out ...

ps: to UPMan: Are you the person to report bugs and/or product suggestions to (I have a BCD996XT), and if so, are the RadioReference forums an appropriate place for such?
 

RoninJoliet

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Here in IL on the Starcom Digital system using the "0" squelch does seem to help some towers from dropouts, I can see the signal start to cough but in a split second very quick is corrected or smoothed over...Thanks for the help "Upman" , always improving and on the job....Ur the best....
 

KC9NEG

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Great topic--thanks for the insight. I can attest to the fact that moving an antenna a couple of feet here in my office can change decoding of our IDPS LSM CQPSK system from very good to nearly unbearable. Those who persist with experimentation receive these systems best (unless you're very lucky!).
 

mancow

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I wonder if it's the abacus IC SDR discriminator that allows for better processing?
 
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