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P25 simulcast multipath interference

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rak313

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In various places, (even in the FAQs here) I have seen reference to interference in simulcast P25 systems caused by interference from multiple transmitters on the same frequency.

The reference below states that the multiple transmitters (on the same frequency) are a feature not a liability - that they are synchronized - and that the combined transmiter's signal allows for fewer transmitter sites (see slides 18-19).

Why would one purposely design a system for 1st responders - that transmits the same signal (on the same frequency) on several physically separated transmitters where mobile receivers will hear the multiple transmitters - unless the system was designed such that the resulting time skew from the multiple signals is small enough to allow proper decoding.

Even if interference from the other transmitters occurs and causes poor eye patterns - would be relatively easy to implement an equalization filter to remove it (as is used in digital cell phones - which have a similar issue - except it's from multipath - not multiple transmitters).

With more and more Police/Fire going to P25, and vendors pushing simulcast systems as more frequency efficient than multicast - it would seem that if scanner vendors don't find a way to receive P25 simulcast reliably - there will be little value in having a scanner.

So I don't get it.
Can someone lighten me?
 

N4DES

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The reason why they do it is to increase the joint probability on the talk-out of the system. LIke a voted system, geographicially spaced transmitters increases the probability of the unit in the field hearing the system.

Actually simulcast and timing in digital is easier and more stable then its analog counterpart, unless you are out of the designed coverage area. An analog system experiences the low "growl" in specific over lap areas where a digital design doesn't have this ( I operate a mixed-mode system and describing an actual situation).

Also using the GPS for timing is very stable along with a contolled (non-public) IP transport.
 

zerg901

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rak313 - can you talk a little about "equalization filters""? What would it cost to include in scanners? Are their intellectual property right issues?

IIRC Constra Costa County California had a design for a trunked system with maybe 12 multicast sites. They converted the design to simulcast, and dropped the number of required freqs in half.
 

jaspence

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P25 simulcast

While I applaud Uniden for their efforts and support, the firmware update did little to change the simulcast situation for me in Washtenaw County. My PSR 800 still gives me the best overall reception day to day.
 

jackj

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What ARE "equalization filters"?

Cell sites are designed so that they have minimal overlapping coverage. Their antennas have both horizontal and vertical patterns that severely limit potential overlap.

Simulcast systems are used to reduce the cost of the license both in terms of dollars and spectrum.
 

rak313

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wow! I just joined this group - and that was my 1st post. Thanks for all the quick responses.

1st - I am not an expert - that is why I posted here - to get some incite.

2nd - 3 days ago - I had never heard of P25. This last Friday night- a police helicopter was overhead - so I turned on my (analog) scanner - and proceeded to hear nothing. It had been 2-3 years since I last had it on - and after some reading I found out my county had gone to the P25 simulcast system.

On the forum on this site i found that local volunteer firefighters were complaining that they had purchased consumer P25 scanners for their home but they would not reliably decode it. So what I really want to know is why cant consumer scanners reliably pick this up - while commercial ones can.

Yes i know the commercial ones cost a lot. But that is (largely) because the are made like tanks - and have a captive market - and must go through large amounts of red tape to design/sell/maintain a system. What I want to know is - what is the technical reason that makes one radio work and the other not?

3) Realize - my knowledge is limited here - an equalization filter restores the channel response reducing distortions in the channel. It is commonly used to reduce multipath

In this case - a (digital) tapped delay line would be used. This implementation is called an finite impulse response (FIR) filter. It has variable tap weights. The input to this filter is the I/Q channels. The I/Q filter outputs are the weighted sum of several delayed versions of the input.

To see how this can work - imagine the signal has one multipath reflection 10 microseconds after the original signal - that is 10% the amplitude. The FIR filter would have a weight of 1 for the tap that corresponds to the input signal - and a tap of -0.1 (10%) at a delay length of 10 microseconds. The output is the sum of both the 2 taps. The 1st tap returns the desired signal - with the reflection. The 2nd tap is a copy of the input - delayed by 10 microseconds - and subtracts (most of) the reflection. It indroduces a reflection of its own (which is only 1% of the original and can be ignored) .

Now the multipath is constantly changing and has many more than 1 reflection, - so how are the taps determined?

Fixed knowledge of the transmitted signal is used to dynamically figure out how to determine the correct tap settings. In cell phone systems a set of the bits are fixed - and convey no information other than how the channel is being distorted - and are used to determine the tap settings. I believe there are similar fixed bits used in P25 - but i'm not sure.

Equalisation

I do not know the architecture of a consumer scanner. So i do not know how difficult it would be to do this. If they use an A/D at the last IF, followed by a DSP - then it should be pretty easy. When I say easy - I don't mean easy to understand - its a very complicated algorithm - i surely don't fully understand it. I mean its "within the power of the hardware".

And for sure -in an era where you can patent a "swipe of a finger movement" - any algorithm involving communication will be covered by someone's patent. Why do you think google bought Motorola? It was not for their thriving smart phone business - IMHO it was to counter sue anyone who sues them over any patent.
 

pdfdems286

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let us not forget. us "unprofessional scanner user's" don't use the same equipment that the end user of one of these systems use's. ie, a commercial grade radio. the trade off would be the cost of building a scanner to professional specs.
 

rak313

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let us not forget. us "unprofessional scanner user's" don't use the same equipment that the end user of one of these systems use's. ie, a commercial grade radio. the trade off would be the cost of building a scanner to professional specs.

I expect scanner companies will fix this soon - followed shortly by 1st responder digital systems turning on encryption.
 

n5ims

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let us not forget. us "unprofessional scanner user's" don't use the same equipment that the end user of one of these systems use's. ie, a commercial grade radio. the trade off would be the cost of building a scanner to professional specs.
Don't forget that the "professional" radios have a very limited frequency range that they cover while a scanner has a very wide frequency range coverage. It's fairly easy to tune circuits for a narrow range, perhaps even adding circuits for several ranges if necessary, if cost isn't a real factor. It's quite difficult and very expensive to have circuits work over a very wide range anywhere near as well as they do over a narrow range of frequencies.

This is probably the #1 reason that "professional" radios work better (over the small range of frequencies they're designed for anyway) than a scanner does. Folks are happy to spend quite a chunk of change for a "professional" radio that only covers a few MHz (think 20 - 40 MHz) while they'd never think of spending even half that amount for a simple scanner with that small of frequency coverage, insisting their scanner cover several hundred MHz instead.
 

rak313

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Don't forget that the "professional" radios have a very limited frequency range that they cover while a scanner has a very wide frequency range coverage. It's fairly easy to tune circuits for a narrow range, perhaps even adding circuits for several ranges if necessary, if cost isn't a real factor. It's quite difficult and very expensive to have circuits work over a very wide range anywhere near as well as they do over a narrow range of frequencies.

This is probably the #1 reason that "professional" radios work better (over the small range of frequencies they're designed for anyway) than a scanner does. Folks are happy to spend quite a chunk of change for a "professional" radio that only covers a few MHz (think 20 - 40 MHz) while they'd never think of spending even half that amount for a simple scanner with that small of frequency coverage, insisting their scanner cover several hundred MHz instead.
I dont think frequency range has anything to do with the simulcast problem. I think its related to signal processing. Maybe the scanners skimp on some of the forward error correction, perhaps the pro radio has adaptive equalization - who knows.

Realize that many have reported that they pick up non-simulcast P25 with no issue, but can't copy P25 simulcast.

Simulcast systems transmit the exact same signal - from multiple transmitters - on the same frequency. How anyone considered this a good thing is beyond me - but the designers do (they make claims of how it lowers the number of transmitter sites needed - and lowers the number of frequencies needed).

Regardless of how good your radio's selectivity is - if the multiple transmitters are on the same frequency and within range - you will receive the multiple signals (and IMHO they will act like an interference pattern - add in some places - subtract in others - depending of the difference in path length between the sites). It's still a mystery to me why the scanner doesn't work as good as the police radio for receiving P25 simulcast.

I'm hoping that someone will "fix" this in a SDR using a PC as the processor and one of the RF USB dongles - as I believe proper signal processing is all that is needed. But (from the youtubes I've seen) as of yet - it seems the SDR approach to P25 decode has yet to yield results that are as good as a scanner.
 

rak313

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Everything I've read in the past 24 hrs suggests that the strongest transmitter in the simulcast systems will capture the receiver in most of the coverage area (and work as well as a single transmitter). In the overlap areas - where multiple transmitters are received within 10-15 db of each other - there will be a delay spread between the multiple received signals.

As long as the delay spread is small (e.g < 20 microseconds - note: the bit time is 208 microsececonds) the BER should be less than 1% and Delivered Audio Quality (DAQ) of 3.4 = "Speech understandable with repetition only rarely required. Some noise/distortion."

I saw a couple of articles on how to lower BER for simulcast systems- but they were concentrated on the transmit side - none on receive. One was a patent on changing the transmit waveforms slightly - to allow detection with longer spread delays (it kept the phase constant for more of the bit time), Another modifies the transmit times (adds slight offset times to the transmitters) such that delay spreads in overlap areas are minimized.

All of these things point to the fact that making a simulcast system work is not simple - but nothing I have found explains why a commercial radio works but a consumer scanner doesn't.

I have seen some posts here suggesting that these simulcast systems are sloppily set up - and that is the reason for poor scanner performance. I don't believe this. I came across a site acceptance test plan for a Motorola system in Florida that broke the 1000 sq mile coverage area into a grid of about 3600 ft by 3600 ft. In order to be sold off it required a DAQ of 3.4 for >95% of the coverage area. So they have to prove they have coverage. I believe this is acceptance testing is typical of critical police/fire/safety systems.

I haven't found any receiver related article on lowering BER due to delay spread in simulcast systems (not that they don't exist - and I will continue to look).

BTW - BER does increase with delay spread - and delay spread is worse with multiple transmitters vs single transmitter (which would be caused by multipath) - so it is harder to receive/decode digital simulcast.
 

W6KRU

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While I applaud Uniden for their efforts and support, the firmware update did little to change the simulcast situation for me in Washtenaw County. My PSR 800 still gives me the best overall reception day to day.
I agree 100% except for me it's a PSR-500 that sets the benchmark.
 

rak313

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In various places, (even in the FAQs here) I have seen reference to interference in simulcast P25 systems caused by interference from multiple transmitters on the same frequency.

The reference below states that the multiple transmitters (on the same frequency) are a feature not a liability - that they are synchronized - and that the combined transmiter's signal allows for fewer transmitter sites (see slides 18-19).
....
Can someone lighten me?

I left out the link - here it is

http://www.simulcastsolutions.com/userfiles/file/simulcastforums/motorola_simulcast_part4.pdf
 

radioalbany

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More on multipath

I have terrible multipath issues with the Contra Costa (CA) EBRCS P25 system. Only part of the issue has to do with simulcast. Even if you're only receiving one site, you can still be receiving both direct and multiple signals reflected from nearby hills. In some places, the best signal may be one of the reflections, not the direct one.

I'm new to the P25 scanner world (996XT), but I'm thinking that a directional receive antenna is the only thing that will deal with that issue, since it will tend to reject signals from other directions. Wilson has an inexpensive ($50) antenna that claims 10 dB gain over 700-2000 mHz or so:

Wide Band Directional Antenna | Store | Wilson Electronics

It's an impressive claim, even for a log-periodic design (which I think it is). I've also looked into narrowband yagis just for the 740 mHz band, but not much luck so far.

Regards,

Andrew Ellis NO6E
Lafayette, CA
 

jsncrso

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The DSPs in scanners are far below the quality of the ones you fine in $4,000 radios. But the biggest thing is that a 2-way radio can communicate back with the tower and adjust its signal parameters as needed. Even still, simulcast can cause issues.

I scan a very difficult P25 simulcast system (so bad, the county is actually putting up a multicast backup system in place...yes, the county radios on the islands have issues sometimes as well due to refraction and inversion layers in the atmosphere common near water). My county, which is mostly water, has 6 tower sites. I use a yagi antenna (the cheap $50 one form ScannerMaster) and aim it at the closest tower (which also happens to be the most geographically separate from the other towers around me) and turn my scanners built in attenuation on. I get 100% decoding on my BCD996XT. So essentially, it all boils down to effectively attenuating the signals from the other towers. Once you do that, you are good to go.
 

Mike_G_D

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rak313 - excellent posts and info! To my knowledge, you are the only other person besides myself to have brought up the equalization processing needed to handle co-channel multiphase distortion. My problem is that I am too hardware-centric and seem to have a bad brain damaged area when it comes to software understanding and especially DSP. I used to work in cellular TDMA subscriber unit design and testing and worked with some really brilliant DSP gurus. One, in particular, used to work almost exclusively on the section of the receiver devoted to exactly what you are describing - the equaliser. As I recall it was literally a whole DSP engine devoted to that one function and he worked on the internal DSP code to perform that function. It was not considered "error correction" as that was another set of processing further down the chain. That is one of my "pet peeves" regarding how this is described in the hobbyist literature - it is only very broadly described as part of the "error correction" processing; it's really a totally separate issue, as you describe. Also, you are correct that the spectrum range of the RF section of the receiver is not really relevant to the multiphase distortion equalization issue - that is taken care of well after the final IF downconversion so has little baring on the RF bandwidth issues (at least as far as phase distortion issues arising externally to the scanner internals are concerned).

I also think that this is an area that typical consumer scanners simply either omit altogether or barely address compared to the actual professional equipment. They do this in the interest of keeping the costs as low as possible and still meet the basic market acceptance - despite all of those complaining about simulcast reception I think that, at least up until recently, it seems like a relatively small number to the purse string controlling bean counters within a small niche market of scanner users and there is simply not enough competition to qualify spending extra design and testing effort in building this in their consumer scanners. It's kind of the same reason that the RF performance is so sub-par on these scanners - it's far easier and cheaper to add flashy digital features like extra memory, display features, PC software extras, etc. Those things also sell more scanners to the less technically inclined users while seemingly esoteric things like "improved internal co-channel multiphase distortion handling at the IF level" yield no marketing value.

I think this might be changing, however - albeit slowly. As more of these systems come online more and more non-technical users (from the point of view of the bean counters and market analysts the most important segment) start to complain the more actual design effort may be devoted to dealing with this problem on future products. The recent Uniden firmware update that actually attempts to address this issue, in my mind, is very significant in that it means that the top US consumer scanner maker is actually both acknowledging the issue AND, for the first time as far as I know, actually admitting to working on a solution! To me, that is extremely significant. Regardless of how well it actually works, at least we know they are admitting to working on the problem rather than simply staying silent on the issue. If GRE can revive itself and stay in the game they now have true competition in this regard so the snowball can get moving; I'm unfortunately unsure of how well GRE is actually doing now in their scanner segment so I can only hope they do stay in the game and provide much needed competition - only one player, no matter how good they are, will simply not move anywhere near as fast as when they have other players to match and/or outrun. It really is too bad we don't have a third major scanner company that caters to the catch-all digital trunking scanner market.

As to why the system designers actually do this I think you've hit all the relevant reasons - greater sustainable coverage areas for subscribers within a given set of frequencies relative to multicast solutions. You would have to hear from real system designers and planners (and many do frequent these forums, especially in the industry/professional threads - it might be good for you to ask in that area) to get more detailed and better answers to that question.

-Mike
 

petnrdx

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Don't anybody take this the wrong way, because it is meant to be a compliment.
This is a surprisingly good / interesting thread.
I will add a couple things.
We build simulcast systems because if we didn't, we would not get the desired (needed) coverage, and would not be able to license enough frequencies to multicast to get similar coverage.
And the actual end user does not want to have to change "channels" as they move throughout a jurisdiction.
The expectation of users is MUCH higher than in previous decades, and with earlier technologies.
Simulcasting from a few to several sites that are LOWER than historically used allows a set of frequencies to be "re-used" at much closer intervals.
And allows the use of lower power repeaters and, even more importantly lower power subscriber radios.
The desired result is significantly improved sharing of the limited resource of radio spectrum.
I have significant experience working with simulcast repeaters.
They are a very good way to go.
Takes a bit of care setting up and maintaining them. but overall a very good "technology".
I too wonder why there appears to be such a discrepancy between one radio and another when listening to simulcast be it digital or analog.
A couple posts above seem to sum it up. Cost.
The earlier P25 firmware in Part 90 radios was pretty poor.
Newer P25 radios ( firmware ) have made significant improvement.
My earlier P25 radios were little better than scanner quality.
Now they are WAY better.
I hope the scanner makers can have a good business reason to make the same improvements.
 
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