P25 simulcast multipath interference

[Mike_G_D]

Excellent post, petnrdx, thank you for the info! Straight from the horse's mouth, as it were!

-Mike

 

[tech020]

Some observations on Simulcast

Having worked for 10years on an early 11 site, 24 channel Simulcast system by Motorola, I offer these tidbits: Simulcast for the most part works fine in dense urban areas by allowing multiple path possibilities back to a system site. Remember transmit sites are also receive sites, so the best signal is processed for simulcast transmit. All sites are time synchronized with GPS backed up with Rubidium time references. Because of spacing of sites as close as 1-2 miles, power levels per channel are closely controlled. Only about 20 watts per channel reaches the antenna due to combiner losses. Multipath did not seem to be a problem. Also, Moto uses LSM (Linear Simulcast Modulation) on Transmitters in simulcast systems. There is a setting in receivers that corresponds. LSM is I believe a proprietary Moto modifications to the C4FM quadrature modulation to enhance the performance. This is probably lacking in scanner DSP because of licensing issues. Having left the position, I now have to use a scanner like everybody else.

 

[KC0CSE]

Having worked for 10years on an early 11 site, 24 channel Simulcast system by Motorola, I offer these tidbits: Simulcast for the most part works fine in dense urban areas by allowing multiple path possibilities back to a system site. Remember transmit sites are also receive sites, so the best signal is processed for simulcast transmit. All sites are time synchronized with GPS backed up with Rubidium time references. Because of spacing of sites as close as 1-2 miles, power levels per channel are closely controlled. Only about 20 watts per channel reaches the antenna due to combiner losses. Multipath did not seem to be a problem. Also, Moto uses LSM (Linear Simulcast Modulation) on Transmitters in simulcast systems. There is a setting in receivers that corresponds. LSM is I believe a proprietary Moto modifications to the C4FM quadrature modulation to enhance the performance. This is probably lacking in scanner DSP because of licensing issues. Having left the position, I now have to use a scanner like everybody else.

start making scanners.....I will buy one...I'm sure I will not be the only one!!

 

[petnrdx]

Interesting discussion.
I have been holding off buying a digital scanner, choosing to monitor with my XTS radios( one for each band ) and / or test equipment.
But it is REALLY hard to tote any P25 test equipment around or four to five handhelds.
So far, what I am reading and hearing is that the scanners, while convenient, lack a bit of RX capability.
I think we all hope that scanners get better BEFORE everything gets encrypted.
Wouldn't it be nice to have a decent RX, that scanned EDACS, P25, NXDN, SMARTNET, on all bands?
But keep the price to maybe $10,000.
Always interesting reading.
Often have to separate the "wheat from the chaff" but no argument from me that there are REALLY knowledgeable people on these forums.
Makes it fun.

 

[Anderegg]

What we need is modified scanner brains that will fit in the case of an old Astro Saber III.......that would make a pretty cool scanner.....assuming we could get Moto quality receiver pieces in there.

I think the Saber case design is the most elegant every produced.....the slim case fits in a back pocket perfectly, and it is very easy to hold.

Paul

 

[Mike_G_D]

Interesting discussion.
I have been holding off buying a digital scanner, choosing to monitor with my XTS radios( one for each band ) and / or test equipment.
But it is REALLY hard to tote any P25 test equipment around or four to five handhelds.
So far, what I am reading and hearing is that the scanners, while convenient, lack a bit of RX capability.
I think we all hope that scanners get better BEFORE everything gets encrypted.
Wouldn't it be nice to have a decent RX, that scanned EDACS, P25, NXDN, SMARTNET, on all bands?
But keep the price to maybe $10,000.
Always interesting reading.
Often have to separate the "wheat from the chaff" but no argument from me that there are REALLY knowledgeable people on these forums.
Makes it fun.

Heh...yeah I'm guessing that many reading that post are stunned into near catatonia after seeing your desired $10K goal!;-). Having worked with many a six figure costing piece of lab equipment, though, I can appreciate what your saying! Still, I rather think that something with the overall feature capability and signal flexibility of what the current scanner consumer is used to getting at around $500 can be made with truly decent RF and IF performance, relatively speaking, for less than $3K and with some tradeoffs maybe even at or around $1K. I know that is still too high for most on here and that is why it is not likely to happen, at least for the common consumer target market segment. Targeting "prosumers" or special use professionals might work but I don't see either Uniden or GRE getting into that area.

If I had the resources and backing I would dive in with gusto to design and prototype a decent scanning receiver beast with decent RF performance! One way to bring the cost down would be to limit the frequency range to, say, 108MHz to 960MHz rather than to try and keep low band and 1.2GHz and above included. DC-to-daylight with high quality and low cost is ridiculous. Most LMR narrowband stuff nowadays is in the 150MHz to 960MHz range and low band could easily be accommodated with what is already out there or even, if really needed, an external upconverter. I keep toying with a modular approach with some form of changeable RF front end modules allowing the user to tailor the unit to a particular range of frequencies and to varying expected RF environments. A "basic" front end would suffice for low RF density rural areas while more robust high dynamic range modules could be made available at higher cost for usage in high RF density environments. The IF section could be common to all configurations with particular attention paid to using quality components and design. At least two (the more the better but good RF quality low noise and fast switching will need to be addressed) good quality low group delay and nicely symmetrical IF bandpass filters would be available to accommodate differing IF bandwidths needed by differing modulation modes and adjacent channel densities. Decent quality VCTCXO's capable of being externally coupled to precision references when needed either for calibration or for continuous use in special cases would be used. For basic "quick and dirty" in the field frequency calibration adjustments some form of on board software could provide the user with advanced calibration functions allowing the radio to use a user chosen "known good signal" as a reference to sync to - something like a continuous broadcasting NOAA station or a well kept trunking control channel. That would provide at least basic frequency calibration good enough for typical hobbyist use and not require returning the unit to the factory for retuning (for typical oscillator drift). I'd use, if at all possible price-wise, low noise oscillators and PLL designs (problem of course is fast frequency switching versus phase noise issue). I'd provide a robust demodulator section with both basic FM discriminator and hardware IQ demodulator available the choice of which regulated by software algorithms which could be user modified and upgradeable via external PC software. The output of the demod section could be diverted to external ports for using PC software demodulation and decoding software if desired. The final IF before the demodulators could also be made externally available for similar reasons and more flexibility. The external couplings would be properly buffered and isolated from the internal signal paths to prevent destructive loading and other coupling effects. Finally, the A/D converter would be high quality and followed by a nice DSP engine (or two) with user upgradeable open source firmware. All of this costs, though, and then there is the need for good inter-stage shielding and robust and well designed coupling (a real problem when considering user changeable modules at UHF and above frequencies). Shielding of receiver sections is another area usually compromised in low cost consumer gear - that metal is not cheap nor is the cost of properly placing and soldering it on-board. And that's just the RF/IF/Demod stuff - there's still the digital micro-controller and audio baseband section.

Anyway I know I got off topic and it's all pipe dream stuff - you just got me going with that $10K pro-scanner desire;-)!

-Mike

 

[Anderegg]

I think that it would be awsome to see like an XTS2500 radio, with all the TX stuff removed, and replaced with additional RX components covering the public safety spectrum, say 136-968 MHz. I would be a customer at the current list price of the XTS2500, and I know they would have a long line of other customers as well!

Paul

 

[bfperez]

I can't imagine it is, but have to ask: Is this simulcast issue unique to P25 systems, or does it affect other systems with digital voice?

I monitor a Motorola system (SNACC-Clark County NV) primarily via the NLV Simulcast site on a BCD396XT and the analog comes through very clear, but the few digital TGs are choppy or simply don't have any audio even with full signal strength registered. If I cup my hand around the antenna or switch it something like a SRH805S, I can get it to output audio much more readily. My understanding of the locations of the Simulcast site locations suggests this happens when I'm somewhat equidistant to a few sites at once.

 

[Mike_G_D]

I can't imagine it is, but have to ask: Is this simulcast issue unique to P25 systems, or does it affect other systems with digital voice?

I monitor a Motorola system (SNACC-Clark County NV) primarily via the NLV Simulcast site on a BCD396XT and the analog comes through very clear, but the few digital TGs are choppy or simply don't have any audio even with full signal strength registered. If I cup my hand around the antenna or switch it something like a SRH805S, I can get it to output audio much more readily. My understanding of the locations of the Simulcast site locations suggests this happens when I'm somewhat equidistant to a few sites at once.

Simulcast system topology could conceivably be applied to pretty much any system with any modulation mode. The same issues would likely surface on any of them - it's just a form of phase distortion similar to multipath but from intended sources. The more your modulation scheme depends on phase changes to contain and transmit information the more affected by phase distortion such signals potentially become. With analog modulation schemes you simply get audio phase distortion artifacts but digital signals may have serious phase errors in their I/Q constellation causing severe bit errors if the phases are not properly equalized.

-Mike

 

[Mike_G_D]

I think that it would be awsome to see like an XTS2500 radio, with all the TX stuff removed, and replaced with additional RX components covering the public safety spectrum, say 136-968 MHz. I would be a customer at the current list price of the XTS2500, and I know they would have a long line of other customers as well!

Paul

The problem, I think, is that the Motorolas/Harris's/Tait's/etc. of the world are not terribly interested in building receive only devices compatible with their digital LMR equipment even for relatively high paying customers like you. Even if they were to do it, I gotta hunch the prices would be higher than you may expect. Also, I doubt you will see Motorola LMR receive only units with NXDN, EDACS, and LTR capability built in nor would you see Harris receivers with Motorola 3600 baud trunk tracking capability; in other words, even if the major LMR manufacturers were to manufacture and market broad band receive only professional grade gear they would likely limit signal demodulation and trunk tracking capabilities to only their respective standard LMR transceiver line supported modes.

On the flip side, the top consumer scanner manufactures, currently, as far as broad coverage trunking capable general purpose scanners are concerned, that would be GRE and Uniden with GRE in an iffy state currently, are not terribly interested in building expensive high quality gear that very few would buy at the prices they would want to sell such gear at and still make a profit.

But, don't get me wrong, I'd like to see such animals on the market too - in the under $3K range, though.

For now, the closest approximations are probably the advanced SDR options that are now available and upcoming. Not too portable yet, though.

-Mike

 

[rak313]

... One way to bring the cost down would be to limit the frequency range to, say, 108MHz to 960MHz rather than to try and keep low band and 1.2GHz and above included. DC-to-daylight with high quality and low cost is ridiculous. ...

... good quality low group delay and nicely symmetrical IF bandpass filters would be available to accommodate differing IF bandwidths needed by differing modulation modes and adjacent channel densities. ...

... I'd provide a robust demodulator section with both basic FM discriminator and hardware IQ demodulator ...

...Finally, the A/D converter would be high quality and followed by a nice DSP engine (or two) with user upgradeable open source firmware. ...


-Mike

Mike ,

I believe the technology to build a high performance, inexpensive P25 scanner (with cost similar to an analog scanner) exists - but may never happen due to the size of the market.

While not exactly the same thing - take a look at this $20 digital TV tuner/decoder.

rtl-sdr and GNU Radio w/Realtek RTL2832U and Elonics E4000

It as a RF tuner chip that goes from 64 MHz to 1.7 GHz using "zero IF" with I/Q outputs that interface to the demodulator chip. The on chip "IF filter" becomes a low pass filter (which serves as an anti-aliasing filter for the off chip A/Ds). The channel shaping - narrow band - filter, and FM discriminator are done (after A/D conversion) in the DSP - eliminating the need for expensive linear phase IF filters.

http://www.superkuh.com/gnuradio/Elonics-E4000-Low-Power-CMOS-Multi-Band-Tunner-Datasheet.pdf

The demodulator chip has I/Q baseband A/Ds (only 8 bits each - but that is plenty good enough) and a DSP. The DSP is used to demodulate/decode the baseband digital TV bitstream.

This demodulator chip has nothing to do with P25 - but it illustrates the processing power that can be put into a low cost chip.


For P25 simulcast multi-transmitter interference - an adaptive equalizer is all that should be needed to make reduce the interference, as this is similar to multipath interference. Adaptive equalization to counteract multipath is not a new concept.

http://wsl.stanford.edu/~ee359/proakis.pdf

What is (relatively) new are low cost chips that have enough processing power to implement it.


Adaptive equalization is used in (2G) cellular (GSM) phones - which must receive a 270 kbit/sec data stream - encoded using GMSK on a 200 kHz channel. While there is no simulcast - there is plenty of multipath to make reception problematic. http://www.radioeng.cz/fulltexts/1999/99_04_06.pdf

Take a look at this 2003 chip set announcement from TI:

Texas Instruments developes UMTS chipset - Telecompaper

from that announcement:

"... A two-chip radio frequency (RF) transceiver subsystem enables the dual-mode GSM/GPRS and WCDMA capabilities of the TCS4105 chipset. Both the GSM/GPRS transceiver, the TRF6151, and the WCDMA transceiver, the TRF6301, feature advanced direct conversion (DC) technology and integrate several external devices such as VCOs, LNAs, PLL loop filters, and synthesizers. ..."

The block diagram of the TI RF tuner portion is very similar to the E4000 tuner from the $20 TV tuner above.

 

[rak313]

... Also, Moto uses LSM (Linear Simulcast Modulation) on Transmitters in simulcast systems. There is a setting in receivers that corresponds. LSM is I believe a proprietary Moto modifications to the C4FM quadrature modulation to enhance the performance. This is probably lacking in scanner DSP because of licensing issues. ...

tech020,

This agrees with what you suggest: The Digital Diversity of APCO-25 from it:

" To add to your list of acronyms, there is another type of modulation scheme that's used with some multi-site Project 25 systems. Linear Simulcast Modulation (LSM) is a trademarked term for a form of CQPSK that provides a way for receivers to properly handle multiple identical transmissions. It's just different enough that the regular C4FM processing doesn't work correctly. "

Now how does that jive with the Project 25 "interoperability" goal?

 

[jsncrso]

Adaptive equalization is used in (2G) cellular (GSM) phones - which must receive a 270 kbit/sec data stream - encoded using GMSK on a 200 kHz channel. While there is no simulcast - there is plenty of multipath to make reception problematic. http://www.radioeng.cz/fulltexts/1999/99_04_06.pdf

You have to remember that cell phone communications frequency hop thousands of times per second over a block of frequencies whereas P25 simulcast systems use only frequency. Also remember that cell phones are 2-way devices that are able to adjust signal propagation with the tower instantly whereas scanners will always be receive only.

Scanners are always going to have compromises and limitations over their 2-way counterparts.

 

[Mike_G_D]

You have to remember that cell phone communications frequency hop thousands of times per second over a block of frequencies whereas P25 simulcast systems use only frequency. Also remember that cell phones are 2-way devices that are able to adjust signal propagation with the tower instantly whereas scanners will always be receive only.

Scanners are always going to have compromises and limitations over their 2-way counterparts.

Not all cellular systems behave in the same manner. Although most of my knowledge of cellular systems is dated to pre-2000, I'm pretty sure that many still hold on one voice frequency while within range of a particular cell site. The system controllers monitor the quality and performs a handoff to the next cell site within range that appears to be getting a better signal - then the frequency changes. Depending on how fast the subscriber is going this can happen at quite a clip but usually not thousands of times a second. Systems that use a rapid frequency hopping spread spectrum methodology are probably what you are thinking of. I am not up to date on the latest cellular modes so some such systems could exist but I am not familiar with them. Also, trunked radio systems including P25 systems do indeed change frequencies while in operation either between each transmission or between each conversation in addition to site roaming changes. In any case, regarding adaptive equalizer systems partly or fully relying on real time communication with the affiliated site controller to perform the phase equalization - while that may be true in some cases, I don't think that's how the system I worked on back in the early to mid nineties (as rak313 stated, adaptive equalization's been around awhile) did things - I recall that it did it all in the subscriber receiver and not based on communication with the site controller though I am not completely sure about that memory-wise.

In any case, it can be done all in the receiver without communication with the site controller. Also, keep in mind that cell phones are one-to-one communications based systems (at least primarily) while LMR systems are one-to-many though one-to-one communications can be facilitated. The relevance here is that using the trunk site controller to adapt its signal to one subscriber could conceivably degrade things for others listening to the broadcast. An aggregate of subscriber input for a given area would have to be a compromise solution.

The point is, as rak313 and others have stated, adaptive equalization is a tried and true system for dealing with multiphase distortion in received radio signals and is perfectly doable in a purely one sided receive only system. Transceivers with system subscriber privileges certainly can augment this in their favor when possible but I think non-subscriber receivers could still certainly benefit from better attention given to this aspect of the receiver IF/demodulator output processing. I still think that this is a part of the receiver signal processing which has been either treated very lightly or completely omitted in consumer scanners and is a large part of the reason why so much issue with simulcast systems is experienced using these types of receivers.

-Mike

 

[jsncrso]

Not all cellular systems behave in the same manner. Although most of my knowledge of cellular systems is dated to pre-2000, I'm pretty sure that many still hold on one voice frequency while within range of a particular cell site. The system controllers monitor the quality and performs a handoff to the next cell site within range that appears to be getting a better signal - then the frequency changes. Depending on how fast the subscriber is going this can happen at quite a clip but usually not thousands of times a second. Systems that use a rapid frequency hopping spread spectrum methodology are probably what you are thinking of. I am not up to date on the latest cellular modes so some such systems could exist but I am not familiar with them. Also, trunked radio systems including P25 systems do indeed change frequencies while in operation either between each transmission or between each conversation in addition to site roaming changes. In any case, regarding adaptive equalizer systems partly or fully relying on real time communication with the affiliated site controller to perform the phase equalization - while that may be true in some cases, I don't think that's how the system I worked on back in the early to mid nineties (as rak313 stated, adaptive equalization's been around awhile) did things - I recall that it did it all in the subscriber receiver and not based on communication with the site controller though I am not completely sure about that memory-wise.

In any case, it can be done all in the receiver without communication with the site controller. Also, keep in mind that cell phones are one-to-one communications based systems (at least primarily) while LMR systems are one-to-many though one-to-one communications can be facilitated. The relevance here is that using the trunk site controller to adapt its signal to one subscriber could conceivably degrade things for others listening to the broadcast. An aggregate of subscriber input for a given area would have to be a compromise solution.

The point is, as rak313 and others have stated, adaptive equalization is a tried and true system for dealing with multiphase distortion in received radio signals and is perfectly doable in a purely one sided receive only system. Transceivers with system subscriber privileges certainly can augment this in their favor when possible but I think non-subscriber receivers could still certainly benefit from better attention given to this aspect of the receiver IF/demodulator output processing. I still think that this is a part of the receiver signal processing which has been either treated very lightly or completely omitted in consumer scanners and is a large part of the reason why so much issue with simulcast systems is experienced using these types of receivers.

-Mike

A lot has changed since the good ol' AMPS days. If each phone only used one frequency, you essentially wouldn't have cell service. Frequency hopping is what allows several thousand phones to use just a handful of frequencies assigned to a cell site. It's what allows carriers to squeeze bandwidth out of the air and allow it to transmit around environmental noise. For example, the TDMA frame duration for a 2G GSM packet is 4.615 milliseconds. This means the phone can move between, transmit, receive, and monitor within that frame, up to 8 times, since each frame is divided into a burst period of just over half of a millisecond, (the "Time Division" part) all of which are on different frequencies. I could type about this more, but I really need to eat! Here is a really informative link about how GSM communications work: ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html


In contrast, a P25 simulcast system stays on one frequency the entire time the link is established between the radio and the tower....much different (at least on the radio link level) than cellular communications.

I will agree with you that multiphase distortion can technically be mostly filtered out with a DSP, the problem is the processing power to determine which bit is the 99%+ correct bit is out of the price range of consumers (and even professional radios as they also have repeated problems with simulcast distortion). This is always going to be a problem (at least for the foreseeable future), since there is no synchronized clock rate to compare the receipt of signals. (Technically is it possible to sync the data with GPS, but that's introducing to many problems for a given solution.)

 

[Mike_G_D]

jsncrso,

This is off topic so I'll try to be brief - I wasn't aware of GSM frequency hopping nature though the doecument you highlight does say a "slow frequency hopping rate" is used. In any case, I worked on TDMA DAMPS which was a good deal newer than GSM and designed for greater capacity within narrower RF channel bandwidths. To my knowledge, our system did not hop during a call unless handed off to another site. It had three voice time slots per RF channel (30KHz). At the time, GSM was not permitted in North America on the 800MHz cellular band. Our chief competition was Qualcomm's CDMA system - which, as everybody knows eventually beat TDMA DAMPS in North America and became the standard, at least for awhile. The 1800MHz PCS bands were new and just starting out. Then GSM was allowed on, I think, all of the bands - that came after I got out of that work.

Anyway, back on topic - don't wish to be contrary but I am going to have to disagree with your last paragraph concerning multiphase distortion handling in receivers. I still maintain it can be done at costs that are not too exorbitant with current technology. But it will certainly add cost - I just think the benefit will justify that cost as more and more simulcast systems are employed. Also, simulcast networks do use an accurate time base to synchronize their co-channel transmitted signals as others have pointed out in this thread. Tech020 indicated GPS with rubidium backups were used which jives with what I have heard from others and logic in general. It is unclear to me but maybe you were actually referring to the subscriber and non-subscriber receivers getting direct time sync data. True, getting that from an external signal (a GPS signal for example) would be problematic but there are other ways to derive timing from transmitted digital signals as well as methods to derive phase distortion and compensate for it. I am not an expert on this by a long shot but I know it can be done as I've worked with those who are experts and have implemented it in very low cost cellular phone units. Obviously, true subscriber radios in simulcast systems do handle it well enough to make it work reliably. I will admit that they can transmit and could dynamically alter their equalizer settings with two way communication with the site controller but I am very curious if that is the cause of the lion's share of their effectiveness in this respect? Tech020 and petnrdx - can either of you chime in here and enlighten us?

-Mike

 

[beachmark]

I hope you guys don't mind me jumping in here but I have been working with the land mobile delay stuff for almost 20 years in both celllular and PS. I'll start with a few tidbits on cellular:

Equalizers were first developed for GSM in the early 1990's to combat geographic multipath of the same signal; i.e., a direct path signal and a delayed version of that signal bouncing off of the Alps, for example. With a 5+ usec symbol period, it only tok a 1/2 mile or less of relative delay to cause issues. The GSM equalizers would combat up to 16 usec (3 symbol periods) of multipath delay in the lab and maybe 12 usec in field conditions.

TDMA in the US followed and needed equalizers for the same reason. The tolerable overlap delay was around 40 usec as I recall, or a bit over one symbol period.

No cellular system technology has ever simulcasted the same signal on the same frequency from multiple sites. In the cell world for AMPS, TDMA, and G1 GSM, the control channels and voice channels were all on different frequencies. So no cellular system ever needed or used equalizers to fight simulcast delays from different sites.

And all cell sites overlap with other cell sites so some overlap was designed in; without it you dropped the call between sites. So the idea that this is not a cell issue due to limited overlap is not true.

(And we can talk about CDMA and UMTS and why they DON'T need equalizers but I'll not start that now.)

And, yes G2 GSM went to frequency hopping as stated. This was done because all the fixed frequency plans per site were becoming somewhere between he** and impossilbe to manage and implement when the site densities got high in the 2000 time frame, and with fast hopping, the occasional time slot collisions between different calls are corrected by link error correction and the vocoders. TDMA was phased out long before they ever got that far.

Now for PS simulcast. For older analog systems the simulcast voice channel growl referred to is caused by small carrier frequency tolerance mismatches between overlapping site radios. This is due to how FM demodulators work and gets louder in the lower frequency ranges, in proportion to 1/f. Think of it as a beat note.

For the analog FM simulcast control channels, there were never any equalizers in the equipment. This problem was managed by making sure that the overlap delay between sites never exceeded 35-40% of the modulation symbol time. The older EDACS for example had a symbol time of apporx 100 usec, and the system site designs and time adjustment between sites was limied to 35 usec and maybe 40 usec of relative overlap delay between adjacent sites in a pinch. With this limited overlap delay, >60% of the symbol time was undistorted and the detection/decision process was reliable.

And PS simulcast sites have been HIGHLY time synchronized from the get-go. In the old days, there were precise time references in each site and landline links between sites to keep the times locked together. GPS has replaced that, but the precise time sync between all sites in a system has always been there in PS simulcast. It is the very essence of making simulcast control channels work, and the setup of the sites was partly to set this time sync to minimize the overlap delay between all site adjacencies.

BTW, This works great in places like FL, IL, and KS< but has some issues in the low mountain areas like VA, as the signals often go where they are not expected; i.e., the simulcast zones overlap s get really messy in hilly areas, and you can't control all overlap zones to the time limit as described above.

With phase P25 1 control channels, the same basic thing is still being applied as far as I know: since the symbol times are around 200 usec on the control channels, an equalizer is not what is used to fight this. You just keep the overlap delays limited, so that the simullcast symbols never overlap enough to casue issues. And for Phase 2, all I can find says that the same tactic is being used; the symbol times are around 160 usec on both control channels and voice channels. I am in "intense P25 delay learn mode" these days, and have yet to run across a qualification test for P25 equipment that requires time overlap delay measurements on base or subscriber units that would indicate any actual equalizer is being used like in GSM or cell TDMA. But I am going to keep digging.

The key to using equalizers or not is how long are the modulation symbol times in the system. GSM is around 5 usec; cell TDMA symbol time was 20 usec if I recall right. The physics of the world mean you DO need equalizers just to manage multipath versions of the same non-simulcast signal bouncing off of mountains with short symbol times. With symbol times of 100-200 usec in PS systems, you don't need to worry about this, you only worry about simulcast-created 'multipath' of 2 separate signals with the same info. In that case, managing the relative overlap delay between simulcast signals to a maximum of 35-40% of symbol time is the trick.

Hope this helps.

Mark B.

 

[Mike_G_D]

Mark B.,

Many thanks for that well written and very informative post! This is what we need - accurate and current info from real folks with the relevant knowledge working in the field. So, from me, I say jump in all you want (and can stand ;-)!

Well, based on your information, then, it looks like I've been barking up the wrong tree as far as attributing scanner simulcast issues to an absence of an equalizer. Now I'm back to wondering more about the quality of the demodulator (discriminator and/or I/Q demodulator or ??) and other aspects of the IF and demod chain that are inherently weaker in scanner designs. Maybe extra jitter contributed by noisy (and less stable) oscillators and/or group delay issues in IF filters...the problem is, for me, aside from being out of practice too long, the bulk of my knowledge extends only up to the final IF with a fair amount in the demod hardware but once it all gets turned into bits it turns into the land of fairies, elves, and unicorns for me.

Anyway, welcome to the boards on RR! And please keep us informed as to what new info you find out in your studies regarding this issue as, at least some of us, are very interested. Don't worry about getting too technical - we'll ask when we're unclear. What you posted made sense based on what I know and understand and was quite clear.

Thanks again for the info!

-Mike

 

[rak313]

The key to using equalizers or not is how long are the modulation symbol times in the system. GSM is around 5 usec; cell TDMA symbol time was 20 usec if I recall right. The physics of the world mean you DO need equalizers just to manage multipath versions of the same non-simulcast signal bouncing off of mountains with short symbol times. With symbol times of 100-200 usec in PS systems, you don't need to worry about this, you only worry about simulcast-created 'multipath' of 2 separate signals with the same info. In that case, managing the relative overlap delay between simulcast signals to a maximum of 35-40% of symbol time is the trick.

Hope this helps.

Mark B.

Mark,

Everything you said in your description makes sense (about not needing an equalizer for 200 usec symbols)- so back my original question that started this thread.

What do you think is the reason consumer scanners work so poorly in simulcast P25 systems (yet some report no issues in non-simulcast P25)?

Thanks for the info.

Rick

 

[beachmark]

Well let me ask a few dumb questions since I really know nothing of the scanner issue, not being a scanner user.....
- Is it correct to assume that this is just on Phase 1 P25 systems? It is important to differentiate between phase 1 and 2 since the modulation type and symbol rate on the control channel downlink changes between phases 1 and 2.
- Is the problem such that the scanner does not catch that a call is being setup via messaging on the control channel, and therefore never goes over to a voice channel to monitor it?
- And just checking, but is this really happening only where there is simulcast or just in some places where users 'think' there is simulcast?
- Are there any details of locations where this is an issue? I can see someone having problems on a high elevation where the scanner sees 3-4 sites and is not in a primary overlap zone where the overlapping simulcast delays have been properly adjusted out. (But a real system radio should also have the issues at such a location.)
Any other references on this would be appreciated.