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Current generation digital scanners, even upgraded to the most recent firmware (where applicable), do a poor job of handling [[P25 CAI]] [[simulcast]] system transmissions. This problem is most evident when monitoring a true [[APCO Project 25]] Simulcast system that use the [[CQPSK]] (Motorola [[LSM]]/Harris WCQPSK) and [[H-DQPSK]] modulation scheme that is not FM modulation based.  This is most often noticed by a breaking up of a fairly strong transmission that one would normally expect to be crystal clear.  In this article, we'll describe and explain the most common reason for this sort of problem, and suggest some ways you might alleviate some of the simulcast distortion your receiver experiences.  
+
Current generation digital scanners, do a poor job of handling ''[[Simulcast]]'' Phase I & Phase II system transmissions. The problem is most evident when a scanner user is monitoring a true [[APCO Project 25]] Simulcast system that use the [[CQPSK]] and [[H-DQPSK]] (Motorola [[LSM]] / Harris WCQPSK) modulation scheme, which is not FM modulation based but is either phase and/or amplitude modulation based.  This is most often noticed when a strong transmission constantly breaks-up, especially when a scanner user is coming from monitoring ''Analog'' signals, where one would have an expectation - to receive the new digital voice to be crystal clear, throughout the transmission.  In this article, we'll describe and explain the most common reason for this sort of problem, and suggest some ways you might alleviate some of the simulcast distortion your scanner experiences, as well as additional solutions that maybe non-scanner based altogether.  
  
 
=Problem=
 
=Problem=
This problem is rooted in the hardware design of all scannersIn scanners, the received signal is the passed through a FM demodulator before being processed by the digital signal processor (DSP) where the digital information is recoveredThis design is cheap to implement, but also strips the critical components of an incoming simulcast signal that is needed to guarantee a successful decode. In other words, part of the signal information lost with no way to recover it.  The end result is the significantly degraded ability for scanners to successfully decode a simulcast signal.
+
Multipath reception describes the situation where reception of digital simultaneously broadcast signal from one or more transmit towers, i.e. over paths of different lengths, from a Site's Tower(s)Generally, single-tower multipath is not a problem; the FM capture effect will pick the strongest signal - as multipath reflections will be of lower power, or the wrong polarizationThough, on a Site's towers that are broadcasting simultaneously on the same frequency, (i.e. digital trunk signals voice and data) there are multiple transmitters on separate towers, with the power from more than one of them is strong enough for your scanner's receiver to pick-up 2 or more of the signals.  
  
==Multipath Misconception==
+
Often, when receiving multipath signals from an analog system you may hear just a bit of wavering in the signal via a receiver and has no trouble detecting the correct signal, let alone your ear/brain focuses on the correct/strongest sound if two or more are presentOlder antenna based analog TV's experienced this by ghosting. Though, shortwave signals suffer this and cause troubles in Single-sideband [[SSB]], due to the nature of the way the signal is handled. Those of you, who have experienced listening to multipath SSB are well aware of how tiresome it is to listen to.
The traditional explanation for simulcast distortion has often been blamed on multipathWhile this explanation is convenient and easier to understand, it is '''not''' the primary cause of simulcast distortion in scanners. Scanners can continue to exhibit symptoms of simulcast distortion even after multipath has been eliminated.
 
  
Multipath reception describes the situation where your receiver receives the signal from one or more transmit sites, i.e. over paths of different lengths. Generally, single-site multipath isn't a problem; the FM capture effect will pick the strongest signal, as multipath reflections will be of lower power, or the wrong polarization. But on Sites that are Simulcast trunking, there are multiple transmitters on separate towers, and the power from more than one of them is strong enough for your receiver to pick-up both up.   
+
The traditional explanation for scanners to have simulcast distortion, in a received digital trunk radio systems, has often been blamed on multipath and/or inter-symbol interference, which are actual and real phenomenon. For some though, they believe the explanation is just a convenient and easy excuse, it's truely '''not''' the only cause of simulcast distortion. Scanners can continue to exhibit symptoms of simulcast distortion even after multipath signals have been minimized. It is most likely because the scanner (receiver) is not using an IQ demodulation technique. Furthermore, as long as there is sufficient overlap in the bit positions, a receiver designed to utilize a IQ demodulator can extract a digitally "Simulcast" System's Site's data & audio in the face of multipath. System's that use "Simulcast" Site(s) i.e. Phase I and II, are specifically designed to synchronize their tower's tranmissions, for their intended coverage area and current users (that are logged-on). So, timing issues are mostly minimized, in real-time by subscriber's transceivers sending data back and forth to the controllers - outputting corrections. Which, helps to constantly maintain the best synchronous information for the subscriber's radios. Though, when the slip is more than one bit time, it can result: in a broken transmission i.e. inter-symbol interference. This generally, occurs on the fringe of an intended coverage area - of a tower(s), where portable use is discouraged and higher output of mobiles are expected/required to be usedIf multipath was a significant issue for actual APCO P25 Phase I and II System use, then a "Simulcast Systems" would not be acceptable for Public Safety use.
  
When receiving multipath signals from an analog system you may hear just a bit of wavering in the signal and your ear/brain has no trouble detecting the correct sounds coming from the speakerOlder antenna based analog TV's experienced this by ghosting. Shortwave signals suffer this and cause more trouble in single sideband [[SSB]] due to the nature of the way the signal is handled. Those of you who have experienced listening to multipath SSB are well aware of how tiresome it is to listen to.
+
=Solutions=
 +
==All Scanners ''Multipath'' ''Mitigation''==
 +
Until the above happens, the only thing a scanner user can do is to attempt to mitigate the problem, depending highly on an individual Users situationMany of the below Mitigating techniques are aimed at reducing the potential contributing factors of Multipath, to the receive and decode problems.  Some, mitigation techniques, could be:
 +
*Scanner Settings
 +
*Scanner Firmware
 +
*Location
 +
**Antenna
 +
**Scanner Location
 +
*Some mitigation techniques are going to be everything but the scanner.
  
The was believed that the signals come from separate towers and are not synchronized when they get to your scanner because the paths are different lengths. This is not correct. As long as there is sufficient overlap in the bit positions, properly designed receiver can extract the audio in the face of this multipath interference. Simulcast systems are specifically designed to synchronize the signal for the intended coverage area so that timing issues are minimized. When the slip is more than one bit time, it results in a broken transmission. This generally only occurs outside the intended coverage area.
+
The variables list may be large, but that shouldn't deter anyone from at least trying to "tweak" specific settings first..:
  
If multipath was a significant issue for actual system users, then simulcast systems would not be acceptable for most public safety users.
+
*Dwell or Hold Time increase, increasing the ability of the scanner to capture and decode all of a Systems voice transmissions & data parameters sent on the Control Channel.
  
=Mitigation=
+
*Limit the amount of Systems - Scanned, or just scan one Site without additional Conventional, Priorities, or Wx, additional Sites or Systems
The solution requires the manufacturers to redesign their hardware so that they can process the signal before the FM demodulation stage of the scanner.  Doing so would eliminate the loss of critical signal information required to successfully demodulate a simulcast signal and allow for scanners to perform much more closely to a commercial radio.  As this problem is the result of the physical hardware design within all scanners, there is little hope for a firmware solution.
 
  
Until the above happens, the only thing a scanner user can do is to attempt to mitigate the problem depending on the situation.  Many of these mitigations are aimed at reducing the potential contribution of multipath to the problem. Some, mitigation techniques, could be in the scanner settings or scanner type; some mitigation techniques are going to be everything but the scanner.
+
*Digital AGC if its ''ON'' try it ''OFF'', because it may be be making Voice transmissions less intelligible, especially if there are multiple errors already in the decode. While it can also, clip loud audio or over-drive quite audio.
  
The variables list may be large, but that shouldn't deter anyone from at least trying to "tweak" specific settings first..
+
*Keep your Firmware updated in your scanner.  Some users report that Simulcast is the cause of all their headaches, when in reality the scanner doesn't have a perfectly written algorithm to decode a particular type of System, Voice, or Band Plan, YET.  Sometimes it may seem that the most current version of Firmware takes a step backwards, but that's due to the fact that each system type is unique.  It may well be what worked better on system 'A' actually causes system 'B' users to notice a backward step.  For some, rolling back to a previous firmware in many models isn't difficult and should be done with recent problems corollary with the newest firmware, but be assured a System's control channel frequency hasn't changed.  Also, don't re-write your programming at the same time as a firmware upgrade because you are creating too many variables to objectively deduce where a new problem arose.
  
Keep your firmware updated in your scanner.  Some users report that simulcast is the cause of all their headaches, when in reality the scanner doesn't have a perfect algorithm written to decode that kind of System, Voice Type, or Band Plan yet.  Sometimes it may seem that the most current version of firmware takes a step backwards, but that's due to the fact that each system type is unique.  It may well be what worked better on system 'A' actually causes system 'B' users to notice a backward step. For some, rolling back to a previous firmware in many models isn't difficult and should be done with recent problems corollary with the newest firmware, but be assured a System's control channel frequency hasn't changed. Also, don't re-write your programming at the same time as a firmware upgrade because you are creating too many variables to objectively deduce where a new problem arose.
+
Remember only one setting or thing should be changed at a time, and always archive and save your beginning settings, whether that be in different folders, or on other media like an additional thumb drive, even snapping pictures or taking video or just on good ole scratch paper. Thanks and Goodluck and happy monitoring...
  
==All Scanners==
+
===Uniden===
 
*Opening Squelch to 0 for Digital Systems. See this post by UPMan: {{Thread|uniden-scanners|267424-loose-open-squelch-improves-lsm-simulcast-reception.html#post1975319|Loose/Open Squelch Improves LSM Simulcast Reception|}}
 
*Opening Squelch to 0 for Digital Systems. See this post by UPMan: {{Thread|uniden-scanners|267424-loose-open-squelch-improves-lsm-simulcast-reception.html#post1975319|Loose/Open Squelch Improves LSM Simulcast Reception|}}
  
*Dwell or Hold Time increase, increasing the ability of the scanner to capture and decode all of a Systems data parameters sent on the Control channel.
+
*P25 Threshold option can be changed after monitoring the Site with Auto to see the best setting for that System to be be decoded on
 
+
**So if, decode is best at 6 in Auto, switch the P25 Decode Threshold for that system to ''Manual'' and ''6''. While 6 isn't a panacea, it's at least a beginning numeric to try starting with, typically up-to 9 in Manual mode will make the biggest difference. Not all scanners allow this ability System/Site specific/individually.  
*Limit the amount of systems scanned, or scan just one site without additional conventional, priorities, or weather.
 
 
 
==Uniden==
 
*Digital AGC if its ''ON'' try it ''OFF'' because it may be be making Voice transmissions less intelligible, especially if there are multiple errors already in the decode.
 
 
 
*P25 Threshold option can be changed after monitoring the Site with Auto to see the best setting for that System to be be decoded on, so if decode is best at 6, switch the P25 decode threshold for that system to ''Manual'' and ''6''. While 6 isn't a panacea, it's at least a beginning numeric to try starting with, typically up-to 9 in Manual mode will make the biggest difference. Not all scanners allow this ability System/Site specific/individually.  
 
  
==GRE\RS\Whistler==
+
===GRE\RS\Whistler===
 
*DSP Level Adapt can be changed in some models and can vary the rate at which the DSP attempts to adjust varying P25 levels.
 
*DSP Level Adapt can be changed in some models and can vary the rate at which the DSP attempts to adjust varying P25 levels.
  
*ADC Gain and DAC Gain could be lowered to help reduce bit error rates. Typically a -2 and -4, respectively, have been used by some users to help the lower the issue. See [[GRE/RS/Whistler based DSP ADC/DAC Adjustments]] for more information.
+
*ADC Gain and DAC Gain could be lowered to help reduce bit error rates. Typically a -2 and -4, respectively, have been used by some users to help the lower the issues. See [[GRE/RS/Whistler based DSP ADC/DAC Adjustments]] for more information, as positive ADC setting can cause internal channel and component cross-talk.
 
   
 
   
==Antenna and Reception==
+
===Antenna and Reception===
This section attempts to address the issues that multipath might contribute to the problem.
+
This section attempts to address the issues that might contribute to the multipath problem.
 
+
*ATT - Attenuation of all the signals sometimes helps, typically the Attenuator in the most  
*ATT - Attenuator -Attenuation of all the signals sometimes helps, typically the Attenuator in the most scanners is via hardware at a standard of -20db.  This is of course due to the fact that if you attenuate all of the signals, your receiver possibly loses the ability to hear the interfering signal from multiple sources.
+
scanners is via hardware at a standard of -20db.  This is of course, due to the fact that if you attenuate all of the signals, your receiver possibly loses the ability to hear the interfering signal. Additional connectors can add low attentuation, as well as there are F-Connectors with fixed 3, 6, 12 decibel attenuators and many other varied attenuation techniques.
  
*In lieu of attenuation, remember that with Digital Signals: the *less* gain your antenna has, the better off you'll be; reducing the overall signal at the receiver input will (generally) increase the signal-to-noise ratio, so the "capture effect" will ameliorate the problem for you.
+
:*In lieu of Attenuation, remember that with Digital Signals: the *less* gain your antenna has, the better off you'll be; reducing the overall signal(s) at the receiver input will (generally) increase the signal-to-noise ratio, so the "capture effect" will ameliorate the problem for you.
 
   
 
   
 
*Attenuation of the secondary signal(s) causing interference via all of an Antenna's nulling properties:  
 
*Attenuation of the secondary signal(s) causing interference via all of an Antenna's nulling properties:  
**A yagi has 2 null spots 130-140 degrees from where it's being pointed.
+
**A Yagi has 2 null spots, typically 90 and 270 degrees from where it's being pointed. So off-center direction to a tower help negate a secondary tower at 40-75 degrees - beam width and attenuation can vary on the design
 
**A Omni-directional, also, has 2 null spots at its tip and bottom. If you have 3 towers and if Tower 1 and Tower 3 form a straight line, then you may be able to most reliably monitor Tower 2 with the antenna laid horizontally in the the line between Towers 1 and 3.
 
**A Omni-directional, also, has 2 null spots at its tip and bottom. If you have 3 towers and if Tower 1 and Tower 3 form a straight line, then you may be able to most reliably monitor Tower 2 with the antenna laid horizontally in the the line between Towers 1 and 3.
 
       2
 
       2
1<-Antenna->3
+
1<-Antenna->3
**Or adding a corner reflector - beam width and attenuation can vary on the design. Search for "cantenna."
+
**Or adding a corner reflector - beam width and attenuation can also, vary on the design. Search for "cantenna."
 +
 
 +
*A yagi antenna pointed at the Site you want to monitor.  If you are receiving all of the signal from only one site, there is no "multipath distortion" to deal with.  This, of course does infer, that there is NOT a second or third Tower in front or slightly offset to the left or right the primary tower you want to monitor (or even a 4th or 5 tower behind the aimed yagi antenna.
 +
***One user actually pointed his Yagi at a tower, with a second tower lying 4-5 miles behind the first. He could never get proper decode, but continued to insist, since a yagi was from one small path, he couldn't be getting multipath; while true, he was still getting multiple mistimed signals (images) causing complete decode failure in his radio.
 +
 
 +
*Scanner Location
 +
**Moving a scanner a few inches left or right and/or up or down can be a big factor, signals can have peaks and nulls, which creates areas of strong and weak signals, larger area's for lower frequencies and higher frequencies have smaller areas of peaks and nulls.
 +
 
 +
**Moving a scanner can also help reduce local interfering signals as many electronics in the house or auto can create interference that overwhelm the front-end or any receiver.
 +
 
 +
*Hairbrained solution get 'Joe' from the next county over to monitor your county and use your scanner to monitor Joe's county. You'll have to use a third-party solution to control and stream Joe's scanner and let Joe do the same with yours.
  
*A Yagi antenna pointed at the site you want to monitorIf you are receiving all of the signal from only one site, there is no "multipath distortion" to deal withThis, of course does infer, that there is NOT a second Tower behind the first Tower for a given Site. (One user actually pointed his Yagi at a tower with a second tower lying 4-5 miles behind the first. He could never get proper decode, but continued to insist, since a Yagi was from one small path, he couldn't be getting multipath; while true, he was still getting multiple signals (images) causing complete decode failure in his radio)
+
===Scanner's real solution===
 +
The real solution: requires the manufacturers to redesign their hardware, so they implement I/Q demodulationDoing so would: eliminate the  possible loss of critical information required for successful reception and decoding of a Simulcast signal. Which, would allow for scanners to perform much more closely to commercial radios and pagersAs this problem is the result of the long-in-the-tooth Triple Heterodyne receiver with only AM and/or FM demodulator without  Phase Modulation. Whereas, there seems to be very little hope for a firmware solution, APCO Phase II modulation uses CQPSK it is supposedly capable of backwards compatible with C4FM demodulators used in Phase I System equipment, and newer digital scanners do have DSPs that can preform some of the comparative and forward-error processing necessary in I/Q demodulation, the solution maybe under there noses or hoods if you will.
  
==Scanner Location==
 
*Moving a scanner a few inches can be a big factor. Signals can have peaks and nulls, which creates areas of strong and weak signals.
 
  
*Moving a scanner can also help reduce local interfering signals. Many electronics in the house can create interference.
+
==Software Defined Radios==
 +
[[Software defined radios]] are made up of a receiver USB device and a Computer to process the incoming System's signals - data and voice streams.  Voice decode, can be superior to scanners, as the signal is processed via a I/Q demodulator, similar to how actual radios would process the incoming signal, less the narrower filtering and stability of commercial-quality oscillating crystals.  Typically limited by only the CPU processing power, and filtering of local spurious signals.
  
*Hairbrained solution get 'Joe' from the next county over to monitor your county and use your scanner to monitor Joe's county. You'll have to use a third-party solution to control and stream Joe's scanner and let Joe do the same with your scanner.
+
To be honest, SDRs can be tedious to set-up, and Phase II cannot be done on a PC/Mac but more modes can be decoded than current scanners. Conceptional, but not currently, these set-ups are not really portable, nor can they decode multiple Systems at one time at least not easily like a scanner.  
  
=Solutions=
+
See [[APCO Project 25#Software Based Decoders]] for additional information.
==Commercial Radios==
 
If this situation was present in commercial radios (the ones actually used by the subscribers), then [[P25 CAI]] would not be acceptable for a public safety use.  Commercial radios greatly benefit from utilizing properly designed hardware where the signal is processed prior to any FM demodulation stage.  This design allows the radio to process the signal without any loss of information, unlike the time tested, AM/FM based designs and implementations, still used in scanners today.  This solution may not be ideal as it can be expensive and extremely complicated to setup correctly.
 
  
 
==Public Safety Pagers==
 
==Public Safety Pagers==
[http://www.unicationusa.com Unication] has developed a couple models of receive only P25 capable public safety pagers. These pager/receivers are intended for commercial use, and therefore have the proper hardware design. They work without any "tweaks" that may be needed for a scanner.
+
[http://www.unicationusa.com Unication] has developed a couple models of "Receive ONLY", P25 capable, Public Safety pagers. These pager/receivers are intended for "Public Safety Users." Therefore, they have use I/Q demodulators and narrower filtering for band specific needs. They work generally without any: "tweaks" that may be needed for a scanner, less better antenni for more reception outside an intend Site's coverage area.
 +
 
 +
To be clear, these units are Commercial-Grade receivers designed for Public Safety Users, not scanners.  Programming as well as operation of these pagers are designed for Public Safety Users; designed to monitor a single trunk System's Site at a time, and possibly a conventional VHF or UHF Dispatch channel.
  
To be clear, these units are commercial receivers designed for public safety users, not scannersProgramming and operation of these pagers are designed for public safety users.  These pagers are designed to monitor a single system at a time.
+
These units are currently limited to Phase 1 capabilitiesIt is reported that a Phase 2 upgrade (license) will be available for purchase at some point in 2018, with beta-testing confirmed Q1 of 2018.
  
These units are currently limited to Phase 1 capabilitiesIt is reported that a Phase 2 license will be available for purchase at some point in 2018.
+
==Commercial Radios==
 +
Commercial radios greatly benefit from using hardware designed to process the signal via a I/Q demodulator and band specific filtering prior to any FM demodulation stage with much narrower and higher quality filtersThis design allows the radio to process the signal with little loss of information and have constant forward error-correction processing algorithms written by the OEM to work nearly perfectly with the System they were intended to be used on. This solution is not be ideal for a typical scanner user, as it can be expensive and extremely complicated to setup correctly and cause can "Life Safety Hazards" for actual Public Safety users when they are set-up incorrectly.  
  
==[[Software Defined Radios]]==
 
Software defined radios are made up of a receive device and a computer to process the incoming signal.  Voice decode can be superior to scanners as the signal is handled properly, similar to how actual radios would process the incoming signal.
 
  
See [[APCO Project 25#Software Based Decoders]] for additional information.
 
  
 
=Related Pages=
 
=Related Pages=
Line 98: Line 106:
  
 
==Other Resources==
 
==Other Resources==
*[http://www.hdtvprimer.com/ANTENNAS/fixes.html This article about HDTV] has some additional information about multipath and other distortion / interference problems. The discussion "maps" well to public safety multipath issues, as well.
+
*[http://www.hdtvprimer.com/ANTENNAS/fixes.html This article about HDTV] has some additional information about multipath and other distortion / interference problems. The discussion "maps" well to Public Safety Multipath issues, as well.
  
 
==External Links==
 
==External Links==
Line 116: Line 124:
 
*http://www.rfwireless-world.com/Terminology/C4FM-vs-CQPSK.html
 
*http://www.rfwireless-world.com/Terminology/C4FM-vs-CQPSK.html
 
*http://www.ece.ualberta.ca/~hcdc/Library/MIMOchClass/ChannelCapacity.pdf
 
*http://www.ece.ualberta.ca/~hcdc/Library/MIMOchClass/ChannelCapacity.pdf
 +
*https://electronics.stackexchange.com/questions/39796/can-somebody-explain-what-iq-quadrature-means-in-terms-of-sdr
 +
  
 
[[Category:RR Glossary]]
 
[[Category:RR Glossary]]

Revision as of 18:03, 8 January 2018

Current generation digital scanners, do a poor job of handling Simulcast Phase I & Phase II system transmissions. The problem is most evident when a scanner user is monitoring a true APCO Project 25 Simulcast system that use the CQPSK and H-DQPSK (Motorola LSM / Harris WCQPSK) modulation scheme, which is not FM modulation based but is either phase and/or amplitude modulation based. This is most often noticed when a strong transmission constantly breaks-up, especially when a scanner user is coming from monitoring Analog signals, where one would have an expectation - to receive the new digital voice to be crystal clear, throughout the transmission. In this article, we'll describe and explain the most common reason for this sort of problem, and suggest some ways you might alleviate some of the simulcast distortion your scanner experiences, as well as additional solutions that maybe non-scanner based altogether.

Problem

Multipath reception describes the situation where reception of digital simultaneously broadcast signal from one or more transmit towers, i.e. over paths of different lengths, from a Site's Tower(s). Generally, single-tower multipath is not a problem; the FM capture effect will pick the strongest signal - as multipath reflections will be of lower power, or the wrong polarization. Though, on a Site's towers that are broadcasting simultaneously on the same frequency, (i.e. digital trunk signals voice and data) there are multiple transmitters on separate towers, with the power from more than one of them is strong enough for your scanner's receiver to pick-up 2 or more of the signals.

Often, when receiving multipath signals from an analog system you may hear just a bit of wavering in the signal via a receiver and has no trouble detecting the correct signal, let alone your ear/brain focuses on the correct/strongest sound if two or more are present. Older antenna based analog TV's experienced this by ghosting. Though, shortwave signals suffer this and cause troubles in Single-sideband SSB, due to the nature of the way the signal is handled. Those of you, who have experienced listening to multipath SSB are well aware of how tiresome it is to listen to.

The traditional explanation for scanners to have simulcast distortion, in a received digital trunk radio systems, has often been blamed on multipath and/or inter-symbol interference, which are actual and real phenomenon. For some though, they believe the explanation is just a convenient and easy excuse, it's truely not the only cause of simulcast distortion. Scanners can continue to exhibit symptoms of simulcast distortion even after multipath signals have been minimized. It is most likely because the scanner (receiver) is not using an IQ demodulation technique. Furthermore, as long as there is sufficient overlap in the bit positions, a receiver designed to utilize a IQ demodulator can extract a digitally "Simulcast" System's Site's data & audio in the face of multipath. System's that use "Simulcast" Site(s) i.e. Phase I and II, are specifically designed to synchronize their tower's tranmissions, for their intended coverage area and current users (that are logged-on). So, timing issues are mostly minimized, in real-time by subscriber's transceivers sending data back and forth to the controllers - outputting corrections. Which, helps to constantly maintain the best synchronous information for the subscriber's radios. Though, when the slip is more than one bit time, it can result: in a broken transmission i.e. inter-symbol interference. This generally, occurs on the fringe of an intended coverage area - of a tower(s), where portable use is discouraged and higher output of mobiles are expected/required to be used. If multipath was a significant issue for actual APCO P25 Phase I and II System use, then a "Simulcast Systems" would not be acceptable for Public Safety use.

Solutions

All Scanners Multipath Mitigation

Until the above happens, the only thing a scanner user can do is to attempt to mitigate the problem, depending highly on an individual Users situation. Many of the below Mitigating techniques are aimed at reducing the potential contributing factors of Multipath, to the receive and decode problems. Some, mitigation techniques, could be:

  • Scanner Settings
  • Scanner Firmware
  • Location
    • Antenna
    • Scanner Location
  • Some mitigation techniques are going to be everything but the scanner.

The variables list may be large, but that shouldn't deter anyone from at least trying to "tweak" specific settings first..:

  • Dwell or Hold Time increase, increasing the ability of the scanner to capture and decode all of a Systems voice transmissions & data parameters sent on the Control Channel.
  • Limit the amount of Systems - Scanned, or just scan one Site without additional Conventional, Priorities, or Wx, additional Sites or Systems
  • Digital AGC if its ON try it OFF, because it may be be making Voice transmissions less intelligible, especially if there are multiple errors already in the decode. While it can also, clip loud audio or over-drive quite audio.
  • Keep your Firmware updated in your scanner. Some users report that Simulcast is the cause of all their headaches, when in reality the scanner doesn't have a perfectly written algorithm to decode a particular type of System, Voice, or Band Plan, YET. Sometimes it may seem that the most current version of Firmware takes a step backwards, but that's due to the fact that each system type is unique. It may well be what worked better on system 'A' actually causes system 'B' users to notice a backward step. For some, rolling back to a previous firmware in many models isn't difficult and should be done with recent problems corollary with the newest firmware, but be assured a System's control channel frequency hasn't changed. Also, don't re-write your programming at the same time as a firmware upgrade because you are creating too many variables to objectively deduce where a new problem arose.

Remember only one setting or thing should be changed at a time, and always archive and save your beginning settings, whether that be in different folders, or on other media like an additional thumb drive, even snapping pictures or taking video or just on good ole scratch paper. Thanks and Goodluck and happy monitoring...

Uniden

  • P25 Threshold option can be changed after monitoring the Site with Auto to see the best setting for that System to be be decoded on
    • So if, decode is best at 6 in Auto, switch the P25 Decode Threshold for that system to Manual and 6. While 6 isn't a panacea, it's at least a beginning numeric to try starting with, typically up-to 9 in Manual mode will make the biggest difference. Not all scanners allow this ability System/Site specific/individually.

GRE\RS\Whistler

  • DSP Level Adapt can be changed in some models and can vary the rate at which the DSP attempts to adjust varying P25 levels.
  • ADC Gain and DAC Gain could be lowered to help reduce bit error rates. Typically a -2 and -4, respectively, have been used by some users to help the lower the issues. See GRE/RS/Whistler based DSP ADC/DAC Adjustments for more information, as positive ADC setting can cause internal channel and component cross-talk.

Antenna and Reception

This section attempts to address the issues that might contribute to the multipath problem.

  • ATT - Attenuation of all the signals sometimes helps, typically the Attenuator in the most

scanners is via hardware at a standard of -20db. This is of course, due to the fact that if you attenuate all of the signals, your receiver possibly loses the ability to hear the interfering signal. Additional connectors can add low attentuation, as well as there are F-Connectors with fixed 3, 6, 12 decibel attenuators and many other varied attenuation techniques.

  • In lieu of Attenuation, remember that with Digital Signals: the *less* gain your antenna has, the better off you'll be; reducing the overall signal(s) at the receiver input will (generally) increase the signal-to-noise ratio, so the "capture effect" will ameliorate the problem for you.
  • Attenuation of the secondary signal(s) causing interference via all of an Antenna's nulling properties:
    • A Yagi has 2 null spots, typically 90 and 270 degrees from where it's being pointed. So off-center direction to a tower help negate a secondary tower at 40-75 degrees - beam width and attenuation can vary on the design
    • A Omni-directional, also, has 2 null spots at its tip and bottom. If you have 3 towers and if Tower 1 and Tower 3 form a straight line, then you may be able to most reliably monitor Tower 2 with the antenna laid horizontally in the the line between Towers 1 and 3.
      2

1<-Antenna->3

    • Or adding a corner reflector - beam width and attenuation can also, vary on the design. Search for "cantenna."
  • A yagi antenna pointed at the Site you want to monitor. If you are receiving all of the signal from only one site, there is no "multipath distortion" to deal with. This, of course does infer, that there is NOT a second or third Tower in front or slightly offset to the left or right the primary tower you want to monitor (or even a 4th or 5 tower behind the aimed yagi antenna.
      • One user actually pointed his Yagi at a tower, with a second tower lying 4-5 miles behind the first. He could never get proper decode, but continued to insist, since a yagi was from one small path, he couldn't be getting multipath; while true, he was still getting multiple mistimed signals (images) causing complete decode failure in his radio.
  • Scanner Location
    • Moving a scanner a few inches left or right and/or up or down can be a big factor, signals can have peaks and nulls, which creates areas of strong and weak signals, larger area's for lower frequencies and higher frequencies have smaller areas of peaks and nulls.
    • Moving a scanner can also help reduce local interfering signals as many electronics in the house or auto can create interference that overwhelm the front-end or any receiver.
  • Hairbrained solution get 'Joe' from the next county over to monitor your county and use your scanner to monitor Joe's county. You'll have to use a third-party solution to control and stream Joe's scanner and let Joe do the same with yours.

Scanner's real solution

The real solution: requires the manufacturers to redesign their hardware, so they implement I/Q demodulation. Doing so would: eliminate the possible loss of critical information required for successful reception and decoding of a Simulcast signal. Which, would allow for scanners to perform much more closely to commercial radios and pagers. As this problem is the result of the long-in-the-tooth Triple Heterodyne receiver with only AM and/or FM demodulator without Phase Modulation. Whereas, there seems to be very little hope for a firmware solution, APCO Phase II modulation uses CQPSK it is supposedly capable of backwards compatible with C4FM demodulators used in Phase I System equipment, and newer digital scanners do have DSPs that can preform some of the comparative and forward-error processing necessary in I/Q demodulation, the solution maybe under there noses or hoods if you will.


Software Defined Radios

Software defined radios are made up of a receiver USB device and a Computer to process the incoming System's signals - data and voice streams. Voice decode, can be superior to scanners, as the signal is processed via a I/Q demodulator, similar to how actual radios would process the incoming signal, less the narrower filtering and stability of commercial-quality oscillating crystals. Typically limited by only the CPU processing power, and filtering of local spurious signals.

To be honest, SDRs can be tedious to set-up, and Phase II cannot be done on a PC/Mac but more modes can be decoded than current scanners. Conceptional, but not currently, these set-ups are not really portable, nor can they decode multiple Systems at one time at least not easily like a scanner.

See APCO Project 25#Software Based Decoders for additional information.

Public Safety Pagers

Unication has developed a couple models of "Receive ONLY", P25 capable, Public Safety pagers. These pager/receivers are intended for "Public Safety Users." Therefore, they have use I/Q demodulators and narrower filtering for band specific needs. They work generally without any: "tweaks" that may be needed for a scanner, less better antenni for more reception outside an intend Site's coverage area.

To be clear, these units are Commercial-Grade receivers designed for Public Safety Users, not scanners. Programming as well as operation of these pagers are designed for Public Safety Users; designed to monitor a single trunk System's Site at a time, and possibly a conventional VHF or UHF Dispatch channel.

These units are currently limited to Phase 1 capabilities. It is reported that a Phase 2 upgrade (license) will be available for purchase at some point in 2018, with beta-testing confirmed Q1 of 2018.

Commercial Radios

Commercial radios greatly benefit from using hardware designed to process the signal via a I/Q demodulator and band specific filtering prior to any FM demodulation stage with much narrower and higher quality filters. This design allows the radio to process the signal with little loss of information and have constant forward error-correction processing algorithms written by the OEM to work nearly perfectly with the System they were intended to be used on. This solution is not be ideal for a typical scanner user, as it can be expensive and extremely complicated to setup correctly and cause can "Life Safety Hazards" for actual Public Safety users when they are set-up incorrectly.


Related Pages

Related Wiki Articles

See also the Cliff effect article on Wikipedia.

Discussions on RR Forums

Other Resources

  • This article about HDTV has some additional information about multipath and other distortion / interference problems. The discussion "maps" well to Public Safety Multipath issues, as well.

External Links