Actions

Difference between revisions of "Simulcast Distortion"

From The RadioReference Wiki

m (Pro's and Cons and small adjustments)
 
(19 intermediate revisions by 4 users not shown)
Line 1: Line 1:
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.  
+
Most current generation digital scanners do a poor job of handling ''[[Simulcast|simulcast]]'' [[APCO Project 25]] systems. This is most often noticed when a strong digital voice transmission constantly breaks-up where it should be clear. This article will describe and explain the most common reason for this sort of problem, suggest some ways you might alleviate some of the simulcast distortion your scanner experiences, and introduce additional solutions that maybe non-scanner based altogether.  
  
=Problem=
+
==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.  
+
To begin to understand the problem requires a basic understanding of the way the digital signals are modulated. Simulcast system use quadrature phase-shift keying (QPSK) in the form of [[CQPSK]] (Motorola [[LSM]] / Harris WCQPSK) for Phase I systems, and [[H-DQPSK]] for Phase II systems. This modulation scheme differs from traditional modulation schemes such as AM and FM. QPSK modulates the phase of the signal rather than the amplitude or frequency. The problem is that traditional scanner hardware was designed to receive the traditional modulation schemes, but not QPSK. The result is that they fall short when attempting to demodulate QPSK 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.
+
Multipath can also be a contributing factor to the issue. Multipath describes the situation where a receiver receives the multiple signals being broadcast from one or more transmit towers. Single tower multipath, such as signal reflection, is generally not a problem as the FM capture effect will pick the strongest signal because the reflect signals are of lower power or the wrong polarization. In a simulcast system, multiple transmitters on separate towers simultaneously broadcasting on the same frequency, which is strong enough to negate the FM capture effect.  
  
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.
+
When receiving multipath signals from an analog system, you may hear just a bit of wavering in the signal, but your ear/brain will focus on the correct sounds. Older antenna based analog TV's experienced this by ghosting. Shortwave signals suffer this and cause troubles in [[Modulation Methods#Amplitude Modulation|single-sideband]] due to the nature of the way the signal is handled. Those that have experienced listening to multipath SSB are aware of how tiresome it is to listen to.
  
=Solutions=
+
When dealing with digital signals, multipath can cause intersymbol interference. A symbol is the digital data being received and intersymbol interference is when that data overlaps, creating ambiguity.
==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:
+
Simulcast systems are specifically designed to synchronize their different site's transmissions for their intended coverage area to minimize the effects of multipath and therefore intersymbol interference. QPSK is used to maximize the symbol dwell time to allow for maximum symbol recovery. A correctly designed radio using properly designed I/Q receiver can recognize the proper timing of the QPSK signal to maximize the correct symbol recovery. Intersymbol interface can still occur on the fringe of an intended coverage area where the synchronization of the signal is not optimized.
 +
 
 +
As scanners do not implement a proper method of handling QPSK, they lack the ability to determine when to properly read the signal for proper data recovery. This compounds with issues of multipath and intersymbol interference, even if it is minimized. The result is the inability of the scanner to correctly recover the data in the signal, which causes poor reception and unintelligible voice. If this was a significant issue for actual system radios, then simulcast would not be acceptable for public safety use.
 +
 
 +
==Solutions==
 +
The solution for scanners requires the manufacturers to redesign their hardware to implement I/Q demodulation. This would allow for the scanner to correctly demodulate the phase changes in the signal, allowing for correct reception of a QPSK signal.
 +
 
 +
There following scanners have implemented this design and should theoretically have superior performance in a simulcast environment.
 +
*'''Uniden''' [[SDS100]]
 +
*'''Uniden''' [[SDS200]]
 +
 
 +
===Multipath Mitigation for Scanners===
 +
Unless a user has a properly designed scanner, the only thing a scanner user can do is to attempt to mitigate the problem. This will depend highly on each individual situation. Many of the below mitigation techniques are aimed at reducing the contributing factors of multipath to this problem.
 +
 
 +
Some, mitigation techniques, could be:
 
*Scanner Settings
 
*Scanner Settings
 
*Scanner Firmware
 
*Scanner Firmware
Line 16: Line 30:
 
**Antenna  
 
**Antenna  
 
**Scanner Location
 
**Scanner Location
*Some mitigation techniques are going to be everything but the scanner.
+
*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..:
+
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.
+
*Opening squelch all the way to 0. 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|}}
  
*Limit the amount of Systems - Scanned, or just scan one Site without additional Conventional, Priorities, or Wx, additional Sites or Systems
+
*Dwell or Hold Time increase, increasing the ability of the scanner to capture and decode all of a systems voice transmissions & data sent on the control channel.
  
*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.
+
*Limit the amount of systems being scanned or just scan one site without additional conventional, priorities, weather, etc.
  
*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.
+
*Digital AGC if its ''ON'' try it ''OFF''. It might be making voice transmissions less intelligible, especially if there are multiple errors already in the decode. It can also clip loud audio or over-drive quiet audio.
  
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...
+
*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. 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. 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.
  
===Uniden===
+
Remember only one setting or thing should be changed at a time. Always archive and save your beginning settings, whether that be in different folders or on other media like an additional thumb drive. Even try snapping pictures, taking video, or keeping track on good ole scratch paper.
*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|}}
 
  
*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
+
====Uniden====
**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.  
+
*P25 Threshold option can be changed after monitoring the Site with Auto to see the best setting for that System to 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.
 +
*Uniden's [[SDS100]] and [[SDS200]] are known to work in many simulcast situations, but are quite pricey
  
===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 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.
+
*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 signal reception issues that might contribute to the multipath.  
 
This section attempts to address the signal reception issues that might contribute to the multipath.  
  
====Attenuation====
+
=====Attenuation=====
*ATT - Attenuation of all the signals sometimes helps, typically the Attenuator in the most  
+
*ATT - Attenuation of all the signals sometimes helps, typically the attenuator in the most scanners is via hardware at a standard of -20db. This helps because if you attenuate all the signals, your receiver possibly loses the ability to hear the interfering signal. Additional connectors can add low attenuation, as well as there are F-Connectors easily available with fixed 3, 6, 12 decibel attenuators and many other varied attenuation techniques.
scanners is via hardware at a standard of -20db. This helps 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 easily available with fixed 3, 6, 12 decibel attenuators and many other varied attenuation techniques.
+
*Attenuation of the secondary signal(s) causing interference via 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, 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 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.
+
**An Omni-directional, also, has 2 null spots at its top 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 line between Towers 1 and 3.
 
       2
 
       2
 
  1<-Antenna->3
 
  1<-Antenna->3
  
:*Or adding a corner reflector - beam width and attenuation can also, vary on the design. Search for "cantenna."
+
*Adding a corner reflector - beam width and attenuation can also vary on the design. Search for "cantenna".
  
====Antenna====
+
=====Antenna=====
Remember that inside the intended coverage area a scanner needs an antenna with *less* gain for digital signals, and you'll be better off; 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. Outside the intended coverage area more gain via a directional or higher up antenna maybe needed, but neither will guarantee reception outside the Systems coverage area.
+
One can try to use an antenna with less gain if they are inside the coverage area to increase the signal-to-noise ratio so that the capture effect may ameliorate the problem. Outside the intended coverage area more gain via a directional or higher up antenna maybe needed, but neither will guarantee reception outside a system's coverage area.
 
   
 
   
*A yagi antenna pointed at a tower 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 of the primary tower you want to monitor (or even a 4th or 5 tower behind the aimed  antenna.
+
*A yagi antenna pointed at a tower you want to monitor. If you are receiving all of the signal from only one site, there should be 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 of the primary tower you want to monitor.  
**Incidentily, 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 his 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, i.e. 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 overwhelms 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.
 
  
 +
=====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.  Peaks and nulls are larger for lower frequencies and smaller for higher frequencies.
 +
*Moving a scanner can also help reduce local interfering signals as many electronics in the house or auto can create interference that overwhelms the front-end for 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.
  
==SDRs==
+
===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.
+
[[Software Defined Radios]] (SDR) 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 like 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.  
+
SDRs can be tedious to set-up, but more modes can be decoded than current scanners. These set-ups are not really portable and cannot really decode multiple systems at once like a scanner.  
  
 
See [[APCO Project 25#Software Based Decoders]] for additional information.
 
See [[APCO Project 25#Software Based Decoders]] for additional information.
  
==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 "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.
+
See [[Unication Voice Pager Receivers]]
 
 
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.
 
 
 
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.
 
 
 
===Unication Pro's and Con's===
 
;PRO
 
*P25 Simulcast reception can't be beat
 
*Two-tone paging (conventional or [some] P25)
 
*Nice display
 
*Nice size/weight
 
*Battery lasts a long time
 
 
 
;CON
 
*No "on the fly" programming
 
*No option to mute encryption
 
*No SKIP / lockout
 
*No real ability to tell the radio to "move along, I don't care about this talkgroup"
 
*Can only monitor a single control channel at a time (no multi-site or multiple system scanning)
 
*Cannot mix scanning of P25 control channel and conventional frequencies.
 
*G4 is only 700/800
 
*G5 adds a band but you have to choose which one (VHF, 380, 400, 450)
 
conventional analog or P25 and/or P25 Phase 1 only (no Motorola, EDACS, LTR, DMR, NXDN, etc.)
 
*Can't assign alpha tags to radio IDs
 
*Programming software (PPS) isn't great
 
*Does not work on EF Johnson P25 systems
 
*No Phase 2 (yet) - upgrade option w/MSRP of $150 (hopefully less on the street)
 
 
 
  
==Commercial Radios==
+
===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.  
+
Commercial radios are specifically designed to be used on simulcast systems and therefore provide superior performance over scanners. They greatly benefit from using hardware designed to process the QPSK signal via an I/Q demodulator and higher quality band specific filtering.
  
 +
This solution is not ideal for a typical scanner user as it can be expensive and extremely complicated to setup correctly and cause life safety hazards for actual Public Safety users when they are set-up incorrectly.
  
 
+
==Related Pages==
Related Pages:
 
;Related Wiki Articles
 
*[[DMA P25 Decoding and Audio Issues]]
 
*[[Uniden Digital AGC Tweak Procedures]]
 
*[[P25 audio decode level adjustment]]
 
*[[GRE/RS/Whistler based DSP ADC/DAC Adjustments]]
 
See also the [https://en.wikipedia.org/wiki/Cliff_effect Cliff effect article] on
 
Wikipedia.
 
 
 
 
;Discussions on RR Forums
 
;Discussions on RR Forums
*{{Thread|voice-control-channel-decoding-software|283705-lsm-why-scanners-suck-real-story.html|LSM: why scanners suck: the real story}}
+
*{{Thread|voice-control-channel-decoding-software.26|283705-lsm-why-scanners-suck-real-story.html|LSM: why scanners suck: the real story}}
*{{Thread|general-scanning-discussion|333609-official-i-want-lsm-work-properly-my-scanner-thread.html|The official "I want LSM to work properly in my scanner" thread}}
+
*{{Thread|general-scanning-discussion.28|333609-official-i-want-lsm-work-properly-my-scanner-thread.html|The official "I want LSM to work properly in my scanner" }}
*{{Thread|industry-discussion|257898-p25-simulcast-multipath-interference.html|P25 simulcast multipath interference}}
+
*{{Thread|industry-discussion.143|257898-p25-simulcast-multipath-interference.html|P25 simulcast multipath interference}}
*{{Thread|project-25-technologies|162181-support-tdma-p25-phase-ii-3.html|Support for TDMA P25 Phase II}}
+
*One individual had several issues with this wiki article as it existed on Jan. 2013. Rather than attempting to rewrite the article, the reader is referred to this {{Thread|new-york-radio-discussion-forum.53|258801-saratoga-county-p25.html#post1903290|NY forum entry|}}
*{{Thread|digital-voice-formats|165679-c4fm-cqpsk-2.html|C4FM or CQPSK?}}
 
*One individual had several issues with this wiki article as it existed on Jan. 2013. Rather than attempting to rewrite the article, the reader is referred to this {{Thread|new-york-radio-discussion-forum|258801-saratoga-county-p25.html#post1903290|NY forum entry|}}
 
 
 
;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.
 
  
 
;External Links
 
;External Links
Line 138: Line 105:
 
*[http://www.lightlink.com/mhp/lsm/ LSM Gallery]
 
*[http://www.lightlink.com/mhp/lsm/ LSM Gallery]
 
*[http://www.lightlink.com/mhp/lsm2/lsm2.html LSM Gallery - Page 2]
 
*[http://www.lightlink.com/mhp/lsm2/lsm2.html LSM Gallery - Page 2]
*[https://en.wikipedia.org/wiki/Intersymbol_interference Intersysmbol Interference]
+
*[https://en.wikipedia.org/wiki/Intersymbol_interference Intersymbol Interference]
 
*http://rfic.eecs.berkeley.edu/~niknejad/ee242/pdf/eecs242_lect3_rxarch.pdf
 
*http://rfic.eecs.berkeley.edu/~niknejad/ee242/pdf/eecs242_lect3_rxarch.pdf
*http://urgentcomm.com/mag/error-control-coding-p25-radios
 
*https://www.maximintegrated.com/en/app-notes/index.mvp/id/641
 
 
*http://www.berk.tc/combas/digital_mod.pdf
 
*http://www.berk.tc/combas/digital_mod.pdf
 
*http://www.rfwireless-world.com/Terminology/ASK-vs-FSK-vs-PSK.html
 
*http://www.rfwireless-world.com/Terminology/ASK-vs-FSK-vs-PSK.html
 
*http://blog.taitradio.com/2015/01/13/comparing-psk-and-fsk-based-digital-modulations-part-1/
 
*http://blog.taitradio.com/2015/01/13/comparing-psk-and-fsk-based-digital-modulations-part-1/
*http://www.dvsinc.com/papers/p25_training_guide.pdf
 
 
*http://www.dtic.mil/dtic/tr/fulltext/u2/a190028.pdf
 
*http://www.dtic.mil/dtic/tr/fulltext/u2/a190028.pdf
 
*https://www.efjohnson.com/resources/dyn/files/972772z218319c9/_fn/Simulcasting+Project+25.pdf
 
*https://www.efjohnson.com/resources/dyn/files/972772z218319c9/_fn/Simulcasting+Project+25.pdf
Line 152: Line 116:
 
*https://electronics.stackexchange.com/questions/39796/can-somebody-explain-what-iq-quadrature-means-in-terms-of-sdr
 
*https://electronics.stackexchange.com/questions/39796/can-somebody-explain-what-iq-quadrature-means-in-terms-of-sdr
  
 +
;Receivers known to work in simulcast environments
 +
* [[BlueTail Technologies P25RX]]
 +
* Uniden [[SDS100 and SDS200 FAQ Page]]
 +
* [[Unication Voice Pager Receivers]]
 +
 +
;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.
 +
 +
;Related Wiki Articles
 +
*[[DMA P25 Decoding and Audio Issues]]
 +
*[[Uniden Digital AGC Tweak Procedures]]
 +
*[[P25 audio decode level adjustment]]
 +
*[[GRE/RS/Whistler based DSP ADC/DAC Adjustments]]
 +
See also the [https://en.wikipedia.org/wiki/Cliff_effect Cliff effect article] on
 +
Wikipedia.
 +
 +
; Scanner School
 +
* [https://www.scannerschool.com/tips-and-tricks-when-dealing-with-simulcast/ Tips and Tricks when working with Simulcast]
 +
* [https://www.scannerschool.com/working-around-simulcast-issues/ Working Around Simulcast Issues]
  
  

Latest revision as of 14:05, 6 April 2021

Most current generation digital scanners do a poor job of handling simulcast APCO Project 25 systems. This is most often noticed when a strong digital voice transmission constantly breaks-up where it should be clear. This article will describe and explain the most common reason for this sort of problem, suggest some ways you might alleviate some of the simulcast distortion your scanner experiences, and introduce additional solutions that maybe non-scanner based altogether.

Problem

To begin to understand the problem requires a basic understanding of the way the digital signals are modulated. Simulcast system use quadrature phase-shift keying (QPSK) in the form of CQPSK (Motorola LSM / Harris WCQPSK) for Phase I systems, and H-DQPSK for Phase II systems. This modulation scheme differs from traditional modulation schemes such as AM and FM. QPSK modulates the phase of the signal rather than the amplitude or frequency. The problem is that traditional scanner hardware was designed to receive the traditional modulation schemes, but not QPSK. The result is that they fall short when attempting to demodulate QPSK signals.

Multipath can also be a contributing factor to the issue. Multipath describes the situation where a receiver receives the multiple signals being broadcast from one or more transmit towers. Single tower multipath, such as signal reflection, is generally not a problem as the FM capture effect will pick the strongest signal because the reflect signals are of lower power or the wrong polarization. In a simulcast system, multiple transmitters on separate towers simultaneously broadcasting on the same frequency, which is strong enough to negate the FM capture effect.

When receiving multipath signals from an analog system, you may hear just a bit of wavering in the signal, but your ear/brain will focus on the correct sounds. Older antenna based analog TV's experienced this by ghosting. Shortwave signals suffer this and cause troubles in single-sideband due to the nature of the way the signal is handled. Those that have experienced listening to multipath SSB are aware of how tiresome it is to listen to.

When dealing with digital signals, multipath can cause intersymbol interference. A symbol is the digital data being received and intersymbol interference is when that data overlaps, creating ambiguity.

Simulcast systems are specifically designed to synchronize their different site's transmissions for their intended coverage area to minimize the effects of multipath and therefore intersymbol interference. QPSK is used to maximize the symbol dwell time to allow for maximum symbol recovery. A correctly designed radio using properly designed I/Q receiver can recognize the proper timing of the QPSK signal to maximize the correct symbol recovery. Intersymbol interface can still occur on the fringe of an intended coverage area where the synchronization of the signal is not optimized.

As scanners do not implement a proper method of handling QPSK, they lack the ability to determine when to properly read the signal for proper data recovery. This compounds with issues of multipath and intersymbol interference, even if it is minimized. The result is the inability of the scanner to correctly recover the data in the signal, which causes poor reception and unintelligible voice. If this was a significant issue for actual system radios, then simulcast would not be acceptable for public safety use.

Solutions

The solution for scanners requires the manufacturers to redesign their hardware to implement I/Q demodulation. This would allow for the scanner to correctly demodulate the phase changes in the signal, allowing for correct reception of a QPSK signal.

There following scanners have implemented this design and should theoretically have superior performance in a simulcast environment.

Multipath Mitigation for Scanners

Unless a user has a properly designed scanner, the only thing a scanner user can do is to attempt to mitigate the problem. This will depend highly on each individual situation. Many of the below mitigation techniques are aimed at reducing the contributing factors of multipath to this problem.

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 sent on the control channel.
  • Limit the amount of systems being scanned or just scan one site without additional conventional, priorities, weather, etc.
  • Digital AGC if its ON try it OFF. It might be making voice transmissions less intelligible, especially if there are multiple errors already in the decode. It can also clip loud audio or over-drive quiet 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. 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. 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. Always archive and save your beginning settings, whether that be in different folders or on other media like an additional thumb drive. Even try snapping pictures, taking video, or keeping track on good ole scratch paper.

Uniden

  • P25 Threshold option can be changed after monitoring the Site with Auto to see the best setting for that System to 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.
  • Uniden's SDS100 and SDS200 are known to work in many simulcast situations, but are quite pricey

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 signal reception issues that might contribute to the multipath.

Attenuation
  • ATT - Attenuation of all the signals sometimes helps, typically the attenuator in the most scanners is via hardware at a standard of -20db. This helps because if you attenuate all the signals, your receiver possibly loses the ability to hear the interfering signal. Additional connectors can add low attenuation, as well as there are F-Connectors easily available with fixed 3, 6, 12 decibel attenuators and many other varied attenuation techniques.
  • Attenuation of the secondary signal(s) causing interference via 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
    • An Omni-directional, also, has 2 null spots at its top 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 line between Towers 1 and 3.
      2
1<-Antenna->3
  • Adding a corner reflector - beam width and attenuation can also vary on the design. Search for "cantenna".
Antenna

One can try to use an antenna with less gain if they are inside the coverage area to increase the signal-to-noise ratio so that the capture effect may ameliorate the problem. Outside the intended coverage area more gain via a directional or higher up antenna maybe needed, but neither will guarantee reception outside a system's coverage area.

  • A yagi antenna pointed at a tower you want to monitor. If you are receiving all of the signal from only one site, there should be 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 of the primary tower you want to monitor.
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. Peaks and nulls are larger for lower frequencies and smaller for higher frequencies.
  • Moving a scanner can also help reduce local interfering signals as many electronics in the house or auto can create interference that overwhelms the front-end for 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.

Software Defined Radios

Software Defined Radios (SDR) 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 like 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.

SDRs can be tedious to set-up, but more modes can be decoded than current scanners. These set-ups are not really portable and cannot really decode multiple systems at once like a scanner.

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

Public Safety Pagers

See Unication Voice Pager Receivers

Commercial Radios

Commercial radios are specifically designed to be used on simulcast systems and therefore provide superior performance over scanners. They greatly benefit from using hardware designed to process the QPSK signal via an I/Q demodulator and higher quality band specific filtering.

This solution is not ideal for a typical scanner user as it can be expensive and extremely complicated to setup correctly and cause life safety hazards for actual Public Safety users when they are set-up incorrectly.

Related Pages

Discussions on RR Forums
External Links
Receivers known to work in simulcast environments
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.
Related Wiki Articles

See also the Cliff effect article on Wikipedia.

Scanner School