From The RadioReference Wiki
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.
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.
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
- 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:
- Opening squelch all the way to 0. See this post by UPMan: 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 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.
- 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
- 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.
- 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.
- Adding a corner reflector - beam width and attenuation can also vary on the design. Search for "cantenna".
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.
- 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
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 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 Cliff effect article on Wikipedia.
- Scanner School
- Discussions on RR Forums
- LSM: why scanners suck: the real story Thread
- The official "I want LSM to work properly in my scanner" Thread
- P25 simulcast multipath interference Thread
- Support for TDMA P25 Phase II Thread
- C4FM or CQPSK? Thread
- 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 NY forum entry
- 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.
- Receivers known to work in simulcast environments
- External Links
- LSM 101
- LSM Gallery
- LSM Gallery - Page 2
- Intersymbol Interference