What is simulcast?

[DisasterGuy]

If you've designed a system in such a way that you're hearing signals from sites on the opposite side of those immediately adjacent to the site covering your location at power levels causing interference, you've failed as a system engineer. If the sites are in a perfect hexagonal honeycomb pattern, you should be able to hear a maximum of 7 sites--the one you're in, and the 6 sites surrounding it. If you're hearing anything beyond that, you've misconfigured the system.

That's exactly my point. In the real world this just doesn't play out in a typical municipal system, hence the reason that you end up needing simulcast systems in order to not require hundreds of pairs or hundreds of sites.

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[jonwienke]

There's never a need for "hundreds" of pairs. Worst case scenario (if you've designed the system sensibly) is (7 * channels/site) pairs, regardless of system size or number of sites.

 

[Project25_MASTR]

That's exactly my point. In the real world this just doesn't play out in a typical municipal system, hence the reason that you end up needing simulcast systems in order to not require hundreds of pairs or hundreds of sites.

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I've actually got a decent example of this. GATRRS as it literally has a mix of everything. In Williamson County, there is a 15 channel simulcast system. In Travis County, there is what we call a two-layer simulcast system as well as 5 or 6 traditional sites. The two-layer system is essentially a 21 channel system with county wide coverage and a 22 channel system with coverage focused in an area with high population density around the lakes. Some sub-sites are co-located but affiliation is driven by talk group restrictions to balance out loading. To the West, you get into part of the VHF portion of the system (what we call Western Counties) where Blanco, Burnet and Llano counties have 11 sites between them. Well in order to meet portable coverage requirements, there is a ton of mobile overlap. In fact, some days you can get all the way out to Concho county or even Waco on a mobile before going out of range. One of the sites provides prime coverage for all three counties though...and of course has the most busy's on it. For reference, we are talking 5 channels or less on the VHF sites.

 

[Ubbe]

Doppler effect isn't honestly that noticeable. Consider a car traveling away (or towards) a tower at 100 mph (160 kph, ~44.4 m/s) and a general frequency of 150 MHz. The actual doppler shift is going to be around 22 Hz.

22Hz up in frequency for the tower in front and 22Hz down for the tower behind.
I thought simulcast needed tower frequencies to be withing a very exact and precise frequency, which the doppler effect will totally destroy. So Mr. 1n32008 are not correct in his statements?

/Ubbe

Originally Posted by kayn1n32008 View Post
Simulcast radio systems very precisely control the phase of the RF signal to ensure that the RF emitted from each of the simulcast transmitters will arrive with before the maximum delay spread value, through out the intended coverage area of the simulcast cell.

 

[kayn1n32008]

22Hz up in frequency for the tower in front and 22Hz down for the tower behind.

I thought simulcast needed tower frequencies to be withing a very exact and precise frequency, which the doppler effect will totally destroy. So Mr. 1n32008 are not correct in his statements?



/Ubbe

Phase, not frequency. Doppler is clearly not an issue with hundreds of existing simulcast systems already in existence.

A couple dozen Hertz is nothing. Most subscriber radios will be +\-400Hz of indicated frequency anyways.


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[Project25_MASTR]

22Hz up in frequency for the tower in front and 22Hz down for the tower behind.

I thought simulcast needed tower frequencies to be withing a very exact and precise frequency, which the doppler effect will totally destroy. So Mr. 1n32008 are not correct in his statements?



/Ubbe

That’s why the sub-sites use CQPSK. It’s more forgiving when it comes to frequency since we are using phase shift keying instead of frequency shift keying.

Subscribers only transmit in C4FM though which gets voted on by all of the sub-sites.


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[Spitfire8520]

From what I've been reading isn't LSM just an acronym for CQPSK?

LSM (Linear Simulcast Modulation) is Motorola's name for their implementation of a CQPSK modulation designed for simulcast. It has since been taken over by the community to refer to all simulcast systems, regardless if it is actually Motorola's LSM. It is similar to how many use the term PL instead of CTCSS.

 

[Ubbe]

Sorry to be so unclear here. It doesn't matter what frequency error the receiver has, it can handle a high degree of frequencies that are offset, it is only the difference in frequency between towers that matters in a simulcast system and is the system specific problem, or advantage, that we try to discuss.

Tower frequencies are controlled by GPS or high stability referencies and need to be precisly set. Something that then only are valid to stationary receiving radios. As soon as a receiving radio are on the move the doppler effect will change the received frequency up or down depending of where the towers are and any calibrated low hertz or phase settings will change in the radio.

In a city enviroment there are buildings that block radio signals at street level and creates many situations where signals from several towers are more or less equal in strenght.

/Ubbe

Phase, not frequency. Doppler is clearly not an issue with hundreds of existing simulcast systems already in existence.

A couple dozen Hertz is nothing. Most subscriber radios will be +\-400Hz of indicated frequency anyways.


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[kayn1n32008]

NOT the frequency. The PHASE of the RF being emitted from the radio. There will always be frequency error, although Doppler effect is negligible, it is there. It is the PHASE of the emitted signals that is critical to making simulcast systems work.


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[DisasterGuy]

NOT the frequency. The PHASE of the RF being emitted from the radio. There will always be frequency error, although Doppler effect is negligible, it is there. It is the PHASE of the emitted signals that is critical to making simulcast systems work.


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This... While frequency alignment plays a role, timing (phase) is what is critical in a simulcast environment and why timing delays are created to minimize TDI. It is also why multi-path (example of a city environment) is problematic. Using LSM provides a greater deal of latitude in tolerable TDI than even analog simulcast. Part of what challenges scanners isn't the LSM itself but rather their inability to handle the TDI. This is the reason that going to the simplest antenna (for some a paper clip) allows them to have clear decode as they only hear one site and don't suffer the TDI.

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[slicerwizard]

That’s why the sub-sites use CQPSK. It’s more forgiving when it comes to frequency since we are using phase shift keying instead of frequency shift keying.

Phase shifting and frequency shifting are essentially the same thing. C4FM and CQPSK differ in where they shift the carrier frequency to. C4FM moves to one of four frequency shifts at each symbol center, whereas CQPSK always returns to the carrier's center frequency at each symbol center. And of even more significance, CQPSK significantly reduces power levels at the symbol edges, which makes it easier for receivers to see the strong symbol centers.

 

[KA1RBI]

CQPSK significantly reduces power levels at the symbol edges

For some details, although with the usual obfuscation, see U.S. Patent 6,061,574.

I can confirm this in operation here through direct observation of a nearby genuine-/\/\ LSM system. It may not be unreasonable to view the variations in power levels at the symbol edges (called "non central portions of the symbol" in the patent) as a sort of low-speed modulating code, which can be a PN sequence according to the patent. The PN sequence is time-shifted so that it's sent at different time offsets at each transmitting site, "so that different transmitters do not adjust their output amplitudes identically at every step of the sequence"...

And, if they do dynamically "adjust" the transmission treatment of the channel, you may rest assured that it's optimized for the benefit of the actual subscriber(s), not for any random third-parties listening in on the call...

73

Max

 

[iMONITOR]

LSM (Linear Simulcast Modulation) is Motorola's name for their implementation of a CQPSK modulation designed for simulcast. It has since been taken over by the community to refer to all simulcast systems, regardless if it is actually Motorola's LSM. It is similar to how many use the term PL instead of CTCSS.

Thank you. That clears of some of the confusion for a lot of us I'm sure. If nothing else LSM is easier to say or type, so I'll go with it!

 

[Ubbe]

NOT the frequency. The PHASE of the RF being emitted from the radio. There will always be frequency error, although Doppler effect is negligible, it is there. It is the PHASE of the emitted signals that is critical to making simulcast systems work.

I probably read this into something that I am not supposed to. To me, phase could be when two signals are of the exact same frequency and only differ from a certain degree of a wavelenght. Or it could be two antennas receiving the same signal and the antennas different distance from the source will make them receive the signal in different phase. If there are two signals being received that have different frequencies the phase will continuosly change between them and have different values. If the signals differ with one hertz they will be received in phase and out of phase in 1 Hz cycles and if the signals are of equal strenght the out of phase signal will totally blank out the RF signal.

But you say there is no relation between phase and frequency in simulcast systems? Are you perhaps refering to phase and FM demodulation?

/Ubbe

 

[belvdr]

I probably read this into something that I am not supposed to. To me, phase could be when two signals are of the exact same frequency and only differ from a certain degree of a wavelenght. Or it could be two antennas receiving the same signal and the antennas different distance from the source will make them receive the signal in different phase. If there are two signals being received that have different frequencies the phase will continuosly change between them and have different values. If the signals differ with one hertz they will be received in phase and out of phase in 1 Hz cycles and if the signals are of equal strenght the out of phase signal will totally blank out the RF signal.

But you say there is no relation between phase and frequency in simulcast systems? Are you perhaps refering to phase and FM demodulation?

/Ubbe

From my [very limited] understanding, phase and frequency have no relation in direct terms. In other words, you cannot calculate phase from frequency or vice versa.

It doesn't sound like it makes any difference whether it's one repeater or several in a simulcast system, other than in a simulcast system they want to ensure each repeater is sending the same signal in phase with each other. Otherwise, if they were much out of phase, it could not be decoded.

 

[kayn1n32008]

I did not say there is no relation between frequency and phase, I said that the phase of the signals need to be precisely controlled. This is to control the delay spread with in the simulcast cell’s intended coverage area.

Of course frequency needs to be relatively precise, but it’s precision is not nearly important as the phase of the signals. It is the phase of the RF that will determine if a simulcast system will work or not.
You how ever, have decided to treat simulcast as if it is an experimental physics.

Go and grab a dozen MSTR VI base stations and see how close they are for frequency stability. Even 0.5ppm stability. You will never get every repeater in a simulcast cell to be less than +/-1Hz in frequency. Not going to happen.

Same with Doppler effect. It just simply is not really a factor in whether a simulcast system will work or not.


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[wa8pyr]

There's never a need for "hundreds" of pairs. Worst case scenario (if you've designed the system sensibly) is (7 * channels/site) pairs, regardless of system size or number of sites.

But as noted above that's one of the reasons for simulcast; using that seven-site example from above and 7 channels/site, you would need 49 frequency pairs. A typical large municipality can usually get away with no more than 20 frequency pairs with simulcast, so you've not only blanketed the area with coverage (and reduced the need for roaming), you've also saved 29 frequency pairs and the associated hardware and costs that went along with them.

And if you're using Phase II you can further reduce the number of frequency pairs needed on the site.

 

[kayn1n32008]

...In a simulcast system they want to ensure each repeater is sending the same signal in phase with each other...

The signals are not necessarily transmitted from the repeaters in phase, but they are transmitted in such a fashion that, upon arrival in the intended coverage area, they arrive in phase or at least with in the desired delay spread.



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[belvdr]

The signals are not necessarily transmitted from the repeaters in phase, but they are transmitted in such a fashion that, upon arrival in the intended coverage area, they arrive in phase or at least with in the desired delay spread.

Thanks for the clarification.

 

[belvdr]

Professional receivers like the Unication, Motorola, Harris et al, do not have this issue but are more expensive and lack the features that scanner users take for granted.

I've been looking at the Unication models because my 436 is deaf in my rural area (it does fine when I get closer to the city).

I've seen this stated before but what features are lacking in the Unication for scanner users? Could it scan local NOAA for alert tones and a P25 Phase 2 system?