How does simulcasting work in a trunked system?

[cjn1962]

Can anyone explain how different transmit/receive sites are coordinated in a wide-area trunking system? For example, the Massachusetts State Police operate a statewide trunk that is divided into several zones. Within each zone there are several transmit/receive sites. I do understand that "voting" allows a site to choose the strongest signal from a mobile/portable and subsequently send it to a transmitter(s). But what happens on the transmit side of the process?

My questions are 1) Does more than one site within a zone transmit simulataneously and if so 2) what prevents a mobile/portable from receiving multiple signals (which may not arrive at the receiver at same time)?

Thanks for any help,

Chris

 

[N_Jay]

Two way to do this.

Different frequencies.
Each site has its own set of frequencies. Typically a channels is only used at a site if a unit on the talkgroup is registered at the site.
Some system just transmit every group (in the zone) at every site.
It depends on the brand of the system, the vintage of the system and some other design considerations as to how a particular system is set up.

Same frequencies (True Simulcasting)
The sites operate on the same frequencies. The transmitters are carefully aligned to send out the exact same signal at the exact same time (Ignoring intentional timing offsets). The coverage design is usually optimized to minimize the areas where the signals overlap and have about the same signal strength.

 

[SCPD]

Actually transmitting the same signal from more than one transmitter at a time on one frequency is what N Jay is calling true simulcasting. When a trunked system simultanously transmits the same signal one more than one transmitter at the same time, but uses different frequencies for each transmitter, it is called multi-casting. The commercial broadcast industry commonly gets this wrong when they say they are "simulcasting" an athletic event on TV and an AM or FM radio station at the same time.

 

[N_Jay]

Actually transmitting the same signal from more than one transmitter at a time on one frequency is what N Jay is calling true simulcasting. When a trunked system simultaneously transmits the same signal one more than one transmitter at the same time, but uses different frequencies for each transmitter, it is called multi-casting. The commercial broadcast industry commonly gets this wrong when they say they are "simulcasting" an athletic event on TV and an AM or FM radio station at the same time.

You are correct as for as Land Mobile Radio is concerned, but Broadcasting has a different definition of "Simulcasting", and it is used correctly in that industry.

It is basically a fictitious word with different meanings per industry.

In the UK, Simulcast (the LMR meaning) is (or was) called Quasi-Synchronous Transmission.

 

[wa8pyr]

My questions are 1) Does more than one site within a zone transmit simulataneously and if so 2) what prevents a mobile/portable from receiving multiple signals (which may not arrive at the receiver at same time)?

Depends on how the system is set up. Wide area systems usually use single sites in specific locations for optimum coverage (Motorola Smartzone and similar); transmissions on Talkgroup X are only heard at sites where a radio is affiliated with (ie, logged in to) that site on that talkgroup.

Simulcast (the same transmission on the same frequency from multiple sites) is usually used in large metro areas to blanket the area with signal and provide better in-building coverage. The sites are synchronized with a reference standard; the transmissions from each site are shifted in frequency very, very slightly to prevent interaction with the signal from other sites. It's unnoticeable to the user on the street unless one site gets out of whack, in which case hetrodyning can be heard.

The signal standard originally was a rubidium device, but GPS is most commonly used today.

Tom WA8PYR

 

[cjn1962]

Thanks Tom, et al.

Tom, the Mass State Police system is a Smartzone system and I was under the impression that each zone utilizes several transceiving sites (according to some online sources that name specific geographic locations for the equipment). Your answer, if I understand it correctly, states that a Smartzone system used a single site per zone. How could one site provide adequate coverage? I live in Smartzone 03, and I can copy the transmissions over a very large area. Can you elaborate a bit more?

Also, thanks very much for the explanation re the heterodyning.

Chris

 

[cjn1962]

How does simulcasting work in a trunked system? Reply to Thread

Ooops. I just re-read your answer and noticed that you used the word "usually."

Chris

 

[studgeman]

If i remember correctly smartzone 3 is central MA zone. Like many statewide systems, the Mass State Police system is divided into simulcast, multicast zones. Zone 3 is a large simulcast zone of multiple syrcronzied transmitters, that is why you can travel around within the zone and never notice when you switch from tower to tower. Now if you were out by Newton, you would be able to pick up 3 different zones. you would hear the central MA zone, on one set of frequencies, the Metro Boston Zone, another set of frequencies, and the Greater Boston Zone, again another set of frequencies. There are some talkgroups which are specific to those zones and others which are broadcast across all zones or only to zones which a radio which has selected that talkgroup has affiliated with. Completely confused yet? By using multiple zones it increases capacity by geographically isolating conversations on the system. Do you really need to hear Troop C in Downtown Boston?

Just one more post on a complex topic!

 

[N_Jay]

I don't know the particulars of the system, but a multi-zone system can have some areas served by multiple sites working in a simulcast configuration.
This is often the case in areas that need high levels of coverage or where they is insufficient channels to enable the total number of sites to be implemented.

 

[ofd8001]

Multi-casting/Simulcasting

The Louisville MetroSafe radio system has got 3 sites up and running for test purposes. When one goes on, all three go on. The coverage is pretty good, even with 50 watt base stations. From what I understand, when it's all said and done in a couple of years, there will be as many as ten sites all talking at the same time.

What I think the ultimate goal of several transmitters transmitting all at once with low power, is to confine the signal to a smaller footprint area, but achieve greater building penetration.

 

[N_Jay]

There are lots of advantages to simulcast. It has been used very successfully for well over 30 years now.

Good building penetration,
wide area coverage prior to automatic multi-zone systems,
reduced frequency requirements,
simple operation for both field and dispatch personnel.


And its a technical challenge to set up (or it used to be).
Darn advances take all the fun out of building radio systems.

 

[wa8pyr]

Tom, the Mass State Police system is a Smartzone system and I was under the impression that each zone utilizes several transceiving sites (according to some online sources that name specific geographic locations for the equipment). Your answer, if I understand it correctly, states that a Smartzone system used a single site per zone. How could one site provide adequate coverage? I live in Smartzone 03, and I can copy the transmissions over a very large area. Can you elaborate a bit more?

Oops. I should have mentioned that Smartzone systems can be comprised of a combination of single sites and simulcast sites. Ohio MARCS is an example; the Columbus site is a 6-site simulcast, while all the others are single sites. As far as the system is concerned they're all the same.

Single sites can provide perfectly good coverage, however, as long as they're properly located.

Tom WA8PYR

 

[N4DES]

Just a clarification that trunking and simulcast are not one of the same and aren't limited in their deployement. There are conventional simulcast systems in existence and as well as trunked systems that aren't simulcast.

 

[KC1UA]

MSP zones have numerous different sites within them that do indeed simulcast. I've heard it when syncronization is lost. Very unfortunate results.

Zone 3 has numerous sites, I believe as many as 8. Your best bet for information besides this website is at:

http://users.adelphia.net/~kc1ho/ematrunk/msp/msp.html

 

[SCPD]

In the UK, Simulcast (the LMR meaning) is (or was) called Quasi-Synchronous Transmission.

So the Brits must call simulcasting QST and that could really confuse us ham operators.

I did not know so much simulcasting was done on trunking systems. I know from experience that multiple repeaters on the same frequency using low tech VHF highband on U.S. Forest Service, BLM, and National Park Service systems really stunk in the overlap areas of two repeaters. All these years of hearing people relating that simulcasting was being used in two way radio systems I assumed they were just using the term incorrectly and thought they really meant multi-casting. It is similar to people who call repeater use duplex, when it, I believe, is really semi-duplex, because duplex is really the use of two frequencies using radios which can transmit and receive at the same time, e.g. cell phones and the older style mobile phones.

Locally I listen to a multi-casted Smartzone system used by Southern California Edison. It covers about 25-30% of California and there are some frequency duplications, but these sites are separated by long distances (100 miles or more) and/or major topography.

 

[N_Jay]

If it sounded that bad, it probably was not a well designed simulcast system.

There is a lot of what we called "Sloppy-cast" out there.

There are also a lot of engineers and techs who think hey know simulcast design and alignment, then tell you how it always sounds bad.

A well designed, well aligned system can sound darn good!

 

[scannernut59]

In simulcast systems, there is a GPS that makes sure the Radio Waves from each site are in time with each other. The basestations have the GPS connected to it for Reference Oscilator.

Scanner Nut

 

[N_Jay]

In simulcast systems, there is a GPS that makes sure the Radio Waves from each site are in time with each other. The basestations have the GPS connected to it for Reference Oscilator.

Scanner Nut

Yes most current simulcast designs use GPS. It typically has two functions. It provides a frequency reference to keep each transmitter carrier frequency aligned. On some systems it also provide a timing reference for the delay (and alignment) of the transmitted signal.

Older systems relied on various forms of high stability oscillators or high stability referenced synthesizers to keep the carriers on frequency, and then relied on static delay elements to keep the transmitted signals aligned.

Depending on how bad the signal was in the overlap area, some system designs also relied on careful antenna designs to minimize the overlap.

 

[KC1UA]

If it sounded that bad, it probably was not a well designed simulcast system.


A well designed, well aligned system can sound darn good!

The Massachusetts State Police system for the most part sounds great and performs quite well. The few times I have noticed a simulcasting issue came during the summer and during significant heat.