Large Area Trunked/Simulcast Voting

[Rlahey]

Explain to me like I'm five...
I understand how trunking sites work, and how the radio samples the control channels to figure out which site to affiliate with... but with lets say a statewide system, if a user is utilizing a simulcast site in a metro area, how does the radio choose with simulcast site to choose? It seems like most simulcast sites utilize the same control channel (or that's my thought process, correct if wrong), how does the radio choose the simulcast site?

 

[GTR8000]

A simulcast "site" is really a collection of physical transmit/receive sites, known as subsites, which together form a simulcast cell. Think of that collection of subsites as a virtual "site", as all subsites transmit and receive on the identical set of frequencies, and therefore have a single site number in the system, which is what is presented to subscribers (and any other receiver/scanner that decodes the control channel).

So at the end of the day, the subscribers don't choose any particular subsite, because it doesn't matter as they're all transmitting the same signal and are synchronized throughout the cell. In fact, there is no uniqueness at all among subsites as far as the subscribers are concerned. If you stood right next to Subsite A and logged the control channel data for 30 seconds, then went 15 miles away to Subsite B and did the same...a comparison would reveal no uniqueness to them. Nothing whatsoever in the decoded data showing which subsite was which; you would only see the one site number announced.

Of course there is a bit more to it on the system side of things. For example, while all of the subsites are simultaneously broadcasting (simulcasting) the same signal, each subsite is also listening to the control channel and traffic channel inputs, and those signals are "voted" by comparators to ensure the best inbound signal makes it through. It's not uncommon for a subscriber to be received by multiple subsites, hence the need for voting to isolate the best signal. The voting is dynamic and so a subscriber in motion that is being voted in from Subsite A could then be voted in from Subsite B or C half a second later, and so on.

 

[Rlahey]

A simulcast "site" is really a collection of physical transmit/receive sites, known as subsites, which together form a simulcast cell. Think of that collection of subsites as a virtual "site", as all subsites transmit and receive on the identical set of frequencies, and therefore have a single site number in the system, which is what is presented to subscribers (and any other receiver/scanner that decodes the control channel).

So at the end of the day, the subscribers don't choose any particular subsite, because it doesn't matter as they're all transmitting the same signal and are synchronized throughout the cell. In fact, there is no uniqueness at all among subsites as far as the subscribers are concerned. If you stood right next to Subsite A and logged the control channel data for 30 seconds, then went 15 miles away to Subsite B and did the same...a comparison would reveal no uniqueness to them. Nothing whatsoever in the decoded data showing which subsite was which; you would only see the one site number announced.

Of course there is a bit more to it on the system side of things. For example, while all of the subsites are simultaneously broadcasting (simulcasting) the same signal, each subsite is also listening to the control channel and traffic channel inputs, and those signals are "voted" by comparators to ensure the best inbound signal makes it through. It's not uncommon for a subscriber to be received by multiple subsites, hence the need for voting to isolate the best signal. The voting is dynamic and so a subscriber in motion that is being voted in from Subsite A could then be voted in from Subsite B or C half a second later, and so on.

Wonderful summarization, that helps me greatly in understanding... now can you get deeper into how the voting systems at the sites work? I have never quite understand that. I understand the concept, not the execution

 

[GTR8000]

In a nutshell, every repeater at each subsite is listening on its designated RX frequency (aka input), just as any normal repeater would do. Each of those frequencies has its own comparator, so if it's a 10 channel simulcast cell, there are 10 comparators. The inbound transmissions on each frequency are then voted on by each comparator, and the strongest signal is selected for re-transmission throughout the cell and/or other sites. If the signal from a transmitting radio improves from one subsite to another, the comparator will start to use that better signal instead, even right smack in the middle of a continuous transmission.

This all happens in the blink of an eye, and a comparator can vote multiple times per second. Modern P25 systems are IP based, and so the transport is extremely fast between the system controller (sometimes known as the core or zone controller), and the various subsites and other standalone sites if applicable.