CLMRN Input Frequencies

[adamfancher]

In an effort to tune in on the other half of the UDP/TCP response packets, I'm attempting to tune in on the input frequency of my local repeater.

My understanding is that the P25 band plan specifies that the TX frequency is 45 MHz less than the RX frequency.

Since I normally tune in to the control channel at 853.6125 which lands me at 808.6125 for an input frequency.

Can anyone tell me if I'm on the right path?

Also, any idea if the input frequencies are repeated? (i.e. would I only be picking up troopers in close proximity to me, as opposed to their signals on 808Mhz being repeated to other towers on the site using the same input frequency)

Thanks in advance.

 

[nhfdcadet]

The DB has the band plan in it. But I don't know how well you'd be able to monitor the input frequencies, you're only going to hear nearby units.

 

[GTR8000]

700 and 800 MHz use a standard offset, no mystery to it. +30 for 700, -45 for 800. And no, the input frequencies are not repeated, that would entirely defeat the purpose of them being inputs to repeaters.

 

[adamfancher]

Yeah that's what I'm going by:


I guess my real remaining question is, if a trooper transmits (data or anything else) on the input frequency on the site above (Troop B) and the nearest tower is in Sharon, does that signal not get repeated to the other towers on the site, on that frequency?

I'm actually located right on top of the Winsted tower, which I would imagine has a preamplified input but then again I really have no idea.

 

[adamfancher]

700 and 800 MHz use a standard offset, no mystery to it. +30 for 700, -45 for 800. And no, the input frequencies are not repeated, that would entirely defeat the purpose of them being inputs to repeaters.

Got it, thanks.

 

[kayn1n32008]

Yeah that's what I'm going by:

View attachment 169464
I guess my real remaining question is, if a trooper transmits (data or anything else) on the input frequency on the site above (Troop B) and the nearest tower is in Sharon, does that signal not get repeated to the other towers on the site, on that frequency?

I'm actually located right on top of the Winsted tower, which I would imagine has a preamplified input but then again I really have no idea.

There are going to be 2 transmissions, the first will be on the input frequency of the control channel. This transmission is going to be very brief. It's the radio telling the controller it's RID and what talkgroup it wishes to use. This transmission is NOT repeated. On the control channel(CC), the controller is going to tell the transmitting radio to either switch to a voice channel(VC) it assigns, or that it is denying the request. If the request is denied, the radio will not transmit further, and emit a 'bonk' the user attempting to access the site. There are various reasons a radio could be denied a VC channel grant, but that is beyond the scope of this thread.

If the controller grants a voice channel, the controller will tell that radio to tune to the VC it assigns, and tell all other radios on that site to also tune to the same VC.

In a multicast trunk system, the controller will also assign(if available) a voice channel on each remote site there are radios affiliated to the same talkgroup.

Once the VC has been assigned, the transmitting radio tunes to the VC and begins to transmit on the repeater input (45MHz below the repeater output for 800MHz, or 30MHz above the repeater output for 700MHz systems.) The users radio will play an audible 'Talk Permit Tone'(aka the triple Beep for Motorola radios) indicating to the user they can now start talking.. The voice traffic is repeated on the assigned VC repeater output for radios affiliated to the same site to receive the voice traffic.

On the multicast sites, once a VC is assigned at the site(s), it is announced on the CC for affiliated radios to tunes to the VC on sites that are carrying this traffic, there is no transmission on the VC input frequency, the voice traffic comes only over the network connection from the site that has the subscriber adio transmitng.

 

[kayn1n32008]

For a simulcast system, all sites in the cell have the exact same receive and transmit frequencies. To the subscriber it appears to be one site, even if it covers a huge geographical area.

For example the city I live close to has a 5 site simulcast cell that covers roughly 264 sq miles. Each of the 5 sites have 24 repeater pairs assigned to it. Depending on. Where you are in the city, if you transmit 1, 2, 3, 4 or 5 sites could potentially hear your transmission.

Our old EDACS system also had additional receiver sites through out the city to improve talk in range for portable users. The P25 simulcast cell may also have remote receivers to improve portable talk-in.

If more than one receiver in the simulcast cell can hear the subscriber, the cell controller will choose the receiver with the lowest bit error rate. It will also do this continously so that none of the transmission is missed or lost.

 

[adamfancher]

Great info, thanks!

 

[kayn1n32008]

I hope thay helps understanding of how these systems work. They are incredibly complex, bit when properly built and implemented, they are absolutely rock solid.

 

[adamfancher]

Yes, that clears a few things up for me.

So negating other towers on the site...a radio transmits to the closest tower, and I'm standing next to the tower with my scanner. The tower can hear the transmission and I can't. Is this a matter of amplification of the input signal at the tower? I did confirm that nothing even close to a signal can be detected on my 806.1375 and 806.6125 input frequencies when there is traffic on the control and voice channels.

I do have 30dB of amplification on my rig but not even seeing weak signals.

 

[GTR8000]

You're dealing with a simulcast cell, which consists of multiple physical TX/RX sites (subsites) all utilizing the identical set of input and output frequencies. The subscribers don't transmit to the closest tower, they simply transmit on the input frequencies, and the comparators for the cell vote to the strongest signal. That's the signal that gets rebroadcast over the traffic channels, and the voting occurs in real time so that a mobile subscriber can seamlessly move between subsites.

Nearly every subsite would have a tower top amp (TTA) amplifying the inbound signals, as well of course of a height advantage vs you on the ground with a rubber ducky antenna on a scanner. So unless the subscriber is known to be very nearby your location, you're probably not going to pick much up on the inputs.

In addition to all of that:

a) Inbound signaling on the control channel input is extremely brief. So brief that a scanner might not even be quick enough to open squelch for it.

b) TDMA inbound signaling is not continuous carrier, but rather very quick on/off/on/off pulsing of the subscriber's power amplifier, which would also likely not be picked up with a scanner.

 

[W1KNE]

Nearly every subsite would have a tower top amp (TTA) amplifying the inbound signals, as well of course of a height advantage vs you on the ground with a rubber ducky antenna on a scanner. So unless the subscriber is known to be very nearby your location, you're probably not going to pick much up on the inputs.

To add to GTR's comment here:
Not to mention the antenna used is very band specific and has elevation gain to it as well. The more narrow the antenna is to the intended frequency range, the more sensitive it will be to the specific signals as well. While there are "800" rubber duckie antennas for the scanner, they're usually a lot more broad band (typically they're 700 to 1300) and have no elevation gain to them.

 

[cg]

I have picked up the inbound traffic but it was on a stand alone site with me at a slightly elevated location. It is nothing special and was similar to what comes over the output. Keep in mind that CLMRN does not do location data for radios (or at least did not when I was in a meeting discussing the system with the state several years ago.
Any PD unit and many FD units around the state use MDTs or IPADs with cellular location showing dispatch and other units where they are. It is not necessary to pipe it over the radios.
That doesn't mean the radios can't do it, just not through CLMRN.

 

[adamfancher]

This is a lot of great information, thanks everybody!