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Old 04-09-2018, 1:24 PM
Join Date: Oct 2015
Posts: 2
Default p25

Hi everyone,
I am totaly new to P25 and will risk asking a stupid question.
If there can be 2 voice channels on the same 12.5 rf bandwith:
Can I have two repeaters each working on each 6.25 bandwidth ?

The intended purpose is to get extended coverage of two repeater locations, but without incurring licencing costs of asking for another 12.5 khz slot.

Thanks anyone for comments and suggestions.
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Old 04-13-2018, 12:35 PM
discombobulated's Avatar
Join Date: Nov 2013
Posts: 115

The way you can have 2 channels on one frequency is by time slots. So you are chopping up data and saying 1 is channel A and 2 is channel B. When you change to channel B on your radio it is still the same frequency but the radio is only listening to the number 2's in the data. Then the radio ignores the 1's and assembles the 2's into voice. So when the repeater transmits it is spitting out 121212121212. In simplified terms.

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Old 04-13-2018, 5:53 PM
scanmanmi's Avatar
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Location: Central Michigan
Posts: 375

I'm a little new like you. Maybe something like this?
Second place is the first loser.
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Old 04-13-2018, 7:33 PM
  Audio Feed Provider
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Default Answer from a hobbiest

I'm not an expert in the field, but I'll attempt to give you an answer since there have only been a few comments so far. There are some fancy terms that I'll be lacking but the general message should be correct.

First, not a stupid question at all. It is a great one and shows your interest in learning the technical details.

12.5K Phase II P25 is TDMA for two voice channels. The repeater is really sending out one combined transmission and the radio listens for which part it is attempting to follow. So both voice channels would need to be from the same physical repeater. Now the two people talking from their handhelds works a little bit different. The input to the repeater comes in from one radio transmitting and then the other radio transmitting, alternating back and forth and using the entire 12.5K bandwidth during its time to transmit or its slot. The portable radio gets it timing by listening to the repeater when it's not transmitting. And also while it's not transmitting it's also possible for the radio to receive commands, like stop transmitting for higher priority traffic.

So to answer you question, no it is not possible to split the locations of a licensed frequency for P25 TDMA..

Which leads to why not use FDMA? FDMA is the opposite of TDMA. Instead of splitting by Time Slots, you split by frequency. That is two separate 6.25khz channels for the same equivalent 12.5khz.

That was the road that P25 Phase II was originally heading down. But it become more advantageous to use TDMA. There are a number of reasons and some technical documents to be found out there. Basically it just ended up being simpler and easier to maintain. If a system needed a total of 20 voice channels, 6.25khz would require 20 FCC licensed frequencies (not counting the control channel) and would require 20 repeaters. With TDMA, you only need 10 licensed frequencies and 10 repeaters for the same 20 voice channels. And probably even more importantly, it prolongs the battery life in handheld radios, as the radio only has to transmit 50% of the time while keyed up, versus a continuous transmission on 6.25 khz.

I'm not aware of any commercial equipment that does 6.25khz P25. Perhaps there were some advanced models before the Phase II specification was finalized. So it could technically be done, but you'd lack mainstream support and still be stuck having to pay for another license.

Finally, a license is usual tied to a fixed repeater tower location.
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