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Digital Comms

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HogDriver

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#1
Question...When you have two users with analog radios on the same frequency, but different PL tones, transmitting at the same time, the signals cover each other. Does the same happen with digital transmissions or do they manage to deconflict because of the digital nature and different color codes, etc.


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#3
The same thing happens with standalone digital stations (such as two P25 users on the same frequency using different NAC codes) unless they utilize some type of frequency hopping or spread spectrum technology or similar.

Shawn
 

cmjonesinc

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#4
If something is using a frequency whether analog voice, digital voice, data, morse or anything and something else is within range using the same frequency you're going to have interference. Minus something like dmr using the same frequency but a different timeslot.
 
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#5
P25 digital also uses separate timeslots on the same frequencies. The reason for going from 12.5mz to 6.25mz separation. Dynamic range not as good but no interference.

Mark
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#8
Question...When you have two users with analog radios on the same frequency, but different PL tones, transmitting at the same time, the signals cover each other. Does the same happen with digital transmissions or do they manage to deconflict because of the digital nature and different color codes, etc.


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If two digital signals such as P25 conventional transmit at the same time they will interfere in the same way that analog signals do. There is some capture ratio, a range by which one or the other signals will be significantly understandable, however with narrowband digital signals, that benefit is minimum. Unlike analog signals where a listener might be aware interference is occurring by the sound, in digital, the listener cannot determine if the difficulty is due to range, interference or that there was any attempt to transmit.

Color Codes, NAC/NID's and Talk Groups do nothing to limit potential co-channel contention.

Some digital techniques such as TDMA and CDMA operate under system rules that permit two or more transmitters to seemingly occupy the same channel. These systems coordinate themselves to avoid interference. DMR (and Tetra) uses TDMA and relies on a master/slave arrangement to assign time slots. In this case a user on slot 1 can operate independently from slot 2. However interference from another station that isn't synchronized to avoid an occupied time slot will cause interference. CDMA is the technique used for WiFi and indeed interference cannot always be avoided as the WiFi bands are over loaded. CDMA works for cellphones because the frequency band is managed by the telephone company who make it their business to avoid interference between their own stations.
 

jwt873

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#9
Some systems avoid interference by preventing transmissions when the channel is busy.

With our local ham radio DMR system, when you press PTT, your radio won't transmit on top of anyone. And, it will produce a 'bonk' tone to let you know the and slot you're trying to transmit on is busy.
 
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#12
Since I was referring to 6.25 mz spread, I was referring to phase II

Mark
WS1095/536/436/996P2/HP1e/HP2e/996XT/325P2/396XT/PRO668/PSR800/PRO652
Do you mean 6.25KHz? If so, P25 Phase 2 is only 6.25KHz Equivilant. TDMA does not use less spectrum than a FDMA channel, it only allows 2 conversations in the same bandwidth.
 
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#13
Some digital techniques such as TDMA and CDMA operate under system rules that permit two or more transmitters to seemingly occupy the same channel. These systems coordinate themselves to avoid interference. DMR (and Tetra) uses TDMA and relies on a master/slave arrangement to assign time slots. In this case a user on slot 1 can operate independently from slot 2. However interference from another station that isn't synchronized to avoid an occupied time slot will cause interference. [...]
I don't see how this relates to timeslots because 2 (DMR) or 4 (TETRA) timeslots are on the same frequency and it doesn't matter if the control channel ("master") is on TS1 and the voice is assigned to another TS ("slave") - if the interferer is on the same frequency it will affect all the timeslots on that transceiver.

-------------------

Digital Simulcast systems (Single Frequency Networks) cope fine with the same frequency being re-used.
 
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#14
I don't see how this relates to timeslots because 2 (DMR) or 4 (TETRA) timeslots are on the same frequency and it doesn't matter if the control channel ("master") is on TS1 and the voice is assigned to another TS ("slave") - if the interferer is on the same frequency it will affect all the timeslots on that transceiver.

-------------------

Digital Simulcast systems (Single Frequency Networks) cope fine with the same frequency being re-used.
Reread the entire context in which I replied to the OP. I am talking about code division or time slot assignment and coordination within a SYSTEM. Yes external interference from an analog or digital system unrelated to the subject system would cause interference. For TDMA systems, in practice to prevent self interference, reuse of a frequency and timeslots would require geographic separation, CDMA systems are somewhat more tolerant from self interference given the enormous numbers of orthogonal codes.

Digital simulcast systems work well to a point where inter-symbol interference occurs due to the sites being spaced too far apart.
 
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#15
Reread the entire context in which I replied to the OP. I am talking about code division or time slot assignment and coordination within a SYSTEM. Yes external interference from an analog or digital system unrelated to the subject system would cause interference. For TDMA systems, in practice to prevent self interference, reuse of a frequency and timeslots would require geographic separation, CDMA systems are somewhat more tolerant from self interference given the enormous numbers of orthogonal codes.
DMR and TETRA use TDMA, there is no way to avoid interference on a particular timeslot since timeslots on the same carrier do not use different frequencies. So it wouldn't matter if the interference comes from the same or other system. I am also not aware of any TETRA or DMR system that does have such a function (with the exception of single frequency networks).

Perhaps your terminology is off since there is no reuse of timeslots in relation to geographic location, at least not in TETRA or DMR.

Else I would be very interested to learn how you think a TDMA system would avoid (internal) interference on individual timeslots.
 
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#16
It depends on two factors. One, whether or not we are talking AM or FM based (AM does not suffer the FM capture effect). Two time division versus frequency division.


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