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FDMA or TDMA for Itinerant freq 464.550

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matt131

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Hello,

I am hoping that I don't open up a big can of worms but I am looking for advice for what digital mode to use on my licensed Itinerant frequency of 464.550.

I am currenly using which I have appropriately licensed MotoTrbo on 464.550. As I think about the FDMA and TDMA digital technologies for the long run would I have done better using FMDA instead of TDMA (MotoTrbo)? I only say that because without a repeater when the FCC says 6.25khz if I use TDMA without a repeater I am really only using 12.5 and not 6.25.

Thanks for the help and comments.
 

Mike_G_D

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12.5 TDMA is 6.25 equivalent as far as the FCC is concerned
But not in simplex mode - in simplex direct unit-to-unit mode without using infrastructure (repeaters, controllers, etc.) the current TDMA MotoTrbo units revert to ONE voice channel per RF channel (just like FDMA digital or standard analog units) so would only have one voice channel per 12.5KHz RF channel. This is partly, as I understand it, because the exact timing signal needed to maintain correct timing for the proper separation of the two time slots used in the normal infrastructure mode which comes from the repeater or site controller and is not available in simplex mode. Of course, in simplex mode, to even really need that TDMA two slot advantage you would need to have many units operating on multiple channels simultaneously within close proximity to each other so the argument can be made that this is not a common need (without infrastructure). The FDMA digital folks would disagree, of course, and here, it is true, they can claim an advantage because FDMA units can (well, the NXDN varieties, anyway) maintain true 6.25KHz channel bandwidth per voice channel. However, this advantage is potentially more of a problem with adjacent channel interference - though, as has been noted in these forums, systems built around RF channels spaced exactly 6.25KHz apart and co-located do exist and are operative so maybe the equipment is good enough to effectively work with this. If so, then I do see that this is an advantage for NXDN systems if close in multi-channel direct mode is needed routinely.

-Mike
 

matt131

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In simple terms I guess my question is, Will I one day have to switch to true 6.25 since I am only using simplex, or is MotoTrbo considered 6.25 repeater or no repeater?

Thanks,
 

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In simple terms I guess my question is, Will I one day have to switch to true 6.25 since I am only using simplex, or is MotoTrbo considered 6.25 repeater or no repeater?

Thanks,
Well, I don't know the legal answer to your question - as I stated earlier, the technical answer is that (as far as I know using the currently available equipment, etc.) MotoTrbo is not true 6.25KHz BW in simplex mode not using infrastructure. As to whether that will mean you will have to replace the equipment in the future once 6.25KHz bandwidth modes are mandatory...maybe.

I am guessing that Motorola will either work out a fix to reduce the modulation bandwidth of MotoTrbo in simplex mode (whether that will be a hardware, firmware, or both fix, I can't guess) or that they will apply for a special waiver from the FCC (maybe they already have done this) to allow their equipment to operate in 12.5KHz channels in simplex mode even after the 6.25KHz channels are mandated. But I really don't know - it's a legal regulations question and I am not knowledgeable in that area (at least relative to my technical knowledge). But given Motorola's strong influence as a two-way radio manufacturer, I think you will be safe for some time - at least until your equipment needs to be replaced from shear old age.

My question to you, though, is why did you go with MotoTrbo at all if you were mainly or only going to use it in simplex mode on itinerant frequencies? You really don't get any added channel capacity that way. In this case, I would have gone fully FDMA and used NXDN equipment if you really needed digital.

-Mike
 

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FDMA cannot be done on a simplex frequency, FDMA is dynamically assigning a channel on an on demand basis. TDMA is taking a single RF frequency and having 2 or more time slots on it and allowing those users to 'share' the one frequency at the same time.
 

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FDMA cannot be done on a simplex frequency, FDMA is dynamically assigning a channel on an on demand basis. TDMA is taking a single RF frequency and having 2 or more time slots on it and allowing those users to 'share' the one frequency at the same time.
Umm...believe me, ...this, as the saying goes, "ain't my first rodeo". I KNOW VERY BLOODY WELL the technical distinctions between FDMA and TDMA - I was involved with the design of such systems! In truth, unless we are talking DSSS (Direct Sequence Spread Spectrum - and, even then, current regulations restrict DSSS systems such as "CDMA" to a certain RF channel bandwidth so, in a sense, they are also, at least partially, FDMA), even so-called "TDMA" systems are actually FDMA + TDMA in RF terms. Fully FDMA only systems such as NXDN (Kenwood's "NEXEDGE" and ICOM's "iDAS") are, ideally, quite capable of yielding twice the capacity of one analog 12.5KHz channel whereas TDMA based MotoTrbo systems cannot, (CURRENTLY), do so. MotoTrbo systems, as I understand it, based on CURRENT technology, cannot, when in simplex mode, using no infrastructure (no repeaters or site controllers and only in direct unit-to-unit mode) revert to ONE and only ONE voice channel per 12.5KHZ RF channel whereas, again, IDEALLY, true fully FDMA-only systems, which are, IDEALLY, capable of true 6.25KHz channel bandwidth, can, IDEALLY, yield TWO voice channels per 12.5KHz bandwidth. "Dynamically assigned" is, IDEALLY, irrelevant, unless we are talking trunked (site controller based) systems and, even so, is, again, IDEALLY, applicable to both FDMA-only and TDMA (actually, FDMA+TDMA) systems.

-Mike
 
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matt131

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I currently only have 2 Trbo radios right now that is why I am asking this question for the long run. I actually am an Icom person. I have always used Icom radios and I really like the F4161DS portables by Icom. You may now ask why I went Trbo. Honestly the answer is due to where I use the radios, Snow storms rain etc, I wanted a water proof radio. Trbo claims to be water proof to IP57 standards.

I guess I just don't want to have to buy new equipment in 6 years if that is when 6.25 may take place.
 

matt131

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I called M and they couldn't answer the question about Trbo 6.25khz in direct radio to radio mode. Can anyone find any information supporting 6.25 in radio to radio mode?

Should I just go FDMA 6.25 on my Itinerant freq?
 

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Perhaps a silly question, but doesn't your FCC license have an emission mode specified, and what would that be?
 

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Mike_G_D said:
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FDMA cannot be done on a simplex frequency, FDMA is dynamically assigning a channel on an on demand basis. TDMA is taking a single RF frequency and having 2 or more time slots on it and allowing those users to 'share' the one frequency at the same time.
Umm...believe me, ...this, as the saying goes, "ain't my first rodeo". I KNOW VERY BLOODY WELL the technical distinctions between FDMA and TDMA - I was involved with the design of such systems! In truth, unless we are talking DSSS (Direct Sequence Spread Spectrum - and, even then, current regulations restrict DSSS systems such as "CDMA" to a certain RF channel bandwidth so, in a sense, they are also, at least partially, FDMA), even so-called "TDMA" systems are actually FDMA + TDMA in RF terms. Fully FDMA only systems such as NXDN (Kenwood's "NEXEDGE" and ICOM's "iDAS") are, ideally, quite capable of yielding twice the capacity of one analog 12.5KHz channel whereas TDMA based MotoTrbo systems cannot, (CURRENTLY), do so. MotoTrbo systems, as I understand it, based on CURRENT technology, cannot, when in simplex mode, using no infrastructure (no repeaters or site controllers and only in direct unit-to-unit mode) revert to ONE and only ONE voice channel per 12.5KHZ RF channel whereas, again, IDEALLY, true fully FDMA-only systems, which are, IDEALLY, capable of true 6.25KHz channel bandwidth, can, IDEALLY, yield TWO voice channels per 12.5KHz bandwidth. "Dynamically assigned" is, IDEALLY, irrelevant, unless we are talking trunked (site controller based) systems and, even so, is, again, IDEALLY, applicable to both FDMA-only and TDMA (actually, FDMA+TDMA) systems.

-Mike
Well if it aint your first rodoe then can you kindly explain how one does FDMA on a SIMPLEX frequency??????????
 

matt131

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Based on readings from papers from M and posts on different sites, TDMA in simplex is NOT 6.25 so this leads me to just not buy anymore Trbo radios and make the switch to Icom and modifly my FCC license.

BUT, then lets say we jump forward 8 years or so and 6.25 is now mandatory, if you repeater goes down and you operate MotoTrbo in simplex you lost your two time slots. ARE YOU NOW OPERATING ILLEGALLY???? This is because you technically are operating 12.5
 

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Based on readings from papers from M and posts on different sites, TDMA in simplex is NOT 6.25 so this leads me to just not buy anymore Trbo radios and make the switch to Icom and modifly my FCC license.

BUT, then lets say we jump forward 8 years or so and 6.25 is now mandatory, if you repeater goes down and you operate MotoTrbo in simplex you lost your two time slots. ARE YOU NOW OPERATING ILLEGALLY???? This is because you technically are operating 12.5
I'm, unfortunately, in the middle of a major power outage and have my networking equipment on limited emergency power so probably won't be checking in for awhile here after this. Anyway, I seriously doubt that if and when 6.25KHz channels become mandatory you will automatically be prohibited from operating any MotoTrbo equipment in simplex mode. For one thing, that kind of operation is low power by nature so is not as likely to cause major issues (at least as much as a high profile repeater system). Also, like I stated before, I rather think Motorola will ask for and likely be granted a special waiver for simplex operation - again, given the low level and power of such operations I think it most likely that the FCC will go easy on such users. Keep in mind that, as I understand it, mandatory 6.25KHz operation is still a ways off (12.5KHz operation becomes mandatory next year, if I understand correctly, but not 6.25KHz for some indefinite period of time). You likely have plenty of time to use your Trbo equipment and then, when the 6.25KHz operation becomes mandatory, can reconsider what type of system you want to change to or otherwise upgrade your equipment. Personally, I wouldn't worry too much at least for the next several years. If you CAN change your equipment you MIGHT want to consider a fully FDMA system, especially if your primary operation is simplex and on itinerant frequencies.

kayne1n32008,

I think you have a confused understanding of FDMA versus TDMA radio operation. FDMA relies entirely on radio frequency separation to fulfill voice channel capacity. Any standard analog FM system is, by nature, an FDMA system. When applied to digital systems, the modulation is digital but all channel separation is still based solely on radio frequency separation. TDMA systems, on the other hand, use their digital mode together with a master timing signal from the site controller to separate, in time, two or more voice data streams per radio channel. This need for a master timing signal from a very stable source makes the system not as practical in simplex mode since most mobile and portable units will not have as stable a timing/frequency source and, really, in order to work as a multi-slot TDMA system, I would think one unit in a group would have to be designated a "master" so as to regulate the assigning of timing slots to voice conversations. When only two units are in operation or when only one voice channel is needed, not having multiple time slots is not a real issue, of course, so this limitation is not usually a major one.

In terms of the two current competing systems that are representative of the TDMA versus FDMA issue, DMR/MotoTrbo for TDMA and NXDN for FDMA, when no infrastructure is used (no site controller or repeater) the current MotoTrbo equipment reverts to one voice channel per 12.5KHz wide radio channel frequency whereas current FDMA NXDN equipment can actually fit two 6.25KHz voice channels into the same space and, apparently, given reports on this site, do so reliably. I admit that this would require tight IF filtering to work in co-located situations but they have apparently licked that issue.

Gotta save power so gotta go.

-Mike
 

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Very nice explaination of FDMA vs. TDMA.

John Rayfield, Jr. CETma

I'm, unfortunately, in the middle of a major power outage and have my networking equipment on limited emergency power so probably won't be checking in for awhile here after this. Anyway, I seriously doubt that if and when 6.25KHz channels become mandatory you will automatically be prohibited from operating any MotoTrbo equipment in simplex mode. For one thing, that kind of operation is low power by nature so is not as likely to cause major issues (at least as much as a high profile repeater system). Also, like I stated before, I rather think Motorola will ask for and likely be granted a special waiver for simplex operation - again, given the low level and power of such operations I think it most likely that the FCC will go easy on such users. Keep in mind that, as I understand it, mandatory 6.25KHz operation is still a ways off (12.5KHz operation becomes mandatory next year, if I understand correctly, but not 6.25KHz for some indefinite period of time). You likely have plenty of time to use your Trbo equipment and then, when the 6.25KHz operation becomes mandatory, can reconsider what type of system you want to change to or otherwise upgrade your equipment. Personally, I wouldn't worry too much at least for the next several years. If you CAN change your equipment you MIGHT want to consider a fully FDMA system, especially if your primary operation is simplex and on itinerant frequencies.

kayne1n32008,

I think you have a confused understanding of FDMA versus TDMA radio operation. FDMA relies entirely on radio frequency separation to fulfill voice channel capacity. Any standard analog FM system is, by nature, an FDMA system. When applied to digital systems, the modulation is digital but all channel separation is still based solely on radio frequency separation. TDMA systems, on the other hand, use their digital mode together with a master timing signal from the site controller to separate, in time, two or more voice data streams per radio channel. This need for a master timing signal from a very stable source makes the system not as practical in simplex mode since most mobile and portable units will not have as stable a timing/frequency source and, really, in order to work as a multi-slot TDMA system, I would think one unit in a group would have to be designated a "master" so as to regulate the assigning of timing slots to voice conversations. When only two units are in operation or when only one voice channel is needed, not having multiple time slots is not a real issue, of course, so this limitation is not usually a major one.

In terms of the two current competing systems that are representative of the TDMA versus FDMA issue, DMR/MotoTrbo for TDMA and NXDN for FDMA, when no infrastructure is used (no site controller or repeater) the current MotoTrbo equipment reverts to one voice channel per 12.5KHz wide radio channel frequency whereas current FDMA NXDN equipment can actually fit two 6.25KHz voice channels into the same space and, apparently, given reports on this site, do so reliably. I admit that this would require tight IF filtering to work in co-located situations but they have apparently licked that issue.

Gotta save power so gotta go.

-Mike
 

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Mike_G_D: The op stated that he is using one simplex channel, and as such, the term FDMA does not truly apply. I believe that Kenwood and Icom will do true 6.25Khz digital voice, rather than Mototurbo 6.25Khz 'equivalent, which as stated is only 12.5Khz in simplex mode. It is nice to see someone thinking ahead to the possibility of future 6.25Khz narrowbanding. The OP would probably be better off going to Icom or Kenwood NXDN and have the ability to just modify programming (rather than have to upgrade equipment and/or firmware by staying with Turbo equipment) and license in the future when it will(probably) be mandated at some point in the future.
 

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Mike_G_D: The op stated that he is using one simplex channel, and as such, the term FDMA does not truly apply. I believe that Kenwood and Icom will do true 6.25Khz digital voice, rather than Mototurbo 6.25Khz 'equivalent, which as stated is only 12.5Khz in simplex mode. It is nice to see someone thinking ahead to the possibility of future 6.25Khz narrowbanding. The OP would probably be better off going to Icom or Kenwood NXDN and have the ability to just modify programming (rather than have to upgrade equipment and/or firmware by staying with Turbo equipment) and license in the future when it will(probably) be mandated at some point in the future.
Power is back on - yeah!

Somehow, I think we are actually agreeing with each other but are having a problem with terminology. Simplex is just an operational mode, in radio terms it implies direct unit-to-unit operation without infrastructure (repeaters, trunking systems, etc.). Unless we are talking very wideband direct sequence spread spectrum (DSSS) pretty much ALL radio traffic is FDMA (Frequency Division Multiple Access) either partially or fully. Even TDMA (Time Division Multiple Access) systems are really FDMA as well as TDMA in that they are using common radio frequency channelizing schemes in addition to the time slot digital encoding. Simplex narrowband radio traffic whether digital or analog as well as repeated and trunked systems all are FDMA since they use radio channelizing schemes as agreed upon by the local governing agency (in the US, the FCC, of course). Project 25 Phase 1 is fully FDMA only whether in simplex, conventional repeated, or trunked mode. Project 25 Phase 2 is two slot TDMA which really means it is FDMA with TDMA capacity enhancement.

The "6.25KHz equivalent" statement in reference to MotoTrbo systems is confusing to a lot of people. Technically, from a purely voice channel capacity standpoint it is accurate but, from a purely radio frequency-centric standpoint it is not. MotoTrbo TDMA systems broadcast a radio carrier which uses the whole 12.5KHz channel width (not exactly but with guard bands it's about right) BUT it DOES carry TWO voice channels on that carrier so...one can say that the capacity is "6.25KHz equivalent" in that it carries two voice channels for every 12.5KHz channel just as a fully FDMA system using 6.25KHz narrowband channels does, ideally. Of course, fully FDMA systems really are true 6.25KHz channelized systems BOTH from a radio channel standpoint AS WELL AS from a voice channel capacity standpoint which is why some proponents of FDMA cry foul with the way TDMA systems are presented. Nevertheless, I think that TDMA is an excellent choice for someone who already has a large system in place and wants to double capacity and conform to the narrower standard but do so with much less radio "plumbing" issues than would be potentially involved with adding the same capability in FDMA only systems. TDMA systems replace added RF complexity with added computational complexity which may make it more attractive from a maintenance standpoint given greater availability of IT personnel versus radio technology maintenance personnel. But, when added capacity is needed without any infrastructure then true FDMA only systems capable of operating within 6.25KHz bandwidths even when multiple simultaneously operating units on adjacent channels are co-located (apparently they can) are a better choice. Yes, as you stated, Kenwood (NEXEDGE) and ICOM (iDAS) NXDN radios both do true 6.25KHz channel bandwidths which is exactly what I have also said. The only thing I think we are disagreeing on is the applicability of the term/acronym "FDMA" to a simplex radio channel. As I said, technically, it is just as applicable as it would be to a repeated or trunked system - in fact standard AM broadcast stations are "FDMA". At this point, I don't know how better to explain it. And, yes, I have worked on the designs of subscriber equipment for both TDMA and FDMA systems.

-Mike
 
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