What's the advantage of TDMA?

[PhillyPhoto]

I understand the basic concept of FDMA vs TDMA, but I was wondering what the actual advantage of TDMA is? I know cell phones have been doing something similar for years, so what's the benefit to radio users?

 

[W8RMH]

Project 25 Phase II TDMA

The Phase II standard is a 2-slot TDMA signal that fits inside a 12.5 kHz wide channel.

This allows existing 12.5 kHz wide license holders to double call capacity by upgrading their infrastructure to Phase II.

 

[PhillyPhoto]

So would systems that transition from Phase I to Phase II be able to reduce their licenses once the transition is complete, or would they keep them to handle increased traffic down the road?

 

[kayn1n32008]

...This allows existing 12.5 kHz wide license holders to double call capacity by upgrading their infrastructure to Phase II.

Not quite double, the control channel is still 12.5KHz, so on a 6 channel system, one whole RF channel is the control, so you end up with 10 voice paths for 6 RF channels. 8 voice paths for 5 RF channels ect. For a TDMA system it will be 2 paths less than double the RF channels. (Edit: two voice paths less than double the RF channels PER SITE.)


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[kayn1n32008]

So would systems that transition from Phase I to Phase II be able to reduce their licenses once the transition is complete, or would they keep them to handle increased traffic down the road?

In theory yes, but with channels hard to come by in places, I doubt an agency would give any up "extra" channels after migration.


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[PhillyPhoto]

Not quite double, the control channel is still 12.5KHz, so on a 6 channel system, one whole RF channel is the control, so you end up with 10 voice paths for 6 RF channels. 8 voice paths for 5 RF channels ect. For a TDMA system it will be 2 paths less than double the RF channels.


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V = 2(F-S) where V is voice paths, F is the total number of frequencies and S is the total number of sites

In theory yes, but with channels hard to come by in places, I doubt an agency would give any up "extra" channels after migration.


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That's what I was figuring. Are the systems taken into account when they apply for frequencies? For example, if a system is going to be Phase II from the get go, would they get less channels than they might otherwise get?

 

[kayn1n32008]

That's what I was figuring. Are the systems taken into account when they apply for frequencies? For example, if a system is going to be Phase II from the get go, would they get less channels than they might otherwise get?

I have no idea.



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[RayAir]

It's just new junk to sell. Got to come up with new schemes every so often so they have new products to hawk.

 

[W4DIK]

Has a purpose

It is not just junk to sell. I can replace a 6 channel LTR system with a 3 repeater Mototrbo system and get the same capacity and save thousands of dollar. Multiply this by 3 systems, and the savings will buy me about 150 radios. That is not chump change.

 

[domes]

How is it saving you money if you already own the 6 FDMA LTR repeaters & mobiles and now the FCC tells you to throw them away and spend more money to replace them with something new?

12.5 TDMA (Mototurbo) may just be an acceptable transition system. The FCC does accept TDMA as being "6.25 efficient" on 12.5 channels but what happens when the 12.5 channels go away? When the FCC began the narrow banding process about 15 years ago they said that 6.25 channelization is their ultimate goal. 12.5 is just the transition phase. The FCC will determine in 2015 if it will be necessary to mandate 6.25 and if so how & when.

Obviously you can double your capacity on existing 12.5KHz channels with either TDMA or 2 VN-FDMA repeaters, but that is not the FCC's concern. They want to cut your spectrum in half again so they can make new 6.25 channels for new licensees. If and How any 12.5 wide radios (TDMA or whatever emission) will exist on the FCC's future 6.25 channelization plan is yet to be determined. So if you have six - 12.5 channels today the FCC may mandate you to six - 6.25 channels, rendering Mototrbo obsolete. Or perhaps they will permit the equivalent spectrum of three - 12.5 channels in the form of 6.25 adjacent pairs. meaning that TDMA systems will actually have to operate "6.25 efficient".

 

[hitechRadio]

How is it saving you money if you already own the 6 FDMA LTR repeaters & mobiles and now the FCC tells you to throw them away and spend more money to replace them with something new?

12.5 TDMA (Mototurbo) may just be an acceptable transition system. The FCC does accept TDMA as being "6.25 efficient" on 12.5 channels but what happens when the 12.5 channels go away? When the FCC began the narrow banding process about 15 years ago they said that 6.25 channelization is their ultimate goal. 12.5 is just the transition phase. The FCC will determine in 2015 if it will be necessary to mandate 6.25 and if so how & when.

Obviously you can double your capacity on existing 12.5KHz channels with either TDMA or 2 VN-FDMA repeaters, but that is not the FCC's concern. They want to cut your spectrum in half again so they can make new 6.25 channels for new licensees. If and How any 12.5 wide radios (TDMA or whatever emission) will exist on the FCC's future 6.25 channelization plan is yet to be determined. So if you have six - 12.5 channels today the FCC may mandate you to six - 6.25 channels, rendering Mototrbo obsolete. Or perhaps they will permit the equivalent spectrum of three - 12.5 channels in the form of 6.25 adjacent pairs. meaning that TDMA systems will actually have to operate "6.25 efficient".

I think you answered your own question. The Mototrbo systems are already 6.25 as far as the FCC is concerned.

 

[domes]

No - TDMA systems are not already considered 6.25 as far as the FCC is concerned. No where in the rules do the say that or use the term "equivalent to 6.25". They do use the term "efficient". I think the poster in #9 probably has it right. He says that he can replace his 6 - 12.5 LTR channels with the 3 - 12.5 trbo channels (or x 2-VN ) and be as efficient. And that might be what the FCC will require in the future.

I'm not trying to knock any TDMA system, I'm just saying that either the FCC is going to narrow band again to 6.25 channels, or they won't. And all TDMA systems require 12.5 channels.

Having said all that. TDMA as a technology is not all that efficient for MA (multiple access) and it does have its faults. 1:1 efficiency (compared to FDMA bandwidth) may be of some cost savings in hardware but the FCC is concerned with limited spectrum and creating new channels for future users. TDMA quickly outlived its usefulness in the Cell industry and was replaced by CDMA which is a much cleaner and more efficient technology. With all of the development in the LTE area and other wide band needs in public safety, I would almost expect that PS channels will actually go wider while other private services will go narrower by necessity.

 

[n5ims]

Perhaps not in the rules, but clearly in the narrowband FAQ they indicate that two voice channels in the wider bandwidth meet the mandate for narrowband migration. VHF/UHF Narrowbanding FAQs

What is Narrowbanding?

Narrowbanding is an effort to ensure more efficient use of the VHF and UHF spectrum by requiring all VHF and UHF Public Safety and Industrial/Business land mobile radio (LMR) systems to migrate to at least 12.5 kHz efficiency technology by January 1, 2013.

More specifically, all existing Part 90 radio systems operating in the 150-174 MHz and 421-512 MHz bands have until January 1, 2013 to convert those systems either to a maximum bandwidth of 12.5 kHz or to a technology that provides at least one voice path per 12.5 kHz of bandwidth or equivalent efficiency.

What does Equivalent Efficiency mean?

Any of the following meet the 12.5 kHz equivalent efficiency requirement:
• One voice path in a 12.5 kHz channel
• Two voice paths in a 25 kHz channel
• Data operations on channels greater than 12.5 KHz must employ data rates greater than 4.8 kbps per 6.25 kHz channel, such as 19.2 kbps per 25 kHz channel

They also indicate that there is no firm date for a 6.25 KHz migration date, although they imply that one is likely sometime in the future.

Has the FCC established a schedule for mandatory migration to 6.25 kHz efficiency?

No. The Commission has not set any date by which licensees must operate in 6.25 kHz efficiency. The current mandate only requires users to migrate to 12.5 kHz efficiency by January 1. 2013.

 

[budevans]

No - TDMA systems are not already considered 6.25 as far as the FCC is concerned. No where in the rules do the say that or use the term "equivalent to 6.25". They do use the term "efficient".

This topic has come up before. I'm pretty sure that I've seen the amended or updated FCC doc/proposal which did use the term equivalent or equivalence. It was issued based on studies showing that the proposed 6.25 bandwidth with todays technology did not remain stable and at this time is unusable.

The original FCC docs calling for 6.25 were based on projected improvements that would make 6.25 work. Those assumptions were made in the 90's and the projected improvements have not happened.

The documents were part of P25 assessment and specifically referenced two slot TDMA (on a 12.5) equivalence as meeting the spectral efficiency mandate.

Sorry I can't point you to the posts, if I remember correctly it was posted around last summer.

 

[hitechRadio]

No - TDMA systems are not already considered 6.25 as far as the FCC is concerned. No where in the rules do the say that or use the term "equivalent to 6.25". They do use the term "efficient". I think the poster in #9 probably has it right. He says that he can replace his 6 - 12.5 LTR channels with the 3 - 12.5 trbo channels (or x 2-VN ) and be as efficient. And that might be what the FCC will require in the future.

I'm not trying to knock any TDMA system, I'm just saying that either the FCC is going to narrow band again to 6.25 channels, or they won't. And all TDMA systems require 12.5 channels.

Having said all that. TDMA as a technology is not all that efficient for MA (multiple access) and it does have its faults. 1:1 efficiency (compared to FDMA bandwidth) may be of some cost savings in hardware but the FCC is concerned with limited spectrum and creating new channels for future users. TDMA quickly outlived its usefulness in the Cell industry and was replaced by CDMA which is a much cleaner and more efficient technology. With all of the development in the LTE area and other wide band needs in public safety, I would almost expect that PS channels will actually go wider while other private services will go narrower by necessity.

The FCC technically could not say that TDMA 6.25e is Equilvalent to FDMA 6.25. As the definition of equilvalent means to be equal or the same. And we all know it is not the same or equal.

That being said, TDMA 6.25e is just as efficient as FDMA 6.25. On a voice/data path view.

2 voice paths for TDMA 6.25e in a 12.5kHz BW.
1 voice path for FDMA 6.25 in a 6.25kHz BW.

I would assume that the the FCC will allow the 6.25e license's to remain even if/or when they would (and I am assuming here) make the ruling to move to 6.25. And that could be many years from now.

Again just assumptions I am making, just as you are making assumptions.

 

[JRayfield]

You state: "The FCC will determine in 2015 if it will be necessary to mandate 6.25 and if so how & when."

So, where is this stated by the FCC? I don't recall having read anything like that from the FCC.

Actually, if you read Part 90 rules, you'll find that the FCC is at least equally concerned, if not more so, with "efficiency", rather than just "bandwidth".

If the FCC does anything to render DMR "obsolete", then it will also render P25 Phase 2 "obsolete". Considering the fact that the Federal Government is promoting P25 for all public safety, and all Federal Government agencies MUST use P25 if they use a digital format, and P25 Phase 2 is the next step in the development of P25, and the FCC has no problems whatsoever with 2-slot TDMA, it's not likely that the FCC is going to "render obsolete" P25 Phase 2 anytime in the foreseeable future.

John Rayfield, Jr. CETma
W0PM

How is it saving you money if you already own the 6 FDMA LTR repeaters & mobiles and now the FCC tells you to throw them away and spend more money to replace them with something new?

12.5 TDMA (Mototurbo) may just be an acceptable transition system. The FCC does accept TDMA as being "6.25 efficient" on 12.5 channels but what happens when the 12.5 channels go away? When the FCC began the narrow banding process about 15 years ago they said that 6.25 channelization is their ultimate goal. 12.5 is just the transition phase. The FCC will determine in 2015 if it will be necessary to mandate 6.25 and if so how & when.

Obviously you can double your capacity on existing 12.5KHz channels with either TDMA or 2 VN-FDMA repeaters, but that is not the FCC's concern. They want to cut your spectrum in half again so they can make new 6.25 channels for new licensees. If and How any 12.5 wide radios (TDMA or whatever emission) will exist on the FCC's future 6.25 channelization plan is yet to be determined. So if you have six - 12.5 channels today the FCC may mandate you to six - 6.25 channels, rendering Mototrbo obsolete. Or perhaps they will permit the equivalent spectrum of three - 12.5 channels in the form of 6.25 adjacent pairs. meaning that TDMA systems will actually have to operate "6.25 efficient".

 

[RRR]

Last I read there is no official timeframe for the proposed 6.25 mandate.

I understand that it may not even happen anytime in the foreseeable future now.

 

[DisasterGuy]

There is when it comes to 700. Some areas including Central Maryland (for the CMARC system) have already submitted waivers.


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[hitechRadio]

There is when it comes to 700. Some areas including Central Maryland (for the CMARC system) have already submitted waivers.


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The 700 band 6.25 mandate has nothing to do with VHF/UHF/800 bands.

The 700Mhz systems that have waivers are P25 Phase1 FDMA systems like (CMARC).

They have waivers because at this time they do not have the budget to move to a P25 Phase2 TDMA 6.25e system to be compliant.
Most of the systems where the early systems to go to the 700 band when APCO was still figuring out what phase2 would be and before Motorola had X2 systems avaliable.

If the where not early systems they may have joined a Phase1 FDMA 800mhz System and added 700mhz sites. Maybe (CMARC) has all XTS subscribers which will not do Phase2 or there RFSS will not do Phase2 or both. I do not know specifically what they have or do not have.



So these Phase1 FDMA systems applying for waivers are using equipment that will not do Phase2 TDMA. And would require millions of dollars to bring the system up to Phase2 TDMA to be compliant and they would not be able to meet the 2016 deadline (only required) in the 700mhz band. Thus the waiver.

Nearly all systems in the 700 band are already Phase 2. 700 systems that are already P25 Phase2 TDMA are already compliant with the 6.25kHz 2016 deadline.

The FCC allowed the Louisiana (LWIN) 700 phase1 system there waiver until 2024.

The FCC has also considered moving or even suspending the 2016 6.25khz deadline for 700mhz systems

 

[902]

No one knows exactly what the outcome of the FCC's 6.25 kHz equivalency will be just yet. A number of concerns have petitioned to have the deadline moved at least, until system costs can be better amortized. In my old job, I had a number of Phase 1-only 700 mobiles and portables that had not even been unpacked yet when people started talking about Phase 2. They're all around 5 years old now, and will be 7 years old at whatever dreamed up compliance/obsolescence date. I'm sure my successor doesn't buy into industry's push for an 18 month replacement cycle either (we were more like 18 years). If one has a strong (and cogent) opinion on the matter, then one should avail themselves of the Commission's participation process. It's free and anyone can do it. I can say without a doubt that even a looming threat of compulsory 6.25 kHz equivalency has devalued the spectrum relative to, say 800 MHz, where there is no such requirement. In other words, I know for a fact places have evaded using 700 MHz and have gone to outlandish (and commodity priced) measures to gather 800 MHz spectrum when there were 700 MHz resources available just because of that hanging Sword of Damocles. They were originally intended to be contiguous bands, anyway.

Although the forum is P25, the term TDMA is agnostic of protocol. TETRA, OpenSky, iDEN, and DMR are also TDMA technologies, some with 2, 3, 4, or 6 timeslots. So had been D-AMPS cellular.

Pros: throughput gain per bandwidth achieving greater efficiency than legacy technologies. Very useful in areas where you're just not going to get any additional spectrum. Ever. (Believe it or not, that's most of the U.S.)

Cons: when coexisting with legacy technologies in a shared environment TDMA technologies may result in high channel occupancy rates that exceed LMR usage patterns, particularly in polling applications. In some cases, deployment of TDMA technologies have forced noise-based incumbents to convert to interference-based methodologies or frequency reassignment - especially in the unpaired VHF environment where an output is inevitably someone else's input.

Inequities: Achieving 6.25 kHz equivalency through TDMA appears to be tolerated by the FCC without FB8 status, while two very narrow FDMA transceivers (4K00 emissions) appear to require FB8 exclusivity and can only be licensed on certain UHF center frequencies.

Sea monster stories: Some TDMA implementations have caused novel problems in certain cases. Some include "super propagation" characteristics well beyond limits thought to interfere (and may really be more channel occupancy issues). These tend to be very poorly documented and have been inconclusive when researched to some extent. No independent stakeholder has committed to thoroughly investigating and documenting exactly what failures (if any) have been reported.

P25 Phase II in action: Seems to work. Probably pretty good.

P25 Phase II in anything other than 7/800 MHz trunked: Unknown how this will affect incumbents, if any. So far, it's not released for conventional (non-trunked) operations, like DMR has been. We'll see, I guess.

Relevancy: LTE's a-comin' and she's a-comin' pretty fast. At least that's if I'm to believe my associates in industry. Some of their bombast may be wishful thinking (they're probably shareholders) and marketing more than substance.