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Asymetrical FDD LTE


richy
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Daft question but is it possible to deploy FDD LTE in an asymmetrical manner. i.e. (5+5) x 5 or 10x5 ? Is the licencing restrictions more of a issue than technical considerations. I understand that MHz for MHz upstream can deliver less MBps in practice but there is normally a significant disparity between upstream and downstream usage but spectrum allocation with FDD is equal in many cases resulting in some amusing speed tests where you see 2mbps down but 6mbps up (this could also be caused by congesting on the backhaul). I recently got a result like this on tmo lte. It seems to a layman that TDD does have the ability to change the balance between upstream and downstream to better fit the usage profile as compared to FDD but is there a way of deploying FDD asymmetrically rather than the current symmetrical profile?

 

Apologies is this is a silly question :) Very interested in your thoughts! It just struck me as one way of better utilizing spectrum given the very high costs of spectrum. I wonder how much this was considered when spectrum was auctioned? 

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Daft question but is it possible to deploy FDD LTE in an asymmetrical manner. i.e. (5+5) x 5 or 10x5 ? Is the licencing restrictions more of a issue than technical considerations. I understand that MHz for MHz upstream can deliver less MBps in practice but there is normally a significant disparity between upstream and downstream usage but spectrum allocation with FDD is equal in many cases resulting in some amusing speed tests where you see 2mbps down but 6mbps up (this could also be caused by congesting on the backhaul). I recently got a result like this on tmo lte. It seems to a layman that TDD does have the ability to change the balance between upstream and downstream to better fit the usage profile as compared to FDD but is there a way of deploying FDD asymmetrically rather than the current symmetrical profile?

 

Apologies is this is a silly question :) Very interested in your thoughts! It just struck me as one way of better utilizing spectrum given the very high costs of spectrum. I wonder how much this was considered when spectrum was auctioned? 

 

As far as I know, it is technically possible.  However, the problem isn't so much technical, as it is licensing.  All FDD spectrum has a downlink and an uplink block that are linked together.  So a 10MHz section of AWS spectrum has a downlink block in 2100MHz and an equal sized uplink block in 1700MHz.  Since there is an equal sized block of downlink and uplink for all current FCC sanctioned FDD wireless spectrum, there is no need for a decreased sized uplink block for any FDD spectrum.

 

That being said, there are some downlink only blocks that are out there.  These aren't designed for 2-way mobile broadband communication.  AT&T owns some.  They are currently not in use.  I can see how someone may in the future try to link some of these downlink only spectrum pieces to an existing underburdened uplink FDD section somewhere.  But I believe this would require a rule change and a new device ecosystem to allow that.

 

The reason why you get a slower download than upload is one of two reasons.  Either the airlink from the site to your device is overloaded, or the backhaul connection from the site to the MSC is overloaded.  Either of these can cause this to occur.

 

Also, to clarify regarding TDD-LTE.  It does not split a 20MHz piece of spectrum into smaller uplink and downlink sections in MHz.  Rather it uses time to split them.  The downlink and uplink occur on the same frequency, just changing back and forth extremely quickly.  A piece of time is allocated to uplink, and then a 3x longer piece of time is used for downlink, then switched back and forth.  That's what TDD is.  Time Division Duplexing.  You may understand this, but it was not clear to me with what you typed.

 

Robert

Edited by S4GRU
Added some more info
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Also, these are not stupid questions.  These are the kind of questions we like.  :)

 

What we dislike is when people don't ask these questions and then talk to us like we are stupid when they're willing to fight with us about things they do not know.  This is a great question.

 

Robert

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Thanks for the explanation. When the FCC sets the licenses do they take into account the usage of adjacent spectrum? I.e. they don't put a block of spectrum that will have a relatively strong strength i.e. site to cellphone next to a weak one like say GPS? I guess I'm asking does the FCC really care what you do or if it is an arbitrary restriction? It just seemed odd to me, if you pay say $4bn for a nation wide chunk of spectrum and build it out and you know your uplink, 50% of your purchase, will sit there mostly unused but your downlink can get saturated. I would be tempted to suggest that if technically possible it would be better to run two downlink signals and one uplink to try and match peoples usage patterns. I will try and dig up some MRTG graphs from a server but normally you see a ratio of 4 or 5 to 1.

 

Sorry re not being clear, I cracked two molars and the painkillers are pretty brutal lol. I'm just glad they're working but its probably an ideal time to try and get my head round this. Thank you for your patience. 

 

TDD seems to have an advantage in that if Sprint had 20MHz and they split it  3:2 and the downlink was saturated they could change it to 4:1 if they wanted whereas with FDD you are stumped. It would frustrate me if I were a carrier so I assumed there was some reason I was just unaware of. Does the FCC consult with carriers prior to auctioning spectrum? They must, and take into consideration usage. I'm just curious where the beginning of the story is, at which point the first decision is made for it to be symmetrical (when the working group designed FDD LTE, when the equipment was designed, when the FCC looked at what spectrum it could free up etc).

 

Given carte blanche to rewrite reality, technological rules and bribe the FCC, if I wanted to use TDD LTE and I bought two 10MHz blocks I would split one into two 5's for 5 up and down and then have the 10MHz as an auxiliary download link, if that makes any sense. 

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I'd like to clarify that we are starting to see asymmetric FDD allocations in the wild now. For example, in South Korea, SK Telecom and KT were allocated 35MHz licenses of DCS spectrum in a 20+15 MHz arrangement (20MHz being used for downlink, and 15MHz for uplink). As a result, SK Telecom and KT deployed 15+15 MHz LTE carriers and then used downlink carrier aggregation to append the extra 5MHz as a supplemental downlink channel.

 

There are some major disadvantages to TDD. One of which is that the temporal ratio must be synchronized across all cell sites operating on the same carrier frequencies. Dissimilar temporal ratios can cause interference. This is especially annoying with mid-band spectrum and low-band spectrum because it's harder to control this problem. It makes it very difficult to choose "optimized" time ratios without intentionally creating coverage islands. You don't fully get to choose your time ratio if there are others playing the same sandbox, basically. Another disadvantage is that there is less sensitivity from the base station end and more uplink power required on the device end to make TDD work because OFDMA is used on both uplink and downlink. And of course, OFDMA signals break down more easily at low power levels. 

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I'd like to clarify that we are starting to see asymmetric FDD allocations in the wild now. For example, in South Korea, SK Telecom was allocated a 35MHz license of DCS spectrum in a 20+15 MHz arrangement (20MHz being used for downlink, and 15MHz for uplink). As a result, SK Telecom deployed a 15+15 MHz LTE carrier and then used downlink carrier aggregation to append the extra 5MHz as a supplemental downlink channel.

 

There are some major disadvantages to TDD. One of which is that the temporal ratio must be synchronized across all cell sites operating on the same carrier frequencies. Dissimilar temporal ratios can cause interference. This is especially annoying with mid-band spectrum and low-band spectrum because it's harder to control this problem. It makes it very difficult to choose "optimized" time ratios without intentionally creating coverage islands. You don't fully get to choose your time ratio if there are others playing the same sandbox, basically. Another disadvantage is that there is less sensitivity from the base station end and more uplink power required on the device end to make TDD work because OFDMA is used on both uplink and downlink. And of course, OFDMA signals break down more easily at low power levels. 

 

Thanks :) There was a discussion on this topic in the lounge a while back and it was mentioned by someone (sorry I forget who) that TDD handoffs are also currently slower which could be an issue with VOLTE among other things.  

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Thanks :) There was a discussion on this topic in the lounge a while back and it was mentioned by someone (sorry I forget who) that TDD handoffs are also currently slower which could be an issue with VOLTE among other things.  

Yeah, they are. Not only does the user equipment have to locate a carrier to handover to, it must determine the temporal ratio and synchronize with it. This makes roaming difficult and slower, too.

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Thanks :) There was a discussion on this topic in the lounge a while back and it was mentioned by someone (sorry I forget who) that TDD handoffs are also currently slower which could be an issue with VOLTE among other things.  

 

There really isn't anything between two TDD sites handing to each other that would cause a problem with VoLTE.  But a change of bands creates a hard handoff which could be problematic for VoLTE.

 

However, Band 41 doesn't need to run VoLTE at all.  All Band 41 LTE will be redundantly overlaid with both Band 25 & 26.  Therefore, if VoLTE was only used on one band, preferably Band 26, then you wouldn't have problems.  And even Band 25 and Band 26 used together probably could be done pretty seamlessly too.

 

Robert

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Thanks for the explanation. When the FCC sets the licenses do they take into account the usage of adjacent spectrum? I.e. they don't put a block of spectrum that will have a relatively strong strength i.e. site to cellphone next to a weak one like say GPS? I guess I'm asking does the FCC really care what you do or if it is an arbitrary restriction? It just seemed odd to me, if you pay say $4bn for a nation wide chunk of spectrum and build it out and you know your uplink, 50% of your purchase, will sit there mostly unused but your downlink can get saturated. I would be tempted to suggest that if technically possible it would be better to run two downlink signals and one uplink to try and match peoples usage patterns. I will try and dig up some MRTG graphs from a server but normally you see a ratio of 4 or 5 to 1.

 

Adjacent spectrum uses are taken into account.  All the time.  No new licenses or changes are made that negatively harm the adjacent license holder.  In the instances that they do, it was because the FCC decided the new purpose was so much more important and beneficial to citizens.  The difference between FDD and TDD are not likely to ever meet this level of scrutiny.

 

If you paid $4BN for spectrum, that was the market rate with these current FCC restrictions.  If you could use it any way you wanted without regard to any other license holder, then the spectrum would be worth far more.

 

TDD seems to have an advantage in that if Sprint had 20MHz and they split it  3:2 and the downlink was saturated they could change it to 4:1 if they wanted whereas with FDD you are stumped. It would frustrate me if I were a carrier so I assumed there was some reason I was just unaware of. Does the FCC consult with carriers prior to auctioning spectrum? They must, and take into consideration usage. I'm just curious where the beginning of the story is, at which point the first decision is made for it to be symmetrical (when the working group designed FDD LTE, when the equipment was designed, when the FCC looked at what spectrum it could free up etc).

 

 

There are advantages and disadvantages to both.  TDD, if used solely for data, works pretty well.  Flexibility in the time ratio between uplink and downlink can be advantageous.  However, most other aspects either are better for FDD or neutral.

 

Given carte blanche to rewrite reality, technological rules and bribe the FCC, if I wanted to use TDD LTE and I bought two 10MHz blocks I would split one into two 5's for 5 up and down and then have the 10MHz as an auxiliary download link, if that makes any sense.

 

Each spectrum block is currently licensed to be a symmetrical block (FDD) or single block (which would require TDD to be used two-way).  A symmetrical dual linked block cannot be used for TDD without authorization from the FCC.  It would also require testing and possibly larger guard bands to protect adjacent license holders.  You cannot place uplink and downlink side by side in most instances.  It will cause interference problems.  You would have to spend a lot of money testing and convincing the FCC that you are harming no other license holders.  The other license holders would still likely file complaints.  The FCC would need to justify that allowing the provider to do this is the greatest common good for the taxpayers.  It is a high burden.  Lightsquared could tell you all about this.

 

Robert

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I was just thinking about this the other day. It would make sense to me for Sprint to divide their 5+5 PCS bands into 7+3 or similar for better downlink with enough uplink to work well.

 

Back of the envelope calculations look like that would give 52.5Mbps downlink and 22.5Mbps uplink on a 7+3 and 127.5Mbps downlink with 22.5Mbps uplink on a 17+3 (I used 37.5 Mbps theoretical max on a 5Mhz carrier to come up with my calculations. Apologies if it doesn't scale that way).

 

Licensing permitting, that would seem to be a better allocation of spectrum resources. I know the devices don't work out this way, with the notable example of Samsung's 5Mhz FDD carrier limitation that I've seen mentioned so many times. Would this be something they could do later with some rebanding efforts, or am I just completely wrong?

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I was just thinking about this the other day. It would make sense to me for Sprint to divide their 5+5 PCS bands into 7+3 or similar for better downlink with enough uplink to work well.

You can't do that in FDD, in the G block you have 5 MHz of downlink and 5 MHz of uplink. 

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They are doing FDD in the G Block already, I think you might mean TDD?

Correct about FDD, but he talking about changing the uplink and downlink to be asymmetrical, which would, I assume, be illegal.

 

EDIT: for clarification Asymmetrical FDD isn't illegal but changing what frequencies are uplink/downlink is.

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I was just thinking about this the other day. It would make sense to me for Sprint to divide their 5+5 PCS bands into 7+3 or similar for better downlink with enough uplink to work well.

 

 

Correct about FDD, but he talking about changing the uplink and downlink to be asymmetrical, which would, I assume, be illegal.

 

EDIT: for clarification Asymmetrical FDD isn't illegal but changing what frequencies are uplink/downlink is.

Considering that the upload and download frequencies are not adjacent, it would be impossible without re-branding the whole PCS spectrum block.

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I didn't realize they weren't adjacent. That said, I know it would be illegal for them to do under the current rules, but I did say "licensing permitting." Apparently it's not just licensing that restricts this.

 

It's still a good idea for FDD in a situation where it would be possible if you could get the FCC on board with it.

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Correct about FDD, but he talking about changing the uplink and downlink to be asymmetrical, which would, I assume, be illegal.

 

EDIT: for clarification Asymmetrical FDD isn't illegal but changing what frequencies are uplink/downlink is.

 

Considering that the upload and download frequencies are not adjacent, it would be impossible without re-branding the whole PCS spectrum block.

So IF the spectrum WAS adjacent, could a carrier change how much of the 10 MHz goes to downlink and how much goes to uplink? Kind of like the 5x5 to 7x3 that afazel was talking about?

 

-Anthony

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So IF the spectrum WAS adjacent, could a carrier change how much of the 10 MHz goes to downlink and how much goes to uplink? Kind of like the 5x5 to 7x3 that afazel was talking about?

 

-Anthony

I do not know, but none of it is, so not really worth the speculation.

 

Here's PCS:

 

Block      Upload                Download

 

Block A: 1850 - 1865 and 1930 - 1945 (30 MHz) - issued by MTAs

Block B: 1870 - 1885 and 1950 - 1965 (30 MHz) - issued by MTAs

Block C: 1895 - 1910 and 1975 - 1990 (30 MHz, 15 MHz or 10 MHz) - issued by BTAs

Block D: 1865 - 1870 and 1945 - 1950 (10 MHz) - issued by BTAs

Block E: 1885 - 1890 and 1965 - 1970 (10 MHz) - issued by BTAs

Block F: 1890 - 1895 and 1970 - 1975 (10 MHz) - issued by BTAs

Block G: 1910 - 1915 and 1990 - 1995 (10 MHz) - issued by EAs

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I didn't realize they weren't adjacent. That said, I know it would be illegal for them to do under the current rules, but I did say "licensing permitting." Apparently it's not just licensing that restricts this.

 

It's still a good idea for FDD in a situation where it would be possible if you could get the FCC on board with it.

The FCC is usually really hesitant about that kind of thing because of interference issues. Just ask DISH.

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Whether symmetric or asymmetric FDD, the uplink and downlink cannot be directly adjacent to one another. Out of band emissions would cause catastrophic self interference between the two links.

 

AJ

About how far apart does uplink and downlink need to be to avoid interference?

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