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 04-16-2018 06:56 PM
high technology
So with the disclaimer that I am not officially any sort of technician, I wanted to throw this out as a question given that users generally download far more than they upload, which causes network congestion that seems to impact all carriers in some manner:

We use FDD LTE primarily in the US, which means that there are separate frequencies that are used for device->tower
(uplink) and tower->device (downlink) communications. While those transmissions can occur simultaneously, the upload frequencies can't be automatically used for downloads (and vice versa).

Band 2 for example, is 1850-1910mhz from device to tower, and 1930-1990 from tower to device. Each of the large 4 carriers in the USA owns a slice of uplink and downlink spectrum of this band, so I am going to use that band as an example. In each case, as I understand, the paired frequencies are 80mhz apart (so if T-Mobile had 1860-1880 for uplink in some location, they would have 1940-1960 for downlink).

Apparently, FDD doesn't necessarily require paired spectrum -- AT&T uses Band 29 which is technically download-only FDD, but has no upload component.

So with ALL of that background, here's my question:

What would it take for a carrier, such as T-Mobile, use their uplink frequencies for downlink to alleviate some of the downlink congestion? I realize they can't just flip a switch, it may need FCC approval, the devices and cell sites might need some updates, we probably would need new band designations, some frequency swaps would be optimal, etc., but it's something I ponder when I get speed test results of 5 Mbps down / 15 mMbps up (which happens occasionally)...

The key question is whether this could be done with only software-based updates for devices and cell sites or whether this would also require hardware updates. For example, could the device modem firmware be updated to support new bands -- let's pretend Band 102 is the new download only spanning all current Band 12 frequencies. If T-Mobile were to swap upload/download with whomever had the adjacent frequencies to get more contiguous spectrum, could they turn their 15+15 into 30mhz of downlink only? If so, then if they carrier aggregated say Band 12 + Band 4 + Band 102, would they effectively have 20+50 LTE instead of 35+35 (assuming they had 5mhz Band 12 and 15 mhz each on Band 2 and Band 4 -- I know their spectrum holdings vary by geography so this numerical example is for illustrative purposes only)?

There is another type of LTE (TDD LTE) that doesn't use paired frequencies -- rather upload/download share spectrum which can be dynamically allocated as needed. Band 35 is the upload portion of Band 2 (1850-1910) and Band 36 is the download portion (1930-1990). Sprint uses this for their 2500mhz Band 41 -- I am putting this out there as a data point, but not sure how it may fit in, whether you can carrier aggregate FDD+TDD, etc... I know the iPhones don't currently support Band 35/36, but again, is this a software update?

So I had no idea where to post this, as it's neither a T-Mobile or iPhone specific issue -- rather it's something that transcends carriers / devices. I opted to post this here, as there are a number of technically inclined people here and T-Mobile provides a good example...
 04-16-2018 08:35 PM
No but they might be able to use a TDD signal where a TDD band overlaps the uplink for an FDD band. For instance, bands 35, 36 & 37 are TDD bands which occupy the same frequencies as band 2's uplink and downlink, and the space in between the uplink and downlink. The technical aspect of if this can actually be done in practice is another matter.
 04-16-2018 09:00 PM
You have to realize that a handset doesn't transmit with the power as the tower, so although theoretically 20x20mhz means equal amounts of bandwidth in each direction, in practice it doesn't work that way because the SNR is worse on the uplink.

The FCC has allowed single direction switches of bandwidth for carrier aggregation - AWS-4 for example. But after the FCC permits it, you have to have a new band class created for it (band 70), then have new phones created that support it. It's not a software update, but an antenna design/modem/software combination that also has to be FCC approved.

If you absolutely need to do this (like Dish did with AWS-4), it's probably more advantageous to do this in greenfield spectrum as compared to trying to refarm existing spectrum (because remember, your band 2 transmissions can't interfere with other carriers band 2 transmissions).

Put it this way - PCS-A block downlink and PCS-H block uplink are only 10 mhz apart, and Dish hasn't been able to get PCS-H block uplink into a band class. It looks like they've left a 15mhz guard band in between PCS-A block downlink and PCS-H block downlink instead because of concerns that 10mhz wouldn't be enough to prevent interference.
 04-16-2018 09:22 PM
Originally posted by Oopsz:

If you absolutely need to do this (like Dish did with AWS-4), it's probably more advantageous to do this in greenfield spectrum as compared to trying to refarm existing spectrum (because remember, your band 2 transmissions can't interfere with other carriers band 2 transmissions).

Exactly. The chance of them make a change on existing spectrum that's under active use is highly unlikely. There's been too much hardware investment by carriers and consumers. The carriers will just have to use what they have whether it's optimum or not. And also look to gain new spectrum that isn't in use yet. That is the future---not switching from FDD to TDD.
 04-19-2018 03:25 AM
I think it's an interesting longer term improvement though. Short term, it's tricky because you have FCC rules that specify uplink and downlink bands, and equipment already using that spectrum (at the cell sites and phone) that assumes non-TDD use. Long term, it would ideally be in best interest to use these bands TDD, increasing peak speeds and overall spectral efficiency.

Short term, these FCC bands (850, 1900, AWS at least) are licensed as uplink and downlink bands, they would be unable to use the spectrum flexibly until this is changed. The FCC of late is rather hand-in-hand with the isps, cell carriers, etc., I could see the FCC allowing TDD quickly if the cell cos wanted to pursue it, and there weren't serious objections during the comment period. If there were objections, lawsuits, etc. over something of course it could drag for years.

I'm really not sure internally how much of the functionality on those Qualcomm chips is fixed-function versus software defined radio. If it's SDR, you may be able to transmit on current receive bands and vice-versa and could potentially upgrade chips with a software update. If there's more fixed-function hardware then the TDD radio might just be hard-wired to support the couple bands it runs on currently. Clearly you could have future chips support TDD on additional bands.
 04-19-2018 12:09 PM
I think it would have been best had the uplink and downlink bands not been symmetrical but rather more spectrum was allocated to downlink. This has been done in a small way with the AWS-3 A1 & B1 blocks and 700 MHz D & E blocks.

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