In the past, carriers have been guarded if not purposely opaque about things like radio network planning, infrastructure rollouts, and other competitive details. With the smartphone boom well underway, many of those curtains are starting to fall as customers get more and more savvy with air interfaces and asking the important questions. This morning on Reddit, a T-Mobile RF engineer started an AMA, no doubt with official T-Mobile backing, to answer some LTE rollout and network modernization questions. Update: I was dropped a line and notified the AMA is being done without any official T-Mobile backing, but has been confirmed, and is nevertheless very interesting. 

I obviously could not resist the temptation to ask a few questions myself, and got some interesting replies. 

My questions: 

• Can you talk briefly about how much traffic on GERAN you see from iPhone customers? How much of a catch-22 is that situation for moving that PCS spectrum dedicated to it over to WCDMA?
• In some markets it seems as though T-Mobile will be unable to run DC-HSDPA on AWS alongside any LTE because of lack of spectrum. Obviously multi-band carrier aggregation (WCDMA carriers on PCS and AWS) is a big part of that future, can you talk about the challenges involved there?
• Will T-Mobile deploy 3GPP Rel 8, or will you guys go right to Rel 10 for LTE?
• How much variance in WCDMA utilization do you see across markets? From your point of view, are caps and the end of unlimited data plans really backed up across the board, or just in a few markets? In my AZ markets (Tucson, Phoenix) where I do a lot of testing, I regularly find I have the sector to myself.
• Traditional PA at bottom architecture, or Remote Radio Head architecture for T-Mobile LTE?
• This is already somewhat obvious, but could you confirm/discuss T-Mobile LTE channel bandwidths?

T-Mobile RF engineer replies:

• We have about a million iPhones on our network now. 99.9% of their traffic is 2G/EDGE only right now, so obviously their load is dwarfed by everything else. The iPhone is a significant part of the modernization project. Once implemented, iPhones will work on U1900 at much higher speeds.
• Spectrum is always a limitation. With the software that exists now, you will not be able to do dual carrier split between AWS and PCS. That's supposed to be fixed in the future, but I don't know a timeline on it.
• It depends on a lot of things. In general, I think caps are stupid, especially the way that we handle them. Events (concerts, sporting events, malls @ Christmas) crush us though.
• We're going with remote radios as much as we can, for LTE, UMTS, and GSM.
• LTE will launch with a single 10 MHz channel, 5 up, 5 down.

So there you have it. Interestingly enough, the 1 million iPhones number seems a bit low, although it's likely this is a ballpark estimate - I suspected an even larger number given the unlocked iPhone 4S sales and AT&T's post-subsidy unlock program. T-Mobile going with Remote Radio Heads (RRH) for LTE, UMTS, and GSM is also interesting - in the USA, AT&T, Sprint, and Clearwire have all gone with RRH deployments which move the power amplifiers up the mast and next to the antennas, dramatically minimizing cable losses which are generally around 2-3 dB depending on length. This leaves Verizon the sole player in the USA using the traditional power amplifier at the bottom of the tower architecture. Finally, there's official confirmation that T-Mobile will be 5 MHz FDD-LTE initially, at launch. For comparison, Verizon is 10 MHz FDD everywhere in the US, and AT&T is 10 MHz FDD or 5 MHz FDD depending on the market. 

Update: The T-Mobile AMA source has dropped a few other interesting tidbits, as the AMA has progressed. Among mundane things like equipment failures, copper theft and vandalism are a major source of problems for T-Mobile in some markets, along with the usual kind of things like animal encroachment. In addition, the source uses a Samsung Galaxy S 3, and noted the use of disguised cell sites. 

Source: Reddit