[XPERIA X1 General] fhsieh:
Why would HTC/SE make a phone that practically every company in the world can use with 3G, but not T-Mobile?

Why would T-mobile chose a 3G band (1700+2100mhz) not used by any other company in the world?

It's really quite complicated, so I'll keep it simple (although the details won't be very clear).

It's neither HTC/SE nor T-mobile's fault. Unlike in Europe, the American government (FCC) does not define or reserve specific bands (1900mhz, 2100mhz, etc) for specific technologies (GSM, UMTS, CDMA, etc). The US government also does not want to reallocate bands that are already being used, even if it's for older technology. Since VoiceStream (Tmo before Deutsche Telekom bought it) didn't have any money to buy spectrum in advance for 3G, that pretty much means that T-mobile has had to settle for whatever bands are leftover.

T-mobile wanted to keep their US 3G bands the same as in Europe, but the lower half (uplink) is already occupied by another technology (which the FCC cannot or will not vacate). So the only choice was to launch 3G on the 1700+2100mhz "AWS" band, or have no 3G at all. And without 3G, AT&T would have a monopoly on UMTS, Tmo would lose business with only 2G, and we as customers would suffer in the long run (especially since without competition, AT&T could keep its prices artificially high).

For the other points, contrary to popular belief, Tmo is not the only carrier in the world that runs a UMTS network in the 1700mhz range. Carrier e-Mobile in Japan runs a UMTS 1700 network. It is slightly different than Tmo's AWS because e-Mobile uses 1700-uplink/1800-downlink (UMTS Band IX) while Tmo uses 1700-uplink/2100-downlink (UMTS Band IV) and is therefore still incompatible. However, as with the G1, it is fully possible to build a radio that can switch between "overlapping" modes (eg: 1700+2100 and 1900+2100* on the G1), it just isn't very cost-effective to make those radios at the moment.

*1900+2100 does not mean AT&T's UMTS 1900mhz network. It's 1900mhz uplink and 2100mhz downlink, and you can see the specific frequency differences here (UMTS Bands I and II).

Keep in mind (and this is a more general point): GSM started off with most phones being only dual-mode (900/1800mhz or 850/1900mhz), but after the technology matured tri- and quad-band radios became commonplace, and now quad-GSM phones are found everywhere. In time we might see a penta-band (850/900/1700/1900/2100) "global" UMTS radio become standard on all phones -- just not with the X1. Then again, by the time we see penta-radios, we will move on to 4G, and with the current split between WiMAX and LTE, we will probably have these same headaches all over again.

So if you want someone to blame, blame the FCC for being unable or unwilling to harmonize the US wireless spectrum with other global standards. Tmo, HTC, and SE are merely just trying to play the cards dealt to them.

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You might also stop to consider: it is still possible for SE to release a Tmo-capable X1 (let's call it the X1t), but the only advantage is that it would work with Tmo, and thus only Tmo customers would buy it (unlike the X1a which can work with any 850/1900/2100mhz carrier, not just AT&T). That makes it cost-prohibitive at the moment. Furthermore, since it would be sold in the American market, the Qualcomm patent restrictions would still apply, and you would end up with the MSM7201A processor which many X1a users have been complaining about. There is just no perfect answer.