[iheartmemcache]

Sorrrrrrta. GSM the spec requires that 45 mhz of tx/rx spacing by convention, but GSM the technology as we colloquially know it, e.g. if you're code-shifting 2G (i.e. via CDMA) you could get away easily in the 900-928 range @ full duplex. No technical limitation at all there. This was briefly discussed in an older thread by myself and a few other chaps here[1] but I guess tons of other people have had the idea before too haha.

The US spec GSM phones operate at (a nominal) 850 mhz, but like you said there are plenty of eurospec (and asian spec.. and US issued quad-band..) which can operate with off-the-shelf with all the (again, nominal) 900 mhz[2] band tech that's being phased out. It will pick up, negotiate the handshake and route just fine. The issue is really just like any other cellular network: getting tens of thousands of antennas up, and having people buy new phones that are quad-band and/or carrying around an eBay phone for what amounts to a novelty.

I'm not on the cutting edge of milliwave or RF anything (all voodoo to me) but as I see Agilent come out with more and more sensitive test gear, I can only presume that the wireless industry is getting way better at tuning YIG's, VXCO's, eliminating side-band tone bleed, and what not. Much like the analog (i.e. 1g, remember those Motorola flip-phones? I do1!) -> 2g shift increased capacity by 3x, I'd have to imagine new encoding techniques and what not are being developed by the PhD's and postdocs. Who else is going to buy a $300k network analyzer otherwise, haha

[1] https://news.ycombinator.com/item?id=11414069

[2] If you talk to old-time HAMs they'll joke about how the FCC really regrets throwing away a few hundred billion dollars worth of band in that spectra alone. You can pack a lot of content into each Hz down down there.

[bogomipz]

Could you clarify a couple of acronyms - YIGs, VXCOs?

Also isn't the real viability issue with this the MSC? Once you've registered with a base station you need to backhaul it to a switch that's capable of routing the call to another hand set. Maybe there are some good affordable options now for a soft swtich that also provides HLR/VLR capabilities? A real class 5 switch like those made by Metswtich are hundreds of thousands of dollars I believe.

[biot]

A search for "tuning yig" led me to this: https://en.wikipedia.org/wiki/Yttrium_iron_garnet

[dfc]

VCXO = voltage-controlled crystal oscillator

[extrapickles]

Good point. Doing that would require at least a firmware change on the handset as I suspect the baseband code is built around the ARFCN (Arbitrary Radio Frequency Channel Number) which enforses that separation. It probably wouldn't be that hard to change the ARFCN formula on a handset, though the easy way would make it not compatible with regular networks.