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Sean Harlow sean at seanharlow.infoIn ITU region 2 there is an ISM and amateur band that overlaps with some of the GSM900 uplink and downlink frequencies. At least in the US, unlicensed "part 15" operation is also permitted there, though part 15 has strict power limits that make it mostly useless beyond a few hundred feet. One could legally implement and test this concept in the US or anywhere else in region 2 with similar laws as long as they can keep it within 902-928 MHz and were either licensed amateurs or keep the power within part 15 specs. That said I had a related idea once about the feasibility of an amateur cellular network, but one of the main questions raised on IRC was about the filtration between TX and RX since the uplink/downlink spacing would have to be narrowed to fit within the available window. This would likely have to be done to an even greater extent to support direct handset to handset communication, thus would be even more of a potential problem. tl;dr: Legally possible if you're west of the Atlantic, but you may have some notable technical problems to overcome. ---------- Sean Harlow sean at seanharlow.info On Jan 25, 2012, at 9:02 AM, Dave Schmidt wrote: >> why should two phones use uplink frequency to communicate directly >> together? why not using downlink frequency? > > Because in most if not all counties transmission on downlink frequency requires a license? > > (Sorry and please correct me if this is some obvious nonsense... to me, the original proposal was looking quite nonsensial because of this, but I'd love to learn if I'm wrong...) > > Dave. >