On Mon, 12 Sep 2011 09:19:29 -0700, Jim Lux
wrote:

Not so much full duplex, but single frequency half duplex, with
negligible time delay (implying 100ms frame time) between Rx and Tx.

Most of the delay will not come from the flight time or mux switching.
It will come from the necessary audio compression. It won't work
without audio compression, which means that some types of
uncompressible pre-randomized data is not going to work (no big deal).
Also, the more compression, the longer the latency.

TDMA cellphone uses split bands probably because it was on top of
existing AMPS systems.

Yep. The problem was that TDMA (IS-54/IS-136) had to be compatible
with the then existing analog cell systems. Therefore, all early TDMA
phones had to offer analog compatibility. Digital only phones weren't
available until about 2003.

There's also little justification for making the change. It will not
double the number of available channels as some pundits have
suggested. Since the return audio now has to be sqeezed into the
previously transmit only channel, the number of users per channel is
cut in half. The result is no capacity change.

There is also a frequency allocation issue (e.g.
no need for new licensing). Having separate forward and reverse bands
also helps with frequency reuse and near-far issues. I hardly think
that hams are going to carpet the country with repeaters to the extent
that cell sites do.

I've looked into butchering cellular handsets into something usable on
ham radio. I have internals on some of the old Motorola flip phones
and bag phones and could probably modify the firmware sufficiently to
turn it into a conventional radios. The fatal flaw was the fixed
45MHz T/R offset. There were simply too many components that would
need to be replaced in order to operate on the smaller offset
available to hams, or on simplex. In addition, it's usually fairly
easy to go down in frequency, but the phones would require going up
from the 850MHz cellular bands to the 915MHz ham band. I gave up on
the idea.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558