How can an antenna be used for both rx and tx?

Hi - I was just thinking about this. How does a device in real time
share an antenna between rx and tx circuits? I'm thinking of something
like a cell phone, for example. My first thought was that the device
would allot a certain amount of time for sending and a certain amount
of time for receiving (say, every other 5 microseconds, or something
like that). But then making every other device match up with that
would be rather difficult methinks. So then my next idea was that the
device is always both sending and receiving and the received signals
are just extracted from the antenna's signal. But then I got to
thinking about exactly how this would be done. You'd want to subtract
the signal being transmitted from the overall signal present at the
antenna - but where would you be getting the antenna signal? Wouldn't
you be getting it from the antenna connection, where the transmitter
circuitry is connected to the antenna? So how would you be able to
pull out the rx signal?

I suspect this is a terribly uninformed question, but I'm really a
robotics/sensors guy - all this RF stuff seems like black magic to
me :)

-Michael

.

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