Hi .

I have an optic solution for you, if you need a radio solution, please ignore this message.

You can use a standart RS232 for the communication,
instead of wires, use a gate of transistors, which drive
a lazer diode.
The lazer beam will be directed to a LDR, Photo Transistor,
Photo Diode ... and will be connect to a gate of transistor.
You will be able to amplify the signal easy, since a chip
lazer diode reach usually to 1000Meters, so the change in
the Photo componment will be drastic.

Goodluck
OK