Alright, I'll tell you EXACTLY what I'm doing... and I'll try to make this as clear as possible!
Basically, what we have is this: I wish to build a receiver that detects if a transmitter is within an immediate radius (15ft). I would like to do this through a two part system:
1) Base Receiver
2) Transmitter
I carry the transmitter on my person. The receiver stays in a fixed spot. When the receiver detects that the transmitter (presumably carried by me in my pocket) is within a close enough range, it will perform a certain action (turn on a light, unlock a door, whatever). When I leave the immediate area of the receiver, it detects the transponder is no longer nearby, and performs another action (lock a door, turn off the light, etc).
Think of this as a "proximity detector." Imagine the practical use of this. When you approach your home door, it unlocks automatically for you and turns on the light inside. When you leave your house, the door automatically locks behind you. No more leaving your keys lying around, locking yourself out, etc. It is an autosensing system that takes care of performing menial tasks. Think of it, perhaps, as a type of long range RFID system.
The way I had imagined this set up was that you would have the transmitter consistently sending a tone coded pulse on a specified frequency. The receiver would listen on the same frequency and filter out all background noise, similar to the function of a squelch. When the transponder comes within sufficient range, the RF power levels would obviously increase, eventually able to transmit clearly through the background noise. This takes care of the proximity issue. Theoretically, the closer the transmitter gets to the base station, the more powerful the RF signal appears to be.
Eventually, when the RF levels get to a preset amount, they are allowed to talk to a decoder which determines if the transmitter is sending the correct tone code. If it detects the correct algorithm, it does whatever action it has been preprogrammed to do in this case. If not, the PIC simply ignores the signal and waits for the correct one.
Is this a little more clear? I hope I didn't manage to mess that one up... I re-read it several times to myself.