Why not just develop a TX/RX system that uses a digital encrypted signal that relies on set algorithms to prevent interference. Kind of like your remote keyless entry for your car works. When you hit the button to pop the trunk, only your trunk opens, despite the thousands of other cars with keyless entry in the area. You could also have it set up so that a person could just buy one TX and then be able to buy as many RX's as they want by linking the RX's up to a computer so that the proper compatible algorithm could be loaded to the RX for decryption.