are you talking about spread spectrum or encoding/encryption.

encoding/encryption might be feasable if you hardcoded in a unique ID (key) into every reciever... then made the radios capable of programming in the key... that way the signal would be encoded and only the one reciver would be able to decrypt the code. I do not know how this would work in the real world but it is certainly an idea ... think of it... no more frequency modules/crystals needed... no more waiting for your channel to free up ... and most of all.. no more "shot down" planes (unless someone programs in your key by mistake)