A physical transmission security layer for wireless multiple access communication systems

A method for securing wireless multiple access channels is proposed. The method is based on the dispersive nature of wireless channels. Specifically, on the fact the channels from distributed nodes in space to a receiver are unique to the receivers´ point in space and change over time. Each node estimates the channel between itself and the receiver. It then transmits a compensated signal yielding a robust joint constellation at the receivers´ point in space. The receiver decodes and separates all nodes at once from the joint constellation. An eavesdropper, placed at least a few wavelengths aside the receiver, experiences a different and unknown channel. It is shown that this impairs his decoding ability for various reasons. The method requires no extra decoding complexity at the receiver, and allows the sources to transmit independently of one another. It is applicable to systems with linear modulation and flat fading channels.