Enhanced PHY-layer security via co-channel underlays

The use of spread spectrum underlay signals has been widely proposed in cognitive radio contexts as a means to provide available spectrum for secondary users. Underlay signals are generally transmitted on a minimal interference basis to prevent disruption of the primary user(s). This paper proposes concepts for integrating the use of arbitrary, digitally modulated signals (primary) with time- and phase-synchronized, co-channel spread spectrum underlay signals, with the singular intent of increasing transmission security of the primary wireless signal via induced self-interference of the conjoined signals. These signals are separable given a priori knowledge of the underlay spreading code, yet remain masked to an uninformed observer. Simulation and proof-of-concept over-the-air results are provided, demonstrating practical feasibility of the approach, while future work will incorporate successive interference cancellation techniques and better quantify performance tradeoffs in hardware.