Experimental Validation of Covert Communication On Software-Defined Radios

Abstract

The fundamental information-theoretic limits of covert, or low probability of detection (LPD),communication have been extensively studied for over a decade, resulting in the square root law (SRL): only ??? covert bits can be reliably transmitted over time-bandwidth product ?, for a constant ? > 0. Attempting to transmit more information either results in detection or decoding issues. The SRL imposes significant constraints on hardware realization of provably-secure covert communication. Thus, experimental validation of covert communication is underexplored: to date, only two experimental studies of SRL-based covert communication are available, both focusing on a quantum optical channel. Here, initial results have been gathered demonstrating provably-secure covert radio-frequency (RF) communication using software-defined radios (SDRs). These validate theoretical predictions and open practical avenues for implementing covert communication systems, as well as raise future research questions. This thesis explores the theoretical and practical realization of covert communications on a discrete-time additive white Gaussian noise (AWGN) channel using SDRs. Careful signal design, power control, and synchronization are proposed to be critical in realizing the limits of covert communication. As such, a high performance radio network has been designed and constructed on the ORBIT Lab’s grid of SDRs for controllable, repeatable testing. An accompanying software package building on radio FPGA firmware and the USRP hardware driver (UHD) resources is used to overcome the unique challenges of covert communication. This thesis derives a lower bound for the probability of error of a maximally advantaged adversary in a covert communications scheme over AWGN channels. The lower bound is experimentally verified using the designed radio system to gather real-world performance data. This outcome validates and extends previous work on covert communications in the discrete-time domain.