Covert Communication System Based on Walsh Modulation and Noise Carriers

Abstract

This study proposes a covert communication system, in which a non-zero-mean normally distributed random process is used as a carrier, and its mean is modulated by a Walsh code carrying M covert bits. The number of combinations is so huge that it is difficult for malicious parties to decode the covert information, even if they are aware of the existence of the transmitting signal. The received signal is multiplied at the receiving end with each Walsh code, and the mean value is computed. The Walsh code corresponding to the largest mean value is the transmitter's modulation code, thus recovering the transmitted covert bits. The system's theoretical symbol error rate and bit error rate are derived under additive white Gaussian noise and quasi-static fading channels, respectively. Simulation results are very consistent with the theoretical derivation. Compared with other existing schemes, the proposed scheme has good security, flexibility, and BER performance, and is very suitable for IoT devices with limited resources and low transmission rate but high concealability requirements.