On Relaying Strategies in Multi-Hop Covert Wireless Communications

Abstract

Multi-hop transmissions are desirable in realizing large-scale long-distance covert wireless communications, since a single-hop transmission cannot fully satisfy the covertness requirement even with high transmit power. Against this background, this work compares amplify-and-forward (AF) and decode-and-forward (DF) relaying strategies by examining their achievable effective throughput taking into the covertness quality-of-service. To this end, we first present a framework of maximizing the effective throughput with the assumption that each relay adopts equal transmit power to seek mathematical tractability. With the number of relays and each relay’s transmit power optimized, our results reveal that DF relaying outperforms AF relaying in the considered multi-hop covert communications, in terms of achieving a higher effective throughput with a smaller optimal number of relays. This is mainly due to that regardless of the same detection performance at the warden Willie for AF and DF relaying under the same condition, AF relays amplify both information and noise signals, while DF relays only forward the information signals. In addition, our examinations show that DF relaying with independent codewords achieves a higher effective throughput with a fewer number of relays relative to the DF relaying with a single-codeword. Furthermore, we find that the optimal number of relays increases as the desired covert communication distance increases or the covertness constraint becomes stringent.