Signal alignment for secure underwater coordinated multipoint transmissions

Although considerable advances in underwater acoustic (UWA) communications and networking have been made in the last decade, research on the secure UWA communications and networking is limited. In this work, we consider the underwater distributed antenna systems (DAS) consisting of geographically distributed antenna elements (DAEs) that are connected via fibers or high-speed radio links, and address the security issue in the coordinated multipoint (CoMP) transmission of DAEs in the presence of an eavesdropper. The information message from DAS to a legitimate user is partitioned into multiple blocks, and each block is assigned to one DAE for transmission. Exploiting the low sound speed in water, the active DAE set and the corresponding transmission schedule, including the transmission starting time and the transmission power of each active DAE, are optimized with an aim of aligning the transmission blocks at the eavesdropper while keeping the blocks well-separated at the legitimate user. A practical design tailored to the orthogonal frequency-division multiplexing (OFDM) modulation is developed. Simulation results are presented to validate the performance of proposed schemes.