On the Physical Layer Security of Underwater Optical Wireless Communication Networks

This article deals with the secrecy analysis of an underwater optical wireless communication (UOWC) system operating under the presence of underwater turbulence and geometric path loss. The considered system consists of either a single transmit laser diode (LD) and multiple receive photodetectors (PD), i.e., single-input multiple-output (SIMO), employing maximal-ratio combining (MRC) technique, or a multiple-input single-output (MISO) with multiple LDs and a single PD using the maximal-ratio transmission (MRT) precoding scheme. A tight approximate expression for the average secrecy capacity (ASC)'s lower bound is provided in terms of the key system parameters. Results indicate that the system's secrecy can be enhanced by either increasing the number of receive PDs or the legal node's PDs size compared to the illegitimate one, whereby the ASC can exceed 4 b/s/Hz when the legitimate PD diameter is twice the eavesdropper's one. In addition, narrow beams and low water turbidity levels provide optimal secrecy performance. Finally, a comparative analysis between the SIMO-MRC and MISO-MRT schemes shows that the latter outperforms its former counterpart.