Analysis of a Mixed Multiuser Underwater Acoustic Communication/Free Space Optical Heterogeneous Relaying System

A mixed multiuser underwater acoustic communication/free space optical (MU-UAC/FSO) air-to-sea heterogeneous relaying system is analyzed in this work, where multiple underwater acoustic communication (UAC) channels aggregate into a free space optical (FSO) channel to obtain maximum capacity. The UAC channels experience kappa-mu shadowed fading and the FSO channel obeys double generalized Gamma fading with the nonzero boresight pointing error. Utilizing the amplify-and-forward protocol, we obtain the approximate statistical distribution of the end-to-end signal-to-noise radio. The expressions of the outage probability (OP), average bit error rate (BER), and average channel capacity (ACC) are derived. Moreover, asymptotic expressions of the OP and average BER are obtained. Relying on the asymptotic results, our findings emphasize that the diversity gain of the MU-UAC/FSO relaying system is determined as the number of users, fading parameters of the UAC and FSO channels, pointing error parameters, and detection types. The results can be simplified to the single-user case. It is shown that the multiuser scheme provides significant diversity gain compared to the single-user scheme. Furthermore, considering the characteristics of UAC channels, such as the Doppler effect, we investigate the performance of an MU-UAC/FSO relaying system in time-varying scenarios.