Performance analysis of mixed FSO-RF transmission in multiuser satellite-aerial-terrestrial networks

In this paper, we study the outage performance of a multiuser satellite-aerial-terrestrial network (SATN) with amplify-and-forward protocol, where the satellite intends to send messages to multiple users with the help of a multi-antenna aerial platform as a relay. Here, we assume that the satellite-aerial link via free space optical (FSO) experiences Gamma-Gamma fading, while aerial-terrestrial links through radio frequency (RF) undergo Nakagami-m fading. To achieve space division multiple access (SDMA) in RF links, we first use the available statistical channel state information (CSI), and propose a beamforming (BF) scheme to suppress the inter-user interference so that all users can be served simultaneously. Then, taking into account the impacts of pointing error in FSO link, we derive an analytical expression for the outage probability (OP) of the SATN. Furthermore, the asymptotical OP expression in high signal-to-noise ratio regime is presented to evaluate the achievable diversity order. It is found that diversity order is related to the FSO channel parameters, RF link severity parameter, and the effect of misalignment. Finally, numerical results show that the proposed BF scheme can achieve almost the same performance as the perfect CSI-based zero-forcing scheme, despite the fact that only statistical CSI is used in our scheme.