Angle-domain secure transmission in air–terrestrial mMIMO system with single antenna ground eavesdropper

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In this paper, we consider a hybrid air–terrestrial network secure transmission scenario, where a low-altitude air platform (AP) equipped with a two-dimensional (2D) rectangular antenna array serves a set of legitimate users (LUs). The entire transmission can be divided into two phases: uplink training and transmission phase, and downlink transmission phase. A single-antenna vehicle eavesdropper (ED) is within the coverage of the AP, and the eavesdropper sends jamming signals in the uplink training and transmission phase and eavesdrops on LUs signals in the downlink transmission phase. During the training phase, a beam extraction based beam-domain (BD) channel estimation and transmission scheme is proposed. The active beam sets (ABS) of the interference channel and the LU channels can be separately extracted by comparing the beam gain vectors from different LUs. The pilot contamination can be eliminated and the channel state information (CSI) of LUs can be purified. By using spatial basis expansion model (BEM), full-dimension CSI of LUs and ED can be obtained, based on which the digital domain beam alignment method is designed to locate the ED. By utilizing the purified full-dimension CSI of LUs and location information of ED, the system achievable rate is analyzed and a geometric programming (GP) based power allocation scheme is proposed to further improve the secrecy capacity performance. The superiority of the proposed scheme is evaluated by simulations.