Joint Power Optimization and AP Selection for Secure Cell-Free Massive MIMO

In this paper, we investigate joint power control and AP selection scheme in a cell-free massive multiple-input multiple-output (CF-mMIMIO) system under an active eaves-dropping attack, where an eavesdropper tries to overhear the signal sent to one of the legitimate users by contaminating the uplink channel estimation. We formulate a joint optimization problem to minimize the eavesdropping spectral efficiency (SE) while guaranteeing a given SE requirement at legitimate users. The challenging formulated problem is converted into a more tractable form and an efficient low-complexity accelerated pro-jected gradient (APG)-based approach is proposed to solve it. Our findings reveal that the proposed joint optimization approach significantly outperforms the heuristic approaches in terms of secrecy SE (SSE). For instance, the 50% likely SSE performance of the proposed approach is 265 % higher than that of equal power allocation and random AP selection scheme.