Figure 3.

Secrecy rate versus transmission power when the BS was equipped with multiple antennas: $M=4$, $M=8$, $M=16$, and $M=32$. (a) Secrecy rate for $M=4$, $M=8$, $N=20$, and $N_s=40$. (b) Secrecy rate for $M=16$, $M=32$, $N=20$, and $N_s=40$.