Best Antenna Placement for Eavesdroppers: Distributed or Co-Located?

Abstract

In this letter, we investigate the best antenna placement for eavesdroppers (Eves), who intend to wiretap information from the legitimate transmitter (Alice). Two configurations of co-located or distributed antennas (CA or DA) are compared to find the most harmful eavesdropper setting, which achieves higher average eavesdropping rate, i.e., the long-term leakage rate to Eves averaged over channel fading and random locations. For $K \to \infty $ , where $K$ is the number of available antennas for Eves, we prove that without a guard zone (i.e., a geometric zone around Alice where Eves cannot be located in), DA Eves are more harmful, while with a nonzero guard zone, CA Eve is superior. For the finite- $K$ case, we derive upper/lower bounds for the average eavesdropping rate. The bounds are then used to find a DA region, in which the DA Eves definitely outperform CA Eve. It is shown that this region depends on the value of $K$ , the radius of the guard zone, and the jamming at Alice.