Full-Duplex Small-Cell Networks: A Physical-Layer Security Perspective

We provide a theoretical study of physical (PHY)- layer security performance in full-duplex (FD) small-cell networks. Here, the multi-antenna base stations (BSs) and user equipments (UEs) follow from the homogeneous Poisson point process (PPP)-based abstraction model. To facilitate FD communications, we take into account (i) successive interference cancellation (SIC) capability at the UE side via guard regions of arbitrary radii, and (ii) residual self-interference (SI) at the BS side using Rician fading distribution with arbitrary statistics. We investigate the small-cell network PHY-layer security performance in the presence of a Poisson field of eavesdroppers (EDs), under the different scenarios of passive and colluding eavesdropping. Considering linear zero-forcing (ZF) beamforming, we characterize the downlink (DL) and uplink (UL) ergodic secrecy rates and derive closed-form expressions for the different useful and interference signals statistics. In certain special cases of interest, we apply non-linear curve-fitting techniques to large sets of (exact) theoretical data in order to obtain closed-form approximations for the different ergodic rates and ergodic secrecy rates under consideration. Our findings indicate that the FD functionality, in addition to enhancing the spectral efficiency (SE), can significantly improve the PHY-layer security performance, especially with the aid of multi-antenna communications and interference cancellation schemes.