Secrecy Outage Analysis for Distributed Antenna Systems in Heterogeneous Cellular Networks

In this paper, we study physical-layer security for a spectrum-sharing heterogeneous cellular network consisting of a macro cell and an underlaying small cell, where a small base station (SBS) and a macro base station (MBS) transmit to their respective small user (SU) and macro user (MU) over their shared spectrum. A common eavesdropper is assumed to tap both the small-cell and macro-cell transmissions. In the macro cell, multiple distributed antennas of MBS are deployed around the far-off MU and only a single antenna is selected to transmit to MU. Meanwhile, SBS directly transmits to SU in the small cell. To improve the spectrum efficiency, we consider underlay spectrum sharing (USS), which allows SBS and MBS to simultaneously access the shared spectrum. We present two USS schemes, namely, the interference-cancelled opportunistic antenna selection (IC-OAS) and interference-limited opportunistic antenna selection (IL-OAS), where a sophisticatedly-designed signal is exploited in the IC-OAS to perfectly cancel out the interference received at MU from SBS. Closed-form expressions of the secrecy outage probability are derived for both the IC-OAS and IL-OAS schemes over Rayleigh fading channels. Numerical results show that the IC-OAS scheme significantly performs better than the IL-OAS scheme for the macro-cell transmission in terms of the secrecy outage probability. Moreover, with an increasing number of distributed antennas, the secrecy outage probabilities of both IC-OAS and IL-OAS decrease substantially.