Secrecy Outage Analysis of Non-Orthogonal Spectrum Sharing for Heterogeneous Cellular Networks

摘要

In this paper, we investigate physical-layer security for a heterogeneous cellular network consisting of a macro cell and a small cell in the presence of a common eavesdropper that intends to tap both the macro cell and small cell. The orthogonal spectrum sharing (OSS) and non-orthogonal spectrum sharing (NOSS) are considered for the heterogeneous cellular network. The OSS allows the macro cell and small cell to access a given spectrum band in an orthogonal manner, whereas the NOSS enables them to access the same spectrum simultaneously and mutual interference exists. We present two NOSS schemes, namely, the interference-limited NOSS (IL-NOSS) and interference-canceled NOSS (IC-NOSS), where the mutual interference is constrained below a tolerable level through power control in the IL-NOSS, and the IC-NOSS exploits a specially designed signal for canceling out the interference received a legitimate user while confusing the eavesdropper. We derive closed-form expressions for an overall secrecy outage probability of the OSS, IL-NOSS, and IC-NOSS schemes by jointly considering the secrecy of both the macro cell and small cell. Furthermore, we characterize the secrecy diversity of the OSS, IL-NOSS, and IC-NOSS schemes through the asymptotic secrecy outage analysis in the high signal-to-noise ratio (SNR) region. It is proved that the OSS and IL-NOSS schemes obtain the same secrecy diversity gain of zero only, and a higher secrecy diversity gain of one is achieved by the IC-NOSS scheme. In addition, numerical results demonstrate that secrecy outage probabilities of the IL-NOSS and IC-NOSS can be minimized through an optimization of the ratio of the SNR of small cell to the SNR of macro cell, called the small-to-macro ratio (SMR). Moreover, with an optimized SMR, the proposed IC-NOSS scheme significantly outperforms the OSS and IL-NOSS schemes in terms of individual secrecy outage probabilities of both the macro cell and small cell.