PHY security improvement of wireless sensor networks in presence of an untrusted user
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This thesis presents a multiuser downlink secure transmission scheme for wireless sensor networks (WSNs) that adopts transmit antenna selection (TAS) at the base station (BS), selection combining (SC) over the legitimate sensor nodes and cooperative jamming at the jammer node. Depending on whether the jammer node has the channel state information (CSI) of both the main and wiretap channels, it explores the zero-forcing beam forming (ZFB) scheme to confuse the eavesdropper while avoiding interference to the legitimate nodes. Subsequently, a new hybrid secure transmission scheme is proposed, denoted as TAS-SC-ZFB, for WSNs. The exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput for this scheme are derived to study the secrecy performance of the system. Using these closed-form expressions, the optimal power allocation factor between the BS and jammer is determined to minimize the secrecy outage probability, while the optimal secrecy rate is decided to maximize the effective secrecy throughput for this scheme. Numerical results are provided to render the theoretical analysis valid and show the impact of the major system parameters on the network's secrecy performance.