| Summary: | 碩士 === 國立中山大學 === 通訊工程研究所 === 101 === Due to the broadcast nature of wireless medium, how to ensure the security of wireless transmission is worth investigating. In addition to the encryption techniques commonly adopted in the application-layer, physical-layer secrecy communications drawn significant research concern recently. Physical-layer secrecy communication is achieved by exploiting spatial property of the physical-layer channel, such that the eavesdroppers can’t decode source important information successfully, and the legitimate transmission becomes more reliable. This thesis considers an amplify-and-forward cooperative network, where the source and destination are equipped with multiple antennas, while multiple relays have single antenna. The relays act a bridge between the source and destination, but we assume that these relays are not reliable and may wiretap the information without authorization. As a result, while being assisted by those untrusted relays, we must avoid relays eavesdropping source information. In order to prevent relay from wiretapping information, the destination generates artificial noise(AN) simultaneously to interfere signal reception at relays, when the source transmits information. Since AN is generated by the destination, the destination can eliminate AN by itself after receiving signal forwarded from the relays to fully eliminate additional interference at the destination. We employ linear precoder at the source and destination to optimize the statistical properties of the transmission signal and artificial noise. On the other hand, the relays scale the received signal by a precoding coefficient before retransmission. Instead of maximizing the secrecy rate, we adopt quality of service(QoS)-based criterion to ensure secrecy transmission implicitly since the secrecy rate is non-convex. More specifically, the precoders at the source, relays and destination are jointly optimazed in terms of maximum SNR of the destination subject to the SINRs at all relays are constrained. Through computer simulation, it shows that secrecy rate is improved obviously through introducing artificial noise and joint optimization of precoders compare with the scheme without AN and the scheme using arbitrary precoders.
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