To improve the secrecy performance of relay networks in the presence of one eavesdropper, the Artificial Noise Precoding (ANP) and Eigen-Beamforming (EB) secure transmission schemes are appilied at the multiple-antenna amplify-and-forward relay, and the new tight closed-form expressions of the Ergodic Achievable Secrecy Rate (EASR) for two schemes are derived. The lower bound of the EASR for ANP is derived with a large antenna array at the relay, and its corresponding asymptotic performance is investigated in the high SNR and low SNR regimes to show valuable intrinsic insights. Analysis and Simulation results show that, in the moderate-to-high SNR regime, ANP achieves remarkable performance gain over EB, while in the low SNR regime, EB outperforms ANP. Moreover, in the high SNR regime, it is optimal to allocate around half of total power to artificial noise for ANP.
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