Secrecy Mutual Information Analysis and Power Control in Amplify-and-forward Relay Networks with Finite-Alphabet Input

In this paper, we compute the secrecy mutual information of Amplify-and-forward (AF) relay networks with finite alphabet input under the total power constraint. Under the assumption that the full CSI is obtained, we propose a beamforming scheme that completely null in the direction of the eavesdropper, on the other hand, we use the strategy of power control to maximize the secrecy mutual information. Besides, we study the average symbol error rate of the proposed strategy. The analysis indicates that the error floor appears in the proposed strategy at high total power. Consequently, we propose a modified scheme that appropriately enhancing the source power to cope with this problem. The modified scheme sacrifices little secrecy mutual information to achieve noteworthy improvement of the symbol error rate, and it eliminates the error floor of the proposed strategy at high total power.