In this letter, an artificial message error-based code scrambling scheme is proposed for secure communications in wiretap channels with channel reciprocity. In the proposed scheme, the artificial message bit errors agreed between the legitimate transmitter and receiver are added to the scrambled message bits at the transmitter prior to the channel encoding procedure, through which the artificial errors are generated by using the reciprocal channel between the legitimate transmitter and receiver. Because of the inaccessibility to the channel state information between the legitimate transmitter and receiver, an eavesdropper would fail to compensate for the artificial errors perfectly. Thus, in addition to decoding errors, the residual artificial errors will also be spread over the descrambled message of the eavesdropper by the error spreading effect of code scrambling. Therefore, unlike the conventional code scrambling scheme, the proposed scheme can provide strong message confidentiality for non-degraded eavesdropping channels, e.g., when the eavesdropper experiences no decoding errors. Furthermore, given that the artificial errors are introduced before the channel encoding procedure, the spread residual errors in the descrambled message remain undetected after the decoding procedures of the eavesdropper. Simulation results confirm that the proposed scheme outperforms the conventional scheme and provides strong message confidentiality in wiretap channels.

In this paper, we propose two secure multiuser multiple-input multiple-output (MIMO) transmission approaches based on interference alignment (IA) in the presence of an eavesdropper. To deal with the information leakage to the eavesdropper as well as the interference signals from undesired transmitters (Txs) at desired receivers (Rxs), our approaches aim to design the transmit precoding and receive subspace matrices to minimize both the total inter-main-link interference and the wiretapped signals (WSs). The first proposed IA scheme focuses on aligning the WSs into proper subspaces while the second one imposes a new structure on the precoding matrices to force the WSs to zero. In each proposed IA scheme, the precoding matrices and the receive subspaces at the legitimate users are alternatively selected to minimize the cost function of a convex optimization problem for every iteration. We provide the feasible conditions and the proofs of convergence for both IA approaches. The simulation results indicate that our two IA approaches outperform the conventional IA algorithm in terms of the average secrecy sum rate.

In this letter, we propose a power allocation scheme to optimize the ergodic secrecy rate of multiple-input single-output (MISO) fading wiretap channels with a probabilistic constraint, using the statistical channel state information (CSI) of the eavesdropper (CSI-E). The analytical expressions of the false secrecy probability are derived and used as constraints in the rate maximization problem. Moreover, we obtain a suboptimal solution by formulating the power allocation problem as a Rayleigh quotient problem.