An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> C. Perfect MCSI Only <s> (STBC) for MIMO Gaussian wiretap channels. It is assumed that the transmitter has the receiver's channel state information, but not that of the eavesdropper. We first propose a full-rate STBC that provides separate decoding complexity (rather than pairwise) at the intended receiver, while requiring an exhaustive search for Maximum Likelihood (ML) decoding at the eavesdropper. Next we make the code more secure by including artificial noise symbols which (for the asymptotically high SNR regime) are aligned with each other and subtracted from the information symbols at the intended receiver, but which can not be cancelled at the eavesdropper. Simulations demonstrate the enhanced physical-layer security that results. <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> C. Perfect MCSI Only <s> This paper investigates the Tomlinson-Harashima precoding (THP) design for secure communications in broadcast multiple-input multiple-output (MIMO) systems in the presence of a passive eavesdropper. We focus on optimizing the nonlinear transceiver to guarantee a certain Quality-of-Service (QoS) level for the intended receiver in terms of mean-squared-error (MSE). The scheme allocates the transmit power in order to achieve the target MSE for the intended receiver, and then uses the remaining available power to transmit artificial noise (AN) to degrade the eavesdropper's channel. With the geometric mean decomposition (GMD) based THP, the lower MSE bound can be achieved. We convert the nonlinear transmit power minimization problem to a standard convex optimization problem. The solution for the problem is obtained by the waterfilling method via the Karush-Kuhn-Tucker (KKT) conditions. Simulation results demonstrate that the proposed nonlinear scheme outperforms existing linear transceiver designs. <s> BIB002 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> C. Perfect MCSI Only <s> In this work, a directional modulation-based technique is devised to enhance the security of a multi-antenna wireless communication system employing M-PSK modulation to convey information. The directional modulation method operates by steering the array beam in such a way that the phase of the received signal at the receiver matches that of the intended M-PSK symbol. Due to the difference between the channels of the legitimate receiver and the eavesdropper, the signals received by the eavesdropper generally encompass a phase component different than the actual symbols. As a result, the transceiver which employs directional modulation can impose a high symbol error rate on the eavesdropper without requiring to know the eavesdropper's channel. The optimal directional modulation beamformer is designed to minimize the consumed power subject to satisfying a specific resulting phase and minimal signal amplitude at each antenna of the legitimate receiver. The simulation results show that the directional modulation results in a much higher symbol error rate at the eavesdropper compared to the conventional benchmark scheme, i.e., zero-forcing precoding at the transmitter. <s> BIB003 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> C. Perfect MCSI Only <s> Wireless communication provides a wide coverage at the cost of exposing information to unintended users. As an information-theoretic paradigm, secrecy rate derives bounds for secure transmission when the channel to the eavesdropper is known. However, such bounds are shown to be restrictive in practice and may require exploitation of specialized coding schemes. In this paper, we employ the concept of directional modulation and follow a signal processing approach to enhance the security of multiuser multi-input multioutput (MIMO) communication systems when a multiantenna eavesdropper is present. Security enhancement is accomplished by increasing the symbol error rate at the eavesdropper. Unlike the information-theoretic secrecy rate paradigm, we assume that the legitimate transmitter is not aware of its channel to the eavesdropper, which is a more realistic assumption. We examine the applicability of MIMO receiving algorithms at the eavesdropper. Using the channel knowledge and the intended symbols for the users, we design security enhancing symbol-level precoders for different transmitter and eavesdropper antenna configurations. We transform each design problem to a linearly constrained quadratic program and propose two solutions, namely the iterative algorithm and one based on nonnegative least squares, at each scenario for a computationally efficient modulation. Simulation results verify the analysis and show that the designed precoders outperform the benchmark scheme in terms of both power efficiency and security enhancement. <s> BIB004 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> C. Perfect MCSI Only <s> In this work, we investigate the security performance obtained by employing a practical precoded orthogonal space time block coding method (POSTBC) in MISO wireless networks. In particular, space time codewords are precoded with an optimum matrix that minimizes the error rate at only the legitimate user (Bob). The acquired results depict that there exists a security gap region in the resulting BER performance as a consequence of using POSTBC. Moreover, we enhance the performance more by developing a new hybrid and green security method called precoding along with partial pre-equalizing (PCPPE). In this method, the transmitted symbols are precoded by a new precoder composed of both the original precoder and a new designed unitary matrix that maps Bob's channel amplitudes or phases estimated over the transmitting antennas into 2D orthonormal matrix. Additionally, three issues associated with the proposed security method have been tackled. Including: the slight increase in the transmit power, the appropriate selection process of the optimal precoding matrix, and the effect of imperfect channel estimation and reciprocity. The comparative simulation results prove that PCPPE method provides a secure link among the legitimate parties without sacrificing Bob's reliability although an eavesdropper is assumed to be fully aware of the used method and the original selected precoding matrix indicator (PMI). <s> BIB005
In the scenarios with a passive eavesdropper, a realistic assumption is that the transmitter does not know anything about the eavesdropper's channel. Under this assumption, BIB003 and BIB004 propose a secure transmission strategy referred to as directional modulation, in which the amplitude and the phase of the transmit signal are adjusted by varying the length of the reflector antennas for each symbol. This scrambles the PSK symbols in all the directions other than that of the legitimate receiver. Other strategies for securing communications without the knowledge of the eavesdropper's channel are proposed in BIB002 and BIB001 . Zhang et al. develop a Tomlinson-Harashima precoding in BIB002 where the transmitter allocates part of its power in order to achieve a target mean squared error (MSE) at the legitimate receiver, and the remaining power is used to transmit artificial noise to degrade the eavesdropper's reception. In BIB001 , a secure spacetime block coding (STBC) scheme is proposed, which enables the legitimate receiver to perform separable decoding, while requiring an exhaustive maximum likelihood (ML) detection at the eavesdropper. Furthermore, the authors combine this scheme with artificial noise injection to ensure a high uncoded BER at Eve. Hamamreh et al. BIB005 propose a practical precoded orthogonal STBC method for MISO wireless networks  TABLE II  PRECODER DESIGN WITH FULL CSI   TABLE III  PRECODER DESIGN WITH PERFECT MCSI AND STATISTICAL ECSI   TABLE IV  TRANSMIT SIGNAL DESIGN WITH PERFECT MCSI AND NO ECSI where space-time codewords are precoded by a matrix that minimizes the error rate at the legitimate receiver and a preequalization step is added to take security requirements into account. Table IV summarizes the existing transmit signal design approaches under perfect MCSI and no ECSI at the transmitter.