An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> D. Other Secrecy Metrics <s> This paper presents a coding scheme for the Gaussian wiretap channel based on low-density parity-check (LDPC) codes. The messages are transmitted over punctured bits to hide data from eavesdroppers. The proposed coding scheme is asymptotically effective in the sense that it yields a bit-error rate (BER) very close to 0.5 for an eavesdropper whose signal-to-noise ratio (SNR) is lower than the threshold SNRE, even if the eavesdropper has the ability to use a bitwise maximum a posteriori (MAP) decoder. Such codes also achieve high reliability for the friendly parties provided they have an SNR above a second threshold SNRB . It is shown how asymptotically optimized LDPC codes are designed with differential evolution where the goal is to achieve high reliability between friendly parties while keeping the security gap SNRB/SNRE as small as possible to protect against passive eavesdroppers. The proposed coding scheme is encodable in linear time, applicable at finite block lengths, and can be combined with existing cryptographic schemes to deliver improved data security by taking advantage of the stochastic nature of many communication channels. <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> D. Other Secrecy Metrics <s> Secure transmission techniques have been receiving growing attention in recent years, as a viable, powerful alternative to blocking eavesdropping attempts in an open wireless medium. This paper proposes a secret transmit beamforming approach using a quality-of-service (QoS)-based perspective. Specifically, we establish design formulations that: i) constrain the maximum allowable signal-to-interference-and-noise ratios (SINRs) of the eavesdroppers, and that ii) provide the intended receiver with a satisfactory SINR through either a guaranteed SINR constraint or SINR maximization. The proposed designs incorporate a relatively new idea called artificial noise (AN), where a suitable amount of AN is added in the transmitted signal to confuse the eavesdroppers. Our designs advocate joint optimization of the transmit weights and AN spatial distribution in accordance with the channel state information (CSI) of the intended receiver and eavesdroppers. Our formulated design problems are shown to be NP-hard in general. We deal with this difficulty by using semidefinite relaxation (SDR), an approximation technique based on convex optimization. Interestingly, we prove that SDR can exactly solve the design problems for a practically representative class of problem instances; e.g., when the intended receiver's instantaneous CSI is known. Extensions to the colluding-eavesdropper scenario and the multi-intended-receiver scenario are also examined. Extensive simulation results illustrate that the proposed AN-aided designs can yield significant power savings or SINR enhancement compared to some other methods. <s> BIB002
Secrecy capacity is the most widely used measure for evaluation of the secrecy performance. However, a complete characterization of the secrecy capacity region is prohibitive in some scenarios. In such cases, alternative metrics can be adopted as surveyed below. 1) Secrecy Outage Probability: Secrecy outage occurs when the instantaneous secrecy capacity falls below a target secrecy rate R s . In delay-critical applications and for scenarios with quasi-static fading, encoding over multiple channel states is not possible. In these cases, secure transmission schemes can be designed such that the probability of occurrence of an outage event is minimized. Obviously, this serves as a weaker notion than secrecy capacity. This is due to the fact that, with this formulation, secrecy is not guaranteed for the entire transmission duration. 2) Error Probability Based Metrics: In a number of works on physical layer security, secure transmission schemes are designed based on constraining the bit error rate at the legitimate receiver and the eavesdropper to pre-specified threshold values. This leads to the definition of a performance metric called "security gap," which quantifies the minimum required difference between the received SNR values at Bob and Eve to ensure that the bit error rate (BER) at Bob is smaller than a threshold while that at the Eve's receiver is larger than another threshold BIB001 . One may note that imposing a high BER at the eavesdropper does not satisfy any of the requirements given in Table I . However, security gap serves as a practical measure of secrecy in a number of applications such as explicit and implementable code design for physical layer secrecy. Some studies (e.g., BIB002 ) consider the difference in the signal-tointerference-plus-noise ratio (SINR) at the legitimate receiver and the eavesdropper as a secrecy metric as well. This is also a weak notion, which does not guarantee secrecy in an information theoretic sense.