An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> Additive noise channels with binary-valued inputs and real-valued outputs are considered. The maximum error probability and the minimum channel capacity achieved by any power-constrained noise distribution are obtained. A general framework which applies to a variety of performance measures shows that the least-favorable noise distribution is, in general, a mixture of two lattice probability mass functions. The framework holds for m-ary input constellations on finite-dimensional lattices. > <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> In this paper, we investigate the secrecy rate of finite-alphabet communications over multiple-input-multiple-output-multiple-antenna eavesdropper (MIMOME) channels. Traditional precoding designs based on Gaussian input assumption may lead to substantial secrecy rate loss when the Gaussian input is replaced by practical finite-alphabet input. To address this issue, we investigate linear precoding designs to directly maximize the secrecy rate for MIMOME systems under the constraint of finite-alphabet input. By exploiting the theory of Karush-Kuhn-Tucker (KKT) analysis and matrix calculus, we first present necessary conditions of the optimal precoding design when instantaneous channel-state information (CSI) of the eavesdropper is known at the transmitter. In this light, an iterative algorithm for finding the optimal precoding matrix is developed, utilizing a gradient decent method with backtracking line search. Moreover, we find that the beamforming design in MIMONE systems, which is a secrecy-capacity-achieving approach for Gaussian signaling, no longer provides the maximum secrecy rate for finite-alphabet input data. This case is substantially different from the Gaussian input case. In addition, we derive the closed-form results on the precoding matrix, which maximizes the secrecy rate in the low signal-to-noise ratio (SNR) region, and reveal the optimal precoding structure in the high-SNR region. A novel jamming signal generation method that draws on the CSI of the eavesdropper to additionally increase the secrecy rate is further proposed. The precoding design with only statistical CSI of the eavesdropper available at the transmitter is also considered. Numerical results show that the proposed designs provide significant gains over recent precoding designs through a power control policy and the precoding design with the Gaussian input assumption in various scenarios. <s> BIB002 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound. <s> BIB003 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> This letter considers the secure MIMO transmission in a wireless environment, in which one transmitter (Alice), one receiver (Bob) and one eavesdropper (Eve) are involved. Apart from the artificial noise (AN) generated by Alice, Bob can also exploit his remaining antenna resources to emit AN to further impair Eve's channel. Such kind of AN can be cancelled by Bob himself by applying the Full-Duplex wireless communication technique. A computable secrecy rate is obtained for this model over a Rayleigh-fading eavesdropping channel. In order to maximize the secrecy rate, a joint optimization scheme is proposed to assign the TX/RX antennas for Bob and to design the beamforming and power allocation for Alice's information and AN signal. Simulation is carried out to illustrate the process of the proposed optimization scheme. <s> BIB004 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> In recent years, there have been increasing demands for the security of wireless multimedia applications. Essentially, wireless networks, compared to traditional wired networks, are more likely to suffer from malicious attacks. Most current security methods consider physical layer and application layer security technologies independently and separately. Usually, physical layer information is dynamic in wireless networks, and application layer information is related to wireless multimedia delivery. Importantly, both of them have significant impact on security performance. In this work, we propose a joint framework involving both the physical and application layer security technologies. Specifically, by exploiting the security capacity and signal processing technologies at the physical layer and the authentication and watermarking strategies at the application layer, the available network resources can be utilized efficiently. In addition, scalable multimedia security services can be maximized within the given multimedia delivery deadlines. In particular, this joint scheme can be implemented easily with low communication overhead, which facilitates its deployment in large-scale wireless multimedia systems. <s> BIB005 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> We focus on linear precoding strategies as a physical layer technique for providing security in Gaussian interference channels. We consider an artificial noise aided scheme where transmitters may broadcast noise in addition to data in order to confuse eavesdroppers. We formulate the problem of minimizing the total mean-square error at the legitimate receivers while keeping the error values at the eavesdroppers above target levels. This set-up leads to a non-convex problem formulation. Hence, we propose a coordinate block descent technique based on a tight semi-definite relaxation and design linear precoders as well as spatial distribution of the artificial noise. Our results illustrate that artificial noise can provide significant performance gains especially when the secrecy levels required at the eavesdroppers are demanding. <s> BIB006 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> Wireless information and power transfer (WIPT) enables more sustainable and resilient communications due to the fact that it avoids frequent battery charging and replacement. However, it also suffers from possible information interception due to the open nature of wireless channels. Compared to traditional secure communications, secrecy wireless information and power transfer (SWIPT) has several distinct characteristic features. Wireless power transfer may increase vulnerability to eavesdropping, since a power receiver as a potential eavesdropper usually has a shorter access distance than an information receiver. On the other hand, wireless power transfer can be exploited to enhance wireless security. This article reviews the security issues in various SWIPT scenarios, with an emphasis on revealing the challenges and opportunities for implementing SWIPT. Furthermore, we provide a survey on a variety of physical layer security techniques to improve secrecy performance. In particular, we propose to use massive MIMO techniques to enhance power transfer efficiency and secure information transmission simultaneously. Finally, we discuss several potential research directions to further enhance the security in SWIPT systems. <s> BIB007 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> The concept of physical layer security builds on the pivotal idea of turning the channel’s imperfections, such as noise and fading, into a source of security. This is established through appropriately designed coding techniques and signal processing strategies. In this vein, it has been shown that fading channels can enhance the transmission of confidential information and that a secure communication can be achieved even when the channel to the eavesdropper is better than the main channel. However, to fully benefit from what fading has to offer, the knowledge of the channel state information at the transmitter (CSIT) is of primordial importance. In practical wireless communication systems, CSIT is usually obtained, prior to data transmission, through CSI feedback sent by the receivers. The channel links over which this feedback information is sent can be either noisy, rate-limited, or delayed, leading to CSIT uncertainty. In this paper, we present a comprehensive review of recent and ongoing research works on physical layer security with CSIT uncertainty. We focus on both information theoretic and signal processing approaches to the topic when the uncertainty concerns the channel to the wiretapper or the channel to the legitimate receiver. Moreover, we present a classification of the research works based on the considered channel uncertainty. Mainly, we distinguish between the cases when the uncertainty comes from an estimation error of the CSIT, from a CSI feedback link with limited capacity, or from an outdated CSI. <s> BIB008 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> The heterogeneous cellular network is believed to be a promising deployment of cellular networks in 5G. This chapter comprehensively studies physical layer security in a multitier HCN where BSs, authorized users and eavesdroppers are all randomly located. We first propose a truncated average received signal power-based secrecy mobile association policy. Under this policy, we investigate and provide tractable expressions for the connection probability and secrecy probability of a randomly located user. We further evaluate the network-wide secrecy throughput and the minimum secrecy throughput per user under both connection and secrecy probability constraints. We prove that the proposed mobile association policy significantly enhances the secrecy throughput performance of the HCN. <s> BIB009 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> Abstract We consider an experimental setup of three Universal Software Radio Peripherals (USRPs) that implement a wiretap channel, two USRPs are the legitimate players Alice and Bob, while the third USRP is the eavesdropper, whose position we vary to evaluate information leakage. The experimented channels are close to slow fading channels, and coset coding of lattice constellations is used for transmission, allowing to introduce controlled randomness at the transmitter. Simulation and measurement results show to which extent coset coding can provide confidentiality, as a function of Eve’s position, and the amount of randomness used. <s> BIB010 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> In this work, Automatic-Repeat-Request (ARQ) and Maximal Ratio Combination (MRC), have been jointly exploited to enhance the confidentiality of wireless services requested by a legitimate user (Bob) against an eavesdropper (Eve). The obtained security performance is analyzed using Packet Error Rate (PER), where the exact PER gap between Bob and Eve is determined. PER is proposed as a new practical security metric in cross layers (Physical/MAC) security design since it reflects the influence of upper layers mechanisms, and it can be linked with Quality of Service (QoS) requirements for various digital services such as voice and video. Exact PER formulas for both Eve and Bob in i.i.d Rayleigh fading channel are derived. The simulation and theoretical results show that the employment of ARQ mechanism and MRC on a signal level basis before demodulation can significantly enhance data security for certain services at specific SNRs. However, to increase and ensure the security of a specific service at any SNR, adaptive modulation is proposed to be used along with the aforementioned scheme. Analytical and simulation studies demonstrate orders of magnitude difference in PER performance between eavesdroppers and intended receivers. <s> BIB011 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> We consider precoder and artificial noise (AN) design for multi-antenna wiretap channels under the finite-alphabet input assumption. We assume that the transmitter has access to the channel coefficients of the legitimate receiver and knows the statistics of the eavesdropper’s channel. Accordingly, we propose a secrecy rate maximization algorithm using a gradient descent-based optimization of the precoder matrix and an exhaustive search over the power levels allocated to the AN. We also propose algorithms to reduce the complexities of direct ergodic secrecy rate maximization by: 1) maximizing a cut-off rate-based approximation for the ergodic secrecy rate, simplifying the mutual information expression, which lacks a closed-form and 2) diagonalizing the channels toward the legitimate receiver and the eavesdropper, which allows for employing a per-group precoding-based technique. Our numerical results reveal that jointly optimizing the precoder and the AN outperforms the existing solutions in the literature, which rely on the precoder optimization only. We also demonstrate that the proposed low complexity alternatives result in a small loss in performance while offering a significant reduction in computational complexity. <s> BIB012 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> B. Challenges and Open Problems <s> Abstract This paper studies the required physical (e.g. relays and jammers) and radio resources (e.g. power) to provide physical layer security for relay and friendly jammer assisted multiple-input and single-output transmissions in the presence of multiple active and passive adversaries. The passive adversaries are half duplex and only able to overhear the transmissions from the legitimate transmitter to the legitimate receiver, while the active adversaries are full duplex and able to jam and eavesdrop simultaneously. Since the channel information between adversaries and other nodes are uncertain, robust optimization methods are considered. In this regard, the main aim is to maximize the worst case secrecy rate subject to the normalized transmit power constraints of legitimate transmitter, friendly jammer and relay, and channel state information uncertainty constraints. Through several examples, we then investigate the required increase in physical and radio resources to maintain secure communication when passive adversaries upgrades themselves to active adversaries. <s> BIB013
Various studies have been carried out on physical layer security with discrete signaling with the objective of bridging the gap between the information theoretic limits of secrecy and practical and implementable secure transmission schemes. However, there still are many important open problems and future challenges to be addressed in this area. Physical layer security with finite-alphabet inputs has been studied only for a limited set of CSI scenarios including the cases with perfect and statistical MCSI and ECSI at the transmitter. Some other important scenarios regarding the CSI uncertainty at different nodes, e.g., noisy estimation of CSI, outdated CSI or limited CSI feedback, which are wellinvestigated under the Gaussian input assumption BIB008 , have not yet been considered for cases with finite-alphabet inputs. It would be interesting and of significant practical value to quantify the losses in the secrecy rates under these different imperfect CSI scenarios in the context of finite-alphabet inputs. While the majority of artificial noise injection strategies rely on transmitting Gaussian distributed noise either by aligning it in the null-space of the main channel or by optimizing its covariance matrix, such artificial noise signals are not necessarily optimal. For instance, it has been shown in BIB001 that the worst case noise for AWGN channels with binary inputs has a discrete distribution. This motivates seeking interference distributions (rather than merely optimizing covariance matrices of Gaussian distributed signals), which maximize secrecy rates. While some steps have been taken in studying physical layer security over finite-input multiuser networks (as surveyed in Section VII), many important scenarios have not yet been tackled. For example, no results have been reported for interference channels with secrecy constraints under the finite-alphabet input assumption (refer to BIB006 for an example with Gaussian inputs). Secrecy issues concerning the networks with simultaneous wireless information and power transfer (SWIPT) is also a topic of recent interest BIB007 . It is demonstrated that SWIPT networks are prone to information leakage at the energy receivers, and hence, the main focus of the existing articles is the characterization of the trade-off between the secrecy capacity/rate and the harvested power in these networks BIB003 -  . Studying this trade-off under the practical discrete signaling assumption is an important subject for future research. Furthermore, physical layer security in heterogeneous networks BIB009 and with full duplex transmitter or receivers (see BIB004 and BIB013 ) are very important, and extending the existing results for these scenarios to the case of finite-alphabet inputs is an interesting direction for future research. Massive MIMO systems provide considerable gains in secrecy capacity. However, for such systems, besides the opportunities offered, many challenges arise in secure transmit signal designs with finite-alphabet inputs. For instance, most of the existing precoder design approaches (such as the ones proposed in BIB002 and BIB012 ) are intractable when the number of antennas is high. Secure mmWave transmission is another promising research frontier. Communicating at the mmWave spectrum considerably changes the channel model with respect to the microwave communication systems, and introduces some new features (e.g., high sensitivity to blockages and considerable differences between the line-of-sight and non-line-of-sight receptions), which motivates development of new frameworks for designing practical secure transmission schemes. Taking into account the hardware constraints in designing secure massive MIMO systems is another topic worth investigating. For example, secure transmission schemes can be proposed with finite-alphabet inputs for multi-antenna transmitters with a single radio frequency (RF) chain (or a few, but less than the number of antennas). Performance of wireless systems can be considerably affected by different RF impairments such as inphase/quadrature-phase (I/Q) imbalance, phase noise or distortions due to the nonlinear power amplifiers, which degrade the reception at the receivers (including the eavesdropper); hence, this behavior can be exploited to enhance secrecy. Moreover, various practical constraints arise in IoT systems (e.g., in terms of complexity and energy efficiency) that should be considered in designing secure transmission schemes. We also note that, designing testbeds and conducting tests by implementing physical layer security schemes is an essential step, which should be taken before these techniques can be adopted by actual communication systems. One example of such experiments is reported in BIB010 where the authors evaluate the secrecy performance of coset coding of lattice constellations with the aid of a software defined radio testbed. Another interesting direction for future work is to further develop cross-layer design based solutions for securing communications as only a limited number of works have been reported on this topic so far. For instance, Hamamreh et al. BIB011 demonstrate the advantages of crosslayer protocol interactions on the achievable secrecy by joint exploitation of automatic-repeat-request (as a MAC layer operation) and maximal ratio combining (as a physical layer operation). The potentials of other layers can also be explored to enhance security. For instance, employing authentication and watermarking strategies at the application layer along with the coding and signal processing at the physical layer BIB005 can lead to considerable secrecy gains. The main problem associated with designing practical coding schemes for physical layer security is the following: Although the security gap is proposed and used as an alternative practical metric, the most widely accepted one (i.e., equivocation) relies on information theoretic quantities, and computing these quantities for finite-length codes over AWGN channels is a highly challenging problem, and can be an important research direction on physical layer security. As a final suggestion for future research, we recommend the problem of precoder design in the scenarios where practical wiretap codes are employed, which would be an important step towards designing implementable secure transmission schemes.