An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> Multi-user MIMO offers big advantages over conventional point-to-point MIMO: it works with cheap single-antenna terminals, a rich scattering environment is not required, and resource allocation is simplified because every active terminal utilizes all of the time-frequency bins. However, multi-user MIMO, as originally envisioned, with roughly equal numbers of service antennas and terminals and frequency-division duplex operation, is not a scalable technology. Massive MIMO (also known as large-scale antenna systems, very large MIMO, hyper MIMO, full-dimension MIMO, and ARGOS) makes a clean break with current practice through the use of a large excess of service antennas over active terminals and time-division duplex operation. Extra antennas help by focusing energy into ever smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include extensive use of inexpensive low-power components, reduced latency, simplification of the MAC layer, and robustness against intentional jamming. The anticipated throughput depends on the propagation environment providing asymptotically orthogonal channels to the terminals, but so far experiments have not disclosed any limitations in this regard. While massive MIMO renders many traditional research problems irrelevant, it uncovers entirely new problems that urgently need attention: the challenge of making many low-cost low-precision components that work effectively together, acquisition and synchronization for newly joined terminals, the exploitation of extra degrees of freedom provided by the excess of service antennas, reducing internal power consumption to achieve total energy efficiency reductions, and finding new deployment scenarios. This article presents an overview of the massive MIMO concept and contemporary research on the topic. <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> The global bandwidth shortage facing wireless carriers has motivated the exploration of the underutilized millimeter wave (mm-wave) frequency spectrum for future broadband cellular communication networks. There is, however, little knowledge about cellular mm-wave propagation in densely populated indoor and outdoor environments. Obtaining this information is vital for the design and operation of future fifth generation cellular networks that use the mm-wave spectrum. In this paper, we present the motivation for new mm-wave cellular systems, methodology, and hardware for measurements and offer a variety of measurement results that show 28 and 38 GHz frequencies can be used when employing steerable directional antennas at base stations and mobile devices. <s> BIB002 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> The small carrier wavelength at millimeter-wave (mm-Wave) frequencies enables featuring a large number of co-located antennas. This paper exploits the potential of large antenna arrays to develop a low-complexity directional modulation technique, Antenna Subset Modulation (ASM), for point-to-point secure wireless communication. The main idea in ASM is to modulate the radiation pattern at the symbol rate by driving only a subset of antennas in the array. This results in a directional radiation pattern that projects a sharply defined constellation in the desired direction and expanded further randomized constellation in other directions. Two techniques for implementing ASM are proposed. The first technique selects an antenna subset randomly for every symbol. While randomly switching antenna subsets does not affect the symbol modulation for a desired receiver along the main direction, it effectively randomizes the amplitude and phase of the received symbol for an eavesdropper along a sidelobe. Using a simplified statistical model, an expression for the average uncoded symbol error rate (SER) is derived as a function of the observation angle. To overcome the problem of large sidelobes in random antenna subset switching, the second technique uses an optimized antenna subset selection procedure based on simulated annealing to achieve superior performance compared with random selection. Numerical comparisons of the SER performance and secrecy capacity of the proposed techniques against those of conventional array transmission are presented to highlight the potential of ASM. <s> BIB003 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> The wireless research community aspires to conceive full duplex operation by supporting concurrent transmission and reception in a single time/frequency channel for the sake of improving the attainable spectral efficiency by a factor of two as compared to the family of conventional half duplex wireless systems. The main challenge encountered in implementing FD wireless devices is that of finding techniques for mitigating the performance degradation imposed by self-interference. In this article, we investigate the potential FD techniques, including passive suppression, active analog cancellation, and active digital cancellation, and highlight their pros and cons. Furthermore, the troubles of FD medium access control protocol design are discussed for addressing the problems such as the resultant end-to-end delay and network congestion. Additionally, an opportunistic decode-andforward- based relay selection scheme is analyzed in underlay cognitive networks communicating over independent and identically distributed Rayleigh and Nakagami-m fading channels in the context of FD relaying. We demonstrate that the outage probability of multi-relay cooperative communication links can be substantially reduced. Finally, we discuss the challenges imposed by the aforementioned techniques and a range of critical issues associated with practical FD implementations. It is shown that numerous open challenges, such as efficient SI suppression, high-performance FD MAC-layer protocol design, low power consumption, and hybrid FD/HD designs, have to be tackled before successfully implementing FD-based systems. <s> BIB004 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> In this paper, we investigate the artificial noise-aided jamming design for a transmitter equipped with large antenna array in Rician fading channels. We figure out that when the number of transmit antennas tends to infinity, whether the secrecy outage happens in a Rician channel depends on the geometric locations of eavesdroppers. In this light, we first define and analytically describe the secrecy outage region (SOR), indicating all possible locations of an eavesdropper that can cause secrecy outage. After that, the secrecy outage probability (SOP) is derived, and a jamming-beneficial range, i.e., the distance range of eavesdroppers which enables uniform jamming to reduce the SOP, is determined. Then, the optimal power allocation between messages and artificial noise is investigated for different scenarios. Furthermore, to use the jamming power more efficiently and further reduce the SOP, we propose directional jamming that generates jamming signals at selected beams (mapped to physical angles) only, and power allocation algorithms are proposed for the cases with and without the information of the suspicious area, i.e., possible locations of eavesdroppers. We further extend the discussions to multiuser and multi-cell scenarios. At last, numerical results validate our conclusions and show the effectiveness of our proposed jamming power allocation schemes. <s> BIB005 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> The Internet of Things (IoT) will feature pervasive sensing and control capabilities via a massive deployment of machine-type communication (MTC) devices. The limited hardware, low-complexity, and severe energy constraints of MTC devices present unique communication and security challenges. As a result, robust physical-layer security methods that can supplement or even replace lightweight cryptographic protocols are appealing solutions. In this paper, we present an overview of low-complexity physical-layer security schemes that are suitable for the IoT. A local IoT deployment is modeled as a composition of multiple sensor and data subnetworks, with uplink communications from sensors to controllers, and downlink communications from controllers to actuators. The state of the art in physical-layer security for sensor networks is reviewed, followed by an overview of communication network security techniques. We then pinpoint the most energy-efficient and low-complexity security techniques that are best suited for IoT sensing applications. This is followed by a discussion of candidate low-complexity schemes for communication security, such as on–off switching and space-time block codes. The paper concludes by discussing open research issues and avenues for further work, especially the need for a theoretically well-founded and holistic approach for incorporating complexity constraints in physical-layer security designs. <s> BIB006 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> Consider a two-way communication scenario where two single-antenna nodes, operating under full-duplex mode, exchange information to one another through the aid of a (full-duplex) multiantenna relay, and there is another single-antenna node who intends to eavesdrop. The relay employs artificial noise (AN) to interfere the eavesdropper's channel, and amplify-forward Alamouti-based rank-two beamforming to establish the two-way communication links of the legitimate nodes. Our problem is to optimize the rank-two beamformer and AN covariance for sum secrecy rate maximization (SSRM). This SSRM problem is nonconvex, and we develop an efficient solution approach using semidefinite relaxation (SDR) and minorization-maximization (MM). We prove that SDR is tight for the SSRM problem and, thus, introduces no loss. Also, we consider an inexact MM method where an approximate but computationally cheap MM solution update is used in place of the exact update in conventional MM. We show that this inexact MM method guarantees convergence to a stationary solution to the SSRM problem. The effectiveness of our proposed approach is further demonstrated by an energy-harvesting scenario extension, and by extensive simulation results. <s> BIB007 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> Wireless networks with directional antennas, like millimeter wave (mmWave) networks, have enhanced security. For a large-scale mmWave ad hoc network in which eavesdroppers are randomly located, however, eavesdroppers can still intercept the confidential messages, since they may reside in the signal beam. This paper explores the potential of physical layer security in mmWave ad hoc networks. Specifically, we characterize the impact of mmWave channel characteristics, random blockages, and antenna gains on the secrecy performance. For the special case of uniform linear array (ULA), a tractable approach is proposed to evaluate the average achievable secrecy rate. We also characterize the impact of artificial noise in such networks. Our results reveal that in the low transmit powerregime, the use of low mmWave frequency achieves better secrecy performance, and when increasing transmit power, a transition from low mmWave frequency to high mmWave frequency is demanded for obtaining a higher secrecy rate. More antennas at the transmitting nodes are needed to decrease the antenna gain obtained by the eavesdroppers when using ULA. Eavesdroppers can intercept more information by using a wide beam pattern. Furthermore, the use of artificial noise may be ineffective for enhancing the secrecy rate. <s> BIB008 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> Compared to traditional antenna arrays system, large-scale antenna arrays with multiple transmit antennas can achieve more accurate beam control ability, and decrease sidelobe leakage. However, sensitive eavesdropping receiver can still receive sidelobe signals and recover it, resulting in the unsafe transmission of information. For secure transmission, traditional directional modulation modulate signals in the RF end, but increasing system complexity. In this paper, we use switched phased array transmission architecture in millimeter-wave frequencies to study physical layer security through antenna selection, which disturb undesired direction constellation and have no effect in desired direction. We also proposed an antenna selection algorithm based on iterative fast Fourier transform (FFT) technique, which selects optimized antenna structure with a low sidelobe to constitute codebook for antenna switching. Compared with the traditional genetic algorithm, the proposed algorithm has lower sidelobe level and computational complexity. The simulation results verify the feasibility and effectiveness of the proposed scheme and algorithm. <s> BIB009 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> Non-orthogonal multiple access (NOMA) is important in 5G, and the users served in NOMA are often paired to avoid the excessive interference. However, for a two-user network, if the channel condition of one user is serious, this network may require the other user to relay this user's signals. In this case, the demands of these users are possibly different. Specifically, the relay user may want the node to increase the spectral efficiency for compensating the cost of relay. On the other hand, because privacy information may be contained in signals, the indirect communication user may primarily focus on his or her information security. Therefore, we propose a novel physical layer scheme to satisfy these demands. Different from the existing relay schemes in NOMA, our scheme has the following characteristics: (i) through power allocation, the relay user can extract his or her signals with spectral efficiency improvement; (ii) through a signal-level method, the relay user can forward the indirect communication user's signals, but he or she is difficult to learn the privacy information in these signals; (iii) through a mechanism, the indirect communication user can attain his or her privacy information. Our scheme is able to support the relay user's demand, and this eavesdropping suppression in our scheme doe not depend on the complicated encryption techniques and positive secrecy rate. <s> BIB010 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VIII. PHYSICAL LAYER SECURITY FOR NEXT GENERATION WIRELESS SYSTEMS <s> In this letter, a secure waveform design for future 5G wireless system is proposed. The developed waveform referred to as secure orthogonal transform division multiplexing (OTDM) waveform, is designed to diagonalize the multi-path channel matrix of only the legitimate receiver (Bob), while degrading eavesdropper’s reception. In particular, instead of using fixed exponential basis functions, generated by IFFT and FFT as in OFDM, orthogonal transform basis functions, which are extracted from the Bob’s channel, are utilized to modulate and demodulate the data symbols securely. The simulation results prove that the proposed design provides a significant practical security gap between the Bob’s and Eve’s performance. The design is shown to be robust against channel imperfection, and it neither sacrifices communication resources nor considers any knowledge on the eavesdropper’s channel. Besides security, the scheme results in a higher SNR, leading to a 3-5-dB gain over OFDM at BER = ${10^{-3}}$ . <s> BIB011
With the advent of the 5G and beyond wireless communication systems, a tremendous growth is expected in the number of connected devices. To meet the high traffic demand, various technologies have emerged during the past years. For instance, massive MIMO BIB001 , millimeter wave (mmWave) communication systems BIB002 , full duplex transmission BIB004 and non-orthogonal multiple access (NOMA)  have attracted significant interest from researchers both in industry and academia. Exploiting large antenna arrays and taking advantage of a spectrum from 30 GHz to 300 GHz away from the almost fully occupied spectral band have proven to provide various benefits, which make these technologies among the key enablers for the next generation wireless systems and beyond. Along with the different aspects of massive MIMO and mmWave communications, investigating their potential in providing physical layer security has been of recent interest (see BIB005 - BIB008 ). The problem of linear precoder design for large-scale (massive) MIMO wiretap channel under the finite-alphabet input assumption is investigated in [63] where GSVD-based precoders are shown to achieve the maximal secrecy rate at high SNR values. The potential of large antenna arrays to realize secure directional modulation at mmWave frequencies is demonstrated in BIB003 where the proposed solution relies on driving only a subset of antennas in the array and choosing the switching configuration such that the desired modulation symbols are received at the legitimate receiver while scrambling the constellation in other (undesired) directions. Fan and Wu BIB009 propose an antenna selection-aided secure mmWave transmission using a switched phased array architecture in large-scale transmit antenna arrays. We note that, different from the solution proposed in BIB003 , which requires modulation at the RF end, the scheme in BIB009 can use traditional baseband modulation schemes. Offering several advantages such as high throughput, low latency and improved connectivity, NOMA is one of the key enabling technologies for next generation wireless networks. It relies on removing the orthogonality (which exists in the conventional practical multiple access techniques), and assigning a specific resource block (e.g., time or frequency) to more than one user. Investigating the security issues of NOMA has been a topic of recent interest. For instance, the secrecy performance of large-scale NOMA networks is studied in  where their secrecy outage probability is derived for both single-antenna and multi-antenna transmitter scenarios. Proposing eavesdropping resilient transmission schemes with NOMA (especially, under a discrete signaling assumption) is an interesting research direction, which is left for future work as also stated in BIB010 . Full duplex wireless communications is another promising technology, which is expected to lead to a considerable increase in spectral efficiency. The main idea behind full duplex communications is to enable the radios to transmit and receive at the same time and within the same frequency band. The research on physical layer security in the scenarios where different nodes are capable of full duplex operation is a hot research topic (for an overview, see  Sec. VII] ). However, only a limited number of the existing studies focus their attention on the practical constellation-constrained scenarios. One example of such studies is reported in BIB007 where secure communications in presence of full duplex relays is investigated under the assumption that the transmitter employs an Alamouti-based rank-two beamforming scheme. Waveform design for 5G has also been a topic of interest during the past years. In addition to an increased bandwidth efficiency and reduced inter-user interference, new waveforms may also provide gains in terms of secrecy. For instance, Hamamreh and Arslan BIB011 propose a secure waveform design for 5G wireless systems referred to as orthogonal transform division multiplexing (OTDM), which relies on replacing Fourier transform blocks in conventional OFDM systems by new blocks allowing for diagonalization of the main channel while degrading the eavesdropper's reception. We conclude this section by noting that the relatively recent paradigm of Internet of things (IoT) is expected to offer connectivity to billions of devices and it will be a centerpiece in the next generation wireless technologies including 5G. In view of the fact that, IoT systems are expected to consist of a large number of devices, the conventional cryptographic solutions, which require key distribution and management, may be too complex for implementation. Accordingly, physical layer security techniques (especially, the ones with lower complexities) can play an important role in safeguarding IoT. A recent survey paper BIB006 provides a thorough discussion of the potentials of physical layer security techniques and pinpoints a number of candidate schemes that can serve as an alternative or complement to the key-based solutions in securing IoT communications.