Physical layer security (PLS) has emerged as a new concept and powerful alternative that can complement and may even replace encryption-based approaches, which entail many hurdles and practical problems for future wireless systems. The basic idea of PLS is to exploit the characteristics of the wireless channel and its impairments including noise, fading, interference, dispersion, diversity, etc. in order to ensure the ability of the intended user to successfully perform data decoding while preventing eavesdroppers from doing so. Thus, the main design goal of PLS is to increase the performance difference between the link of the legitimate receiver and that of the eavesdropper by using well-designed transmission schemes. In this survey, we propose a conceptual, generic, and expandable framework for classifying the existing PLS techniques against wireless passive eavesdropping. In this flexible framework, the security techniques that we comprehensively review in this treatise are divided into two primary approaches: signal-to-interference-plus-noise ratio (SINR)-based approach and complexity-based approach. The first approach is classified into three major categories: first, secrecy channel codes-based schemes; second, security techniques based on channel adaptation; third, schemes based on injecting interfering artificial (noise/jamming) signals along with the transmitted information signals. The second approach (complexity-based), which is associated with the mechanisms of extracting secret sequences from the shared channel, is classified into two main categories based on which layer the secret sequence obtained by channel quantization is applied on. The techniques belonging to each one of these categories are divided and classified into three main signal domains: time, frequency and space. For each one of these domains, several examples are given and illustrated along with the review of the state-of-the-art security advances in each domain. Moreover, the advantages and disadvantages of each approach alongside the lessons learned from existing research works are stated and discussed. The recent applications of PLS techniques to different emerging communication systems such as visible light communication (VLC), body area network (BAN), power line communication (PLC), Internet of things (IoT), smart grid, mm-Wave, cognitive radio (CR), vehicular ad-hoc network (VANET), unmanned aerial vehicle (UAV), ultra-wideband (UWB), device-to-device (D2D), radio-frequency identification (RFID), index modulation (IM) and 5G non-orthogonal multiple access (NOMA) based-systems, are also reviewed and discussed. The paper is concluded with recommendations and future research directions for designing robust, efficient and strong security methods for current and future wireless systems.

An efficient physical layer security technique, referred to as OFDM with subcarrier index selection (OFDM-SIS), is proposed for safeguarding the transmission of OFDM-based waveforms against eavesdropping in 5G and beyond wireless networks. This is achieved by developing a joint optimal subcarrier index selection (SIS) and adaptive interleaving (AI) design, which enables providing two levels (sources) of security in time division duplexing (TDD) mode: one is generated by the optimal selection of the subcarrier indices that can maximize the signal-to-noise ratio at only the legitimate receiver, while the other is produced by the AI performed based on the legitimate user’s channel that is different from that of the eavesdropper. The proposed scheme not only provides a remarkable secrecy gap, but also enhances the reliability performance of the legitimate user compared with the standard OFDM scheme. Particularly, a gain of 5–10 dB is observed at a bit error rate value of 10⁻³ compared with standard OFDM as a result of using the adaptive channel-based subcarrier selection mechanism. Moreover, the proposed technique saves power, considers no knowledge of the eavesdropper’s channel, and provides secrecy even in the worst security scenario, where the eavesdropper can know the channel of the legitimate link when an explicit channel feedback is used as is the case in frequency division duplexing systems. This is achieved while maintaining low complexity and high reliability at the legitimate user, making the proposed scheme a harmonious candidate technique for secure 5G ultra reliable and low latency communications (URLLC) services.

A new modulation technique, named as Orthogonal Frequency Division Multiplexing (OFDM) with Subcarrier Number Modulation (SNM), is proposed for efficient data transmission. In this scheme, the information bits are conveyed by changing the number of active subcarriers in each OFDM subblock. The idea of this scheme is inspired from the integration of OFDM with Pulse Width Modulation (PWM), where the width of the pulse represents the number of active subcarriers corresponding to specific information bits. This is different from OFDM with Index Modulation (OFDM-IM), where the information bits are sent by the indices of the subcarriers instead of their number. The scheme is shown to provide better spectral efficiency than that of OFDM-IM at comparable Bit Error Rate (BER) performances. Another key merit of the proposed scheme over OFDM-IM is that the active subcarriers can be located in any position within the subblock, and thus enabling channel-dependent optimal subcarrier selection that can further enhance the system performance.

This paper provides a comparative study on the performance of different modulation options for orthogonal frequency division multiplexing (OFDM) in terms of their spectral efficiency, reliability, peak-to-average power ratio, power efficiency, out-of-band emission, and computational complexity. The modulation candidates are classified into two main categories based on the signal plane dimension they exploit. These categories are: 1) 2-D signal plane category including conventional OFDM with classical fixed or adaptive QAM modulation and OFDM with differential modulation, where information is conveyed in changes between two successive symbols in the same subcarrier or between two consecutive subcarriers in the same OFDM symbol and 2) 3-D signal plane category encompassing: a) index-based OFDM modulation schemes which include: i) spatial modulation OFDM, where information is sent by the indices of antennas along with conventional modulated symbols and ii) OFDM with index modulation, where the subcarriers' indices are used to send additional information; b) number-based OFDM modulation schemes which include OFDM with subcarrier number modulation, in which number of subcarriers is exploited to convey additional information; and c) shape-based OFDM modulation schemes which include OFDM with pulse superposition modulation, where the shape of pulses is introduced as a third new dimension to convey additional information. Based on the provided comparative study, the relationship and interaction between these different modulation options and the requirements of future 5G networks are discussed and explained. This paper is then concluded with some recommendations and future research directions.

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 = ${10^{-3}}$ .

Automatic-repeat-request (ARQ) as a MAC layer mechanism and artificial noise (AN) as a physical layer mechanism along with the help of maximal ratio combining (MRC), are jointly designed to achieve secrecy. Basically, a special AN, which does not require null-space in the channel, is designed based on the quality of service requirements and the channel condition between the legitimate parties and injected to the data packet. If the same packet is requested by the legitimate receiver (Bob), an AN canceling signal is properly designed and added to the next packet. Then, an AN-free packet is obtained by using MRC process at Bob, while deteriorating the eavesdropper's performance. Furthermore, two simple closed-form expressions of the achievable secure throughput are derived. The first one is given in a closed-form for the case of ARQ scheme without AN, while the second one is given in an upper-bound form for the case of ARQ with AN. Moreover, this paper addresses two critical security-associated problems: 1) the joint design of secrecy, reliability, throughput, delay and the tradeoff among them, and 2) the increase in the peak-to-average power ratio (PAPR) due to the added AN. Finally, the proposed design is extended to OFDM to demonstrate its capability in not only enhancing the secrecy due to the frequency selectivity of the channel, but also in reducing the PAPR and out-of-band emission of OFDM-based waveforms, while maintaining secrecy.