In this paper, we propose a novel, energy-efficient directional modulation scheme to enhance the physical-layer security of wireless communication systems. The proposed analytical method designs a uniform linear array (ULA) in the spatial frequency domain, allowing for arbitrary distortion of the constellation pattern. In the secure directions, the design ensures low bit error rates (BERs), while in unsecure angles, it enforces a threshold BER to disrupt potential eavesdroppers. The proposed scheme is not only simple to design but also provides flexible trade-off between the mainlobe and sidelobes, ensuring efficient energy consumption. Furthermore, we show that the new scheme has low computational complexity when optimizing antenna weightings, and while we only consider 4- and 16-QAM in this paper, the proposed method can be generalized to higher order QAM modulation schemes. Numerical results validate the effectiveness of the design in achieving the desired BER thresholds and energy efficiency, demonstrating its suitability for real-world applications.

A remotely operated road vehicle (RORV) refers to a vehicle operated wirelessly from a remote location. In this paper, we report results from an evaluation of two safety mechanisms: safe braking and disconnection. These safety mechanisms are included in the control software for RORV developed by Roboauto, an intelligent mobility solutions provider. The safety mechanisms monitor the communication system to detect packet transmission delays, lost messages, and outages caused by naturally occurring interference as well as denial-of-service (DoS) attacks. When the delay in the communication channel exceeds certain threshold values, the safety mechanisms are to initiate control actions to reduce the vehicle speed or stop the affected vehicle safely as soon as possible. To evaluate the effectiveness of the safety mechanisms, we exposed the vehicle control software to various communication failures using a software-in-the-loop (SIL) testing environment developed specifically for this study. Our results show that the safety mechanisms behaved correctly for a vast majority of the simulated communication failures. However, in a few cases, we noted that the safety mechanisms were triggered incorrectly, either too early or too late, according to the system specification.

In this work, we propose a joint security approach based on physical layer security (PhySec) and noisy ciphertext to improve the secrecy rate of a relay network with cooperative jamming via two trusted relays. While the secrecy capacity has been considered as the maximum limit for a perfect secrecy at the physical layer, we demonstrate that the consideration of error- prone ciphertext in encryption and its interaction with PhySec allows us to transmit above the PhySec capacity without any leakage. By formulating and solving a non-convex encryption- aided secrecy rate maximization problem, it is shown that beyond PhySec capacity performances can be achieved even in a simple jamming scenario in which the two jammers just send independent jamming signals. The optimal encryption-aware power allocation and the secrecy rate maximization solution are also established in the case of common jamming signals to further increase the secrecy rate. Numerical results are finally provided to demonstrate the superiority of the proposed joint security framework over traditional PhySec.

This paper presents ComFASE, a communication fault and attack simulation engine. ComFASE is used to identify and evaluate potentially dangerous behaviours of interconnected automated vehicles in the presence of faults and attacks in wireless vehicular networks. ComFASE is built on top of OM-NET++ (a network simulator) and integrates SUMO (a traffic simulator) and Veins (a vehicular network simulator). The tool is flexible in modelling different types of faults and attacks and can be effectively used to study the interplay between safety and cybersecurity attributes by injecting cybersecurity attacks and evaluating their safety implications. To demonstrate the tool, we present results from a series of simulation experiments, where we injected delay and denial-of-service attacks on wireless messages exchanged between vehicles in a platooning application. The results show how different variants of attacks influence the platooning system in terms of collision incidents.

In this work, we evaluate the safety of a platoon of four vehicles under jamming attacks. The platooning application is provided by Plexe-veins, which is a cooperative driving framework, and the vehicles in the platoon are equipped with cooperative adaptive cruise control controllers to represent the vehicles' behavior. The jamming attacks investigated are modeled by extending ComFASE (a Communication Fault and Attack Simulation Engine) and represent three real-world attacks, namely, destructive interference, barrage jamming, and deceptive jamming. The attacks are injected in the physical layer of the IEEE 802.11p communication protocol simulated in Veins (a vehicular network simulator). To evaluate the safety implications of the injected attacks, the experimental results are classified by using the deceleration profiles and collision incidents of the vehicles. The results of our experiments show that jamming attacks on the communication can jeopardize vehicle safety, causing emergency braking and collision incidents. Moreover, we describe the impact of different attack injection parameters (such as, attack start time, attack duration and attack value) on the behavior of the vehicles subjected to the attacks.

In traditional physical layer security paradigm, no leakage of confidential information to the eavesdropper is tolerated regardless of whether the message is encrypted or not. This will result in an achievable secure transmission rate that is significantly smaller than the channel capacity. This article presents a novel approach that allows treatment of physical layer security in conjunction with encryption to achieve a flexible trade-off of system resources. We propose a novel framework to model the interplay between secured transmission rate and error probability in error prone ciphertexts. To this end, we use the concept of rate-equivocation region to establish such a connection. To clearly describe the application of our framework, we consider the case of 4-node Gaussian wiretap channel. For such a channel, we characterize the rate-equivocation region in different scenarios, and then use it to study the achievable rate of encryption-aware physical layer security. The obtained results show that, for a fixed transmission power, the prior knowledge of encryption can significantly increase the secured transmission rate. In addition, encryption-aware physical layer security can achieve a target transmission rate at a reduced transmission power compared to the conventional encryption-agnostic physical layer security.

In this paper, we investigate a trellis-coded space shift keying modulation scheme, called trellis-coded space-time shift keying (TC-STSK), which combines trellis-coded modulation with space shift keying. By analyzing the rank criterion using a graph theoretic approach, we maximize the number of unique differences between pairwise codewords as the code design goal. We discuss the code design principles for achieving diversity, and propose a design algorithm that is mostly suitable for rate-1/n codes, where n is the number of the code output bits. We then propose a diversity adaptation mechanism by selecting only the codewords with desired diversity, and use it to design higher rate codes that achieve full diversity. We also provide the code designs and a modified soft Viterbi decoder for such schemes. Simulation results show that the proposed scheme can achieve diversity orders expected from the theoretical analysis, and that increasing the number of shift registers and transmit antennas improves the performance of the TC-STSK scheme. The results also show that TC-STSK outperforms a space-time trellis code with the same bit rate and the number of shift registers.

We derive the asymptotic ergodic secrecy sum rate (ESSR) of block diagonalization precoding for physical-layer security in downlink multi-user wireless networks that use artificial noise. First, we derive the ESSR of the system. Then, we derive the power allocation and the asymptotic ESSR as the number of antennas increases. After, we derive an approximate expression for the optimum ratio of the number of transmit antennas to the number of receive antennas (Ψ). We then introduce the user dropping factor (UDF) as a means to derive the optimum number of users in the system under different scenarios and then derive the asymptotic optimum UDF, which gives the optimum number of scheduled users. We also study the effect of imperfect channel state information and derive a closed-form ESSR. Simulation results show that these precoders outperform channel inversion (CI) and regularized CI schemes in terms of the ESSR at all SNRs. We also show that as the number of receive antennas of each user increases, the asymptotic ESSR gap also increases.

We analyze asymptotic performance of regularized block diagonalization (RBD) precoding along with artificial noise (AN) used for physical layer security in downlink multi-user wireless networks. We derive the secrecy and asymptotic secrecy sum-rates for this scheme. We also derive closed form expressions for the asymptotic power allocation to the AN signal and legitimate users. Simulations show RBD precoding outperform regularized channel inversion (RCI) in terms of secrecy sum-rate with a margin of 0.5 bits/s/Hz.

We derive the asymptotic secrecy sum-rate of block diagonalization (BD) precoding for physical layer security in downlink multi-user wireless networks using artificial noise (AN). We first derive the secrecy sum-rate assuming that the channel of the eavesdropper is not known at the transmitter and then the asymptotic secrecy sum-rate is derived. The optimum power allocation for asymptotic secrecy sum-rate is derived after that. Simulation results show that the proposed precoders outperform channel inversion (CI) and regularized channel inversion (RCI) schemes in terms of the secrecy sum-rate at all SNRs. We also show that as the number of receive antennas of each user increases, the sum-rate gap between the proposed scheme and existing methods increases.

We study the application of spatial modulation (SM) in overlay cognitive radio (CR) networks, in which the primary and secondary networks work concurrently over the same spectrum band. We assume that the direct link between the primary transmitter and receiver is not available, i.e. broken, and that the CR transmitter assists the primary network as a relay to amplify-and-forward the transmitted symbols of the primary. SM is used in the secondary to split the transmission space into two amplitude-phase modulation (APM) and spatial domains. In the proposed scheme, the secondary transmitter retransmits the primary symbols in APM domain, while its own information is transmitted by the index of transmitting antenna, similar to space shift keying, without causing any interference to the primary receiver. We analyze the performance of the optimal detectors at both the primary and secondary receivers for the case of phase shift keying modulation in terms of the average symbol error rate (ASER). We also study the asymptotic behavior of the ASER at both the primary and secondary at high signal-to-noise ratios. Simulation results show that the SM can be effectively used in overlay CR systems.

In this paper, the performance of suboptimal detection of spatial modulation (SM) based on the so-called maximum ratio combining (MRC) is investigated. In this detection scheme, in contrast to maximum likelihood (ML) detection, transmit antenna index and amplitude-phase modulation(APM) symbols are detected separately in two stages. Therefore, the detection complexity of SM can be significantly reduced. We analyze the instantaneous and average symbol error rate (SER) of such a detection scheme and compare it with that of the optimal ML detection. Also, we propose an adaptive space modulation scheme to minimize the SER of MRC detection by utilizing the channel state information. Finally, the problem of constellation breakdown of SM in the presence of the MRC detection for the special case of the phase-shift keying (PSK)-modulated SM is investigated. The derived analytical results are further validated using extensive simulations.

A layered spatial modulation (LSM) scheme is proposed for the downlink of multiuser wireless communication systems. The proposed scheme falls in the class of the well-known point-to-point SM as it employs only one antenna in each transmission interval. However, it is designed based on assigning different sets of transmit antennas, rather than single antenna elements, to transmit different symbols of each user. We first study the error performance of the proposed LSM. In order to improve the overall error performance of the system, we employ a graphical approach to adaptively allocate antennas to different users. We also propose some techniques to increase the spectral efficiency of LSM. Extending the proposed method to the generalized LSM, we also compare the maximum transmission rate of the proposed scheme with that of the existing SM-based multiuser schemes, and show that this technique can significantly improve the spectral efficiency of the SM-based multiuser communications.

Conventional amplitude-phase modulations (APMs) are designed such that the minimum Euclidean distance between constellation points is maximized. Unlike these modulations, the error performance of spatial modulation (SM) is not only a function of the Euclidean distances but also the energy of each symbol, i.e. constellation point. Therefore, conventional APM schemes that are designed solely based on the notion of Euclidean distance are not necessarily suitable for SM transmission. In this paper, the constellation design for SM is investigated and it is shown that, by a proper constellation design, a significant performance gain is obtained compared to the well-known phase shift keying (PSK) or quadrature amplitude modulation (QAM). It is shown that in many cases multi-ring star-QAM is a suitable constellation for SM. However, when the number of transmit antennas is large, the numerically derived constellation converges to the conventional phase shift keying (PSK) constellation, especially at small number of receive antennas.

We study the feasibility of cognitive radio (CR) communication in the presence of a K-user multi-input multi-output (MIMO) interference channel as the primary network. Assuming that the primary interference network has unused spatial degrees of freedom (DoFs) , we first investigate the sufficient condition on the number of antennas at the secondary transmitter under which the secondary system can communicate while causing no interference to the primary receivers. We show that, to maximize the benefit, the secondary transmitter should have at least the same number of antennas as the spatial DoFs of the primary system. We then derive the secondary precoding and decoding matrices to have zero interference leakage into the primary network while the signal-to-interference plus noise ratio (SINR) at the secondary receiver is maximized. As the success of the secondary communication depends on the availability of unused DoFs, we then propose a fast sensing method based on the eigenvalue analysis of the received signal covariance matrix to determine the availability of unused DoFs or equivalently spatial holes. Since the proposed fast sensing method cannot identify the indices of inactive primary streams, we also provide a fine sensing method based on the generalized likelihood ratio test (GLRT) to decide the absence of individual primary streams. Simulation results show that the proposed CR sensing and transmission scheme can, in practice, provide a significant throughput while causing no interference to the primary receivers, and that the sensing detects the spatial holes of the primary network with high detection probability.

The problem of joint precoder design for the source and relay nodes of a cooperative cognitive radio (CR) system is investigated. We consider a non-regenerative relaying secondary CR system with underlay spectrum sharing methodology with the primary users. Based on an expression for the achievable throughput of secondary system, we develop an optimization problem to maximize the throughput subject to power constraints at the secondary source and relay, and an interference power constraint at the primary due to secondary source and relay transmission. First, we derive the structure of each of the relay and source precoding matrices separately assuming the other is known. Then, we propose an iterative technique to jointly design the source and relay precoding matrices. The results are also extended to the case where the direct source-destination link is available. Through simulation, we demonstrate the effectiveness of our proposed approach in improving the achievable throughput, and compare its performance with that of numerical optimization, source only precoding, relay only precoding, and a naive amplify-and-forward method.

Spatial modulation (SM) is a new transmission technique for multi-antenna systems in which the transmit antennas are used to modulate the signal. In this paper, the symbol error rate (SER) performance of SM is investigated. In SM, a signal domain modulation (i.e. amplitude-phase modulation) and an antenna domain modulation (i.e. space shift keying) are combined together to achieve a certain transmission rate while exploiting the properties of two independent modulation domains. A key question is the fine balance between the constellation sizes in the two domains when a constant rate is targeted. For a fixed rate, there are many ways to assign constellation vectors to the spatial and signal domains. In this paper, we investigate optimal constellation breakdown between space and signal domains. The analysis is based on the union bound of the error probability of SM with two typical APM schemes, i.e. phase-shift keying (PSK) and square quadrature amplitude modulation (S-QAM). It is shown that, at any transmission rate, there exists an optimal APM dimension in which the SER is minimized. Furthermore, a trade-off between the number of transmit antennas and the transmit power is introduced.

In this paper, we study the problem of relay precoder design in a multi-antenna non-regenerative relay cognitive radio (CR) system assuming underlay spectrum sharing methodology with the primary users (PUs). An expression for the capacity of the secondary system is derived assuming fixed source covariance matrix. We then consider maximizing this capacity expression subject to a power constraint at the relay and an interference power constraint at the PU due to the secondary relay. We then derive the structure of the relay precoding up to a square matrix that should be chosen optimally. A constrained optimization problem is derived to find this square matrix to maximize the capacity equation. The solution to this optimization problem involves an exhaustive search and the complexity increases with the number of relay antennas. We, therefore, propose an alternative sub-optimal approach with significantly lower complexity to derive the structure of the relay precoding matrix. Through simulation, we compare the effectiveness of the optimal and the sub-optimal approaches to improve the achievable throughput of the CR system. It is shown that both approaches provide a significant improvement in the throughput of the system at different power and interference regimes.

In this paper, we investigate the channel regularization and vector perturbation techniques at the source and linear precoding at the relay to enhance the performance of a dual-hop amplify and-forward (AF) relay downlink system. At first, an optimal regularized channel inversion scheme via regularization parameter is proposed. The joint channel regularization and linear precoder design at the relay is then investigated using the mean-square-error (MSE) criterion. Since dealing directly with such a criterion is quite involved, we propose a simple iterative solution. Our simulation results show that the proposed solution significantly outperforms the conventional channel inversion method. To further improve the performance of the regularization scheme, we further incorporate vector perturbation into channel regularization using a sphere encoder at the source. Numerical results show that the bit-error-rate (BER) performance of the regularized perturbation technique outperforms other well-known precoder designs in dual-hop AF relay downlink systems.

This paper combines the idea of relay and antenna selection with that of partial channel state information (CSI) based precoding. We consider a non-regenerative two-hop relay network where all the nodes are equipped with multiple antennas. It is assumed that multiple relay nodes are available for cooperation and that the source and each individual relay have the full CSI of the source-relay channel. It is further assumed that each relay knows the spatial correlation matrix of relay-destination channel. The structure of the optimum precoders to minimize a tight lower-bound on the average received minimum mean square error (MMSE) at the source and the relays are then introduced. Using these precoders and based on a computationally efficient proposed method, the best relay and antenna subsets at the source and the relay are then selected to further minimize the MMSE. By simulation, we show that the proposed scheme provides a significant performance gain in terms of the bit error rate (BER).