The security and reliability requirements for communication systems are increasingly stringent due to the exponential growth in demand for information exchange. Key stream generators are critical components of secure communication systems. However, chaotic-based key stream generators are susceptible to inherent dynamic degradation phenomena, which pose significant security risks. To address this vulnerability, this paper proposes a novel key stream generator based on a chaotic multi-state cellular automaton (CA). We develop a chaotic 8-state CA using a two-dimensional logistic tent modular map to generate both the chaotic rule space and cellular space. The state data of the CA can be updated in parallel, and each state can be directly used as a key stream through simple binary conversion. Furthermore, the Lyapunov exponent theoretical calculation method is extended to multi-state CA, thereby verifying the chaotic behavior of the proposed 8-state CA. Finally, the obtained results show that the proposed key stream generator is not affected by dynamic degradation, and can achieve a maximum expandable key space of 27 2 8. The generator successfully passes the NIST and TestU01 standard random number tests. This study provides a novel framework for designing key stream generators and demonstrates the potential application of multi-state CA in secure communication.

Industrial information transmitted through data centers (DCs) is increasingly exposed to more widespread and powerful attack threats. In this article, we propose a chaotic encryption scheme that utilizes a reversible hybrid evolution life-like cellular automata (CA) and 4D modulation to enhance industrial information security. A chaotic permutation based on the logistic-adjusted-sine map and 4D modulation is applied. Subsequently, the rule selection method of the life-like CA is optimized. The reversible hybrid evolution life-like CA is used for block data diffusion, and a bidirectional diffusion mechanism is designed for the block diffusion. Finally, the 4D encrypted data with a bit rate of 224 Gb/s has been successfully verified over an 80 km standard single-mode fiber. The results indicate that the proposed encryption scheme demonstrates strong resistance to cryptanalytical attacks. Attackers cannot obtain valid data when the optical signal-to-noise ratio of the eavesdropping channel is below 18 dB. This scheme provides robust security guarantees for industrial information transmission in DCs.

Emerging intelligent reflecting surfaces (IRSs) significantly improve system performance, but also pose a huge risk for physical layer security. Existing works have illustrated that a disco IRS (DIRS), i.e., an illegitimate IRS with random time-varying reflection properties (like a “disco ball”), can be employed by an attacker to actively age the channels of legitimate users (LUs). Such active channel aging (ACA) generated by the DIRS can be employed to jam multi-user multiple-input single-output (MU-MISO) systems without relying on either jamming power or LU channel state information (CSI). To address the significant threats posed by DIRS-based fully-passive jammers (FPJs), an anti-jamming precoder is proposed that requires only the statistical characteristics of the DIRS-based ACA channels instead of their CSI. The statistical characteristics of DIRS-jammed channels are first derived, and then the anti-jamming precoder is derived based on the statistical characteristics. Furthermore, we prove that the anti-jamming precoder can achieve the maximum signal-to-jamming-plus-noise ratio (SJNR). To acquire the ACA statistics without changing the system architecture or cooperating with the illegitimate DIRS, we design a data frame structure that the legitimate access point (AP) can use to estimate the statistical characteristics. During the designed data frame, the LUs only need to feed back their received power to the legitimate AP when they detect jamming attacks. Numerical results are also presented to evaluate the effectiveness of the proposed anti-jamming precoder against the DIRS-based FPJs and the feasibility of the designed data frame used by the legitimate AP to estimate the statistical characteristics.

Difficulties in controlling IRSs to form the optimized passive beamforming have rarely been considered in intelligent reflecting surface (IRS)-aided systems, which are summarized as follows: 1) sending the optimized passive precoding vectors to the IRS controller incurs significant control overheads; 2) implementing the optimized passive precoding needs to set massive modes in the IRS control circuit. To address these issues, we investigate codebook-based passive beamforming for multi-IRS-aided millimeter-wave (mmWave) multi-user multiple-input single-output (MU-MISO) systems, where the control overheads are reduced to several scalars and the number of modes set in the IRS control circuit is reduced to that of codewords. Moreover, we formulate a joint passive and active precoding problem in the multi-IRS-aided mmWave MU-MISO system as a mixed-integer nonlinear programming (MINLP) problem, and then develop a generalized Benders decomposition (GBD)-based joint passive and active precoding algorithm. The proposed algorithm offers near-optimal performance ($\ge99.9$%) with significantly-reduced computational complexity. Simulation results show that the proposed algorithm achieves energy savings of up to 50% and 95%, compared to the benchmark by the maximum ratio transmission and that without IRSs, respectively. In addition, the energy savings increase with the number of reflecting elements packed on each IRS as well as that of codewords.

Recently, reconfigurable intelligent surfaces (RISs) have emerged as a potential technique for future 6G communications. Considering the practical hardware constraints of RISs, e.g., the availability of only quantized phase shifts for reflecting elements, we investigate codebook-based passive beamforming, and then develop a two-phase precoding algorithm for multi-RIS-aided multi-user multiple-input single-output (MU-MISO) systems, where the required pilot overhead is much less than that for training the perfect channel state information (CSI). Compared with the maximum ratio transmission (MRT), we propose a more efficient codebook-based passive beamforming scheme based on the sum reward maximization. To verify the feasibility of the proposed reward-maximization-based passive beamforming, we compare the average sum rates achieved by the proposed method, the MRT method, as well as the exhaustive method. Further, we design a feasible set with a few codewords to reduce the computational complexity of the exhaustive method. Moreover, the obtained results based on different codebooks are given to illustrate the generality of the proposed scheme.

Reconfigurable intelligent surface (RIS) with massive passive reflecting elements can greatly improve the wireless propagation environment. Although the RIS is easily introduced into the existing multiple-input multiple-output (MIMO) systems, effectively supporting MIMO technologies is still a major challenge in millimeter-wave cloud radio access networks (mmWave C-RANs) with fiber-wireless fronthaul. Herein, an optical true time delay pool-based hybrid beamforming (OTTDP-HBF) scheme, enabling centralized analog beamforming control, is proposed for the RIS-aided mmWave C-RAN. In the OTTDP-HBF scheme, the physical implementation and the computational processing of analog beamforming are decoupled from all active antenna units (AAUs) and centrally deployed. For the RIS-aided mmWave CRAN with OTTDP-HBF, we develop a two-stage precoding (TSP) algorithm, where the complex joint optimization problem of hybrid precoding matrices and the phase-shift matrix for the RIS is formulated as an alternating optimization (AO) problem and a sparse reconstruction problem. For an AAU equipped with 8 antennas, a designed example of the proposed OTTDPHBF is presented, where the performance can be significantly improved by introducing the RIS into mmWave C-RANs. Moreover, the obtained results of different schemes show that the proposed TSP algorithm is effective for the RIS-aided mmWave C-RAN with OTTDP-HBF.

Line-of-sight wireless communications can benefit from the simultaneous transmission of multiple independent data streams through the same medium in order to increase system capacity. A common approach is to use conventional spatial multiplexing with spatially separated transmitter/receiver antennae, for which inter-channel crosstalk is reduced by employing multiple-input-multiple-output (MIMO) signal processing at the receivers. Another fairly recent approach to transmitting multiple data streams is to use orbital-angular-momentum (OAM) multiplexing, which employs the orthogonality among OAM beams to minimize inter-channel crosstalk and enable efficient (de)multiplexing. In this paper, we explore the potential of utilizing both of these multiplexing techniques to provide system design flexibility and performance enhancement. We demonstrate a 16 Gbit/s millimeter-wave link using OAM multiplexing combined with conventional spatial multiplexing over a short link distance of 1.8 meters (shorter than Rayleigh distance). Specifically, we implement a spatial multiplexing system with a 2 × 2 antenna aperture architecture, in which each transmitter aperture contains two multiplexed 4 Gbit/s data-carrying OAM beams. A MIMO-based signal processing is used at the receiver to mitigate channel interference. Our experimental results show performance improvements for all channels after MIMO processing, with bit-error rates of each channel below the forward error correction limit of 3.8 × 10-3. We also simulate the capacity for both the 4 × 4 MIMO system and the 2 × 2 MIMO with OAM multiplexing. Our work indicates that OAM multiplexing and conventional spatial multiplexing can be simultaneously utilized to provide design flexibility. The combination of these two approaches can potentially enhance system capacity given a fixed aperture area of the transmitter/receiver (when the link distance is within a few Rayleigh distances).

We present the design and performance characterization of a thin metamaterial plate for generation of orbital angular momentum (OAM) modes of a millimeter-wave beam, which can carry independent data streams over the same physical medium. The plate has a thickness of 1.56 mm, and consists of 3.06 × 0.68 mm rectangular apertures with spatial variant orientations. It generates OAM beams l = +1 and l = +3 with mode purity larger than 77.5% over a bandwidth of 6 GHz (25-31 GHz). We then use these streams to experimentally demonstrate a 16-Gbit/s millimeter-wave wireless communications link using two multiplexed OAM modes, each carrying a 2-Gbaud 16-QAM signal. A channel crosstalk less than -20 dB over a bandwidth of 4 GHz (26-30 GHz) and biterror-rates (BER) less than 3.8 × 10-3are achieved.

This paper reports an experimental demonstration of a 16 Gbit/s millimeter-wave communication link using MIMO processing of 2 OAM modes on each of two transmitter/receiver antenna apertures. Two groups of multiplexed OAM beams, each containing OAM beams of ℓ =1 and +3 are generated and transmitted through two transmitter apertures respectively. The two transmitter apertures are separated with a certain distance such that the two groups of OAM beams are spatially overlapping at the receiver aperture plane. Each channel carries 1-GBaud 16-QAM signals at the same carrier frequency of 28 GHz. Our experimental results show that MIMO equalization processing can help mitigate the interferences from the other OAM channels and the BER performance of each channel improves significantly after MIMO processing. Our results indicate that OAM multiplexing and traditional spatial multiplexing combined with MIMO processing can be compatible and complementary with each other.