Chaotic waveforms, characterized by their noise-like properties and potential security advantages, have emerged as a promising solution for physical layer security (PLS) in wireless communication schemes. However, conventional differential chaos shift keying (DCSK) schemes suffer from limitations such as low data rate, energy inefficiency, and vulnerability to frequency-selective fading (FSF) channels. To address these challenges, this paper proposes a new matrix index technique for multi-carrier DCSK scheme, referred to asMIT-MC-DCSKscheme, tailored for robust and secure transmission over FSF channels. The proposed MIT-MC-DCSK scheme enables time-frequency diversity to mitigate the negative effect of fading while enhancing bit error rate (BER) performance. Theoretical BER expressions of the proposed scheme are rigorously derived over FSF channels, which are validated by extensive Monte Carlo simulations. Experimental results demonstrate that the proposed MIT-MC-DCSK scheme achieves superior BER performance and higher transmission efficiency than the state-of-the-art benchmark schemes. Furthermore, the proposed MIT-MC-DCSK scheme significantly reduces information leakage and improves secrecy capacity by leveraging matrix reconstruction technique, making it resilient against eavesdropping. Thanks to these advancements, the MIT-MC-DCSK scheme appears to be a robust candidate for secure and reliable communication in FSF scenarios.

Affine frequency division multiplexing (AFDM) exhibits strong robustness against time and frequency dispersion in doubly dispersive channels (DDCs), enabling reliable communication under high mobilities. However, in the multi-user uplink scenario, inter-user channel delay and Doppler differences in the discrete affine Fourier transform (DAFT) domain manifest as inevitable multi-user interference (MUI). To address this issue, building upon the DAFT and AFDM, we propose a novel uplink multiple access scheme termed as DAFT-spread affine frequency division multiple access (DAFT-s-AFDMA). In our proposed scheme, DAFT spreading is performed by each user to multiplex the transmitted symbols over the DAFT domain, which includes a pre-chirp parameter that can be flexibly adjusted to reduce the peak-to-average power ratio (PAPR) of the AFDM system. Accordingly, we derive new guidelines for setting the DAFT parameters and the asymptotically tight upper bounds on the average bit error rate, revealing the insights of PAPR reduction. Furthermore, a low-complexity cross-domain expectation propagation (CD-EP) detector is proposed, capitalizing on the sparsity of DAFT domain effective channel matrix and the corresponding symbol domain constellation constraints to enhance the error performance. Simulation results show that the proposed CD-EP detector outperforms both conventional Gaussian message passing (GMP) and minimum mean square error (MMSE) detectors with a much lower complexity, and also verify the superiority of DAFT-s-AFDMA to plain AFDMA across various scenarios of high-mobility DDCs.

Affine frequency division multiplexing (AFDM) is a novel modulation technique based on chirp signals that has been recently proposed as an effective solution for highly reliable communications in high-mobility scenarios. In this paper, we focus on the design of robust index modulation (IM) schemes under the multiple-antenna AFDM transmission framework. To this end, the cyclic delay diversity (CDD) technique is employed to harvest the transmit diversity gain. As a result, we propose two novel AFDM-IM schemes with transmit diversity, termed as CDD-AFDM-IM-I and CDD-AFDM-IM-II. We analyze the full diversity conditions and parameter settings of the proposed CDD-AFDM-IM schemes for both integer and fractional Doppler cases over linear time-varying (LTV) channels. Moreover, we prove that IM enables AFDM to have stronger diversity protection when the full diversity condition is not satisfied. Asymptotically tight upper bounds on the average bit error rates (BERs) of the proposed schemes with maximum-likelihood (ML) detection are derived in closed-form. Furthermore, we propose a low-complexity double-layer message passing (DLMP) algorithm for practical large-dimensional signal detection in the proposed CDD-AFDM-IM systems. Comparison with existing detections shows that the proposed DLMP algorithm achieves a better tradeoff between the BER performance and the computational complexity. Finally, BER simulation results confirm that our proposed CDD-AFDM-IM schemes with both the ML and DLMP detections outperform the benchmark schemes over the LTV channels.

Affine frequency division multiplexing (AFDM) and orthogonal AFDM access (O-AFDMA) are promising techniques based on chirp signals, which are able to suppress the performance deterioration caused by Doppler shifts in high-mobility scenarios. However, the high peak-to-average power ratio (PAPR) in AFDM or O-AFDMA is still a crucial problem, which severely limits their practical applications. In this paper, we propose a discrete affine Fourier transform (DAFT)-spread AFDMA scheme based on the properties of the AFDM systems, named DAFT-s-AFDMA to significantly reduce the PAPR by resorting to the DAFT. We formulate the transmitted time-domain signals of the proposed DAFT-s-AFDMA schemes with localized and interleaved chirp subcarrier allocation strategies. Accordingly, we derive the guidelines for setting the DAFT parameters, revealing the insights of PAPR reduction. Finally, simulation results of PAPR comparison in terms of the complementary cumulative distribution function (CCDF) show that the proposed DAFT-s-AFDMA schemes with localized and interleaved strategies can both attain better PAPR performances than the conventional O-AFDMA scheme.

Affine frequency division multiplexing (AFDM) is a new multicarrier technique based on chirp signals tailored for high-mobility communications, which can achieve full diversity. In this paper, we propose an index modulation (IM) scheme based on the framework of AFDM systems, named AFDM-IM. In the proposed AFDM-IM scheme, the information bits are carried by the activation state of the subsymbols in the discrete affine Fourier (DAF) domain in addition to the conventional constellation symbols. To efficiently perform IM, we divide the subsymbols in the DAF domain into several groups and consider both the localized and distributed strategies. An asymptotically tight upper bound on the average bit error rate (BER) of the maximum-likelihood detection in the existence of channel estimation errors is derived in closed-form. Computer simulations are carried out to evaluate the performance of the proposed AFDM-IM scheme, whose results corroborate its superiority over the benchmark schemes in the linear time-varying channels. We also evaluate the BER performance of the index and modulated bits for the AFDM-IM scheme with and without satisfying the full diversity condition of AFDM. The results show that the index bits have a stronger diversity protection than the modulated bits even when the full diversity condition of AFDM is not satisfied.

A three-dimensional constellation-basedM-ary differential chaos shift keying modulation system, referred to as3D-M-DCSK system, is proposed in this paper. In particular, we exploit the excellent cross-correlation properties of chaotic signals to intelligently combine the 3D constellation mapping and the DCSK scheme, thus increasing the minimum Euclidean distance (MED) between the adjacent mapped symbols. In this way, the date rate and transmission reliability of conventional two-dimensional constellation mapping can be effectively improved. The theoretical expressions for the symbol error rate (SER) of the proposed 3D-M-DCSK system are derived under the additive white Gaussian noise (AWGN) channel as well as the multipath Rayleigh fading channel. Additionally, the peak-to-average-power-ratio (PAPR) performance of the proposed 3D-M-DCSK system is carefully analyzed. Simulation results show that the proposed 3D-M-DCSK system achieves outstanding improvement in terms of transmission reliability and PAPR performance compared to the state-of-the-artM-DCSK system in both AWGN and multipath Rayleigh fading channels.

In this paper, we propose a reconfigurable intelligent surface (RIS)-assisted frequency-modulated (FM) differential chaos shift keying (DCSK) scheme with simultaneous wireless information and power transfer (SWIPT), called RIS-FM-DCSK-SWIPT scheme, for low-power, low-cost, and high-reliability wireless communication networks. In particular, the proposed scheme is developed under a non-linear energy-harvesting (EH) model which can accurately characterize the practical situation. The proposed RIS-FM-DCSK-SWIPT scheme has an appealing feature that it does not require channel state information, thus avoiding the complex channel estimation. We further derive the closed-form theoretical expressions for the energy shortage probability and bit error rate (BER) of the proposed scheme over the multipath Rayleigh fading channel. In addition, we investigate the influence of key parameters on the performance of the proposed transmission scheme in two different scenarios, i.e., RIS-assisted access point (RIS-AP) and dual-hop communication (RIS-DH). Finally, we carry out various Monte-Carlo experiments to verify the accuracy of the theoretical derivation, illustrate the performance advantage of the proposed scheme, and give some design insights for future study.

To realize high energy-efficient, high spectrum-efficient, and high-throughput data transmission, a multi-carrier differential chaos shift keying communication system with hybrid index modulation, referred to asHIM-MC-DCSK system, is proposed in this paper. In the HIM-MC-DCSK system, we intelligently integrate the carrier-number-index technique and carrier-index technique into the MC-DCSK modulation in order to significantly boost the transmission efficiency in wireless communication systems. Especially, we use a pair of orthogonal signals to represent the active and inactive subcarriers in the HIM-MC-DCSK system so as to exploit all the subcarriers to transmit information bits. In addition, the theoretical bit-error-rate (BER) expressions of the HIM-MC-DCSK system are derived over additive white Gaussian noise (AWGN) and multipath Rayleigh fading channels. Also, the data rate, spectrum efficiency, energy efficiency, and complexity of the proposed system are carefully analyzed. Monte-Carlo simulations not only verify the accuracy of the theoretical analysis but also illustrate the superiority of the proposed system.

This paper describes the design and manufacturing of soft artificial skin with an array of embedded soft strain sensors for detecting various hand gestures by measuring joint motions of five fingers. The proposed skin was made of a hyperelastic elastomer material with embedded microchannels filled with two different liquid conductors, an ionic liquid and a liquid metal. The ionic liquid microchannels were used to detect the mechanical strain changes of the sensing material, and the liquid metal microchannels were used as flexible and stretchable electrical wires for connecting the sensors to an external control circuit. The two heterogeneous liquid conductors were electrically interfaced through flexible conductive threads to prevent the two liquid from being intermixed. The skin device was connected to a computer through a microcontroller instrumentation circuit for reconstructing the 3-D hand motions graphically. The paper also presents preliminary calibration and experimental results.