Integrated Transmit Waveform and RIS Phase Shift Design for LPI Detection and Communication

Since cognition has become an important topic in Electronic Warfare (EW) systems, Electronic Support Measures (ESM) are used to monitor, intercept and analyse radar signals. Low Probability of Intercept (LPI) radars is preferred to be able to detect targets without being detected by ES systems. Because of their properties as low power, variable frequency, wide bandwidth, LPI Radar waveforms are difficult to intercept with ESM systems. In addition to intercepting, the determination of the waveform types used by the LPI Radars is also very important for applying counter-measures against these radars. In this study, a solution for the LPI Radar waveform recognition is proposed. The solution is based on the training of Support Vector Machine (SVM) after applying Principal Component Analysis (PCA) to the data obtained by Time-Frequency Images (TFI). TFIs are generated using Choi-Williams Distribution. High energy regions on these images are cropped automatically and then resized to obtain uniform data set. To obtain the best result in SVM, the SVM Hyper-Parameters are also optimized. Results are obtained by using one-against-all and one-against-one methods. Better classification performance than those given in the literature have been obtained especially for lower Signal to Noise Ratio (SNR) values. The cross-validated results obtained are compared with the best results in the literature.

An improved method of reducing the input impedance of a four-arm sinuous antenna is demonstrated for a wide gain bandwidth. The proposed antenna has a dual-linear polarized response for electronic support measures (ESM) systems and is designed with a planar substrate. The structure for reducing the input impedance is developed based on the transmission and coupled line theory and is shown to be in excellent agreement with theoretical predictions. The proposed antenna achieves an impedance bandwidth in the range of 0.45–6 GHz with a reference impedance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100 ~\Omega $ </tex-math></inline-formula> . Simulations for comparison according to the difference in input impedance of designed antennas with feeding structures are also presented. Finally, the performance of the fabricated antenna with wideband balun is verified by measuring co- and cross-polarization radiation patterns. As a result, the proposed antenna achieves a wide gain bandwidth (>2.5 dBi) and a cross-polarization isolation (>−13 dB) of more than 12:1 in the frequency range over 0.6 GHz, which can be used in ESM systems.

Energy efficiency (EE) has become a key performance indicator in designing future wireless networks. In this regard, we propose a novel two reconfigurable intelligent surfaces (RISs) assisted coordinated multipoint (CoMP) network, where two RISs cooperate to reduce energy consumption by constructing an intelligent radio environment. Aiming to maximize this system’s energy efficiency (EE), the joint optimization problem on the cooperative beamforming among base stations (BSs) and passive beamforming among RISs, subject to each user’s data rate requirement, is formulated. An alternative optimization algorithm is proposed by decoupling it into a cooperative beamforming subproblem among BSs and a cooperative reflective coefficients design subproblem among RISs. Dinkelbach’s transform and complex fractional programming algorithms are also invoked to transform these two subproblems into convex optimization problems and obtain the optimal solution for each sub-problem. Numerical results validate the proposed algorithm’s feasibility, fast convergence, and flexibility. It also shows that the proposed scheme with cooperative beamforming design for both BSs and RISs outperforms that without RIS and that with randomly initialized RISs.

Joint LPI waveform and passive beamforming design for FDA-MIMO-DFRC systems

This work analyzes a technique, based on the concept of cross-correlation applied to Electronic Support Measures (ESM) Receivers, to detect noise radar signals, a class of waveforms with low probability of intercept (LPI) features, and to evaluate the related performance with relevant comparisons.

This paper covers development trends in communication and radar electronic warfare (EW). Threat scenario and role of EW sensors in developing Electronic Order of Battle (EOB) are described. EW is subdivided into Electronic Support Measure (ESM), Electronic Counter Measure (ECM) and Electronic Counter-counter Measure (ECCM) systems. Developments in communication ESM and ECM systems and their constituents are described. Under COM ECM, jammer system configuration and realisation of the constituent subsystems, jamming strategies used for effectively countering various types of communications are described. Developments in critical subsystems and configurations of typical radar EW systems are discussed. Finally the status of the indigenous developments in ESM, ECM and integrated systems are described.

This paper proposes a partition-based reconfigurable intelligent surface (RIS)-assisted multi-user multiple-input single-output (MU-MISO) symbiotic radio (SR) system. Unlike conventional RIS designs in SR systems that utilize all elements for secondary transmissions, which restricts its ability to support the primary transmission, the RIS is divided into two subsurfaces: one enhances primary transmissions while the other transmits IoT information via spectrum sharing. We formulate a joint optimization problem to minimize transmit power under rate constraints for both primary and IoT transmissions through active beamforming and passive RIS phase-shift design. To tackle the non-convex constraints and variable coupling in the formulated problem, we propose efficient optimization techniques, including alternating optimization and difference-of-convex methods. Furthermore, we propose a low-complexity interference-free scheme leveraging partitioned RIS to eliminate inter-user interference, which is unachievable with conventional RIS design. Simulation results reveal that compared to the conventional scheme, which can be viewed as a special case of partition-based RIS where all the reflecting elements are used to transmit IoT information, partition-based RIS demonstrates superior performance, thereby validating the advantages of RIS partitioning in SR systems.

Reconfigurable intelligent surface (RIS) is promising for future 6G wireless communications. However, the increased number of RIS elements results in the high overhead for channel acquisition and the non-negligible power consumption. Therefore, how to improve the system capacity with limited RIS elements is essential. Unlike the classical regular RIS whose elements are arranged on a regular grid, in this paper, we propose an irregular RIS structure to improve the system capacity. The key idea is to irregularly configure a given number of RIS elements on an enlarged surface, which provides extra spatial degrees of freedom compared with the regular RIS. In this way, the received signal power can be enhanced, and thus the system capacity can be improved. Then, we formulate a joint topology and precoding optimization problem to maximize the capacity for irregular RIS-aided communication systems. Accordingly, a joint optimization algorithm with low complexity is proposed to alternately optimize the RIS topology and the precoding design. Particularly, a tabu search-based method is used to design the irregular RIS topology, and a neighbor extraction-based cross-entropy method is introduced to optimize the precoding design. Simulation results demonstrate that, subject to the constraint of limited RIS elements, the proposed irregular RIS can significantly enhance the system capacity.

The low probability of intercept (LPI) radar waveform is reflected in a lower pulse power and complex structure of the pulse series. The previous one ensures that it avoids being intercepted by the electronic measures system (ESM) carried by the goal; the latter makes it difficult for ESM to separate and identify. As for pulse position jitter pulse array, because of pulse position's random jitter in a specific pulse interval of the pulse repetition interval (PRI), the spectrum broadens and spectral density reduces, making it hard for the ESM to intercept. Meanwhile, because the waveform doesn't have a fixed PRI, it's also difficult to be sorted or identified Therefore pulse position jitter waveform is a rational LPI waveform. Combined with a design about the LPI radar signal waveform, this paper carries out a research on PRI jitter waveform, and provides a waveform generating and echo signal processing method; then the paper simulates the LPI characteristics and comes to a conclusion which has an important applying value .

This paper investigates a reconfigurable intelligent surface (RIS) aided communication scenario in which the RIS and the transmitter jointly send information to the receiver. While most studies of the RIS focus on its ability to beamform and boost the received signal-to-noise ratio (SNR), it is known that if the information data stream is also available at the RIS and can be modulated through the adjustable phases at the RIS, significant improvement in the multiplexing gain of the overall channel is possible. To achieve the theoretic multiplexing gain, this paper provides a symbol level precoding (SLP) approach for modulating data through the phases of the RIS. The SLP strategy is to set the received signal to be as close to the desired points in a constellation as possible. The idea is to directly make the received signal to be the symbols corresponding to the information to be transmitted and to design the transmitter and the reflective coefficients of the RIS so that the desired symbols are synthesized at the receiver. This strategy is formulated as an optimization problem and an augmented Lagrangian approach with the Riemannian conjugate gradient descent method is proposed to solve the optimization problem efficiently. Numerical simulation results show that the proposed method can achieve the theoretical multiplexing gain.

Although the reconfigurable intelligent surface (RIS) technology is envisioned promising to enhance communication from all aspects, including physical-layer security, increasing concerns have lately been cast onto its defects—the severe “double-fading” loss and its confined-to-half-space coverage. Diverse novel RIS architectures have recently emerged to partially overcome these shortcomings, yet perfect solution is still absent. This paper proposes a novel double-faced active (DFA)-RIS structure to surmount the above two prominent defects simultaneously. Furthermore, we utilize the DFA-RIS to promote secrecy performance via jointly designing access point (AP)’s beamforming and DFA-RIS configuration towards maximizing sum secrecy rate (SR). The optimization problem is highly challenging due to the constraints deriving from the DFA-RIS architecture, especially the presence of power splitting parameters. By leveraging majorization–minimization (MM) and penalty dual decomposition (PDD) methods, we develop an efficient solution that updates all variables via convex optimization techniques. Our proposed solution is significant and general as it is applicable to all other cutting-the-edge RIS architectures to maximize sum SR, which has not yet been thoroughly worked out. Numerical results verify the convergence and effectiveness of our proposed algorithm and demonstrate that our proposed DFA-RIS architecture outperforms all other state-of-the-art RIS techniques to enhance communication security.

This paper studies the coexistence of enhanced mobile broadband (eMBB) and ultra-reliable and low-latency communication (URLLC) services in a cellular network that is assisted by a reconfigurable intelligent surface (RIS). The system model consists of one base station (BS) and one RIS that is deployed to enhance the performance of both eMBB and URLLC in terms of the achievable data rate and reliability, respectively. We formulate two optimization problems, a time slot basis eMBB allocation problem and a mini-time slot basis URLLC allocation problem. The eMBB allocation problem aims at maximizing the eMBB sum rate by jointly optimizing the power allocation at the BS and the RIS phase-shift matrix while satisfying the eMBB rate constraint. On the other hand, the URLLC allocation problem is formulated as a multi-objective problem with the goal of maximizing the URLLC admitted packets and minimizing the eMBB rate loss. This is achieved by jointly optimizing the power and frequency allocations along with the RIS phase-shift matrix. In order to avoid the violation in the URLLC latency requirements, we propose a novel framework in which the RIS phase-shift matrix that enhances the URLLC reliability is proactively designed at the beginning of the time slot. For the sake of solving the URLLC allocation problem, two algorithms are proposed, namely, an optimization-based URLLC allocation algorithm and a heuristic algorithm. The simulation results show that the heuristic algorithm has a low time complexity, which makes it practical for real-time and efficient multiplexing between eMBB and URLLC traffic. In addition, using only 60 RIS elements, we observe that the proposed scheme achieves around 99.99% URLLC packets admission rate compared to 95.6% when there is no RIS, while also achieving up to 70% enhancement on the eMBB sum rate.

Incorporating rate-splitting multiple access (RSMA) into integrated sensing and communication (ISAC) presents a significant security challenge, particularly in scenarios where the location of a potential eavesdropper (Eve) is unidentified. Splitting users' messages into common and private streams exposes them to eavesdropping, with the common stream dedicated for sensing and accessible to multiple users. In response to this challenge, this paper proposes a novel approach that leverages active reconfigurable intelligent surface (RIS)-aided beamforming and artificial noise (AN) to enhance the security of RSMA-enabled ISAC. Specifically, we first derive the ergodic private secrecy rate (EPSR) based on a mathematical approximation of the average Eve channel gain. An optimization problem is then formulated to maximize the minimum EPSR, while satisfying the minimum required thresholds on the ergodic common secrecy rate, the radar sensing and the RIS power budget. To address this non-convex problem, a novel optimization strategy is developed, whereby we alternatively optimize the transmit beamforming matrix for the common and private streams, rate splitting, AN, RIS reflection coefficient matrix, and radar receive beamformer. Successive convex approximation (SCA) and Majorization-Minimization (MM) are employed to convexify the beamforming and RIS sub-problems. Simulations are conducted to showcase the effectiveness of the proposed framework against established benchmarks.

Reconfigurable intelligent surface (RIS) is capable of improving the wireless system performance by steering the reflected signal in the desired direction. One of the major challenges is that both the transceiver and RIS have to be jointly optimized, where the optimization problems have to be reformulated for different system models and scenarios. To circumvent this challenge, new low-complexity antenna selection (AS) algorithms for transceiver-agnostic RIS configuration are proposed. Given a multiple-input multiple-output (MIMO) channel, the proposed RIS-AS opts for accurately aligning the RIS both with the transmit antenna (TA) and receive antenna (RA) for the sake of maximizing the MIMO channel’s overall output power. The proposed RIS-AS only has to configure the RIS alone, i.e. without iterations with the transceiver optimization. As a result, the proposed RIS-AS has the compelling benefit that they are generically applicable, regardless of the specific transceiver architecture. Our simulation results confirm that the proposed RIS-AS is capable of supporting any MIMO configuration, regardless of their closed/open-loop, single-/ full-RF and multiplexing-/diversity-oriented setups.

This paper accounts for recent efforts by CelsiusTech Electronics AB to study a concept of an LPI, low probability of intercept, short range air search radar giving accurate 3D designation of targets to, e.g. electro-optical tracking and fire control equipment. This work has mainly been performed in 1995-96; earlier efforts are summarized. The described effort also includes the application of super resolution algorithms for estimating the elevation angle of the target. LPI has become an important consideration in modern radar because of advances in radar warning receivers, RWR, and electronic support measures, ESM. An outline of how to achieve the best possible LPI is also given.