This paper investigates the problem of direction-of-arrival (DoA) estimation in integrated orbital angular momentum (OAM) radar-communication systems. Unlike previous studies, we consider a more realistic and challenging scenario where coherent signals result from mainlobe overlap among echoes from multiple targets. To tackle this issue, we propose an iterative DoA estimation framework based on the estimation of signal parameters via rotational invariance techniques (ESPRIT) algorithm. Specifically, the coherent echo signals are forcibly decoupled to form a data set comprising multiple independent components, each associated with the DoA of a specific target. A novel iterative estimation strategy is then introduced, wherein a recursive ESPRIT algorithm is sequentially applied to each component during each iteration to extract the corresponding DoA information. The estimated angles are subsequently used to update the data set for the next iteration. This alternating process continues until the DoA estimates converge to the true values. Simulation results demonstrate the effectiveness and robustness of the proposed method under coherent echo conditions.

To ensure high array gain, RIS is evolving towards the extra-large RIS (XL-RIS). However, XL-RIS encounters significant challenges in high pilot overhead. In this paper, we consider a downlink XL-RIS communication system, which is used for assisting the data transmission from a single base station (BS) to multiple user equipments (UEs). We first propose a beam training strategy by turning on only a few RIS elements to reduce pilot overhead, so that the RIS can determine the UE areas. Then, the position information of the UEs is able to be obtained by estimating the far-field channels between the UEs and RIS. After that, all elements are activated for high- throughput data transmission, in which the near-field XL-RIS beamforming is performed based on the connection between the near-field and the far-field channels that we disclose with the positional information of UEs, ultimately forming multiple beams directed at different UEs with optimal power allocation.

Consider a communication system in which a single-antenna user equipment exchanges information with a multi-antenna base station via a reconfigurable intelligent surface (RIS) in the presence of spatially correlated channels and electromagnetic interference (EMI). To exploit the attractive advantages of RIS technology, accurate configuration of its reflecting elements is crucial. In this paper, we use statistical knowledge of channels and EMI to optimize the RIS elements for 1i) accurate channel estimation and 2) reliable data transmission. In both cases, our goal is to determine the RIS coefficients that minimize the mean square error, resulting in the formulation of two non-convex problems that share the same structure. To solve these two problems, we present an alternating optimization approach that reliably converges to a locally optimal solution. The incorporation of the diagonally scaled steepest descent algorithm, derived from Newton’s method, ensures fast convergence with manageable complexity. Numerical results demonstrate the effectiveness of the proposed method under various propagation conditions. Notably, it shows significant advantages over existing alternatives that depend on a suboptimal configuration of the RIS and are derived on the basis of different criteria.

Orbital angular momentum with index modulation (OAM-IM) presents an innovative information modulation scheme for OAM wireless communications, which is expected to achieve superior spectral efficiency (SE) and energy efficiency (EE) by utilizing a set of activated OAM modes to carry additional information based on the principle of IM. However, in the existing OAM-IM communication systems, the accurate alignment between the transmitter and receiver is required to avoid a larger performance penalty. To break the bottleneck, we present a novel misaligned OAM multi-mode multiplexing with IM (OAM-MMUX-IM) scheme based on uniform circular arrays (UCAs), including the generation of OAM-MMUX-IM signals and the multi-mode OAM detection. At the transmit UCA, the OAM-IM signals are designed to carry additional information by activating a subset of the available OAM modes. According to the characteristics of IM, the multi-layer back-propagation neural network (BPNN) is applied to the misaligned receive UCA for the first time to achieve the OAM multi-mode detection. The simulation results from MATLAB show that the presented OAM-MMUX-IM scheme can flexibly identify multiple OAM modes in the misaligned OAM system and achieve excellent SE and error performance.

As a new degree of freedom (DOF), orbital angular momentum (OAM) has the potential of greatly enhancing the channel capacity of wireless communication systems. However, the OAM beams diverge with the increase of transmission distance and OAM mode, which makes the OAM beams modulated with information difficult to be received with a limited aperture array. Therefore, there is still a big challenge for achieving the expected extremely high data rate with orthogonal multiplexed OAM beams in a long-distance communication link. To reduce the divergence of OAM beams, the spherical Luneberg lens is proposed for the uniform circular array (UCA)-based OAM communication system to realize OAM beam convergence. However, due to the non-negligible UCA aperture relative to the Luneberg lens, we find through electromagnetism (EM) simulation that the 0-mode OAM beam becomes more divergent after passing through Luneberg lens. To solve this problem, we propose a hybrid circular array with a center element (CAC) and spherical Luneberg lens (HCCL) structure. When the HCCL structure is applied in the OAM wireless communication system, the divergence angles of generated OAM beams are significantly reduced, and the received signal-to-noise ratio (SNR) is expected to be significantly improved. Besides, the multiplexed OAM modes in the proposed HCCL-based OAM communication system keep orthogonal. Therefore, the channel capacities of the proposed HCCL-based OAM communication systems are obviously better than that of the existing UCA-based OAM system in the long-distance transmission. Furthermore, both numerical analysis and simulation results validate that the performance of the OAM system equipped with the HCCL structure both at transmitter and receiver is significantly better than that of the system equipped with the HCCL structure at one side

Radio frequency-orbital angular momentum (RF-OAM) is a novel approach of multiplexing a set of orthogonal modes on the same frequency channel to achieve high spectrum efficiencies. Since OAM requires precise alignment of the transmit and the receive antennas, the electronic beam steering approach has been proposed for the uniform circular array (UCA)-based OAM communication system to circumvent large performance degradation induced by small antenna misalignment in practical environment. However, in the case of large-angle misalignment, the OAM channel capacity cannot be effectively compensated only by the electronic beam steering. To solve this problem, we propose a hybrid mechanical and electronic beam steering scheme, in which mechanical rotating devices controlled by pulse width modulation (PWM) signals as the execution unit are utilized to eliminate the large misalignment angle, while electronic beam steering is in charge of the remaining small misalignment angle caused by perturbations. Furthermore, due to the interferometry, the receive signal-to-noise ratios (SNRs) are not uniform at the elements of the receive UCA. Therefore, a rotatable UCA structure is proposed for the OAM receiver to maximize the channel capacity, in which the simulated annealing algorithm is adopted to obtain the optimal rotation angle at first, then the servo system performs mechanical rotation, at last the electronic beam steering is adjusted accordingly. Both mathematical analysis and simulation results validate that the proposed hybrid mechanical and electronic beam steering scheme can effectively eliminate the effect of diverse misalignment errors of any practical OAM channel and maximize the OAM channel capacity.

Orbital angular momentum (OAM) radars are able to estimate the azimuth angle and the rotation velocity of multiple targets without relative motion or beam scanning. Moreover, OAM wireless communications can achieve high spectral efficiency (SE) by utilizing a set of information-bearing modes on the same frequency channel. Benefitting from the above advantages, in this paper, we design a novel radar-centric joint OAM radar-communication (RadCom) scheme based on uniform circular arrays (UCAs), which modulates information signals on the existing OAM radar waveform. In details, we first propose an OAM-based three-dimensional (3-D) super-resolution position estimation and rotation velocity detection method, which can accurately estimate the 3-D position and rotation velocity of multiple targets without beam scanning. Then, we derive the posterior Cramér-Rao bound (PCRB) of the OAM-based estimates and, finally, we analyze SE of the integrated system. To achieve the best trade-off between imaging and communication, the transmitted integrated OAM beams are optimized by means of an exhaustive search method. Both mathematical analysis and simulation results show that the proposed radar-centric joint OAM RadCom scheme can accurately estimate the 3-D position and rotation velocity of multiple targets while ensuring the SE of the communication receiver, which can be regarded as an effective supplement to existing joint RadCom schemes.

Orbital angular momentum (OAM) at radio frequency (RF) provides a novel approach of multiplexing a set of orthogonal modes on the same frequency channel to achieve high spectrum efficiencies. However, there are still big challenges in the multi-mode OAM generation, OAM antenna alignment and OAM signal reception. To solve these problems, we propose an overall scheme of the line-of-sight multi-carrier and multi-mode OAM (LoS MCMM-OAM) communication based on uniform circular arrays (UCAs). First, we verify that UCA can generate multi-mode OAM radio beam with both the RF analog synthesis method and the baseband digital synthesis method. Then, for the considered UCA-based LoS MCMM-OAM communication system, a distance and AoA estimation method is proposed based on the two-dimensional ESPRIT (2-D ESPRIT) algorithm. A salient feature of the proposed LoS MCMM-OAM and LoS MCMM-OAM-MIMO systems is that the channel matrices are completely characterized by three parameters, namely, the azimuth angle, the elevation angle and the distance, independent of the numbers of subcarriers and antennas, which significantly reduces the burden by avoiding estimating large channel matrices, as traditional MIMO-OFDM systems. After that, we propose an OAM reception scheme including the beam steering with the estimated AoA and the amplitude detection with the estimated distance. At last, the proposed methods are extended to the LoS MCMM-OAM-MIMO system equipped with uniform concentric circular arrays (UCCAs). Both mathematical analysis and simulation results validate that the proposed OAM reception scheme can eliminate the effect of the misalignment error of a practical OAM channel and approaches the performance of an ideally aligned OAM channel.

Orbital angular momentum (OAM) at radio frequency (RF) provides a novel approach of multiplexing a set of orthogonal modes on the same frequency channel to achieve high spectral efficiencies. However, OAM communication system requires perfect alignment of the transmit and the receive antennas and this harsh precondition greatly challenges practical multiplexing gain of OAM system. In this paper, we propose a UCA-based multi-mode OAM reception method including the beam steering with the estimated angle of arrival (AoA) and the amplitude detection with the estimated distance. Both mathematical analysis and simulation results validate that the proposed OAM reception method can completely eliminate the effect of the misalignment error of a practical OAM channel and approaches the performance of an ideally aligned OAM channel.