Radio Frequency Orbital Angular Momentum Research Articles

Optical integrated reconfigurable mode generator-multiplexer for radio-frequency orbital angular momentum.

Radio-frequency orbital angular momentum (RF-OAM) mode multiplexing has received increasing attention in next-generation (6G) wireless communication as a new spatial multiplexing dimension to further extend data capacity. In particular, the circular antenna array (CAA) based RF-OAM generation using photonic methods is a promising approach due to its flexible reconfigurability, low propagation loss, high operating bandwidth, and high pattern quality. However, most current reports focus either on the bulk optics-based implementations or on band-limited delay line-based approaches. In this paper, we present a novel phased array-based integrated photonic RF-OAM generator-multiplexer for a four-antenna CAA system supporting three RF-OAM modes at 16 GHz for both fast mode switching and mode multiplexing. It consists of a specially designed interleaved optical filter and a four-element phased array based on cascaded phase shifters. By changing the pair of wavelengths used, the output RF-OAM modes can be quickly switched between fundamental mode and ±1 modes. A bandwidth of 1 GHz was experimentally demonstrated. Mode multiplexing can be achieved when all three wavelength pairs are loaded.

Hybrid Mechanical and Electronic Beam Steering for Maximizing OAM Channel Capacity

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.

Optical-controlled Fast Switching of Radio Frequency Orbital Angular Momentum Beams With Different Mode and Radiation Direction

The wireless imaging radar need more orbital angular momentum (OAM) modes to irradiate targets alternately to obtain higher azimuth resolution. For that, this present paper investigates the fast-switching technique of radio frequency orbital angular momentum (RF-OAM) beam with the use of an optical-controlled system. Compared with the method using pure electrical device, the optical-controlled method offers conveniences in processing same-frequency radio signal, which will contribute to make the system compact. Moreover, the fast-responding speed of the optical device provide a fast speed of switching. The fast-switching system of RF-OAM beams with different mode and radiation direction is proposed and constructed, and proof-of-concept experiments are performed. The proposed system is realized by employing a program-controlled optical switch and the wavelength multiplexing technique integrated into an optical-controlled circular antenna array (CAA). In the demonstration experiments, the switching between +1-mode and −1-mode RF-OAM beams without steering control is detected within 20 ms, then the system achieves the switching between a +1-mode OAM beam with radiation direction −3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> and a −1-mode OAM beam with radiation direction 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> at the same rate.

Secure Transmission of Radio Orbital Angular Momentum Beams Based on the Frequency Diverse Array

The circular phased array is one of the main methods to generate radio frequency orbital angular momentum (RF-OAM) beams with high purity and capability of OAM-beams steering, but OAM beams generation and steering are only related to angle, not range dimensions. Therefore, the mode of OAM beams can be easily received in a wide range and intercepted by unintended eavesdroppers in the beam axis. As a promising solution, a range-angle dependent OAM beams transmission scheme is highlighted by the frequency diverse array (FDA) for the sake of the OAM secure transmission in this paper. In order to realize the range-angle dependent OAM beams, a special frequency interval between adjacent antenna elements in the FDA is designed and illustrated. The modes of OAM beams generated by the FDA can only be observed in the legal target location. Numerical results show that the scheme can effectively realize the safe transmission of OAM beams, i.e. , reducing the possibility of the OAM beams being tapped, improving the OAM-beams transmission performance, and enhancing the energy efficiency with low complexity.

Misalignment Measurement of Orbital Angular Momentum Signal Based on Spectrum Analysis and Image Processing

Misalignment measurement is important in radio frequency orbital angular momentum (RF-OAM)-related communication and radar applications. Misalignment causes distorted vortex wavefront and OAM spectra dispersion, therefore a conventional gradient method cannot be used directly. We propose a misaligned measurement method for RF-vortex beams based on image processing. Theoretical analysis and numerical simulations of the misaligned OAM wavefront demonstrate that the misalignment information can be measured by extracting the features of the OAM mean value distribution and the OAM variance value distribution. It is found in this paper that the extrema of mean OAM distribution and OAM variance distribution correspond to the tilted singularity point and original pivot point, separately. These extrema points can be found by image processing algorithms that recognize round speckles. A proof-of-concept experiment is presented, which employs a 2-D scanning probe to measure a 10°-tilted OAM beam with mode +1. The tilt measurement error is only 0.39°.

2D optically controlled radio frequency orbital angular momentum beam steering system based on a dual-parallel Mach-Zehnder modulator.

An optically controlled system for generating and continuously steering radio frequency (RF) signals with double orbital angular momentum (OAM) modes is proposed and experimentally demonstrated. The optical carrier's utilization efficiency can be doubled through the distinct electro-optical modulation, which is based on two single-sideband modulation operations on a single optical carrier through a customized dual-parallel Mach-Zehnder modulator. A constructive antenna phase feeding method of a circular antenna array for collectively forming and steering an OAM radio beam is proposed and illustrated. A proof-of-concept experiment is conducted to generate and steer a dual-mode RF-OAM beam to two different two-dimensional (2D) directions, independently and simultaneously. One 17GHz OAM beam with mode L=1 is continuously steered to 2D directions (:, 0°, 0°), (:, 0°, 1.70°), (:, 0°, 3.87°), (:, 0°, 6.17°), and(:, 0°, 7.80°), with vortex properties, where ":" means "any value of." Meanwhile, the 19GHz OAM beam with mode L=-1 carried is steered from (:, 0°, 0°) to (:, 0°, -6.72°), and the constellations are obtained successfully.

Beam Steering for the Misalignment in UCA-Based OAM Communication Systems

Radio frequency-orbital angular momentum (RF-OAM) is a technique that provides extra degrees of freedom to improve spectrum efficiency of wireless communications. However, OAM requires perfect alignment of the transmit and the receive antennas and this harsh precondition greatly challenges practical applications of RF-OAM. In this letter, we first investigate the effect of non-parallel misalignment on the channel capacity of the RF-OAM communication system equipped with uniform circular array. Then, we propose a transmit/receive beam steering approach to circumvent the large performance degradation in not only non-parallel case, but also off-axis and other general misalignment cases. The effectiveness of the beam steering approach is validated through both mathematical analysis and numerical simulations.

Generation of coupled radio frequency orbital angular momentum beam with an optical-controlled circular antenna array

A circular antenna array (CAA) principle for generating radio frequency orbital angular momentum (RF-OAM) beams with dual-opposite-states is proposed in an optical-controlled system. At this condition, the pre-existing restrictions of antenna amount for CAA system is extended. It is worth noting that the simulative beam patterns show almost no distortion in this superposition states case. In the following control experiment, transmission characteristics, including Error Vector Magnitude (EVM), Signal to Noise Ratio (SNR) and receiving power, are all improved significantly in each proposed coupled OAM channel, compared to ones in single-state OAM beam. The EVM can be decreased from 4.82% to 4.59% in average, and the receiving power is increased by 2.7 dB at the same transmission condition, which indicates effective propagation distance can be increased.

Mode Measurement of a Dual-Mode Radio Frequency Orbital Angular Momentum Beam by Circular Phase Gradient Method

A measurement method of dual-mode radio frequency orbital angular momentum (RF-OAM) beams is proposed, dedicated to meeting the practical needs of the orbital angular momentum mode-division multiplexing (OAM-MDM) in free-space communication. Theoretical analysis and numerical simulations are conducted to demonstrate that a dual-mode OAM beam can be unambiguously measured by gathering the phase gradient function of a circle around the OAM beam axis and calculating its average value and PV (peak/valley) factor. A proof-of-concept experiment is presented, which employs a single receiving antenna on a rotatable plane to measure mode +1 and -2 of a dual-mode OAM beam.

OAM multiple transmission using uniform circular arrays: Numerical modeling and experimental verification with two digital television signals

AbstractIn this work we present the outcomes of a radio frequency orbital angular momentum (OAM) transmission between two antenna arrays performed in a real‐world context. The analysis is supplemented by deep simulative investigations able to provide both a preliminary overview of the experimental scenario and a posteriori validation of the achieved results. As a first step, the far‐field OAM communication link is tested at various frequencies and the corresponding link budget is studied by means of an angular scan generated by the rotation of the receiving system. Then, on the same site, two digital television signals encoded as OAM modes (ℓ = 1 and ℓ =− 1) are simultaneously transmitted at a common frequency of 198.5 MHz with good mode insulation.