Orbital Angular Momentum Communication Systems Research Articles

Analysis and Design of Broadband OAM Array Antenna

This paper presents a Uniform Circular Array (UCA) antenna of crossed-dipole that can excite vortex waves in a wide frequency range from 3.5 GHz to 8.4 GHz. In the design process, the theoretical derivation of the influence on the Orbital Angular Momentum (OAM) when the antenna elements are arrayed in Co-directional Antenna Array (CAA) and Rotational Antenna Array (RAA) is given respectively. The fixed mode of the dipoles CAA is achieved by feeding each element with equal amplitude and 90∘ phase difference produced by the broadband feeding network. Furthermore, the proposed broadband OAM array antenna has been fabricated and measured to verify the predicted properties. The vortex electromagnetic wave with +1 mode could be excited in the bandwidth of 82.35%. Simulated and measured results are in good agreement. The proposed OAM array antenna is simple in design principle, compact in structure and low in profile, making this array antenna an excellent candidate for broadband OAM communication systems.

On the design of multi-vortex beam multiplexers using programmable metasurfaces

This paper presents an analytical procedure for designing orbital angular momentum (OAM) multiplexers with the capability of generating multiple multiplexed beams with different directions and OAM mode indices. The presented design method provides independent control of the power of each output OAM mode in a multiplexed beam. As a proof of concept, a metasurface with four incident beams and two multiplexed scattered beams with different directions and OAM modes is designed. It is based on a 2/2 bits phase–amplitude programmable metasurface which is numerically simulated, and the results are in good agreement with analytical predictions. The presented approach paves the way for design and implementation of multifunctional devices specially used in OAM communication systems.

Multi-User Orbital Angular Momentum Based Terahertz Communications

Terahertz (THz) wireless communications are commonly regarded as one of the key technologies of 6G communication. Combined with THz, the newly exploited physical layer transmission dimension orbital angular momentum (OAM) that multiplexes a set of orthogonal modes on the same frequency channel, can unleash its potential in achieving high spectrum efficiency. Currently, most of the research on radio OAM communications focus on the point-to-point scenario in microwave and millimeter-wave (mmWave) bands. In this paper, we propose a uniform circular array (UCA)-based THz multi-user OAM (MU-OAM) communication system including downlink and uplink transmission schemes that simplifies the signal detection to the despiralization and amplitude detection (AD). A salient feature of the proposed MU-OAM communication scheme is lower computational complexity than the traditional MU-MIMO scheme without sacrifacing bit error rate (BER) and achievable sum rate performances, which is validated by mathematical analysis and numerical simulations.

3D Printed Multi-Beam OAM Antenna Based On Quasi-Optical Beamforming Network

In this paper, a multi-beam orbital angular momentum (OAM) antenna is proposed. By employing 3D printed millimeter-wave quasi-optical beamforming network (BFN), the emitted OAM wave can realize passive beam scanning capability. The antenna is composed of a pillbox BFN, a slotted waveguide antenna (SWA) array, and several full-dielectric phase elements (PEs) on the top of the SWA array. The required phase distribution of the OAM beams is turned by the phase shifts of the PEs. The pillbox BFN can generate the input waves with different phase gradients for the SWA array, and thus steer the beam direction. Moreover, the folded structure of the pillbox BFN can greatly decrease the size of the antenna. The proposed topology can be easily expanded to realize more beam number, higher directivity, and other OAM modes for its periodical structure. To verify the performance of the proposed antenna, a prototype is fabricated and measured. Two types of 3D printing technology are used, i.e., selective laser melting (SLM) for the pillbox BFN and the SWA array, and fused deposition modeling (FDM) for the PEs. The application of the 3D printed method greatly helps to decrease the fabrication difficulty. The measured results show that the antenna can achieve five +1 mode beams with ±20° 3 dB beam coverage range. Moreover, due to the full-metal structure of the BFN and the SWA array, the proposed design can achieve the minimum efficiency of 83.2%, which is much better than the other printed circuit broad (PCB) based OAM antennas. The advantages of low-cost beam scanning capability and the extensibility make the proposed antenna topology a good choice for future OAM communication and sensor systems.

Channel Polarization Scheme for Ocean Turbulence Channels in Underwater Visible Light Communication

Underwater wireless communication technology plays an important role in marine environment monitoring and ecological protection. Underwater optical wireless communications (UWOCs) can currently achieve a transmission distance of hundreds of meters, and the rate can reach hundreds of Mbps or even Gbps, with low power consumption and high-speed features. In addition, UWOC also has the advantages of a small transceiver size and strong anti-electromagnetic interference ability, which is especially suitable for scenarios where underwater volume and power consumption are relatively limited. However, UWOC systems face problems such as unstable transceiver ends, ocean turbulence, and so on, resulting in reduced communication reliability and limited transmission distance. Establishing a stable and reliable communication link is critical to extending the communication distance of the UWOC system. In this paper, a model of ocean turbulence channels is established based on the power spectrum inversion method. The transmission characteristics of orbital angular momentum (OAM) light in an ocean turbulence channel are studied, then the mode selection of OAM light is determined. At the same time, the polarization coding technique is applied to the underwater OAM communication system for the first time. The simulation results show that this scheme can effectively extend the communication distance and reduce the system bit error rate.

Hybrid Circular Array and Luneberg Lens for Long-Distance OAM Wireless Communications

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

Enhancing Performance of Air–Ground OAM Communication System Utilizing Vector Vortex Beams in the Atmosphere

The modified uplink and downlink atmospheric turbulence channel models were established and employed to assess the system performance of air–ground orbital angular momentum (OAM) communication. The advantage of the vector vortex beam taking the place of the scalar one in the OAM communication system operated in the atmospheric turbulence was verified, that vector vortex beam can guarantee the more homogeneous energy in the circular hollow beam profile and the less phase distortion on signal OAM in the turbulence, which can reduce OAM crosstalk and improve OAM communication performance, especially small topological charge in strong turbulent regime. With the increase in turbulence strength, the vortex beam with a larger topological charge suffered more OAM mode crosstalk, and the average BER of the OAM communication system increased. Bessel–Gaussian (BG) beams with larger beam shape parameters had the strong capability of turbulence disturbance rejection in short-distance atmospheric applications, conversely, Laguerre–Gaussian (LG) beams with suitable parameter selection were preferred for long-distance atmospheric applications. Additionally, compared to the downlink channel, the transmission of OAM mode and the related communication system in the uplink channel are dramatically deteriorated due to atmospheric turbulent effects.

Wideband Blind Calibration for UCA OAM Communication System

This letter presents a wideband blind calibration method for uniform circular array (UCA) orbital angular momentum (OAM) systems without the prior knowledge of the positions and attitudes of the transmit and receive UCAs. The basic idea is to rewrite the maximum-likelihood estimation (MLE) for all parameters as an optimization problem over a Lie group which can represent the positions and attitudes of the UCAs. Then an accelerated Levenberg-Marquardt algorithm is implemented to solve this optimization problem by utilizing the sparsity in the iteration process. Simulation results show that the proposed wideband blind calibration method can estimate the gain/phase errors and the positions and attitudes of the transmit and receive UCAs simultaneously and improve the achievable spectral efficiency of the UCA OAM communication system efficiently.

Broadband beam steering for misaligned multi-mode OAM communication systems

Orbital angular momentum (OAM) at radio frequency (RF) has attracted more and more attention as a novel approach of multiplexing a set of orthogonal OAM modes on the same frequency channel to achieve high spectral efficiency (SE). However, the precondition for maintaining the orthogonality among different OAM modes is perfect alignment of the transmit and receive uniform circular arrays (UCAs), which is difficult to be satisfied in practical wireless communication scenarios. Therefore, to achieve available multi-mode OAM broadband wireless communication, we first investigate the effect of oblique angles on the transmission performance of the multi-mode OAM broadband system in the non-parallel misalignment case. Then, we compare the UCA-based RF analog and baseband digital transceiver structures and corresponding beam steering schemes. Mathematical analysis and numerical simulations validate that the SE of the misaligned multi-mode OAM broadband system is quite low, while analog and digital beam steering (DBS) both can significantly improve the SE of the system. However, DBS can obtain higher SE than analog beam steering especially when the bandwidth and the number of array elements are large, which validates that the baseband digital transceiver with DBS is more suitable for multi-mode OAM broadband wireless communication systems in practice.

Achievable Rate Maximization for Intelligent Reflecting Surface-Assisted Orbital Angular Momentum-Based Communication Systems

The orbital angular momentum (OAM)-based communication systems may face severe transmission problems when the transmit and receive uniform circular array pairs are blocked. In this paper, a promising technique named intelligent reflecting surface (IRS) is proposed to help alleviate blockages and provide alternative line-of-sight links. To maximize the achievable rate of the IRS-assisted OAM communication systems, we optimize its transmit power allocation along with the IRS's reflecting phase shifts, and propose an alternative optimization-based algorithm to solve the resulting optimization problem with coupled variables and non-convex structure. Specifically, the proposed algorithm obtains a closed-form solution to the transmit power allocation by applying the majorization-minimization and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\ell _1$</tex-math></inline-formula> -ball projection approaches, and obtain the locally optimal solution to the IRS's reflecting phase shifts by applying the weighted minimum mean square error-based fixed point iteration approach. Simulation results demonstrate the superiority of our proposed algorithm over existing baseline algorithms and also show its robust stability to the oblique angle errors.

Data-defined naïve Bayes (DNB) based decision scheme for the nonlinear mitigation for OAM mode division multiplexed optical fiber communication.

In this manuscript, a data-defined naïve Bayes (DNB)-based decision scheme for nonlinear mitigation is presented for an orbital angular momentum (OAM) mode-division multiplexed optical fiber communication system. Due to the inherent nonlinearity characteristic of opto-electronic devices, the strong nonlinear impairments are deemed inevitable in OAM mode-division multiplexed transmission, leading to severely nonlinear effects. A DNB algorithm based on the prior probability distribution is adopted to mitigate the strong device nonlinearity of the OAM communication system, which is hard to solve using the conventional approaches due to the complex theoretical model of opto-electronic devices. An experiment using eight-mode OAM with a 32GBaud Nyquist QPSK signal optical fiber communication system is carried out with ring core fiber (RCF) transmission over 10 km to verify the effectiveness of the proposed scheme. The experimental results demonstrate that the nonlinear effects on OAM transmission can be effectively mitigated using a DNB-based decision with a bit error rate (BER) reduction of at most 66%. Moreover, compared with other nonlinear decision algorithms based on machine learning, such as support vector machine (SVM) or k-nearest neighbors (KNN), the digital signal processing complexity of the DNB algorithm is significantly reduced.

Receiving sensitivity improvement by wide-spectrum OAM with OTDM and polarization multiplexing in weak atmospheric turbulence.

In this paper, a wide-spectrum orbital angular momentum (OAM) system with a polarization and optical time division multiplexing (OTDM) free-space transmission system is experimentally demonstrated. To enhance the system transmission performance in atmospheric turbulent channel, a wide-spectrum laser and an OAM beam are used. The wide-spectrum laser can be generated by utilizing pumped laser to pump nonlinear fiber, and OAM can be generated with a special light modulator. Furthermore, OTDM and polarization multiplexing methods are used to enhance the communication rate from 4 Gbit/s to 32 Gbit/s. With the use of the wide-spectrum laser and the OAM beam, the receiving scintillation index (SI) can be reduced, and detection sensitivity can be improved. It is the first time a wide-spectrum OAM communication system performance has been studied. It is shown that under weak atmospheric turbulence condition, the SI can be reduced by 38% and the receiving sensitivity can be improved by 3.18 dB via wide-spectrum OAM beams.

Analyze UCA Based OAM Communication From Spatial Correlation

Orbital Angular Momentum(OAM) of Electromagnetic(EM) wave can be seen as a new degree of freedom(DoF) for wireless communication besides time and frequency. Uniform Circular Antenna Array(UCA) fed with signal of equal amplitude and equal phase difference on each antenna element is a good OAM generation method on radio frequency(RF) band. Previous researches have shown that UCA based OAM communication system is essentially a special case of Multiple-input Multiple-output(MIMO) system and elaborated the viewpoint from different perspectives. In this paper, we demonstrate the above viewpoint from a new perspective. Specifically, for UCA based OAM communication in ideal(line of sight(Los) and coaxial) channel, we systematically analyze the variations of radiation pattern and spatial correlation of UCA under different radius and distance settings. To extend our demonstration, we first simulate the channel capacity variation with the adjustment of UCA radius and distance, and then simulate the channel capacity of both UCA and uniform linear antenna array(ULA) based communication systems, and find that ULA can also achieve similar capacity performance as UCA when spatial distance scale is similar. As is known that MIMO communication system provides spatial DoF besides time and frequency, we can conclude that mode DoF of UCA based OAM essentially originates spatial DoF of UCA based on the analysis in the paper.

Superhigh-Resolution Recognition of Optical Vortex Modes Assisted by a Deep-Learning Method.

Orbital angular momentum (OAM) has demonstrated great success in the optical communication field, which theoretically allows an infinite increase of the transmitted capacity. The resolution of a receiver to precisely recognize OAM modes is crucial to expand the communication capacity. Here, we propose a deep learning (DL) method to precisely recognize OAM modes with fractional topological charges. The minimum interval recognized between adjacent modes decreases to 0.01, which as far as we know is the first time this superhigh resolution has been realized. To exhibit its efficiency in the optical communication process, we transfer an Einstein portrait by a superhigh-resolution OAM multiplexing system. As the convolutional neuron networks can be trained by data up to an infinitely large volume in theory, this work exhibits a huge potential of generalized suitability for next generation DL based ultrafine OAM optical communication, which might even be applied to microwave, millimeter wave, and terahertz OAM communication systems.

Wavelength and polarization multiplexed optical vortex demultiplexer

Optical vortex beams with orbital angular momentum (OAM) have recently received considerable attention in high-speed optical communication due to their spatial orthogonality. For realizing undamaged demultiplexing, an all-dielectric multiplexed metasurface for demultiplexing wavelength-division multiplexing and polarization-division multiplexing OAM light was proposed. This OAM demultiplexer works in the near-infrared spectral region and the transmitted vortex beams carry original topological charges and phase distribution. It is worth mentioning that this demultiplexer is vastly different from previous demultiplexers, in that it not only eliminates the cumbersome process of converting vortex beams into Gaussian beams but also maintains the original property of the transmitted vortex beams. The presented demultiplexer has flexible and compact features, which are suitable for OAM communication systems. Also, the designed wavelength- and polarization-multiplexed metasurface expands the scope of metasurface compactness multifunctional photonics, which has applications in information security and anti-counterfeiting.

Tailoring polarization states of multiple beams that carry different topological charges of orbital angular momentums.

In the past few years, orbital angular momentum (OAM) has aroused great interest in the scientific communities, because it shows great potential for enhancing capacities of radio and optical communication systems. Here, we propose anisotropic metasurfaces to generate multiple OAM vortex beams at microwave frequencies. A phase compensation theory is presented, in order to determine the phase distributions on metasurfaces, This theory enables independent control of beam numbers, polarizations, orientations, and topological charges of OAM vortex beams, respectively. The metasurface is composed of anisotropic elements, whose reflection phases can be engineered separately in different polarization directions. The scheme is validated by both simulation and experimental results and shows great potential for the polarization division multiplexing in OAM communication systems.

Adaptive turbulence compensation with a hybrid input-output algorithm in orbital angular momentum-based free-space optical communication.

Atmospheric turbulence is a major challenge for practical orbital angular momentum (OAM)-based free-space optical (FSO) communication systems that causes intermodal crosstalk and degrades the performance of the system. Herein, we propose a hybrid input-output algorithm (HIOA)-based adaptive optics (AO) system to compensate for distorted OAM beams. The principle and parameters of the HIOA-based AO system in an OAM-based FSO system are analyzed, and the performance is discussed. The simulation results indicate that the HIOA-based AO system can effectively correct distorted OAM beams and that the HIOA improves the compensation performance and convergence speed compared to the traditional Gerchberg-Saxton algorithm. Moreover, we analyze the compensation performance based on different probe beams. Using an OAM beam with state l=1 as a probe beam can yield better correction effects than a Gaussian beam. This work verifies the feasibility of using an HIOA for adaptive turbulence compensation and provides new insights into OAM communication systems.

Generation of radio vortex beams with designable polarization using anisotropic frequency selective surface

We propose a strategy to convert a linearly polarized wave from a single point source to an orbital angular momentum (OAM) wave by arbitrary polarization via an anisotropic frequency selective surface (FSS) in the microwave frequency. By tailoring the geometries of FSS elements, reflection-phases in x and y polarizations are engineered and encoded independently, which allows us to design the eventual polarization state of the generated OAM vortex beam by elaborately selecting individual coding sequences for each polarization. Two types of FSSs are designed and experimentally characterized to demonstrate the capability of OAM generation with circular and linear polarizations, respectively, showing excellent performance in a wide bandwidth from 14 to 16 GHz. This method provides opportunities for polarization multiplexing in microwave OAM communication systems.