High-capacity millimetre-wave communications with orbital angular momentum multiplexing

Orbital angular momentum (OAM) multiplexing has recently been proposed as a solution to the ultimate goal of increasing the channel capacity of wireless communication links because of the existence of infinite orthogonal modes. It can be combined with the existing conventional multiplexing techniques to boost up the data rate multiple times for future wireless communication systems. We analyze the fundamental behavior of the channel capacity of OAM channels based on two concepts for the most common Laguerre-Gaussian OAM beams: paraxiality and orthogonality. We also confirm our general theory by applying the proposed method to the evanescing OAM beams, circular transverse electric beams. Our study is based on calculating the paraxial estimator of these OAM beams and relating it as a parameter that limits the degree of freedom in OAM channels. In particular, we investigate the dependency on physical parameters, such as transmitter and receiver sizes, in our calculation to define the capacity of OAM channels.

Orbital angular momentum (OAM) multiplexing technique has recently emerged and generated widespread interests since the OAM was discovered as a property of electromagnetic wave and acoustic wave. It was widely acknowledged that OAM multiplexing can achieve very high effective degrees of freedom (EDOF) and improve the spectral efficiency in optical, radio and acoustic communications. However, in the field of free-space optical (FSO) communications, it was demonstrated that OAM multiplexing is not the optimal multiplexing technique and the spatial bandwidth product (SBP) limits the EDOF. Is there any EDOF limits of OAM multiplexing in radio and acoustic communications? Could OAM multiplexing be safely scaled to far field? Here, we discover that the azimuthal resolution of OAM mode generator in OAM multiplexing limits its EDOF. Furthermore, we also verify that the OAM multiplexing in radio and acoustic communication fails to enable a long distance transmission and high EDOF simultaneously incurred by the inherently imperfect OAM mode generator.

Orbital Angular Momentum (OAM) multiplexing is a technology of communication systems that enables high-capacity optical communication networks. One of the most important determinants of this technology is the channel capacity, loss of power, and Bit Error Rate (BER) accompanying the transmission. This article proposed an Orbital Angular Momentum (OAM)/Spatial Domain Multiplexing (SDM) Gigabit-capable Passive Optical Network (G-PON) architecture for a Multiple-Input Multiple-Output (MIMO) communication system that supports (OAM/SDM G-PON) technology. The proposed architecture is used to multiplex the downstream OAM channels and the upstream SDM channels, and an OAM multiplexer/demultiplexer (OAM-MUX/DEMUX) is used to multiplex and demultiplex the OAM channels. In the OAM/SDM G-PON system, the signal will propagate through three different mediums, each having its own nature in influencing the power of the signal that passes through that medium. The experiment involves bidirectional transmissions with a DS/US data rate of 2.4 Gbps and Binary Phase Shift Keying (BPSK) downstream and 1.2 Gbps upstream. The observed results showed that the bit-error rate (BER) is a function of coupling angles and increases with the increase in the OAM ring size.

As a pathbreaking approach to generate orbital angular momentum (OAM) beam, uniform circular array (UCA) has been extensively researched and applied. However, the impact of the imperfect UCA (IUCA) converted from a UCA with the position of antenna elements on the purity of the generated OAM beam is rarely considered. In order to verify the effect of position deviation on the OAM beam, this letter deduces the analysis model of the IUCA and the OAM mode purity model. The OAM mode purity refers to the weight of the intensity of the "expected OAM mode." Numerical simulation results show that the OAM beam generated by IUCA is a superposition of OAM beams with a series of different modes, and this results in a certain OAM beam received being contaminated. Compared with the radius deviation, the azimuth deviation of antenna elements has more obvious influence on the OAM mode purity. Moreover, low-mode OAM beams are more suitable for long-distance transmission. Properly increasing the antenna array radius can improve the purity of high-order OAM modes.

In the realm of free-space optical (FSO) communication systems, the utilization of orbital angular momentum (OAM) multiplexing has garnered significant attention. However, OAM beams are subject to atmospheric turbulence (AT) during transmission in free space, resulting in distortions of the phase front, intermodal crosstalk, and mode-dependent loss (MDL). While techniques such as adaptive optics and digital signal processing can mitigate these effects, inconsistencies in OAM mode performance persist due to MDL. Some studies have evaluated an MDL-impaired mode division multiplexing system for non-unitary multiple input multiple output (MIMO) channels based on maximum likelihood detection. Nevertheless, the computational complexity remains considerably higher compared to simpler yet suboptimal advanced MIMO decoders, such as successive interference cancellation (SIC) with zero-forcing/minimum mean square error (ZF/MMSE). On the other hand, an OAM mode and radial index are two distinct indices of Laguerre-Gaussian (LG) modes, with the potential to carry a large amount of information. To the best of our knowledge, no prior studies have explored the impact and mitigation strategies for MDL induced by atmospheric turbulence in an FSO system utilizing both OAM mode and radial index multiplexing with MMSE-SIC receiver. This paper introduces OAM multiplexing holography to generate OAM modes. We perform both analytical and numerical analyses of the orbital angular momentum spectra associated with distorted single-mode and multiplexed LG beams. This is achieved by generating holograms, incorporating random phase screens along the propagation path, and varying the refractive index structure constants. We mathematically develop and simulate an MMSE-SIC receiver for OAM multiplexed FSO for different radial values for the first time and compared its BER to Zero Forcing-SIC and K-best detectors. Additionally, orthogonal space-time block coding is employed at the transmitter to enhance the overall performance of the system.

With the development of various emerging technologies, the capacity of current wireless communication system is gradually saturated, and terahertz (THz) and orbital angular momentum (OAM) have hereby attracted much attention as two key technologies to increase the communication capacity. In this article, a THz-OAM wireless communication system operating at 100 GHz based on a transmissive metasurface is proposed for the first time to the best of our knowledge. The operation principle of multiplexing metasurface, which allows simultaneous generation of two coaxial OAM beams at different modes of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {l}$ </tex-math></inline-formula> = 1 and −1, is studied. To meet the requirements of wireless communication and OAM multiplexing, a C-shaped transmissive unit is designed with characteristics of broad bandwidth, low transmission loss, and insensitive to the change of incident angle. Experimental measurements show that the generated two OAM beams feature high mode purity, providing the excellent orthogonality of modes. The generated two OAM modes are also employed as two OAM channels to transmit two independent communication signals with ON–OFF keying (OOK) modulation, and a data rate of 10 Gbit/s is achieved at a transmission distance of 300 mm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100\boldsymbol {\lambda }$ </tex-math></inline-formula> of 100 GHz) with bit error rates (BERs) below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.8\,\,\boldsymbol {\times }\,\,\boldsymbol {10^{-3}}$ </tex-math></inline-formula> . This work verifies the feasibility of the multiplexing metasurface supporting wireless communication and makes THz-OAM wireless communication systems potential for future large capacity wireless services.

Traditional wireless links employing OAM (orbital angular momentum) beams make use of their spatial orthogonality in order to increase their spectral efficiency, by carrying a separate data stream on each beam. This requires that the OAM antenna axes are aligned with the line of sight of the link, and are capable of radiating and receiving high-purity orthogonal OAM beams. Due to the deep spatial nulls of these beams (except the zero-order OAM beam) on their axes, the received signal strengths are much lower than from conventional beams of comparable-sized antennas, limiting the signal-to-noise ratio (SNR) and the reach of such links, or requiring very large receive antennas in terms of wavelengths.Here we describe an alternative technique for recovering received OAM beams based on pseudo-Doppler techniques [1]-[3] which avoid the above limitations of traditional OAM links. In this work, the receiving antennas are not aligned with the OAM beam axes, so the received signals are stronger, the receivers and their antennas are fewer and simpler, and the reach is longer than in traditional wireless OAM links. We demonstrate for the first time, by simulations and experiments performed on an outdoor 100m link at 28 GHz, that it is possible to recover 4 independent data streams carried in a common 20MHz channel on 4 co-aperture OAM beams using the technique developed in [1], with only 2 receiver chains. The transmitting OAM antenna radiates all OAM beams with the same cone angles and the 2 receiving antennas are set on their peaks off-axis in the far field, 20cm apart and tangent to the OAM cones, unlike in traditional OAM wireless links [4]. Salient aspects of the pseudo-Doppler OAM recovery algorithm, its simulation in an end-to-end link, and key design features of the demonstration hardware and signal-processing are presented.

Orbital angular momentum (OAM) beams, have attracted great attention in recent years. An OAM beam with a phase singularity is characterized by a helical phase front which provides an additional degree of freedom for wide amount of classical and quantum optical applications. However, despite many attempts to generate and manipulate OAM beams, a robust, reliable and scalable technique to directly address generation, multiplexing and low-loss transmission of the distinct OAM beams is still in great demand. Here, we review the development of all-fiber, ring core photonics lantern mode multiplexer to generate high quality OAM beams up to the second order within a broad spectral range of >550 nm. Our device is a 5-mode mode selective photonic lantern (MSPL) with an annular refractive index profile which is fully compatible with well-established ring core and vortex transmission fibers. Through the excitation of pairs of degenerate linearly polarized (LP) modes of the MSPL, we demonstrate the generation of high quality OAM beams up to the second order. In addition, we demonstrate multiplexing of two OAM modes (OAM+1+ OAM-2) to verify complex beam pattern generation of our all fiber devices. Furthermore, by splicing the end-facet of the photonic lantern to a ring core fiber, we achieve low-loss coupling of OAM modes while maintaining high contrast spiral phase patterns. These results demonstrate the potential of photonic lanterns for generating complex optical beams.

Atmospheric turbulence mitigation using spatial mode multiplexing and modified pulse position modulation in hybrid RF/FSO orbital-angular-momentum multiplexed based on MIMO wireless communications system

Traditional multi-mode orbital angular momentum (OAM) beams suffer from low energy efficiency in detection since the divergence angle tends to increase with increasing order of the OAM mode. Thus, OAM beams with equal divergence angle are highly desirable. In this paper, a frequency-diverse multi-mode (FDMM) OAM modulator with approximately equal divergence angle is proposed. The modulator is composed of an OAM modulation metasurface and a spatial filter (SF) with narrow passband. Firstly, the OAM modulation metasurface is designed based on multiple ring-like regions with different radii in the metasurface aperture. Each mode of OAM beam is generated at a corresponding frequency point in one region. Thus, the metasurface aperture can generate different modes of OAM beams at different frequencies (i.e., FDMM OAM beams) simultaneously. Then, the SF with narrow passband is proposed through synthesizing multi-layer frequency selective metasurfaces, aiming to reduce interferences between different modes of OAM beams. Finally, the FDMM OAM modulator is realized by superposing the SF on the OAM modulation metasurface. The FDMM OAM modulator can generate six modes of OAM beams (i.e., ±3 , ±2 and ±1 modes) at 12.5 GHz, 13.0 GHz and 13.6 GHz, respectively. A multi-mode OAM coincidence imaging model based on the FDMM OAM modulator is established. The simulation and experiment results show that the proposed FDMM OAM modulator can be used for target reconstruction and the coincidence imaging performance can be improved assisted by an optimization algorithm.

This contribution presents the design of a 60-GHz planar antenna system for radiative near-field region line-of-sight (LoS) link, based on an orbital angular momentum (OAM) multiplexing. The antenna system is composed of a radial line slot array (RLSA) fed by a Butler network, made in post-wall waveguide technology, and located underneath the array. The network can launch different progressive phase states in the RLSA parallel plate waveguide (PPW) to radiate three different OAM beams. The design concept has been verified by prototyping and testing a 20 free-space wavelengths radius antenna. The measurements show that the proposed antenna can generate simultaneously the three OAM-mode states n = 0, −1, and +1.

There is a continuing growth in the demand for data bandwidth, and the multiplexing of multiple independent data streams has the potential to provide the needed data capacity. One technique uses the spatial domain of an electromagnetic (EM) wave, and space division multiplexing (SDM) has become increasingly important for increased transmission capacity and spectral efficiency of a communication system. A subset of SDM is mode division multiplexing (MDM), in which multiple orthogonal beams each on a different mode can be multiplexed. A potential modal basis set to achieve MDM is to use orbital angular momentum (OAM) of EM waves. In such a system, multiple OAM beams each carrying an independent data stream are multiplexed at the transmitter, propagate through a common medium and are demultiplexed at the receiver. As a result, the total capacity and spectral efficiency of the communication system can be multiplied by a factor equal to the number of transmitted OAM modes. Over the past few years, progress has been made in understanding the advantages and limitations of using multiplexed OAM beams for communication systems. In this review paper, we highlight recent advances in the use of OAM multiplexing for high-capacity free-space optical and millimetre-wave communications. We discuss different technical challenges (e.g. atmospheric turbulence and crosstalk) as well as potential techniques to mitigate such degrading effects.This article is part of the themed issue 'Optical orbital angular momentum'.

Orbital angular momentum (OAM) multiplexing of electromagnetic (EM) waves is of great significance for high-speed wireless communication and remote sensing. To achieve high-efficiency OAM multiplexing for multi-channel incident EM waves, this paper presents a novel angle-dispersive meta-atom structure, which can introduce the required anti-symmetric phase dispersion as well as high transmission efficiency for OAM multiplexing. These meta-atoms are then arranged delicately to form an angle-dispersive metasurface working at the X band, which enables three-channel OAM multiplexing by converting highly directional transverse-magnetic (TM) waves incident from 0 and ±45° to coaxial OAM beams with l = 0 and ±2 modes, respectively. The simulation and experimental results reveal that the proposed metasurface can convert a higher proportion of energy to the required OAM modes compared to the conventional OAM multiplexing metasurfaces, which can significantly improve the coaxial transmission efficiency of multi-channel OAM multiplexing.

Orbital angular momentum (OAM) multiplexing is a recently considered solution for enhancing wireless and free-space optical communications channel capacity, whether implemented separately or in combination with existing multiplexing techniques. The theoretically infinite number of paraxially propagating and mutually orthogonal OAM modes is expected to increase the channel capacity. However, the orthogonality for different OAM modes has been shown to decrease for far link range distances, and the paraxiality of the OAM beams is not very good for small radiating sources. Based on the current knowledge, OAM beams are most likely to be used for short-range communications. Many models of the electromagnetic (EM) fields carrying the OAM neglect the fact that the OAM beam sources could be electrically large or introduce other approximations that are appropriate for far-field analysis only. An in-depth analysis of the short-range properties of OAM EM fields is still lacking. To address this problem, we propose the use of the infinitesimal (Hertz) dipole method customized for the analysis of the OAM EM fields. This technique can model the positioning and basic radiation properties of separate antennas or antenna sub-arrays that are the building blocks of OAM arrays exactly and efficiently. Similar modeling can represent the OAM sources for free-space optical communications. We focus here on the uniform circular antenna arrays and provide an in-depth analysis of what can and cannot be expected, in the best case, in their utilization. We assume low losses, which is a common assumption for many methods, except for computationally much more demanding full-wave simulations. The obtained results indicate the need to simultaneously optimize the transmission of all planned OAM modes and allow estimates of the link distances that could provide adequate OAM wave reception in various cases.

Terahertz (THz) beams with abundant spectrum resources and low beam divergence can be combined with orbital angular momentum (OAM) beams characterized by orthogonal propagation to multifold the capacity of wireless communication systems. However, in order to support the explosive growth of massive communication data traffic, THz OAM must be further enhanced with other multiplexing technologies. Therefore, it is imperative to develop new multifunctional devices. In this article, we propose a novel broadband dual-polarized reflective metasurface, which can generate two coaxially propagating OAM beams with distinct modes for each polarization. Because of the geometric symmetry, the metasurface shows the same electromagnetic response with respect to both <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${x}$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${y}$ </tex-math></inline-formula> -polarized waves, thus possessing the dual-polarized property. Simulated and measured results verify that the metasurface can realize the incorporation of OAM multiplexing and polarization multiplexing in a broad frequency range of 90–110 GHz. The crosstalks from other OAM modes and polarizations are lower than −16 dB and −21 dB, respectively. Based on the metasurface, a THz wireless communication system was demonstrated by multiplexing four independent OAM beams with each channel carrying a 6-Gbaud quaternary phase shift keying (QPSK) signal, so that an aggregate 48 Gbit/s data transmission with bit error rate (BER) below 3.8e-3 was achieved. This work verifies the feasibility of metasurface for multidimensional multiplexing in the THz region, meaning the metasurface may serve as a promising candidate for ultrahigh speed THz communication systems.