Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

/pdf/optical-vortices-30-years-on-oam-manipulation-from-2yknluuuga.pdf

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Unmanned aerial vehicles (UAVs) have stroke great interested both by the academic community and the industrial community due to their diverse military applications and civilian applications. Furthermore, UAVs are also envisioned to be part of future airspace traffic. The application functions delivery relies on information exchange among UAVs as well as between UAVs and ground stations (GSs), which further closely depends on aeronautical channels. However, there is a paucity of comprehensive surveys on aeronautical channel modeling in line with the specific aeronautical characteristics and scenarios. To fill this gap, this paper focuses on reviewing the air-to-ground (A2G), ground-to-ground (G2G), and air-to-air (A2A) channel measurements and modeling for UAV communications and aeronautical communications under various scenarios. We also provide the design guideline for managing the link budget of UAV communications taking account of link losses and channel fading effects. Moreover, we also analyze the receive/transmit diversity gain and spatial multiplexing gain achieved by multiple-antenna-aided UAV communications. Finally, we discuss the remaining challenge and open issues for the future development of UAV communication channel modeling.

/pdf/a-comprehensive-survey-on-uav-communication-channel-modeling-41snqetr75.pdf

A Comprehensive Survey on UAV Communication Channel Modeling

Time, polarization, and wavelength multiplexing schemes have been used to satisfy the growing need of transmission capacity. Using space as a new dimension for communication systems has been recently suggested as a versatile technique to address future bandwidth issues. We review the potentials of harnessing the space as an additional degree of freedom for communication applications including free space optics, optical fiber installation, underwater wireless optical links, on-chip interconnects, data center indoor connections, radio frequency, and acoustic communications. We focus on the orbital angular momentum (OAM) modes and equally identify the challenges related to each of the applications of spatial modes and the particular OAM modes in communication. We further discuss the perspectives of this emerging technology. Finally, we provide the open research directions and discuss the practical deployment of OAM communication links for different applications.

/pdf/communicating-using-spatial-mode-multiplexing-potentials-3l7esc1h6e.pdf

Communicating Using Spatial Mode Multiplexing: Potentials, Challenges, and Perspectives

The standard development of 5G wireless communication culminated between 2017 and 2019, followed by the worldwide deployment of 5G networks, which is expected to result in very high data rate for enhanced mobile broadband, support ultra-reliable and low-latency services and accommodate massive number of connections. Research attention is shifting to future generation of wireless communications, for instance, beyond 5G or 6G. Unlike previous papers, which discussed the use cases, deployment scenarios, or new network architectures of 6G in depth, this paper focuses on a few potential technologies for 6G wireless communications, all of which represent certain fundamental breakthrough at the physical layer-technical hardcore of any new generation of wireless communications. Some of them, such as holographic radio, terahertz communication, large intelligent surface, and orbital angular momentum, are of revolutionary nature and many related studies are still at their scientific exploration stage. Several technical areas, such as advanced channel coding and modulation, visible light communication, and advanced duplex, while having been studied, may find more opportunities in 6G.

Potential Key Technologies for 6G Mobile Communications

Orbital angular momentum (OAM)-based mode division multiplexing (MDM) is an emerging physical layer solution for short-distance line-of-sight wireless communications by providing high spectral efficiency with a considerably low detection complexity. Similarly, index modulation (IM) schemes have been extensively studied over the past few years due to their attractive features, such as improved error performance, low complexity, and high energy efficiency. In this paper, we propose the scheme of orbital angular momentum with index modulation (OAM-IM) by using the activated OAM modes themselves to carry additional information through the principle of IM. A general guideline is presented for the construction of optimal OAM-IM schemes along with the formulation of a low-complexity maximum likelihood detector. It is shown via computer simulations as well as theoretical bit error probability calculations that the proposed OAM-IM schemes can achieve considerably better error performance than the reference OAM-MDM schemes, while having the same level of detection complexity.

Orbital Angular Momentum With Index Modulation

Orbital angular momentum (OAM) has been widely studied in fibre and short-range communications. The implementation of millimetre waves with OAM is expected to increase the communication capacity. Most experiments demonstrate the distinction of OAM modes by receiving all of the energy in the surface vertical to the radiation axis in space. However, the reception of OAM is difficult in free space due to the non-zero beam angle and divergence of energy. The reception of OAM in the space domain in a manner similar to that in optical fibres (i.e., receiving all of the energy rings vertical to the radiation axis) is impractical, especially for long-distance transmission. Here, we fabricate a prototype of the antenna and demonstrate that rather than in the space domain, the OAM can be well received in the time domain via a single antenna by rotating the OAM wave at the transmitter, i.e., the radio wave with rotational OAM. The phase and frequency measured in the experiment reveal that for different OAM modes, the received signals act as a commonly used orthogonal frequency division multiplexing (OFDM) signal in the time domain. This phase rotation has promising prospects for use in the practical reception of different OAMs of millimetre waves in long-distance transmission.

/pdf/millimetre-wave-with-rotational-orbital-angular-momentum-2yjlcx4gdr.pdf

Millimetre Wave with Rotational Orbital Angular Momentum

The optimal interelement space of the multiple-input–multiple-output (MIMO) antenna array varies with the attitude of the aircraft, as well as the relative position to the ground station (GS), i.e., the relative distance, the azimuth, and the altitude. However, the interelement space is not changeable in case of the traditional antenna layout because the antennas are physically deployed in the fixed positions on-board. In this letter, the interpolated MIMO antenna array is obtained with two orthogonal linear antenna arrays. The resultant interpolated airborne MIMO antenna array can adapt to the attitude and the relative position of the aircraft to GS, which can perfectly enlarge the capacity of the MIMO transmission in aeronautical communications.

Interpolated Airborne MIMO Antenna Array

Union modulation scheme is proposed and analyzed based on trellis-coded modulation, orbital angular momentum (OAM), and quadrature amplitude modulation (QAM). Specifically, source data symbols are mapped to a 3-D constellation diagram consisting of OAM modes and QAM constellation. Moreover, the constellation point is jointly detected by an OAM single-point detector and a pruned Viterbi decoder at the receiver. Compared with standalone modulation schemes, larger Euclidean distance with lower bit error rate (BER) and lower complexity can be obtained. Furthermore, the simulation results show that the single-point detection is more robust than the phase-gradient detection at a large receiving distance.

Trellis-Coded OAM-QAM Union Modulation with Single-Point Receiver

Background Poor sleep quality is related to depression. However, the investigation of the neural basis for poor sleep quality in individuals with major depression (MD) is limited. Methods Resting state functional and structural MRI data were derived from 114 MD individuals and 74 normal controls (NCs). Fractional amplitude of low-frequency fluctuation (fALFF) and gray matter volume (GMV) were used to measure function and structure of the brain. Pittsburgh Sleep Quality Index (PSQI) was performed to evaluate subjective sleep quality. Correlations were carried out to investigate links of PSQI score with brain imaging indices in MD and NCs, separately. We also examined the differences in fALFF and GMV of brain regions related to PSQI score between MD and NCs. Results In contrast to NCs, MD individuals had higher PSQI score. The higher PSQI score was associated with lower fALFF and lower GMV in bilateral precuneus in MD individuals. Moreover, the MD individuals exhibited increased fALFF in bilateral precuneus compared with NCs. However, the correlation between subjective sleep quality and neuroimaging parameters was not significant in NCs. Conclusion The implication of these findings is that the function and structure of precuneus provides a neural basis for subjective poor sleep quality in MD. Understanding this may lead to better intervention of depression and associated sleep complaints.

/pdf/the-function-and-structure-of-precuneus-is-associated-with-1aj2t4h4.pdf

The Function and Structure of Precuneus Is Associated With Subjective Sleep Quality in Major Depression

Due to the high flight speed of the aircraft and long range to access the ground station (GS), the channel state information (CSI) is not accurately obtained by direct feedback from the receiver to the transmitter. This makes it difficult to achieve stable transmission when the feedback codebook selection scheme is directly employed in the multiple-input multiple-output (MIMO) for the air-to-ground (A/G) communications. Therefore, in this paper, a stable transmission scheme for the A/G MIMO transmission is proposed to handle these issues. Two contributions are proposed: 1) an open-loop calibrated channel estimation scheme based on navigation filtering to offer a more accurate CSI estimation; and 2) an adaptive precoding scheme based on the correlation coefficient for stable transmission. The simulations confirm the validity and efficiency of the proposed schemes. For the 4 $\times$ 4 MIMO transmission, compared with the traditional scheme of a single antenna, the spectrum efficiency can be improved by 2 $\sim$ 4 times with a stable transmission.

Stable A/G MIMO Transmission Aided by Open-Loop Calibrated Channel Estimation and Adaptive Precoding

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

Lu Ma

Author Tools

Chat about Author