Antenna Theory And Design

Orbital angular momentum (OAM) has aroused a widespread interest in many fields, especially in telecommunications due to its potential for unleashing new capacity in the severely congested spectrum of commercial communication systems. Beams carrying OAM have a helical phase front and a field strength with a singularity along the axial center, which can be used for information transmission, imaging and particle manipulation. The number of orthogonal OAM modes in a single beam is theoretically infinite and each mode is an element of a complete orthogonal basis that can be employed for multiplexing different signals, thus greatly improving the spectrum efficiency. In this paper, we comprehensively summarize and compare the methods for generation and detection of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications and technical challenges of OAM in communications, including free-space optical communications, optical fiber communications, radio communications and acoustic communications. To complete our survey, we also discuss the state of art of particle manipulation and target imaging with OAM beams.

Orbital Angular Momentum Waves: Generation, Detection, and Emerging Applications

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.

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

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

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

Orbital Angular Momentum Waves: Generation, Detection and Emerging Applications

Radio orbital angular momentum (OAM) provides another perspective of spatial multiplexing to improve the spectrum efficiency. However, multipath induces severe intra- and interchannel crosstalk. To solve the problem in a uniform circular array (UCA)-based OAM system, we first incorporate the effect of sign changing of OAM reflection in modeling the multipath OAM channel. Then, we propose a transceiver architecture for broadband OAM orthogonal frequency-division multiplexing (OFDM) wireless communication systems, which uses baseband digital 2-D fast Fourier transform (FFT) rather than existing radio frequency analog phase shifters to generate and receive the OAM-OFDM signal, thus reducing energy consumption and hardware cost. At last, a flexible 2-D FFT algorithm is developed. Analysis and simulation results show that compared with the traditional row–column FFT algorithm, the proposed 2-D FFT algorithm could reduce the multiplication complexity by $\frac{1}{4}MN\log _2N$ , where $N$ and $M$ are the number of UCA antenna elements and the number of subcarriers, respectively.

A 2-D FFT-Based Transceiver Architecture for OAM-OFDM Systems With UCA Antennas

The orbital angular momentum (OAM) is considered as a new degree of freedom (DOF) that could be utilized to enhance the capacity of wireless communication systems. However, it is known that uniform circular array (UCA)-based OAM in line-of-sight channels is a special case of the multiple-input multiple-output (MIMO) system. In this letter, we investigate the performance of UCA-based OAM communications in a “spatial keyhole” channel and reveal that in contrast to traditional MIMO, UCA-based OAM may provide additional DOF. In particular, the phenomenon that diffraction at the “spatial keyhole” does not change the helical phase structure of OAM modes provides a promising way to overcome the keyhole effect for traditional multi-antenna wireless communications.

On the Performance of OAM in Keyhole Channels

Orbital angular momentum (OAM) provides extra degrees of freedom to improve spectrum efficiency. However, the harsh requirement that transmit antenna(s) and receive antenna(s) need aligning throws out a great challenge to its application. To investigate the reason of performance degradation caused by misalignment, we first consider uniform circular array (UCA) based OAM system and set up two free space channel models respectively for two misalignment case, i.e., non- parallel and off-axis. Then, the capacities of single- mode and multi-mode OAM system are analyzed and compared under different oblique angles and axis deviations. Analysis and simulation results show that OAM transmission especially the multi-mode OAM is quite sensitive to the misalignment between transmitter and receiver. At last, we propose a misalignment-robust receiving method through extracting the phases of channel state information (CSI) and implementing joint phase compensation and signal detection. Simulation results show that proposed receiving scheme is more robust to misalignment than conventional receiving scheme.

Misalignment-Robust Receiving Scheme for UCA-Based OAM Communication Systems

MmWave systems with most prior work focused on its narrowband hybrid analog/digital precoding, however, will likely operate on wideband channels with frequency selectivity. Therefore, in this paper we investigate wideband angular beamforming schemes for mmWave massive MIMO-OFDM systems. First, for the RF analog beamforming, the optimal beamforming of an unconstrained antenna array (UAA) is given as the performance benchmark. Then, the optimal angular beamforming (OAB) and a simple dominant angular beamforming (DAB) of a shared antenna array (SAA) are compared in received SNR and implementation cost. Second, for the baseband digital precoding, the space-frequency vector perturbation (SFVP) precoding is proposed to collect both spatial and multi-path diversity. Finally, analytical and simulation results show that: a) DAB is a cost- effective RF beamforming scheme under LOS channel environment; b) the proposed hybrid DAB-SFVP beamforming scheme achieves the array gain equaling the number of transmit antennas $N_t$ and diversity gain equaling the product of the number of RF chains $K$ and the number of temporal resolvable clusters ℒ.

Hui Xu

Papers

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

A 2-D FFT-Based Transceiver Architecture for OAM-OFDM Systems With UCA Antennas

On the Performance of OAM in Keyhole Channels

Misalignment-Robust Receiving Scheme for UCA-Based OAM Communication Systems

Hybrid Beamforming for Broadband Millimeter Wave Massive MIMO Systems