Mode division multiplexing (MDM) using orbital angular momentum (OAM) is a recently developed physical layer transmission technique, which has obtained intensive interest among optics, millimeter-wave, and radio frequency due to its capability to enhance communication capacity while retaining an ultra-low receiver complexity. In this paper, the system model based on OAM-MDM is mathematically analyzed and it is theoretically concluded that such system architecture can bring a vast reduction in receiver complexity without capacity penalty compared with conventional line-of-sight multiple-inmultiple-out systems under the same physical constraint. Furthermore, a 4×4 OAM-MDM communication experiment adopting a pair of easily realized Cassegrain reflector antennas capable of multiplexing/demultiplexing four orthogonal OAM modes of l = -3, -2, +2, and +3 is carried out at a microwave frequency of 10 GHz. The experimental results show high spectral efficiency as well as low receiver complexity.

Based on metallic traveling-wave ring-slot structure, an easily realized four-orbital angular momentum (OAM)-mode antenna capable of generating four coaxially propagating waves with OAM modes of l = -3, -2, +2, and +3, respectively, is experimentally demonstrated at 10 GHz. The feeding network is simply implemented using two 90° hybrid couplers. A ring-focus double-reflector with a favorable focus-diameter-ratio is specially designed to gain a high directivity. The main characteristics of the antenna-such as S-parameters, directivity, and near-field radiation-are presented both in simulation and experiment. Employing a pair of four-OAM-mode antennas, the experiment shows that the near-field OAM-based radio communication channels are naturally isolated.

A generalized signal model for multiple orbital angular momentum (OAM) mode-based radio communication combined with a novel partial arc sampling receiving (PASR) scheme for compactly receiving is proposed and mathematically analyzed. Furthermore, a near-field line-of-sight PASR radio data link using four OAM modes is experimentally demonstrated to quadruple the spectral efficiency without adding signal processing complexity at the receiving end. Experiment results show that all the four OAM channels are recovered with bit-error rates below 3.8 × 10 ^-3 , indicating the robustness of the four-OAM-mode-PASR data link.

The design of a radio communication system employing orbital angular momentum (OAM) mode multiplexing is described. Rotman lenses followed by circular antenna arrays are used to transmit and receive radio frequency beams carrying OAM. In our experiment, two 3 × 3 Rotman lenses are fabricated to feed the three-element circular arrays with certain phase gradients for OAM modes multiplexing and demultiplexing. The OAM-carrying beams of mode number l = +1, 0, and -1 are generated at 9.7 GHz, with each channel carrying an independent data stream of 40 Mbit s ^-1 quadrature phase shift keying signal in a bandwidth of 20 MHz. And all three channels are recovered with bit error rates below the forward error correction limit of 3.8 × 10 ^-3 .

A plane spiral orbital angular momentum (OAM) waves-based 4 × 4 multiple-in-multiple-out (MIMO) communication system for line-of-sight channel is proposed and experimentally demonstrated in comparison with conventional MIMO. The transmitting OAM waves are propagating along the transverse direction; hence there is no singularity or divergence problem. Compared with the conventional MIMO, this system could achieve lower spatial correlations and a higher capacity gain while the OAM antennas as the elements of the transmitting array are sorted in an ordered sequence. The measured bit-error-rate performance clearly demonstrates the feasibility of the proposed system, and the causation of the improved performance is also detailedly discussed. This technology can be applied in near-field communications or short-range MIMO systems to realize higher capacity communications.

Conventional security screening systems cannot operate in real time and often suffer from false alarms due to the profile-only imaging without further analysis on the object material properties. Reflectarrays were recently proposed as devices capable of performing real-time security screening using a single-frequency radar signal. This article presents first a physical optics (PO)-based simulation method to facilitate the design of a multireflectarray system. Then, an object material characterization method using geometrical optics (GO) is derived for such a system. The characterization process not only retrieves the complex relative permittivity of the object but also improves its profile reconstruction in terms of a more accurate thickness. Both simulations and experiments are carried out to verify the effectiveness and efficiency of the proposed methods. Primary results show great potentials for security screening, especially in scenarios where the inspection of human bodies for threat materials, such as narcotics and explosives, is required.