Orbital angular momentum (OAM) in electromagnetic (EM) waves can significantly enhance spectrum efficiency in wireless communications without requiring additional power, time, or frequency resources. Different OAM modes in EM waves create orthogonal channels, thereby improving spectrum efficiency. Additionally, OAM waves can more easily maintain orthogonality in line-of-sight (LOS) transmissions, offering an advantage over multiple-input and multiple-output (MIMO) technology in LOS scenarios. However, challenges such as divergence and crosstalk hinder OAM’s efficiency. Additionally, channel modeling for OAM transmissions is still limited. A reliable channel model with balanced accuracy and complexity is essential for further system analysis. In this paper, we present a quasi-deterministic channel model for OAM channels in the 5.8 GHz and 28 GHz bands based on measurement data. Accurate measurement, especially at high frequencies like millimeter bands, requires synchronized RF channels to maintain phase coherence and purity, which is a major challenge for OAM channel measurement. To address this, we developed an 8-channel OAM generation device at 28 GHz to ensure beam integrity. By measuring and modeling OAM channels at 5.8 GHz and 28 GHz with a modified 3D geometric-based stochastic model (GBSM), this study provides insights into OAM channel characteristics, aiding simulation-based analysis and system optimization.

Orbital angular momentum (OAM) carried electromagnetic waves have the potential to improve spectrum efficiency in optical and radio-frequency communications due to the orthogonal wavefronts of different OAM modes. However, OAM beams are vortically hollow and divergent, which significantly decreases the capacity of OAM transmissions. In addition, unaligned transceivers in OAM transmissions can result in a high bit error rate (BER). The Talbot effect is a self-imaging phenomenon that can be used to generate optical or radio-frequency OAM beams with periodic repeating structures at multiples of a certain distance along the propagation direction. These periodic structures make it unnecessary for the transceiver antennas to be perfectly aligned and can also alleviate the hollow divergence of OAM beams. In this paper, we propose Talbot-effect-based fractal OAM generation and detection schemes using a uniform circular array (UCA) to significantly improve capacity and BER performance in unaligned OAM transmissions. We first provide a brief overview of fractal OAM. Then, we propose the fractal OAM beam generation and detection schemes. Numerical analysis and simulations verify the effectiveness of our proposed fractal OAM generation scheme and also demonstrate improved capacity and BER performance compared to normal OAM transmissions. We also analyze how the receive UCA radius and the distance between the UCAs impact the capacity and BER performances.

Orbital angular momentum (OAM) carried electro-magnetic waves can be used for optical and radio-frequency communications to improve spectrum efficiency thanks to the orthogonal wavefronts of different OAM modes. However, OAM beams are vortically hollow and divergent, which seriously decreases the capacity for OAM transmissions. Moreover, unaligned-transceiver-based OAM transmissions can lead to a high bit error rate. In this paper, we propose the fractal OAM generation and detection schemes, which can alleviate the hollow divergence of OAM beams and greatly improve the capacity performance for unaligned OAM transmissions. We first briefly introduce the fractal OAM phenomenon. Then, we propose the fractal OAM beam generation and detection schemes. Simulations verify our proposed fractal OAM generation scheme. The improved capacity performance of our proposed fractal OAM compared with normal OAM transmissions are also validated via simulations.

Simultaneous wireless information and power transfer (SWIPT) can enhance the spectrum and power efficiencies of wireless communications networks. Line-of-sight (LOS) transmission is a typical SWIPT scenario. However, the strong channel correlation limits the spectrum and energy efficiencies of SWIPT in the LOS channel. Due to the orthogonal wavefronts, orbital angular momentum (OAM) waves can facilitate the SWIPT in LOS channels. With the assistance of the reconfigurable intelligent surface (RIS), both the energy efficiency and capacity can be further improved for the OAM-SWIPT systems. In this paper, we model the RIS-assisted OAM-SWIPT transmission and derive the optimal reflection coefficients and power splitting ratio for it. We first give the system and channel models. Then, we propose the transmission scheme. Based on the transmission scheme, we formulate the capacity and energy harvesting (EH) trade-off problem. We solve the problem by developing an alternating optimization algorithm. Simulations validate the capacity and EH enhancements brought by the RIS for OAM-SWIPT.

Since the relatively low frequency band has been extensively used, the design paradigm is shifted to the high frequency band such as the promising millimeter wave (mmWave) band in the fifth generation (5G) beyond and the expected terahertz in the sixth generation (6G). Due to the severe electromagnetic degradation for high frequency band, line-of-sight (LOS) transmission for short-range high-speed scenarios has been receiving much attention. In this paper, we introduce the nonplanar electromagnetic waves (NEWs) based wireless communications, which can provide a considerable enhancement for channel capacity. After giving a brief definition of the NEWs, a specific case of the NEWs which is based on the Zadoff-Chu (ZC) phase shift matrix is proposed and analyzed. This kind of NEWs is non-hollow with its orthogonal beams sharing the same radiation pattern except with different rotations along the azimuth. Simulation results are given to validate the capacity enhancement and analyze the robustness performance of NEWs based wireless communications.

Due to its low energy consumption and simplicity, near field communication (NFC) has been extensively used in various short-range transmission scenarios, for example, proximity payment and NFC entrance guard. However, the low data rate of NFC limits its application in high rate demanded scenarios, such as high-resolution fingerprint identification and streaming media transmission as well as the future promising high rate indoor communications among pads, phones, and laptops. In this paper, we model and analyze the performance of the orbital angular momentum based NFC (OAM-NFC) system, which can significantly increase the capacity of NFC. We first give the OAM system model. With coils circularly equipped at the transmitter and receiver, OAM-NFC signals can be transmitted, received, and detected. Then, we develop the OAM-NFC generation and detection schemes for NFC multiplexing transmission. We also analyze the OAM-NFC channel capacity and compare it with those of single-input-single-output (SISO) as well as multi-input-multi-output (MIMO) NFC. Simulation results validate the feasibility and capacity enhancement of our proposed OAM-NFC system. How different variables, such as the transceiver misalignment, the numbers of transceiver coils, and transceiver distance, impact the OAM-NFC capacity are also analyzed.

Due to its low energy consumption and high security, near field communication (NFC) has been extensively used in various short-range non-contact transmission scenarios such as the proximity payment and NFC entrance guard. However, the low data rate of NFC limits its application in high rate demanded scenarios, such as high-resolution fingerprint identification and streaming media transmission. In this paper, we propose the orbital angular momentum (OAM) based NFC system to significantly increase the capacity for NFC systems. With coils circularly equipped at the transmitter and receiver, OAM signals can be transmitted, received, and detected. Then, we analyze the mutual inductances between the transmit and receive coils to derive the OAM-NFC magneto-inductive channel matrix. Based on the channel matrix, we develop the OAM-NFC transmission and detection schemes for NFC multiplexing transmission. We also compare the capacity of our proposed OAM-NFC system with those of SISO NFC system and MIMO NFC system. Simulation results validate the feasibility and capacity enhancement of our proposed OAM-NFC system. How the number of the transceiver coils impacts the capacity of OAM-NFC system is also evaluated.

Orbital angular momentum (OAM), which is inherently possessed by the electromagnetic (EM) beams, bridges a new way for multiple access using multiple orthogonal OAM-modes in wireless communications. Uniform circular array (UCA), as a convenient antenna structure for supporting the transmission and reception of OAM based signals, has been paid much attention in recent years. However, the OAM beams generated by the UCAs are centrally hollow and divergent, limiting the efficient reception as well as the long-distance transmission for OAM based signals. To solve this problem, in this paper we propose the Zadoff-Chu (ZC) sequence based scrambling scheme to generate the non-hollow OAM beams while maintaining the orthogonality among different OAM-modes. Then, based on the properties of applying discrete Fourier transform (DFT) for circulant matrices and zero-free sequences, we develop the scrambling-tolerant detection scheme to efficiently obtain the transmit symbols. Performance evaluations show the non-hollow irregular OAM beams and validate that our developed ZC sequence based scrambling scheme works well for long-distance transmission in OAM based wireless communications.

In the past decade, more and more researchers have concentrated on orbital-angular-momentum (OAM) based radio vortex wireless communications, which is expected to provide orthogonality among different OAM-modes. The uniform circular array (UCA) is considered as one promising antenna structure for OAM based radio vortex wireless communications. However, most studies regarding UCA focus on the scenario where the transmit and receive UCAs are aligned with each other. In this paper, we investigate the OAM based radio vortex wireless communications with non-coaxial UCA, i.e., the UCA transceivers are parallel but non-coaxial. We study the channel model and develop the mode-decomposition scheme to decompose the OAM-modes. Then, we discuss the impact of included angles on the channel model under non-coaxial scenario. Numerical results are presented to evaluate our developed scheme and show that the spectrum efficiency of the non-coaxial UCA transceiver in some cases is larger than that of the aligned UCA transceiver based radio vortex wireless communications.