The vortex electromagnetic wave carrying multiple orthogonal orbital angular momentum (OAM) modes in the same frequency band can be applied to the field of wireless communications, which greatly increases the spectrum efficiency. The uniform circular array (UCA) is widely used to generate and receive vortex electromagnetic waves with multiple OAM-modes. However, the maximum number of orthogonal OAM-modes based on UCA is usually limited to the number of array-elements of the UCA antenna, leaving how to utilize more OAM-modes to achieve higher channel capacity with a fixed number of array-elements as an intriguing question. In this paper, we propose anN-dimensional quasi-fractal UCA (ND QF-UCA) antenna structure in different fractal geometry layouts to break through the limits of array-elements number on OAM-modes number. We develop theN-dimensional OAM modulation (NOM) and demodulation (NOD) schemes for OAM multiplexing transmission with the OAM-modes number exceeding the array-elements number, which is beyond the traditional concept of multiple antenna based wireless communications. Then, we investigate different dimensional multiplexing transmission schemes based on the corresponding QF-UCA antenna structure with various array-element layouts and evaluate the optimal layout type and dimension to obtain the highest channel capacity with a fixed number of array-elements. Simulation results show that our proposed schemes can obtain a higher spectrum efficiency, surpassing those of alternative array-element layouts of QF-UCA and the traditional multiple antenna systems.

Transmission at millimeter-wave (mmWave) frequencies is promising in the sixth generation (6G) and beyond systems due to abundant spectrum resources. Reconfigurable intelligent surface (RIS) assisted mmWave communications has been envisioned as an effective technique to effectively compensate for propagation loss and increase the capacity of line-of-sight (LOS) multiple-input-multiple-output (MIMO) wireless communication. To decrease the complexity of transceivers, the uniform circular array (UCA) is an effective antenna structure for RIS-assisted mmWave MIMO wireless communications when the transmit and receive UCAs are aligned. However, the complexity increases when the transceivers are not perfectly aligned and it is difficult to ensure the strict alignment in practice. To achieve low-complexity transceivers and increase the spectrum efficiency for RIS-assisted mmWave MIMO wireless communications, in this paper we investigate the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications. The channel model is derived for the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications. Then, a beamforming scheme at the transmit UCA, a phase compensation scheme at the spherical RIS, and a predetection scheme at the receive UCA, which can decrease the complexity of transceivers, are developed in the misaligned scenario. Our proposed fast symbol-wise maximum likelihood (ML) detection scheme can recover the signals with low complexity. Furthermore, we propose an algorithm to achieve the optimal spherical RIS design for increasing the spectrum efficiency of the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications. Simulation results are presented to illustrate the theory for the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications.

Reconfigurable intelligent surface (RIS) has attracted much attention to assist millimeter-wave (mmWave) multiple-input-multiple-output (MIMO) wireless communications over line-of-sight (LOS) channels for performance improvement due to low hardware cost and low power consumption. The uniform circular array (UCA) is an effective antenna structure with low-complexity transceivers for RIS-assisted mmWave MIMO wireless communications where the transceivers are aligned. However, it is difficult to achieve strict alignment between the transceivers in practice. To address the challenges, in this paper we investigate the UCA based sphere RIS-assisted mmWave MIMO wireless communications with misaligned transceivers and sphere RIS. The channel model is investigated for the UCA based sphere RIS-assisted mmWave MIMO wireless communications. Furthermore, a precoding scheme at the transmitter, a phase compensation scheme at the sphere RIS, and a predecoding scheme at the receiver, which can convert the channel matrix into an equivalent circulant matrix, are developed. Then, our proposed fast symbol-wise maximum likelihood (ML) detection scheme can recover the signals with low complexity. Simulation results are presented to illustrate the theory for the UCA based sphere RIS-assisted mmWave MIMO wireless communications.

The development of orbital angular momentum (OAM)-based radio vortex transmission presents a promising opportunity for increasing the capacity of wireless communication in correlated channels due to its inherent orthogonality among different OAM modes. One of the most popular schemes for high-efficient OAM transmission is the digital baseband associated with uniform circular array (UCA) based transceiver. However, the periodicity of complex-exponential feed makes the maximum number of orthogonal signals carried by multiple OAM modes generally restricted to the array-element number of UCA antenna, which poses an open question of how to employ more OAM modes given a fixed number of array elements. Furthermore, signals modulated with high-order OAM modes are difficult to be captured by the receiver due to their serious divergence as propagating in free space, thus severely limiting the capacity of radio vortex communications. To overcome the above challenges, in this paper based on the partly element-overlapped fractal geometry layout and effectively using low-order OAM modes, we propose the quasi-fractal UCA (QF-UCA) antenna based OAM multiplexing transmission. We perform the two-dimension OAM modulation (TOM) and demodulation (TOD) schemes with the orthogonal OAM mode number exceeding the array-element number, which is beyond the traditional concept of multiple antennas based wireless communications. Simulation results show that our proposed scheme can achieve more number of orthogonal multiplexing streams than the maximum number of orthogonal multiplexing corresponding to traditional multiple antenna systems.

The vortex electromagnetic wave carried by multiple orthogonal orbital angular momentum (OAM) modes in the same frequency band can be applied to the field of wireless communications, which greatly increases the spectrum efficiency. The uniform circular array (UCA) structure is widely used to generate or receive vortex electromagnetic wave with multiple OAM-modes. However, the maximum number of orthogonal OAM-modes based UCA is usually limited to the number of array-elements of the UCA antenna, leaving how to utilize more OAM-modes given a fixed number of array-elements as an intriguing question. In this paper, we propose a quasi-fractal UCA (QF-UCA) antenna structure in different fractal geometry layouts to break through the limits of array-element number on OAM-modes number. We develop the two-dimensional OAM modulation (TOM) and demodulation (TOD) schemes for OAM multiplexing transmission with the OAM-modes number exceeding the array-element number, which is beyond the traditional concept of multiple antennas based wireless communications. Then, we study four different types of array-element layouts for QF-UCA and evaluate the optimal layout to obtain the maximum number of OAM-modes with fixed number of array-elements. Simulation results show that our proposed scheme can obtain the maximum number of orthogonal streams, which is larger than those corresponding to other array-element layouts of QF -UCA and the traditional multiple antenna systems.

Due to high bandwidth and small antenna size, millimeter-wave (mmWave) integrated line-of-sight (LOS) multiple-input-multiple-output (MIMO) systems have attracted much attention. Reconfigurable intelligent surfaces (RISs), which have the potential to change the characteristics of incident electromagnetic waves with low power cost, can improve the performance for the MIMO mmWave wireless communications. Uniform circular array (UCA) is an effective antenna structure with low complexity transceiver. In this paper, UCA based RIS-assisted MIMO mmWave wireless communications with transmit UCA, the RIS UCAs, and receive UCA are investigated. Since the rotation angles between the transceiver make the channel matrix noncirculant, an algorithm is developed to derive the ranges of the rotation angles based on an acceptable error and reduce the impact of rotation angles on channel matrix. Then, we propose a low-complexity precoding scheme at the transmitter, phase designs at the RIS UCAs, and a phase compensation scheme at the receiver, which can convert the channel matrix into an equivalent circulant channel matrix with a small error. Then, a fast symbol-wise maximum likelihood (ML) detection scheme is proposed to recover the signals with low computational complexity. Simulation results are presented to illustrate the theory.

Orbital angular momentum (OAM), which is considered as a novel way to achieve high capacity, has been attracted much attention recently. OAM signals emitted by uniform circular array (UCA) are widely treated as going through the Bessel-form channels. However, the channel gains corresponding to the Bessel-form channels are with low signal-to-noise-ratio (SNR) on OAM-modes and it is difficult to achieve high capacity using all OAM modes. To achieve maximum capacity offered by OAM multiplexing for wireless backhaul communications, in this paper we propose the optimal UCA design, which selects the optimal OAM-modes and radius of receive UCA. We formulate the capacity maximization problem and divide it into two subproblems for obtaining the corresponding optimal UCA design for OAM multiplexing based wireless backhaul communications. In particular, the optimal radius of the receive UCA is firstly derived. Then, we propose a mode selection scheme to choose appropriate OAM-modes for data transmission to maximize the capacity. Extensive simulations obtained validate that the capacity of OAM multiplexing can be significantly increased with our developed scheme.

Orbital angular momentum (OAM) has attracted much attention for radio vortex wireless communications due to the orthogonality among different OAM-modes. To maintain the orthogonality among different OAM modes at the receiver, the strict alignment between transmit and receive antennas is highly demanded. However, it is not practical to guarantee the transceiver alignment in wireless communications. The phase turbulence, resulting from the misaligned transceivers, leads to serious intermode interference among different OAM modes and therefore fail for signals detection of multiple OAM modes at the receiver. To achieve practical OAM based wireless communications, in this paper we investigate the radio vortex wireless communications with misaligned transmit and receive antennas. We propose a joint Beamforming and Pre-detection (BePre) scheme, which uses two unitary matrices to convert the channel matrix into the equivalent circulant matrix for keeping the orthogonality among OAM-modes at the receiver. Then, the OAM signals can be detected with the mode-decomposition scheme at the misaligned receiver. Extensive simulations obtained validate and evaluate that our developed joint BePre scheme can efficiently detect the signals of multiple OAM-modes for the misaligned transceiver and can significantly increase the spectrum efficiency.

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.

The plane wave based wireless communications have becoming more and more matured, along with the well utilization of the traditional resources such as time and frequency. To further increase the capacity for rapidly increasing capacity demand of wireless communications, it is potential to use the twist wave, which has the orbital angular momentum (OAM). In this paper, we discuss the OAM based wireless communications in the aspect of orthogonality, degree of freedom (DoF), and capacity, where both the transmitter and the receiver use uniform circular array (UCA) antennas. In particular, we compare OAM based wireless communications with multiple-input-multiple-output (MIMO) based wireless communications in terms of DoF and capacity. Numerical results are presented to validate and evaluate that the DoF of OAM based wireless communications is greater than or equal to that of correlated MIMO based wireless communications when the transmitter and the receiver antennas are aligned well. The OAM based wireless communications can achieve larger capacity than the correlated MIMO in high signal-to-noise ratio (SNR) region under line-of-sight scenario.