This paper proposes a guard interval (GI) control scheme using the theoretical system capacity for unmanned aerial vehicle (UAV) multi-user multiple-input and multiple-output orthogonal frequency-division multiplexing (MU-MIMO-OFDM) Tomlinson-Harashima precoding (THP) systems. Considering the need for coverage extension assumed in sixth-generation mobile communication systems (6G), we consider UAVs as users as well. Therefore, variation in delay spread between users is assumed to increase due to differences in the communication distance and flight height. In such scenarios, GI length is typically determined from the longest delay among all mobile stations (MSs) to eliminate inter-symbol interference (ISI). However, this leads to significant degradation in transmission efficiency. The proposed scheme adopts a GI length that maximizes the theoretical system capacity derived from the correlation calculation while allowing some ISI, which reduces transmission overhead. Moreover, considering the assumed spatial correlation, particularly in UAV communications, THP is adopted as the MU-MIMO precoding method. The effectiveness of the proposed scheme is demonstrated via computer simulations in comparison with the traditional scheme adjusted to the largest multipath delay.

The aim of this paper is to further investigate the communication with Orbital Angular Momentum (OAM) waves between two OAM antenna arrays. The antenna arrays consist of individual dipoles, which together form a Uniform Circular Array (UCA). In this paper, two configurations that have already been discovered are assumed, in which there is no crosstalk between the individual modes. On the one hand, the vertical alignment already examined by many papers and on the other hand a horizontal alignment in which the dipoles have a certain rotation around the axis. The first investigation deals with the frequency dependency in OAM communication in the given setup. In a further investigation, the influence of the radius of the antenna arrays is conducted. The simulations will focus on the transmission of the individual OAM modes and the crosstalk between different OAM modes.

This study investigates the theoretical system capacity of single-carrier orbital angular momentum (OAM) multiplexing considering the radiation pattern and polarization in the presence of ground reflection. Assuming that the adoption of a highly directional antenna is required to overcome the large propagation loss in the extremely high-frequency bands, we incorporate the impact of radiation pattern into the systemcapacity analysis of single-carrier broadband OAM multiplexing. Moreover, because the impact of reflected waves depends on the polarization plane, which determines the reflection coe fficient, we demonstrate its di fference in the system capacity by assuming vertical and horizontal polarization. Regarding the analytical evaluation in the presence of reflected wave, the consideration of either the radiation pattern or di fference in the polarization planes has been unfortunately lacking. As for radio propagation, we utilize the 3GPP technical report (TR) 38.901 as a radiation pattern and clarify its impact with and without frequency domain equalization (FDE) compared to the isotropic-antenna case using analytical approaches.

This study investigates the system capacity of dual-polarized orbital angular momentum (OAM) multiplexing that employs single-carrier with frequency domain equalization (FDE) in urban street canyon environments. To overcome the impact of multipaths, two types of FDE, designed with and without the consideration of the cross-polarization interference (XPI), are introduced. We derive the effects of the residual inter-mode interference (IMI), inter-symbol interference (ISI), and XPI when applying such FDE and incorporate their impacts into the system capacity analysis. Moreover, we adopt the urban street canyon model based on a ray-tracing simulation, which enables performance evaluation in a realistic multipath environment. The numerical results show that although the computational complexity is increased, the consideration of the XPI in the FDE weight design improves the system capacity, and its effectiveness is remarkable in the cases of short link distances.

This paper investigates the effect of multiplexing-based least squares (LS) channel estimation on uplink large-scale MU-MIMO-OFDM in the presence of terminal mobility. In uplink large-scale MU-MIMO, the same number of pilot symbols as that of mobile stations (MSs) is generally required, which results in the consumption of a substantial amount of resources for channel estimation. This overhead can be reduced by using the LS channel estimation in user multiplexing because this method has only to use a limited number of subcarriers, with a multipath length at most. In addition, the achieved overhead reductions can be effectively utilized for improving the tracking capability of channel estimation under varying channels. Since our performance evaluation focuses on mobile communications systems such as 6G, uplink large-scale MU-MIMO is assumed, and Gaussian belief propagation (GaBP) is applied to detect multiple uplink signals. The effectiveness of this method relative to that of the traditional LS method with the same number of pilot symbols is demonstrated under time selective fading through computer simulations.

With the growing demand for wireless mobile services and the development of fifth-and sixth-generation mobile communication systems, the throughput analysis of broadband wireless transmission in extremely high-frequency bands is becoming increasingly important. This study investigates the system capacity of millimeter-wave (mmWave) and terahertz (THz) broadband communications considering delayed waves and radiation patterns, as well as the molecular absorption and rain attenuation. In our performance evaluation, single-carrier with frequency domain equalization (SC-FDE) is adopted as a practical transmission method for extremely high-frequency bands, and the 3GPP technical report (TR) 38.901 is used as the channel and antenna model to evaluate the impact of delay spread and radiation pattern. Compared with the case of an isotropic antenna, the proposed method incorporates the frequency selectivity and directionality into the performance evaluation according to the line-of-sight (LoS) and non-line-of sight (NLoS) conditions.

This paper investigates the system capacity of polarized orbital angular momentum (OAM) multiplexing by comparing it with OAM multiplexing without polarization for the same number of antenna elements. Since the same number of antenna elements is assumed for both approaches, their performances can be compared under the same number of simultaneous OAM streams, thereby allowing the effectiveness of polarization on UCA-based OAM multiplexing to be examined. Numerical results, obtained through computer simulations, provide the mode selectivity and system capacity with respect to link distance and carrier frequency.

Orbital angular momentum (OAM) multiplexing suffers from inter-mode interference (IMI) due to a beam axis misalignment. However, the traditional IMI cancellation method assumes that the IMI rises from all OAM modes, which results in heavy computational loads. This paper proposes an efficient IMI cancellation method for OAM multiplexing in the presence of a beam axis misalignment. In the proposed method, considering that the interference from adjacent OAM modes is dominant in such a misalignment, IMI cancellation is performed by considering only the neighboring OAM modes. This method can reduce the computational cost while retaining the transmission performance. Moreover, the minimum mean square error (MMSE) is adopted as the equalizer criterion to suppress noise enhancement. The effectiveness of the proposed method as compared with the traditional IMI cancellation method is demonstrated in terms of the system capacity and computational complexity through computer simulations.

This paper proposes a mode group selection method for orbital angular momentum (OAM) multiplexing to suppress inter-mode interference (IMI) owing to the misalignment of the beam axis. We have proposed an IMI suppression method employing even-numbered OAM modes, and its effectiveness has been successfully confirmed. Odd-numbered OAM modes are robust and used in addition to even-numbered modes to improve the robustness against the IMI, and the appropriate mode group is selected based on the system capacity. The effectiveness of the proposed method is demonstrated in terms of system capacity via computer simulations.

This paper investigates the system capacity of polarized orbital angular momentum (OAM) multiplexing considering the impact of polarization interference. Dipole antenna is considered as the practical radiation pattern and the composition of the received mode signal is clarified by analyzing the effect of cross-polarization interference from other modes, as well as both cross-polarization interference in a certain mode and co-polarization interference from different modes. Based on the interference analysis, system capacity is precisely evaluated by comparison with the isotropic antenna case, employing a link distance and carrier frequency parameter. The numerical results show that cross-polarization interference from other modes rises at the regular mode interval, presenting a significant impact on system capacity, particularly for relatively short link distances or low carrier frequencies.

This paper presents the theoretical system capacity of orbital angular momentum (OAM) multiplexing that employs single-carrier with frequency domain equalization (SC-FDE) in the presence of ground reflection. Considering that inter-symbol interference (ISI) remains even after inserting a guard interval (GI), in addition to inter-mode interference (IMI), we derive the effect of such residual IMI and ISI, and incorporate their impacts into the analysis of the system capacity. Moreover, in FDE, zero-forcing (ZF) and minimum mean square error (MMSE) are considered as the factors affecting the system capacity because the effect of noise enhancement differs among these two factors. In this study, we demonstrate theoretical results based on the two-ray ground reflection model by using parameters of link distance and carrier frequency. The numerical results show that OAM multiplexing employing SC-FDE improves the system capacity compared with the case without FDE regardless of the link distance, and its effectiveness is remarkable in the cases of relatively lower carrier frequencies.

This paper proposes a method that employs successive interference cancellation (SIC) to further enhance the capacity of uniform circular array (UCA)-based orbital angular momentum (OAM)- MIMO systems. The central feature of the proposed approach is to exploit the additional space diversity benefit in the remaining streams by means of SIC owing to the large difference in the received power among different UCAs in each OAM mode. Therefore, it is expected that the proposed method can further stabilize the performance of UCA-based OAM-MIMO along with the communication distance. Our numerical results show that the proposed method outperforms the traditional method without SIC in terms of the system capacity owing to the large received power difference among different UCAs.

This paper proposes a method that employs successive interference cancellation (SIC) to further enhance the capacity of uniform circular array (UCA)-based orbital angular momentum (OAM)-MIMO systems. The main feature of the proposed method is to stabilize the performance of UCA-based OAM-MIMO systems by conducting SIC based on the results of parallel interference cancellation (PIC), which exploits the additional full space diversity benefit. The development of the proposed approach is motivated by the feature that large differences in the received power of different UCAs are advantageous to SIC. Moreover, the impact of the misalignment of the beam axis on the effectiveness of the proposed method is also investigated by considering a more realistic scenario. Our numerical results show that the effectiveness of the proposed method increases with increasing number of UCAs, and its advantage is retained within a certain amount of the misalignment of the beam axis.

This paper proposes a method that employs successive interference cancellation (SIC) to further enhance the capacity of uniform circular array (UCA)-based orbital angular momentum (OAM)-MIMO systems. The central feature of the proposed approach is to exploit the additional space diversity benefit in the remaining streams by means of SIC owing to the large difference in the received power among different UCAs in each OAM mode. Therefore, it is expected that the proposed method can further stabilize the performance of UCA-based OAM-MIMO along with the communication distance. Our numerical results show that the proposed method outperforms the traditional method without SIC in terms of the system capacity owing to the large received power difference among different UCAs.

This paper proposes an inter-mode interference suppression method that employs only even-numbered modes for uniform circular array (UCA)-based orbital angular momentum (OAM) multiplexing. Given that interference from adjacent OAM modes becomes severe when the beam axis is misaligned, the proposed method uses only the even-numbered modes while leaving the odd-numbered modes unused, which effectively prevents the inter-mode interference. Moreover, we verify the effectiveness of the proposed method by applying it to multiple-input and multiple-output (MIMO) with zero-forcing (ZF) alone or ZF with successive interference cancellation because severe inter-mode interference is caused by transmitting multiple streams from multiple UCAs in each mode. The effectiveness of the proposed method compared with the traditional method, which uses all OAM modes, is demonstrated in terms of the system capacity through computer simulations.

This paper investigates the influence of the number of uniform circular arrays (UCAs) on the system capacity in orbital angular momentum (OAM) multiplexing. In UCA-based OAM, although multiple UCAs can alleviate the difference in the received signal power among OAM modes, the maximum number of OAM modes is limited. The primary contribution of our work is to evaluate the waterfilling-based system capacity of UCA-based OAM with the number of UCAs at constant transmit power and number of antenna elements, to demonstrate the benefit of multiple UCAs according to the communication distance. Our numerical results show that the multiple UCAs stabilize the system capacity irrespective of the communication distance, whereas the single UCA is effective particularly in short-range communications.

This paper proposes an inter-symbol interference (ISI) suppression scheme employing periodic signals for network MIMO-OFDM systems. Network MIMO-OFDM suffers from the ISI due to long time difference of arrival (TDOA) especially in large cells. In the proposed scheme, the periodic signals generated by only the even-numbered sub-carriers are adopted for ISI-affected users while the traditional non-periodic signals are used for ISI-free users. Moreover, to realize the transmission mode selection between the periodic and non-periodic signals, the theoretical derivation method of the BER is newly derived considering the impact of channel coding. The effectiveness of the proposed method is demonstrated in comparison with the traditional non-periodic transmission by means of computer simulations.