The terahertz (THz) band above 100 GHz is a promising resource for high capacity wireless transmission in future wireless networks due to the availability of ultra-wide bandwidth over 10 GHz. In order to realize ultra-high-capacity wireless transmission by exhaustively utilizing the resource, we are focusing on orbital angular momentum (OAM) multiplexing transmission technology using uniform circular array (UCA) in the sub-THz band. The OAM have a larger beam divergence than plane waves, resulting in shorter transmission distance range, thus, it is effective to widen the beamwidth and reduce the divergence by using reflector. However, it is necessary to consider shielding and scattering by the array antenna itself and other structures and precisely maintain the orthogonality between OAM modes in practice. In this paper, we propose off-axis double-reflector antenna designs for rotationally symmetric array antenna such as UCAs to extend the transmission distance by enlarging their effective array size while reducing the self-shielding and scattering problem. Experimental end-to-end transmission evaluation shows the proposed reflector can properly maintain the orthogonality between OAM modes, and our off-axis double-reflector design is feasible for Tbps-class long distance OAM-MIMO multiplexing transmission.

This study investigates the system capacity of an orbital angular momentum multiple-input multiple-output (OAM-MIMO) with a parabolic reflector, and assesses its effectiveness compared to a traditional uniform circular array (UCA)-based OAM-MIMO without a parabolic reflector. Specifically, we analyzed the system capacity when deploying a parabolic reflector on the transmitting side, considering parameters such as the number of multiplexed streams and carrier frequency. Furthermore, we examined the potential of OAM-MIMO with successive interference cancellation (SIC), which is crucial because of the differences in the received signal power among multiple UCAs that create spatial diversity. Our performance evaluation maintains a fixed total number of antenna elements and total transmission power, ensuring a fair assessment that considers the tradeoff between the stream power and bandwidth expansion effect resulting from an increased number of multiplexed streams.

Mobile traffic growth requires the advancement of not only the wireless access networks but also their backhaul and fronthaul. Terabit-class wireless backhaul and fronthaul can be an alternative to optical fiber transmission and will be one of the key technologies to construct a more flexible and less expensive network infrastructure for sixth-generation mobile networks (6G). However, it is a challenge to provide an extremely high-capacity wireless link for point-to-point connection without spatial multiplexing gain obtained by the multi-path rich environment. We demonstrated the world’s highest wireless transmission data rate of 1.58 Tbps in the sub-terahertz (sub-THz) band for 6G backhaul and fronthaul networks on the basis of the orbital angular momentum (OAM) multiplexing technology with a wideband Butler matrix. Terabit-class wireless transmission was achieved by designing a wideband 8×8 Butler matrix with two types of 3-dB couplers for the structure without crossover and differential phase shifters that give the desired phase difference over wide bandwidth. Our Butler matrix is capable of multiplexing eight OAM beams and show a high mode isolation of greater than 15 dB and low insertion loss of less than 1.5 dB from 135 to 170 GHz. We implemented the Butler matrices in our OAM multiplexing transmission system, in which the physical-layer data rate of 1.58 Tbps wireless transmission was confirmed with eight OAM modes and dual polarization using the 32 GHz bandwidth.

This paper presents the performance analysis and experimental demonstrations of our orbital angular momentum-multiple-input multiple-output (OAM-MIMO) multiplexing system. OAM is a fixed orthogonal basis set, so OAM multiplexing has a high affinity to analog processing. We extend OAM multiplexing to OAM-MIMO multiplexing technology, which effectively combines the advantage of OAM multiplexing with that of MIMO-based digital signal processing with multiple uniform circular arrays (multi-UCAs) for line-of-sight wireless transmission. Basically, OAM-MIMO is classified as a practical form of analog-digital hybrid MIMO technology. Our multi-UCA-based OAM-MIMO multiplexing transmission system has two hybrid analog-digital architectures. We evaluated its performance through a comparison with different antenna arrangements and configurations. We implemented antennas of quadruple UCAs, with each UCA having a broadband Butler matrix circuit that generates and separates OAM modes as an analog part of hybrid MIMO on a 28-GHz frequency band. We experimentally demonstrated 130-Gbit/s wireless data transmission with 11 streams using five OAM modes (0, ±1, ±2) at a distance of 10 m. We also demonstrated simultaneous use of OAM-MIMO and polarization multiplexing and achieved wireless transmission over 200 Gbit/s with 21 streams. These results indicate the practicality and effectiveness of our system for high-capacity wireless communication.

The adoption of a parabolic antenna is effective in further improving the transmission performance of orbital angular momentum (OAM) multiplexing. Although it has been demonstrated that the propagation attenuation for OAM multiplexing using a parabolic antenna does not necessarily depend on the OAM mode for a relatively short link distance, the influence of the link distance on propagation attenuation has not been clarified comprehensively. In this study, we investigate the transmission performance of OAM multiplexing using a parabolic antenna over a wide range of link distances. In detail, the received power in the use of a parabolic antenna or uniform circular array (UCA) is analyzed using the link distance, carrier frequency, and receiving antenna radius, which clarifies the effectiveness of planar reception created by the parabolic antenna on the received power. Based on these results, the impact of the OAM mode on propagation attenuation is demonstrated at different link distances.

We propose an OAM-MIMO multiplexing system that effectively combines orbital angular momentum (OAM) and the advantage of multiple input multiple output (MIMO) based digital signal processing with multiple uniform circular arrays (UCAs) for point to point (P2P) line of sight (LOS) wireless transmission. We also implement transmitting (Tx) and receiving (Rx) antennas with quadruple UCAs, and each UCA has a broadband Butler matrix circuit for generating and separating OAM modes in the 28-GHz frequency band. We experimentally successfully demonstrate 120-Gbps wireless data transmission using five OAM modes (0, ±1, ±2) and 2-Gbaud adaptive modulation and coding with maximum modulation order of 256-QAM in the 27.5-29.5-GHz range. Our experimental results show the first step toward the next stage for super-high-capacity wireless communications.

This paper presents wireless OAM (orbital angular momentum) and OAM-MIMO (multi-input and multi-output) multiplexing in a 28 GHz frequency band. We have implemented transmission (Tx) and reception (Rx) antennas consisting of multiple uniform circular arrays (UCAs) to confirm the feasibility. Each UCA can concurrently transmit or receive five OAM mode signals (0, ±1, ±2). Using implemented antennas, we rectified mode-selective Rx SNR degradation caused by the inherent nature of OAM beams. With a combination of antenna selection and receiver diversity we have achieved a 45 Gbps transmission rate using five OAM modes. We also experimentally demonstrated the effectiveness of the OAM-MIMO multiplexing by using a total of eleven OAM modes (three OAM 0 modes and two sets of OAM ±1 and ±2 modes). Experimental results reached a new milestone in point-to-point transmission rates by achieving 100 Gbps at 10 m transmission distance.

A channel modeling method is developed and proposed to practically model the various effects of mode couplings for optical communication systems with multi-core fiber (MCF) and/or few-mode fiber (FMF). The method is similar to existing channel modeling methods in which strong coupling is exclusively considered but extends them to cover both strong and weak couplings. With the method, the entire channel is divided into K statistically independent sections where each section may represent an optical device or the span of a few-mode fiber transmission line that maintains coherence. Each section is modeled by a channel matrix model comprising full random unitary matrices and a differential mode delay (DMD) matrix. To further enhance the modeling of weak coupling, the matrix is modified by segregating it into partial block matrices and inserting correlation terms among blocks. The method is evaluated with data obtained in two representative cases: midrange (40 km) and long-haul (527 km) transmissions. Its effectiveness and practicality are validated from agreement obtained between simulation and measured results for the two cases.

The Flexible Wireless System (FWS) has been proposed as a networked system for a User-Centric Wireless Networks (UCWN). UCWNs allow users to make network connections easily at all times without being conscious of any upgrades or differences in wireless systems. The FWS is a unified wireless platform that simultaneously deals with various types of wireless signals. It consists of flexible access points and a wireless signal processing platform. Various types of wireless signals are received at a distributed flexible access point and transferred to a server in the wireless signal processing platform through the wired access line. Transferred signals are separated and demodulated at the server. To achieve highly flexible and efficient radio wave data transfer between the access point and the server, this paper proposes a sparse-matrix-based compressed sensing method under the framework of the belief propagation and cavity method. Low cost implementation using interlevers and adders is also proposed. An empirical study with real data shows the proposed method achieves greater efficiency and reduced calculation cost compared to the conventional compressed sensing method.

The “Flexible Wireless System (FWS)” has been proposed as a networked system for the “User-Centric Wireless Network (UCWN)”. The UCWN will allow users to make network connections easily at all times without being conscious of any upgrades or differences in wireless systems. The FWS is a unified wireless platform that simultaneously deals with various types of wireless signals. It consists of flexible access points and a wireless signal processing platform. Various types of wireless signals are received at a distributed flexible access point and transferred to a server in the wireless signal processing platform through the wired access line. Transferred signals are separated and demodulated at the server. To achieve highly flexible and efficient radio wave data transfer between the access point and the FWS server, we consider compression of transfer data. In this paper, we propose 1-bit compressed sensing with smoothed edge detection, which enhances compression and reconstruction performance. This paper shows the performance of the proposed method by using computer simulations to explore the method's validity for transferring compressed radio wave data.

Recently developed compressed sensing theory enables signal acquisition and reconstruction from incomplete information with high probability provided that the signal is sparsely represented in some basis. This paper applies compressed sensing for spectrum sensing in a networked system. To tackle the calculation and communication cost problems, this paper also applies structured compressed sensing and 1-bit compressed sensing. Measurement using the partial random circulant matrices can reduce the calculation cost at the sacrifice of a slightly increased number of measurements by utilizing the fact that a circulant matrix is decomposed by multiplications of structured matrices. This paper investigates the tradeoff between calculation cost and compression performance. 1-bit compressed sensing extracts only sign data (1-bit quantization) from measured data, and reconstructs the original signal from the extracted sign data. Therefore, 1-bit compressed sensing can save communication costs associated with spectrum sensing in a networked system. This paper evaluates the efficiency of 1-bit compressed sensing. In addition, this paper also proposes a block reconstruction algorithm for 1-bit compressed sensing that uses the block sparsity of the signals. Empirical study shows that partial random circulant matrices work as efficient as completely random measurement matrices for spectrum sensing and that 1-bit compressed sensing can be used for spectrum sensing with greatly reduced communication costs.