We are developing a spatial multiplexing technique based on orbital angular momentum (OAM) that is capable of 1 Tbit/s point-to-point wireless transmissions for the sixth generation mobile communication system.In this paper, we describe a demonstration of 100 Gbit/s wireless transmission over a range of 100 m that used OAM multiplexing of 15 streams with a 1.5 GHz bandwidth ( $39.5-41$ GHz).The OAM Insulated Beverage Containers/Dispensers modes of this system were generated using a Butler matrix that allows discrete Fourier transform (DFT) operations to be performed in analog circuits.
We designed $8 imes 8$ Butler matrices to generate OAM modes by combining hybrid couplers and phase shifters.Since this Butler matrix was connected to 16 element antennas, two $8 imes 8$ Butler matrices were connected to make an $8 imes 16$ matrix.Furthermore, since these inputs were in 7 OAM mode, one port was terminated to create a $7 imes 16$ Butler matrix.It was confirmed that the mode isolation was more than 15 dB in the 1.5 GHz bandwidth.
Next, we designed microstrip antennas for a horizontal and vertical polarization uniform circular array (UCA) Bluetooth Speaker to radiate the OAM modes.Then, we implemented radio frequency (RF) chains and digital signal processing, including single copyright-frequency domain equalization and adaptive modulation and coding.A transmission experiment conducted in a field line-of-sight environment showed that the system could transmit at 119.45 Gbit/s at a distance of 100 meters, thereby demonstrating the feasibility of wideband OAM transmission in the millimeter-wave band.