Mode Selection Method Suitable for Dual-Circular-Polarized OAM Transmission

This communication proposes an analog orbital angular momentum (OAM) transmission method based on dual-circular polarization (CP), where the low-gain modes are excluded to enhance the achievable data rate. The transmitter and receiver antennas, which face each other directly, are identical circular arrays, each of which has four circular microstrip antennas with dual CP. A discrete Fourier transform (DFT) feed network is used at both sides to realize quasieigenmode transmission, where mode orthogonalization is achieved by using just analog circuits; digital interference cancellation is not used. The key idea of the proposed method is its mode selection technique, which simply chooses some of the highest modes and distributes all available power to the chosen modes equally. An analysis shows that this method increases the achievable rate by 2 bits/s/Hz compared to short-range multiple input and multiple output (SR-MIMO) when the number of the ports at each side is 8 and the distance between transmitter and receiver antennas is 10λ0(λ0is the wavelength in vacuum) or more. Also, it is found that the achievable rate of the proposed scheme is asymptotic to that of SR-MIMO with water-filling.