Optimization-based mode selection scheme for OAM millimeter wave system in the off-axis misalignment case

Millimeter-wave transmission combined with Orbital Angular Momentum (OAM) has the advantage of reducing the loss of beam power and increasing the system capacity. However, to fulfill this advantage, the antennas at the transmitter and receiver must be parallel and coaxial; otherwise, the accuracy of mode detection at the receiver can be seriously influenced. In this paper, we design an OAM millimeter-wave communication system for overcoming the above limitation. Specifically, the first contribution is that the power distribution between different OAM modes and the capacity of the system with different mode sets are analytically derived for performance analysis. The second contribution lies in that a novel mode selection scheme is proposed to reduce the total interference between different modes. Numerical results show that system performance is less affected by the offset when the mode set with smaller modes or larger intervals is selected.