Optimal Power Allocation Algorithm for Multi-Mode OFDM-OAM Communication Systems in Multipath Channel

Vortex wave carrying orbital angular momentum (OAM), an important property of electromagnetic wave, has bridged a new way to achieve high-capacity radio-frequency communications using different OAM-modes. However, the system performance of OAM multiplexing with orthogonal-frequency-division multiplexing (OFDM) technologies has not been thoroughly investigated in multipath channel environment. In this paper, we first built up the uniform circular array (UCA)-based multi-mode OFDM-OAM communication system, where the reflected paths has been generated based on ideal specular reflector. Based on the multi-mode OFDM-OAM communication system model, we also derived the two-path OAM channel model with the direct and reflected paths. Then, we proposed a robust optimal power allocation algorithm by decomposing the whole optimization problem into inner-layer OAM-modes optimization and outer-layer OFDM subcarriers optimization in order to control the allocated power flexibly based on the channel quality differences between OAM-modes. The simulation results demonstrate that the proposed algorithm can significantly increase the channel capacity for multi-mode OFDM-OAM communications in multipath channel as compared with the traditional equal power allocation method and has a considerable low computational complexity than the binary search water-filling method when the number of OFDM subcarriers is large enough.