Near-Field Communications Based on Orbital Angular Momentum: Channel Modeling and Precoding Design

Orbital angular momentum (OAM) technology can provide an additional degree of freedom in the spatial domain and solve the problem of spectrum shortage in the sixth generation (6G) networks. In this paper, a near-field OAM channel model based on the electromagnetic information (EMI) theory is proposed, in which the dyadic Green’s function approach and discrete Fourier basis functions are utilized to accurately describe the characteristics of near-field channel and OAM signal propagation in practical scenarios, respectively. To improve the performance of misaligned transmission, a near-field OAM misalignment precoding scheme is presented according to the characteristics of the proposed channel model. Specifically, phase errors are compensated and the optimal power is assigned according to the channel condition, which can mitigate the channel capacity loss caused by the the inter-mode interference resulting from the misalignment. Numerical results show that the benefit brought by the polarization along the propagation direction is predominantly significant at short distances, and decreases rapidly to zero as the distance increases. Results also show that the presented scheme outperforms the conventional fixed parameter scheme and effectively reduces the effects of the misalignment in practical scenarios.