Capacity evaluation on the long-distance orbital angular momentum non-orthogonal transmission

Electromagnetic (EM) wave with Orbital Angular Momentum (OAM) can achieve high channel capacity via OAM mode multiplexing. However, due to the beam divergence angle, it is impossible to receive the whole phase front in long-distance. Consequently, partial aperture receiving gradually draws attention in the long-distance OAM transmission. In this paper, the channel capacity of such long-distance non-orthogonal transmission is analyzed. Specifically, the long-distance OAM non-orthogonal transmission system structure and the mathematical model of the channel capacity are established. Meanwhile, simulations are carried out to evaluate the channel capacities. Compared with the plane wave received by antenna array, the OAM non-orthogonal transmission can also result in a significant channel capacity gain. Similar to the Multiple-Input and Multiple-Output (MIMO) system, the capacity gain non-linearly decreases when the multiplexed OAM modes increase, which means a limited number of OAM modes is sufficient and appreciated in the practical applications.