Hybrid Circular Array and Luneberg Lens for Long-Distance OAM Wireless Communications

As a new degree of freedom (DOF), orbital angular momentum (OAM) has the potential of greatly enhancing the channel capacity of wireless communication systems. However, the OAM beams diverge with the increase of transmission distance and OAM mode, which makes the OAM beams modulated with information difficult to be received with a limited aperture array. Therefore, there is still a big challenge for achieving the expected extremely high data rate with orthogonal multiplexed OAM beams in a long-distance communication link. To reduce the divergence of OAM beams, the spherical Luneberg lens is proposed for the uniform circular array (UCA)-based OAM communication system to realize OAM beam convergence. However, due to the non-negligible UCA aperture relative to the Luneberg lens, we find through electromagnetism (EM) simulation that the 0-mode OAM beam becomes more divergent after passing through Luneberg lens. To solve this problem, we propose a hybrid circular array with a center element (CAC) and spherical Luneberg lens (HCCL) structure. When the HCCL structure is applied in the OAM wireless communication system, the divergence angles of generated OAM beams are significantly reduced, and the received signal-to-noise ratio (SNR) is expected to be significantly improved. Besides, the multiplexed OAM modes in the proposed HCCL-based OAM communication system keep orthogonal. Therefore, the channel capacities of the proposed HCCL-based OAM communication systems are obviously better than that of the existing UCA-based OAM system in the long-distance transmission. Furthermore, both numerical analysis and simulation results validate that the performance of the OAM system equipped with the HCCL structure both at transmitter and receiver is significantly better than that of the system equipped with the HCCL structure at one side.