Mutual Coupling Reduction for Ultra-Dense Multi-Band Plasmonic Nano-Antenna Arrays Using Graphene-Based Frequency Selective Surface

Terahertz (THz) band provides huge bandwidth but the molecular absorptions in these frequencies cause high path losses in long-distance communications. Recently, multi-band ultra-massive MIMO (UM-MIMO) systems based on graphene-based plasmonic nano-antennas have been proposed to overcome the distance problem. In the UM MIMO systems, the mutual coupling effect is a challenging problem because of the ultra-dense integration of the multi-band nano-antenna arrays. In this paper, a graphene-based frequency selective surface (FSS) is proposed to reduce the coupling effects in dense plasmonic nano-antenna arrays for multi-band UM MIMO systems. The performance of the proposed structure is evaluated by full-wave simulation for different cases. The results show that the FSS structure has a wide stopband (-15 dB bandwidth, approximately 43%-50%) from 1.1 to 1.7 THz. By inserting the FSS structure between nano-antennas, a high isolation coefficient of -25 dB with a 15 dB fall and an envelope correlation coefficient of less than 0.01 are achieved. The field distributions and the radiation patterns are also presented to confirm that the proposed FSS structure improves the performance of the array with negligible influence on the antenna itself. Moreover, the receiving mode simulation of the array is performed with the FSS structure. It is also asymptotically shown that the mutual coupling experienced by the nano-antenna with the FSS structure is negligible even in the presence of a very large number of closely integrated elements. Finally, the technological issues are discussed for practical implementations.