OAM Communications in Multipath Environments

Due to the orthogonality of orbital angular momentum (OAM) modes, OAM multiplexing technique has drawn considerable attention for capacity enhancement. OAM multiplexing techniques in line of sight (LoS) as well as multipath environments are discussed in this chapter. Moreover, the OAM multiplexing technique can be combined with other multiplexing technique to further improve the performance of the system. Overall, the OAM multiplexing system performs well in short-LoS environments. Nevertheless, multipath complicates the OAM communication in that there exist strong inter- and intra-channel interferences due to reflection, diffraction, and scattering. It was believed that the OAM multiplexing technique is not suitable for communications in multipath environments. However, as will be shown in this chapter, when combined with spatial equalization and orthogonal frequency division multiplexing (OFDM) techniques, OAM multiplexing can be easily adapted to a rich multipath environment. Moreover, thanks to the spatial equalizer, the OAM multiplexing system supports high order modulation and provides comparable performance to the conventional multiple-input multiple-output (MIMO) systems in multipath environments. In other words, the OAM multiplexing can be used in not only the LoS environment but also the multipath environment.