Concentric UCAs Based Low-Order OAM for High Capacity in Radio Vortex Wireless Communications

Due to the potential capacity-boosting for wireless communications, radio vortex wireless communication (RowComm) over orthogonal states/modes of orbital angular momentum (OAM) has been paid much attention to in recent years. A uniform circular array (UCA), as an efficient and convenient antenna structure, can transmit/receive multiple OAM beams with different OAM-modes simultaneously when the transmitter and receiver are aligned. However, for high-order OAMmodes, the OAM beams are divergent accompanied by severe attenuation. Thus, it is difficult to directly use high-order OAM-modes to achieve high capacity for RowComms. To obtain high capacity potentially offered by OAM-modes, in this paper we transform the singular UCA into the concentric UCAs, where high capacity can be achieved using multiple parallel low-order OAM-modes instead of all high-order OAM-modes, to increase the capacity of transmitter-receiver aligned Row-Comms. In particular, we study two cases: 1). concentric UCAs based RowComms without co-mode interference; 2). concentric UCAs based RowComms with co-mode interference. We propose a co-mode-interference-free and a co-mode-interference-contained mode-decomposition schemes to recover transmitted signals for these two cases, respectively. Additionally, we develop optimal power allocation schemes to maximize the capacity for these two cases. Numerical simulations are presented to validate and evaluate that our developed concentric UCAs based low-order RowComms can significantly increase the capacity as compared with that of singular UCA based RowComms.