Abstract
Orbital angular momentum (OAM) provides extra degrees of freedom to improve spectrum efficiency. However, the harsh requirement that transmit antenna(s) and receive antenna(s) need aligning throws out a great challenge to its application. To investigate the reason of performance degradation caused by misalignment, we first consider uniform circular array (UCA) based OAM system and set up two free space channel models respectively for two misalignment case, i.e., non- parallel and off-axis. Then, the capacities of single- mode and multi-mode OAM system are analyzed and compared under different oblique angles and axis deviations. Analysis and simulation results show that OAM transmission especially the multi-mode OAM is quite sensitive to the misalignment between transmitter and receiver. At last, we propose a misalignment-robust receiving method through extracting the phases of channel state information (CSI) and implementing joint phase compensation and signal detection. Simulation results show that proposed receiving scheme is more robust to misalignment than conventional receiving scheme.
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