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On the Performance of Delay Line Based OAM Communications

Delay line based uniform circular antenna array (UCA) is commonly used for orbital angular momentum (OAM) generation. It is known that delay line is preferred for Multiple-input Multiple-output(MIMO) beamforming than the ideal phase shifter(PS), as the latter causes the problem of beam squint. Different from traditional MIMO beamforming, we theoretically prove in this paper that ideal PS is preferred for UCA based OAM communications, while delay line causes vortex distortion. Specifically, through the derivation of baseband equivalent model, we demonstrate that vortex distortion of delay line is a time-varying process, which is affected by OAM mode and signal bandwidth. To verify our analysis, we first simulate instantaneous distortion, and then simulate isolation and spectrum efficiency in different OAM mode and bandwidth configurations. The simulation results indicate that for UCA based OAM commu-nication systems using delay line, higher OAM mode and higher bandwidth greatly limit mode isolation and spectrum efficiency. Our research also applies to delay line based customized OAM antennas, such as helicoidal parabolic antenna, traveling wave circular loop antenna and so on. The research result is of great significance to OAM based wireless communications.