Abstract
Orbital Angular Momentum (OAM) is a typical spatial mode of an Electro-Magnetic (EM) wave. Correctly detecting the OAM mode is fundamental and of foremost importance when applying the phenomenon to wireless transmission in free space. It is found that when rotating an OAM wave, a rotational Doppler shift that is proportional to the rotation speed and the OAM mode number can be observed. This property can be used for OAM detection, i.e., different OAM modes are identified by measuring the corresponding rotational Doppler frequency shifts. In previous work, this method was implemented by mechanically rotating the OAM wave, resulting in a small frequency shift. Since the high-speed mechanical rotation is hard to manufacture in a real system, it brings limitations to the bandwidth for each OAM wave. In this paper, we report on an OAM mode detection method based on digitally rotating a virtual antenna. The transmitter and receiver are physically fixed, but the Virtual Rotational Antenna (VRA) is obtained by interpolating the signals received from transverse-mounted receiving antennas. A large rotational Doppler shift occurs as a consequence of using digital processing, resulting in more capability for wideband wireless data transmission with the larger shifted frequency.
Highlights
Note that the frequency shift is caused by the rotation of the Orbital Angular Momentum (OAM) wave relative to the detection point
In our previous work (Fig. 1a), the method for detecting the OAM mode was implemented by rotating the SPP at the transmitter and detecting the frequency shift at the receiver
As discussed in the introduction, this frequency shift is caused by the relative rotation between the OAM wave and the detector
Summary
Note that the frequency shift is caused by the rotation of the OAM wave relative to the detection point It will work when rotating the detecting antenna azimuthally with respect to the beam axis of the OAM wave while keeping the OAM wave stationary. We demonstrate an equivalent rotation method implemented digitally at the receiving side using a Virtual Rotational Antenna (VRA) This is done by interpolating the signals received from transverse-deployed receiving antennas. We built a novel interpolation scheme to realize the VRA This OAM detection method is verified using a sinusoidal signal without modulated symbols, it can detect the data encoded on the OAM by restraining the rotation period of the virtual antenna within the symbol period (see Fig. S4 in Supplementary Information). It should be emphasized that, in this paper, only the OAM detection method for single mode is studied and highlighted, leaving the simultaneous detection of multiple OAM modes in multiplexing for future research
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