Multiple OAM Signal Recovery Using Pseudo- Doppler Technique with Two mm-Wave Receivers

Abstract

Traditional wireless links employing OAM (orbital angular momentum) beams make use of their spatial orthogonality in order to increase their spectral efficiency, by carrying a separate data stream on each beam. This requires that the OAM antenna axes are aligned with the line of sight of the link, and are capable of radiating and receiving high-purity orthogonal OAM beams. Due to the deep spatial nulls of these beams (except the zero-order OAM beam) on their axes, the received signal strengths are much lower than from conventional beams of comparable-sized antennas, limiting the signal-to-noise ratio (SNR) and the reach of such links, or requiring very large receive antennas in terms of wavelengths.Here we describe an alternative technique for recovering received OAM beams based on pseudo-Doppler techniques [1]-[3] which avoid the above limitations of traditional OAM links. In this work, the receiving antennas are not aligned with the OAM beam axes, so the received signals are stronger, the receivers and their antennas are fewer and simpler, and the reach is longer than in traditional wireless OAM links. We demonstrate for the first time, by simulations and experiments performed on an outdoor 100m link at 28 GHz, that it is possible to recover 4 independent data streams carried in a common 20MHz channel on 4 co-aperture OAM beams using the technique developed in [1], with only 2 receiver chains. The transmitting OAM antenna radiates all OAM beams with the same cone angles and the 2 receiving antennas are set on their peaks off-axis in the far field, 20cm apart and tangent to the OAM cones, unlike in traditional OAM wireless links [4]. Salient aspects of the pseudo-Doppler OAM recovery algorithm, its simulation in an end-to-end link, and key design features of the demonstration hardware and signal-processing are presented.