Comment on 'Encoding many channels on the same frequency through radio vorticity: first experimental test'

TL;DR

This paper critiques a recent experiment claiming to encode multiple channels via radio vortex modes, showing it is a standard MIMO setup and explaining why the observed signals are due to near-field effects rather than vortex encoding.

Contribution

The paper demonstrates that the Venice experiment is a typical MIMO system, clarifies that vortex modes are unnecessary for channel encoding, and explains the near-field conditions enabling signal decoding.

Findings

The Venice experiment is equivalent to known MIMO techniques.

Vortex modes are not required for channel encoding.

Near-field effects enable decoding despite line-of-sight constraints.

Abstract

We show that the public experiment held in Venice by F. Tamburini et al and reported in (2012 New J. Phys. 14 033001) can be regarded as a particular implementation of Multiple-Input Multiple-Output (MIMO) communications, hence bringing no advantages with respect to known techniques. Moreover, we explain that the use of a 'vortex' mode (orbital angular momentum OAM l = 1) at one of the transmit antennas is not necessary to encode different channels since only different patterns -or similarly different pointing angles- of the transmit antennas are required. Finally, we identify why this MIMO transmission allowed decoding of two signals despite being line-of-sight. This is due to the large separation between the receiving antennas, which places the transmit antennas in the near-field Fresnel region of the receiving 'array'. This strongly limits the application of this technique in practice, since, for a fixed separation between receiving antennas, the detectable signal power from any additional vortex mode decays at least as 1/r^4.