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

We show that the public experiment held in Venice by 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 and, therefore, has no advantages over established techniques. Moreover, we explain that the use of a ‘vortex’ mode (orbital angular momentum 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 the 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 severely 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/r⁴.