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/r(4).