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 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. 
Michele Tamagnone, Christophe Craeye and Julien Perruisseau-Carrier
physics.optics