标题： 对“通过无线电涡旋在同一种频率上编码多个信道：首次实验测试”的评论 显示英文标题

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

摘要： 我们证明了F. Tamburini等人在威尼斯举行的公开实验，并在(2012 New J. Phys. 14 033001)中报道，可以被视为多输入多输出(MIMO)通信的一种特殊实现，因此相对于已知技术没有优势。 此外，我们解释了在发射天线之一使用“涡旋”模式（轨道角动量OAM l = 1）并不是编码不同信道所必需的，因为只需要发射天线的不同图案——或者类似的不同指向角度。 最后，我们确定了为什么这种MIMO传输允许解码两个信号，尽管是视距传输。 这是由于接收天线之间的大距离，使得发射天线位于接收“阵列”的近场菲涅尔区域。 这极大地限制了该技术在实际中的应用，因为对于固定的接收天线间距，任何额外涡旋模式的可检测信号功率至少以1/r^4的速度衰减。 显示英文摘要

摘要： 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.