Vortex Waves and Channel Capacity: Hopes and Reality

Several recent contributions have envisioned the possibility of increasing currently exploitable maximum channel capacity of a free-space link, both at optical and radio frequencies, by using vortex waves, i.e., carrying orbital angular momentum (OAM). Our objective is to disprove these claims by showing that they are in contradiction with very fundamental properties of Maxwellian fields. We demonstrate that the degrees of freedom (DoFs) of the field cannot be increased by the helical phase structure of electromagnetic vortex waves beyond what can be done without invoking this property. We also show that the often-advocated over-quadratic power decay of OAM beams with distance does not play any fundamental role in the determination of the channel DoF.