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Is orbital angular momentum (OAM) based radio communication an unexploited area?
Ove Edfors, Member, IEEE, and Anders J Johansson, Member, IEEEAbstract—We compare the technique of using the orbital angular momentum (OAM) of radio waves for generating multiple channels in a radio communication scenario with traditional multiple-in-multiple-out (MIMO) communication methods. We demonstrate that, for certain array conﬁgurations in free space, traditional MIMO theory leads to eigen-modes identical to the OAM states. From this we conclude thatcommunicating over the sub-channels given by OAM states is a subset of the solutions offered by MIMO, and therefore does not offer any additional gains in capacity. Index Terms—Antenna arrays, antenna radiation patterns, channel capacity, free-space propagation, MIMO, orbital angular momentum, radio communication.
I. I NTRODUCTION T WAS recently shown that the photon orbital angular momentum (OAM)can be used in the low frequency radio domain and is not restricted to the optical frequency range . These ﬁndings and the claimed prospects for opening a new frontier in wireless communications, with “promise for the development of novel information-rich radar and wireless communication concepts and methodologies” , motivates the investigation in this paper. Here we focus on the wirelesscommunication aspects of  and the follow-up paper . We start by identifying the conditions under which electromagnetic waves with speciﬁc OAM characteristics1 are generated in   and continue by comparing with properties of traditional communication using multiple-in-multiple-out (MIMO) antenna systems   . We pay special attention to the singular value decomposition (SVD) basedderivation of channel capacity for MIMO systems , when applied to MIMO systems under free-space propagation conditions. Spatial multiplexing under free-space conditions may seem like a contradiction, but this very concept has been investigated in various forms for almost a decade    . When restricting ourselves to using circular antenna arrays, the SVD-based analysis in combinationwith properties of circulant matrices   can deliver the same beam-forming and the same OAM properties as in  . The beam forming process for all eigenmodes/OAM states can be performed by a discrete Fourier transform (DFT)
Manuscript received February 24, 2011; revised May 26, 2011. Both authors are with the Department of Electrical and Information Technology, Lund University, Sweden.c 2011 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. 1 These radiowaves with speciﬁc OAM characteristics are often called “twisted radio beams”   in popular science contexts.
Fig. 1. Illustration of the creation of Laguerre-Gaussian laser beams from planar laser beams by using transparent spiral phase plates introducing a linear phase delay with azimuthal angle. OAM state k implies a 2πk phase delay over one revolution. Three different phase platesare illustrated in gray, for OAM state 0, 1 and 2. The colored surfaces are contour surfaces indicating where the phase of the laser beams is zero.
, which was also observed in . The analysis also reveals that the eigenmodes of the resulting MIMO system are not necessarily unique, making OAM radio communication a sub-class of traditional MIMO communication with circular antenna arrays....