Photon Orbital Angular Momentum (OAM) is a new technique to increase wireless channel capacity. However, not everyone agrees that this is a conceptually new area in radio communication and they make the case that OAM is a special mode of MIMO communications. This view was presented in a recent IEEE Transactions on Antennas & Propagation paper.
The argument made is that OAM is a unique mode of MIMO theory which is obtained under certain conditions and therefore not a conceptually new area. These conditions are obtained when a circular antenna array configuration is used in free space communication which results in MIMO eigen-modes that are similar to the OAM states. In this case, the argument is that OAM is a sub-class of traditional MIMO communication.
Aside from the debate on whether OAM is a form of MIMO, it was shown that for uniform circular arrays the gain falls quickly for most OAM states which render them unusable for communication. Furthermore, the greater the distance away from the antenna system, the fewer number of states that would be available. (This distance is defined by the Rayleigh distance which is proportional to the antenna aperture and inversely proportional to wavelength.) In all, at long distance away from the antenna, only one OAM state remains viable leading to a small gain over a SISO system that is due to the array gain. There is no multiplexing gain since there is only one OAM mode is to carry information. However, for short distance, OAM achieves close to the full capacity predicted by MIMO. The larger the array, the longer the distance over which multiplexing gain is reached.
The tradeoffs that OAM provides place limits on what can be achieved in practice. One can conclude that OAM will be more valuable at higher frequency bands (e.g. millimeter waves; > 30 GHz) where the smaller antenna size allows for greater number of antennas in the same physical area, or a larger array. Such a solution would be most valuable for indoor communications.