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. Read more of this post