Back in October I wrote a blog post on radio vortices after reading an article that suggested Twisting Radio Waves Could Give Us 100x More Wireless Bandwidth. I postulated that this might be a viable way to get additional independent paths for a point-to-point MIMO radio link.
I was wrong!
Shortly after I wrote that blog post, Ove Edfors pointed me to a paper that he wrote with Anders J. Johansson, Is orbital angular momentum (OAM) based radio communication an unexploited area? In this paper they prove that radio vortices are a subset of MIMO. Specifically, they show that OAM radio communication, i.e. using radio vortices, is a sub-class of traditional MIMO communication with circular antenna arrays. So, if you have a set of antenna elements that can create radio vortices, the calculations inherent in a MIMO radio system will automatically create and use vortices to the extent they are useful.
Unfortunately they also proved vortices don't give us the additional independent paths I'd hoped for. To get spatial multiplexing gain, MIMO needs additional independent paths, a.k.a. multi-path. Multipath is easy to come by in an office environment (due to reflections from walls, ceiling, floor, filing cabinets, etc.). But in an outdoor point-to-point link with directional antennas there are no reflections, so the only independent paths we've been able to exploit are those resulting from polarization differences (e.g. horizontal vs vertical). This remains the case - radio vortices don't help.
At very short distances, with widely spaced antenna elements at either end, you can get multiple independent paths, just due to the separate fields radiated by separate elements, but this separation gets jumbled as soon as the two ends of a link are separated by more than the Rayleigh distance, as shown in this graph.
At 5.8 GHz with a total antenna apeture of 30 cm, the Rayleigh distance is about 3.5 meters. Since I'm interested in point-to-point links that are typically over 50 meters, we remain limited to two spatial streams. There are no additional paths due to twisted vortices.