Orbital angular momentum (OAM) modes for 2-D beam-steering of circular arrays

In the context of increasing the data-transmission capacity of next-generation wireless links using multi-input multi-output (MIMO) techniques involving antenna arrays, the concept of so-called orbital angular momentum (OAM) states or modes has attracted considerable attention. It is a concept borrowed from physics and optics, applied to much longer RF wavelengths, with the inevitable consequences of generally larger physical dimensions, so that the MIMO capacity is not achieved in a line of sight exceeding the Rayleigh distance from the antenna. However, it is recognized that OAM modes are readily generated by circular arrays with linear phase excitations, and are really the axial patterns of what were known as phase-modes used for steering beams or nulls of circular arrays in the azimuth dimension in past applications. Here the phase-modes of a circular array are used to steer its beam in both the azimuth and elevation dimensions, using 3 or 4 phase-shifters and 3 simple hybrids for an arbitrary number of array elements. The beam-steering is effected in the far field, in polar coordinates all around the array axis, and radially over 1 or 2 beam-widths. The radial range can be extended by combining higher-order phase-modes. Such OAM-based beam-steering means are particularly suited to backhaul links for small cells using mm-wave antennas in future 5G wireless networks. Because RF phase-shifters are expensive and do not scale well at millimeter wavelengths, their numbers must be minimized to fit them in the antenna structure and to keep down costs. The steering range is adequate to compensate for motion of small-cell mounting platforms which are typically nonrigid street fixtures such as sign- and lamp-posts.