Minimal-hardware 2-D steering of arbitrarily large circular arrays (combining axial patterns of phase-modes)

This paper describes a very simple beam steering system which can steer the axial beam of any arbitrarily-large circular array in 2 dimensions, using 3 hybrids and 3 or 4 variable phase shifters, depending on the variant of the method. It is particularly suited to mm-wave arrays for 5G backhaul links among small cells, where the nodes would typically be mounted on non-rigid platforms which would require a beam-alignment and tracking capability over some limited range in azimuth and elevation around the beam axis, to compensate for platform motions. By minimizing the number of expensive RF components irrespective of the array size, this technique also facilitates low cost hardware implementation which is particularly important for large-scale deployments in future 5G wireless networks. The method makes use of the axial radiation patterns of phase-modes, which are generated by an additional 4 hybrids and imposed on an arbitrary number of antenna elements by a simple "pill-box" radial TEM waveguide feed structure. Phase modes have been used in past applications, but almost exclusively in the azimuthal plane of circular ring arrays, whereas the present technique employs their radiation patterns in the axial direction, orthogonal to the plane of the array. Higher-order phase modes differing in order by "1" may be similarly combined to increase the radial steering range, e.g. for mobile satellite antennas. An adaptive beam-tracking algorithm for such beam steerers is also formulated. Mathematical developments and simple system design illustrate the concept and some possible variations. Full-wave electromagnetic simulations at a scaled frequency in the 5 GHz band are presented to show its feasibility, and scalability to mm-wave or other wavelengths, and its applicability to filled circular arrays with circular polarizations.