Zero-Forcing Beamforming for Beam Sweeping with Reconfigurable Holographic Surfaces

Reconfigurable holographic surfaces (RHSs) for multi-beam steering applications have been receiving significant attention due to their low fabrication cost and power consumption. RHSs have the advantage of exploiting more antenna elements by configuring only their radiation levels, as opposed to the typical phased antenna arrays which enable the control of each element's phase and amplitude. RHSs can be utilized to realize holographic beamforming with single-feed or multi-feeds, where digital beamforming can also be implemented using multi-feeds. In this paper, first, the holographic beamforming problem is formulated by means of the conventional beamforming expressions using the array factor and the holographic beamforming weights. Then, a zero-forcing beamforming method is developed for a single-feed RHS to generate multi-beam and multi-null patterns. Additionally, a bisectional null scanning approach for beam sweeping is presented to exploit the advantages of RHS for forming wide nulls in the beamspace. Simulation results imply a substantial advantage of such single-feed RHS for high-accuracy beam sweeping.