RIS-Based Steerable Beamforming Antenna with Near-Field Eigenmode Feeder

Abstract

We present a novel and hardware-efficient space-fed antenna system that exploits the eigenmodes of the over-the-air propagation matrix from a small active multi-antenna feeder (AMAF) to a large reflective intelligent surface (RIS), both configured as standard linear arrays and placed in the near-field of each other. We demonstrate the flexibility of the proposed architecture by showing that it is capable of generating beam shapes for multiple applications, such as very narrow beams with low side lobes for space division multiple access communications, flat-top wide-angle patterns for short-range automotive sensing and sectorial beaconing, and monopulse-like patterns for radar angular tracking. A key parameter in our designs is the AMAF-RIS distance which must be optimized and it is generally much less than the Rayleigh distance. For a given AMAF array size, the optimal AMAF-RIS distance increases as a function of RIS size. The AMAF-RIS loss is almost exactly compensated by the larger aperture gain of the RIS, resulting in an almost constant RIS gain for increasing RIS sizes. This allows different beam angle selectivities to be selected for the same center beam gain. Active RF amplification is done at the AMAF only, thus resulting in a much lower hardware complexity than conventional active phased arrays for the same beamforming performance.