Flat-Top Beamforming with Efficient Array-Fed RIS

TL;DR

The paper tackles flat-top beam shaping for RIS-fed, uniformly weighted small active arrays to improve power distribution over wide angular sectors. It presents a pragmatic, low-dimensional design that leverages the AMAF-RIS principal eigenmode, aligning PEM taper to a FIR-based flat-top envelope via an $F/D$ choice, binary RIS phase groupings, and a phase-perturbation widening function, with optional refinement through a non-convex optimizer. The approach yields flat-top beams with controllable width and low ripple, demonstrated on linear and planar RIS configurations, and shows favorable energy efficiency compared with large constant-modulus active arrays due to reduced splitter losses and compact PA utilization. The work promises practical, energy-efficient, scalable beamforming for beaconing, broadcast signaling, and hierarchical beamforming in next-generation networks while mitigating environmental impact.

Abstract

Flat-top beam designs are essential for uniform power distribution over a wide angular sector for applications such as 5G/6G networks, beaconing, satellite communications, radar systems, etc. Low sidelobe levels with steep transitions allow negligible cross sector illumination. Active array designs requiring amplitude taper suffer from poor power amplifier utilization. Phase only designs, e.g., Zadoff-Chu or generalized step chirp polyphase sequence methods, often require large active antenna arrays which in turns increases the hardware complexity and reduces the energy efficiency. In our recently proposed novel array-fed reflective intelligent surface (RIS) architecture, the small ($2 \times 2$) active array has uniform (principal eigenmode) amplitude weighting. We now present a pragmatic flat-top pattern design method for practical array (RIS) sizes, which outperforms current state-of-the-art in terms of design superiority, energy efficiency, and deployment feasibility. This novel design holds promise for advancing sustainable wireless technologies in next-generation communication systems, including applications such as beaconing, broadcast signaling, and hierarchical beamforming, while mitigating the environmental impact of high-energy antenna arrays.

Figures (6)

• Figure 1: RIS fed by an AMAF placed in its near field. $F/D < 1$.
• Figure 2: Matching RIS PEM magnitude profile to that from "firpm" FIR filter for a flat top beam shape.
• Figure 3: Comparison of beam patterns: (a) beams from steps 2 and 3, and (b) tunable width flat top beams from step 4.
• Figure 4: Confined flat top beam from Step 2.
• Figure 5: Azimuth widened flat top beam from Step 4.

Theorems & Definitions (1)

• Remark 1