Power Efficiency, Overhead, and Complexity Tradeoff in IRS-Assisted Communications -- Quadratic Phase-Shift Design

TL;DR

This paper introduces a quadratic phase-shift design for IRS-assisted communications that improves power efficiency in small codebooks by balancing overhead and complexity, outperforming linear designs.

Contribution

It proposes a novel quadratic phase-shift design for IRS codebooks, addressing the power efficiency tradeoff and providing a method to optimize phase-shift coefficients.

Findings

Quadratic phase-shift design outperforms linear baseline in power efficiency.

Small codebooks benefit from higher-order phase variations.

The proposed design reduces overhead while maintaining performance.

Abstract

In this paper, we focus on large intelligent reflecting surfaces (IRSs) and propose a new codebook construction method to obtain a set of predesigned phase-shift configurations for the IRS unit cells. Since the overhead for channel estimation and the complexity of online optimization for IRS-assisted communications scale with the size of the phase-shift codebook, the design of small codebooks is of high importance. We show that there exists a fundamental tradeoff between power efficiency and the size of the codebook. We first analyze this tradeoff for baseline designs that employ a linear phase-shift across the IRS. Subsequently, we show that an efficient design for small codebooks mandates higher-order phase-shift variations across the IRS. Consequently, we propose a quadratic phase-shift design, derive its coefficients as a function of the codebook size, and analyze its performance. Our simulation results show that the proposed design yields a higher power efficiency for small codebooks than the linear baseline designs.