Near- and Far-Field Communications with Large Intelligent Surfaces

TL;DR

This paper analyzes the uplink spectral efficiency of two single-antenna UEs communicating with a large intelligent surface, emphasizing the importance of near-field modeling and comparing MR and MMSE schemes.

Contribution

It provides a detailed analysis of near-field effects on spectral efficiency with large LIS arrays, highlighting the necessity of near-field models and the performance of MMSE over MR.

Findings

Near-field channel models are essential for accurate SE evaluation.

Interference persists even as LIS size grows large.

MMSE outperforms MR in large LIS scenarios.

Abstract

This paper studies the uplink spectral efficiency (SE) achieved by two single-antenna user equipments (UEs) communicating with a Large Intelligent Surface (LIS), defined as a planar array consisting of $N$ antennas that each has area $A$. The analysis is carried out with a deterministic line-of-sight propagation channel model that captures key fundamental aspects of the so-called geometric near-field of the array. Maximum ratio (MR) and minimum mean squared error (MMSE) combining schemes are considered. With both schemes, the signal and interference terms are numerically analyzed as a function of the position of the transmitting devices when the width/height $L = \sqrt{NA}$ of the square-shaped array grows large. The results show that an exact near-field channel model is needed to evaluate the SE whenever the distance of transmitting UEs is comparable with the LIS' dimensions. It is shown that, if $L$ grows, the UEs are eventually in the geometric near-field and the interference does not vanish. MMSE outperforms MR for an LIS of practically large size.