LoS MIMO-Arrays vs. LoS MIMO-Surfaces Marco Di Renzo Davide Dardariy and Nicol o Decarliz Universit e Paris-Saclay CNRS CentraleSup elec Laboratoire des Signaux et Syst emes 91192 Gif-sur-Yvette France

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LoS MIMO-Arrays vs. LoS MIMO-Surfaces

Marco Di Renzo∗, Davide Dardari†, and Nicol`

o Decarli‡

∗Universit´

e Paris-Saclay, CNRS, CentraleSup´

elec, Laboratoire des Signaux et Syst`

emes, 91192 Gif-sur-Yvette, France

†University of Bologna, Department of Electrical, Electronic, Information Engineering (DEI), 40126 Bologna, Italy

‡Italian National Research Council, Institute of Electronics, Computer, Telecommun. Engineering, 40136 Bologna, Italy

marco.di-renzo@universite-paris-saclay.fr

Abstract—The wireless research community has expressed

major interest in the sub-terahertz band for enabling mobile

communications in future wireless networks. The sub-terahertz

band offers a large amount of available bandwidth and, therefore,

the promise to realize wireless communications at optical speeds.

At such high frequency bands, the transceivers need to have

larger apertures and need to be deployed more densely than at

lower frequency bands. These factors proportionally increase the

far-ﬁeld limit and the spherical curvature of the electromagnetic

waves cannot be ignored anymore. This offers the opportunity to

realize spatial multiplexing even in line-of-sight channels. In this

paper, we overview and compare existing design options to realize

spatial multiplexing in line-of-sight multi-antenna channels.

Index Terms—Sub-terahertz, line-of-sight, multiple-input

multiple-output, metamaterials, holographic surfaces.

I. INTRODUCTION

The wireless research community has recently expressed

major interest in investigating the opportunities offered by the

sub-terahertz (sub-THz) band (30-300 GHz) for future mobile

communications [1]. At these high frequencies, point-to-point

multiple-input multiple-output (MIMO) links can support the

transmission of multiple data streams even in line-of-sight

(LoS) channels, by capitalizing on the large aperture of the

transceivers, the short transmission range between them, and

the small wavelength that characterizes sub-THz signals [2],

[3]. The transmission of multiple data streams on the same

physical resource is usually referred to as spatial multiplexing,

multimode communication, or high-rank transmission. At sub-

THz frequencies, the electromagnetic waves exhibit a distinct

spherical wavefront, which shifts traditional design paradigms

based on far-ﬁeld beamforming antenna-arrays towards near-

ﬁeld focused electrically-large surfaces [4], [5], offering the

opportunity of integrating communication and radar sensing

in a single transceiver as well [6].

LoS MIMO communication is not a new ﬁeld of research

and several works exist in the literature, e.g., [7], [8], [9].

However, conventional spatial multiplexing MIMO schemes

have inherently relied on the underlying existence of rich-

scattering channels. In point-to-point MIMO links character-

ized by large-aperture transceivers (also known as extra-large

MIMO or XL-MIMO), short distances among the antenna

arrays (short-range MIMO), and high frequencies, however,

multipath propagation may not be rich enough but, at the

same time, multipath propagation becomes not essential to

support multimode communications. In light of this emerging

trend in wireless communications, which is fueled by the

Fig. 1. Possible options to realize spatial multiplexing in LoS MIMO

channels: (top) MIMO-surfaces; (center) half-wavelength spaced MIMO-

arrays; (bottom) optimally-spaced MIMO-arrays.

interest in utilizing the sub-THz frequency band, in this

article we describe the main existing options to realize spatial

multiplexing LoS MIMO communications, and discuss their

advantages and limitations. For brevity, we focus our attention

on high signal-to-noise ratio (SNR) scenarios, where spatial

multiplexing is the desired choice. In general, it is known that

the rank of the channel needs to be optimized as a function

of the SNR [10].

Speciﬁcally, we discuss three main design principles and

architectures (see Fig. 1): (1) MIMO-surfaces (also known as

holographic surfaces) that are virtually continuous electromag-

netic objects whose array elements are spaced less than half-

wavelength; (2) half-wavelength spaced MIMO-arrays, which

is nowadays the typical implementation; and (3) optimally-

spaced MIMO-arrays, in which the element spacing is larger

than half-wavelength. For ease of discussion, we refer to

linear antenna-arrays and, therefore, we consider MIMO-lines

instead of MIMO-surfaces. MIMO-lines and MIMO-surfaces

are tightly intertwined [7], [11]. With the term antenna we

refer to each of the three implementations, considering the

antenna-array as a whole electromagnetic object.

II. SPATIAL MULTIPLEXING IN LOS MIMO CHANNELS

The possibility of supporting spatial multiplexing in LoS

MIMO channels depends on the interplay of

1) large-aperture antennas

2) short-range links

3) high carrier frequencies

and, speciﬁcally, on their relationship according to the concept

of Fraunhofer distance of an antenna, which determines the

arXiv:2210.08616v2 [cs.IT] 7 Feb 2023

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LoSMIMO-Arraysvs.LoSMIMO-SurfacesMarcoDiRenzo,DavideDardariy,andNicoloDecarlizUniversit´eParis-Saclay,CNRS,CentraleSup´elec,LaboratoiredesSignauxetSystemes,91192Gif-sur-Yvette,FranceyUniversityofBologna,DepartmentofElectrical,Electronic,InformationEngineering(DEI),40126Bologna,ItalyzItalianNatio...

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LoS MIMO-Arrays vs. LoS MIMO-Surfaces Marco Di Renzo Davide Dardariy and Nicol o Decarliz Universit e Paris-Saclay CNRS CentraleSup elec Laboratoire des Signaux et Syst emes 91192 Gif-sur-Yvette France

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