Advanced Tri-Sectoral Multi-User Millimeter-Wave Smart Repeater Kai Dong Silvia Mura Marouan Mizmizi Dario Tagliaferri and Umberto Spagnolini

2025-04-27 0 0 1006.57KB 6 页 10玖币

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Advanced Tri-Sectoral Multi-User Millimeter-Wave

Smart Repeater

Kai Dong, Silvia Mura, Marouan Mizmizi, Dario Tagliaferri, and Umberto Spagnolini

Dipartimento di Elettronica, Informazione e Bioingegneria (DEIB), Politecnico di Milano, Milan, Italy.

Email: {kai.dong, silvia.mura, marouan.mizmizi, dario.tagliaferri, umberto.spagnolini}@polimi.it

Abstract—Smart Repeaters (SR) can potentially enhance the

coverage in Millimeter-wave (mmWave) wireless communica-

tions. However, the angular coverage of the existing two-panel

SR is too limited to make the SR a truly cost-effective mmWave

range extender. This paper proposes the usage of a tri-sectoral

Advanced SR (ASR) to extend the angular coverage with respect

to conventional SR. We propose a multi-user precoder optimiza-

tion for ASR in a downlink multi-carrier communication system

to maximize the number of served User Equipments (UEs) while

guaranteeing constraints on per-UE rate and time-frequency

resources. Numerical results show the beneﬁts of the ASR against

conventional SR in terms of both cumulative spectral efﬁciency

and number of served UEs (both improved by an average factor

2), varying the system parameters.

Index Terms—Millimeter-wave, Smart Repeaters (SR), Am-

plify&Forward (AF), Tri-sectoral, Multi-User.

I. INTRODUCTION

The 5G network rollouts are in full swing globally, with

standardization advancing to address new market verticals

such as automotive, energy, and industrial internet of things

[1]. One of the main innovations of 5G and the primary

feature of future 6G networks is the high frequency, e.g.,

mmWave (30 -100 GHz), sub-THz, and THz bands [2].

Radio propagation at these frequencies is subject to a strong

path attenuation and is severely affected by link blockage [3].

Hence, communication is efﬁcient only for a limited range.

Thereby, a denser deployment of network infrastructures is

required. However, exclusively employing Base Stations (BSs)

represents a questionable strategy with prohibitive deployment

and maintenance costs. The recently introduced Reconﬁg-

urable Intelligent Surfaces (RISs) represent a low-cost solu-

tion to complement the network infrastructure, extending its

coverage [4] and bypassing blocked links [5]. However, RISs

are not fully mature for large-scale deployment, as several

technological challenges need to be addressed, e.g., the real-

time conﬁguration in high mobility [6].

Alternatively, Smart Repeaters (SR), a.k.a. network-

controlled repeaters, are being considered by the Third Genera-

tion Partnership Project (3GPP) in Release 18 for 5G networks

deployment to extend the coverage provided by the BSs [7].

The SRs can be regarded as the evolution of the classical

RF repeaters based on Amplify-and-Forward (AF) operation,

which is low-cost and easy to deploy [8]. The SRs use side

information to achieve a more intelligent AF operation in a

system with Time-Division Duplex (TDD) access and beam-

forming operation [9]. From the hardware perspective, SR

consists of two beamforming antennas (e.g., phased antenna

arrays), one oriented toward the serving BS and the other

toward the service area to be covered, as illustrated in Fig.

1a.

The key challenge addressed in this paper is the design

of the precoding/combining at the SR to enable multi-user

functionality while extending the network coverage. The recent

literature focuses on hybrid Multiple-Input Multiple-Output

(MIMO) architectures with several RF chains, which enable

high communication efﬁciency while increasing the deploy-

ment costs and the system complexity and introducing a delay

due to base-band processing. The authors in [10] proposed

a single-user multi-stream precoder and combiner in an AF

relay based on a hybrid antenna array. The proposed solution

requires multiple RF chains and Channel State Information

(CSI) at the relay. A similar architecture is considered in

[11]–[13]. Nevertheless, the channel from the BS to the SR

is highly sparse at mmWave bands [14], which restricts the

number of spatial streams that can be transmitted. In fact, in

practical deployments, the BS and the SR are mounted in over-

elevated positions, resulting in a Line-of-Sight (LoS) dominant

channel condition, which allows for a single spatial stream

transmission. This paper proposes a tri-sectoral Advanced SR

(ASR), composed of an antenna array oriented toward the BS

and two antenna arrays oriented toward two separated serving

areas, each having a ﬁeld of view of 120 deg, as depicted

in Fig. 1b. The two arrays serving the UEs can operate in

time or frequency division duplexing. The ASR is more than

just two SR as the ASR multi-user precoder is optimized to

maximize the number of served UEs in the coverage area

with constraints on the time-frequency employed resources

and on the minimum per-UE rate requirement. The proposed

solution is evaluated numerically against the conventional

two-panel SR, for an equal number of antennas, showing a

signiﬁcant gain in the achieved cumulative Spectral Efﬁciency

(SE) and the number of served UEs. In particular, the ASR

doubles the SE and the number of served UEs with respect to

the conventional SR at medium-to-low UE density in space.

Moreover, with a suitable trade-off between the maximization

of served UEs and the minimization of the employed slots,

the gain is even more evident.

Organization: The paper is organized as follows: Section II

outlines the system model, Section III describes the ASR de-

sign, with multi-user precoder optimization. Numerical results

are in Section IV while Section V concludes the paper.

arXiv:2210.04859v1 [eess.SP] 10 Oct 2022

dbr

(a) Conventional two-panel SR

ASR

dbr

(b) Advanced tri-panel SR

Fig. 1: Top view of the BS-UE downlink scenario assisted by: (a)

conventional SR; (b) proposed ASR. The conventional SR covers an

angular sector of 120 deg, while the ASR coverage is doubled. The

remaining 120 deg is covered by the BS.

Notation: Bold upper- and lower-case letters describe ma-

trices and column vectors. Matrix transposition and hermitian

are indicated respectively as ATand AH.is the element-

wise product between matrices, Inis the identity matrix

of size n. With a∼ CN(µ,C)we denote a multi-variate

circularly complex Gaussian random variable awith mean µ

and covariance C.1Ndenotes an all-ones column vector of

size N,E[·]is the expectation operator, while R,Cand Bstand

for the set of real complex, and Boolean numbers, respectively.

δnis the Kronecker delta.

II. SYSTEM MODEL

Let us consider an Orthogonal Frequency Division Mul-

tiplexing (OFDM) system with Nsubchannels, where a set

K(cardinality K) of User Equipments (UEs) is connected

to a BS through an ASR, as depicted in Fig. 1b (the direct

link between UEs and BS can be blocked). The KUEs are

uniformly and randomly distributed in the service areas A1

and A2of the ASR panels P1and P2, respectively, while ASR

panel P0is connected to the BS. Without loss of generality,

both BS and ASR are equipped with Uniform Linear Arrays

(ULA) of Mband Mrelements, respectively, while UEs are

equipped with a single antenna each. In the q-th time slot,

ruled by the BS, Kq≤KUEs (in the set Kq⊆ K) shall

be served through the ASR. On the BS-ASR link (panel P0

09/09/22 2

⋮

𝑀!𝑀"

⋮

ASR

sq,n

xq,n

qq,n

f(1)

f(2)

⋮

𝑀"

⋮

𝑀"

UE1

UEk

UE2

UEK

h(1)

h(2)

h(k)

h(K)

y(K)

q,n

y(k)

q,n

y(2)

q,n

y(1)

q,n

Hbr

Fig. 2: Block diagram of the ASR-aided DL system model.

in Fig. 1b), the k-th UE, k∈ Kq, is assigned by the BS

with the set of N(k)

q⊆ {1, ..., N}subchannels (of cardinality

|N(k)

q|=N(k)

q), such that

N(k)

q∪ N(`)

q=∅for k6=`(1)

and

k∈Kq

N(k)

q≤N(2)

where (1) and (2) denote the frequency multiplexing of the

KqUEs as for OFDM. On the n-th subcarrier, the symbol

to be transmitted is sq,n, uncorrelated between subcarriers,

i.e., Esq,ns∗

q,m=σ2

sδn−m. The signal received at the ASR

antenna P0can be expressed as

xq,n =wH

rHbrfbsq,n +wH

rnq,n =β sq,n + ˜nq,n (3)

where: Hbr ∈CMr×Mbdenotes the channel matrix between

the BS and ASR, fb∈CMb×1and wr∈CMr×1are the

frequency-independent Tx and Rx beamforming vectors at the

BS and the ASR, respectively, nq,n ∈CMr×1is the spatial

noise vector across ASR antennas, uncorrelated in space,

time and frequency, i.e., E[nq,nnH

`,m] = σ2

nIMrδq−`δn−m.

Notice that fband wrare ﬁxed and can be computed during

deployment of the BS and ASR, due to the static BS-P0link,

thus the channel amplitude βis constant.

The signal xq,n in (3) is divided into two streams, one for

each ASR panel (P1or P2), such that

xq,n =gqqq,nxq,n (4)

where qq,n ∈B2×1is a selection vector that allocates the

signal xq,n to either P1or P2, depending on the scheduling

strategy and ASR operating mode while gqis the ampliﬁcation

factor. The signal xq,n is then beam-forwarded to the UEs over

P1or P2. Hence, the signal received by the k-th UE can be

expressed as

y(k)

q,n =h(k)

qFqxq,n +z(k)

q,n

=gqβh(k)

qFqqq,nsq,n +gqh(k)

qFqqq,n ˜nq,n +z(k)

q,n

(5)

where h(k)

q∈C1×2Mris the channel vector between the ASR

antennas and the k-th UE, Fq∈C2Mr×2is the frequency-

independent precoding matrix applied at the ASR, and z(k)

q,n is

the additive Gaussian nose at the UE, such that E[z(k)

q,nz(k),∗

`,m ] =

σ2

zδq−`δn−m.

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AdvancedTri-SectoralMulti-UserMillimeter-WaveSmartRepeaterKaiDong,SilviaMura,MarouanMizmizi,DarioTagliaferri,andUmbertoSpagnoliniDipartimentodiElettronica,InformazioneeBioingegneria(DEIB),PolitecnicodiMilano,Milan,Italy.Email:{kai.dong,silvia.mura,marouan.mizmizi,dario.tagliaferri,umberto.spagnolini...

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Advanced Tri-Sectoral Multi-User Millimeter-Wave Smart Repeater Kai Dong Silvia Mura Marouan Mizmizi Dario Tagliaferri and Umberto Spagnolini

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