1 5G NR-LTE Coexistence Opportunities Challenges and Solutions

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5G NR-LTE Coexistence:

Opportunities, Challenges, and Solutions

Sneihil Gopal†‡, David Grifﬁth‡, Richard A. Rouil‡and Chunmei Liu‡

†Department of Physics, Georgetown University, USA

‡National Institute of Standards and Technology (NIST), USA

Emails: {sneihil.gopal, david.grifﬁth, richard.rouil, chunmei.liu}@nist.gov

ABSTRACT

5G New Radio (NR) promises to support di-

verse services such as enhanced mobile broadband

(eMBB), ultra-reliable low-latency communication

(URLLC), and massive machine-type communica-

tion (mMTC). This requires spectrum, most of

which is occupied by 4G Long Term Evolution

(LTE). Hence, network operators are expected to

deploy 5G using the existing LTE infrastructure

while migrating to NR. In addition, operators must

support legacy LTE devices during the migration, so

LTE and NR systems will coexist for the foreseeable

future. In this article, we address LTE-NR coexis-

tence starting with a review of both radio access

technologies. We then describe the contributions by

the 3rd Generation Partnership Project (3GPP) to

solving the coexistence issue and catalog the major

coexistence scenarios. Lastly, we introduce a novel

spectrum sharing scheme that can be applied to the

coexistence scenarios under study.

I. INTRODUCTION

The proliferation of multimedia applications and

the rapid digitalization of industries has led to an ex-

ponential growth in mobile data trafﬁc in the recent

years [1]. To cater to this increasing demand for data

trafﬁc, 3rd Generation Partnership Project (3GPP),

the global standards development organization, is

currently specifying a ﬁfth generation (5G) of radio

interface referred to as New Radio (NR) [2]. 5G NR

will support increasing trafﬁc demand and wireless

connectivity for a wide range of new applications

and use cases, such as automotive, healthcare, public

safety, and smart cities.

U.S. Government work, not subject to U.S. Copyright.

Fig. 1: 5G NR services, associated applications and

their respective requirements.

Speciﬁcally, as illustrated in Figure 1, NR will

support three main use cases: enhanced mobile

broadband (eMBB), ultra-reliable low-latency com-

munication (URLLC), and massive machine-type

communication (mMTC), each with different re-

quirements and applications. eMBB and URLLC

services require high data rates and high reliability

with low latency, respectively, which in turn require

large bandwidths such as what is available above

3 GHz. In addition, eMBB and mMTC services

require good coverage to ensure network access for

most users, which can be best achieved by operating

below 2 GHz. To provide high data rate, low latency,

and good coverage, 5G NR will operate in two

spectral bands: Frequency Range 1 (FR1) from

410 MHz to 7.125 GHz, previously known as sub-

6 GHz band, and Frequency Range 2 (FR2) from

24.25 GHz to 52.6 GHz, also known as mmWave

band.

Because almost all accessible low frequency

bands are occupied by existing 4G Long Term Evo-

lution (LTE), operators have two choices: acquire

arXiv:2210.03906v2 [cs.NI] 11 Oct 2022

new spectrum or refarm the existing spectrum from

LTE to NR. Both options are expensive and, since

majority of the trafﬁc in the near future will be

carried by LTE networks, refarming low frequency

bands from LTE without a corresponding increase

in NR devices will lead to congestion in the LTE

bands and degrade network performance. Therefore,

operators must leverage the existing 4G infrastruc-

ture as they migrate to NR, while providing services

to legacy devices. As a result, LTE and NR networks

will coexist for the foreseeable future.

In this article, we discuss the LTE-NR coexis-

tence issue with a focus on eMBB, mMTC, and

URLLC, in the context of Public Safety Communi-

cations (PSC) networks, Machine-Type Communi-

cations (MTC) networks, and Vehicle-to-Everything

(V2X) networks. We discuss the coexistence tech-

niques being developed by 3GPP, and examine the

major coexistence scenarios that can occur between

LTE and NR devices. Lastly, we propose a spectrum

sharing scheme and discuss how a network operator

could use the approach to support static and dy-

namic resource allocation for coexisting networks.

In Section II, we review the evolution of radio

access technologies from LTE to NR. In Section III,

we address the LTE-NR coexistence issue in differ-

ent applications under study, describe the scenarios

for each of them, and discuss 3GPP’s proposed

solutions. Lastly, in Section IV, we introduce a

novel and generic spectrum sharing scheme that is

applicable to the coexistence scenarios under study.

II. LTE TO NR: ANOVERVIEW

3GPP introduced LTE in Rel 8 with high spectral

efﬁciency, variable bandwidths up to 20 MHz, and

peak downlink (DL) and uplink (UL) data rates

of 300 Mbit/s and 75 Mbit/s, respectively. Rel 9

introduced Multiple-Input Multiple-Output (MIMO)

beam forming, multicast/broadcast services, and

location-based services.

In Rel 10, Rel 11, and Rel 12, also known as LTE-

Advanced, 3GPP introduced several new features.

In Rel 10, features such as carrier aggregation,

UL multiple antenna transmission, relaying, and

enhancements to multicast/broadcast services were

introduced, enabling peak DL and UL data rates

of 3 Gbit/s and 1.5 Gbit/s, respectively. Rel 11

introduced Coordinated Multi-Point (CoMP) trans-

mission/reception to improve coverage, cell-edge

throughput, and spectral efﬁciency. Rel 12 included

support for MTC and public safety services such as

Device-to-Device (D2D) communication, which is

an enabling feature for V2X communication.

Rel 13 and its successors are known as LTE-

Advanced (LTE-A) Pro. Rel 13 introduced extended

support for MTC through Narrowband-Internet of

Things (NB-IoT) and enhanced MTC (eMTC), en-

hancements to D2D to support advanced proxim-

ity services for public safety services, and spec-

tral efﬁciency enhancements via Full-Dimensional

multiple-input multiple-output (FD-MIMO). LTE-A

Pro brought enhancements in multiple dimensions.

It supported higher level of carrier aggregation and

Licensed-Assisted Access (LAA), which led to the

introduction of Gigabit LTE offering data rates up

to 2 Gbit/s. It introduced enhancements to NB-IoT

and MTC. It also expanded the reach of cellular

technology to vehicular communication and intro-

duced Cellular V2X (C-V2X), which is a crucial

element of autonomous driving. The enhancements

in Rel 15 made LTE-A Pro meet the International

Telecom Union’s (ITU) IMT-2020 requirements,

which entitled it to be referred to as 5G.

3GPP in Rel 15 designed 5G NR to address

a variety of usage scenarios requiring enhanced

data rates, latency, coverage, and reliability. The

key features of NR include ultra-lean design, spec-

trum ﬂexibility including operation in high fre-

quency bands, interworking between high and low-

frequency bands, and advanced antenna technolo-

gies. Rel 16 focused on URLLC and Industrial Inter-

net of Things (IIoT)-related enhancements, NR on

unlicensed bands (NR-U), and NR V2X. Recently,

3GPP completed Rel 17 which includes enhanced

support for IIoT, proximity services, and network

automation, as well as sidelink (SL) enhancements

for V2X and public safety. It extends NR operations

to frequencies beyond 52 GHz, which is anticipated

to lead to speciﬁcations in Rel 18. Figure 2 sum-

marizes the evolution of radio access technologies

from LTE to NR and beyond.

III. LTE-NR COEXISTENCE

5G NR will operate in FR1 and FR2 bands, and

coexist with legacy users, including LTE and radar

systems.In this article, we consider only LTE-NR

coexistence, with a focus on different applications of

eMBB, mMTC, and URLLC, i.e., PSC, MTC, and

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15GNR-LTECoexistence:Opportunities,Challenges,andSolutionsSneihilGopalyz,DavidGrifthz,RichardA.RouilzandChunmeiLiuzyDepartmentofPhysics,GeorgetownUniversity,USAzNationalInstituteofStandardsandTechnology(NIST),USAEmails:fsneihil.gopal,david.grifth,richard.rouil,chunmei.liug@nist.govABSTRACT5GNewRad...

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