LTE Backhaul E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 595

Veröffentlichungsjahr: 2015

Beschreibung

The aim of this book is to enable network planners to realize and maintain cost efficient LTE backhaul networks, which meet the necessary performance requirements. Through an introduction to the technology background, the economical modelling, the dimensioning theory, planning and optimization processes and relevant network management aspects, the reader shall obtain all relevant information to achieve good backhaul results in their own network environment.  It is aimed at network planners and other experts with responsibilities for LTE IP network dimensioning, LTE network planning, providing and managing leased lines, business management, LTE IP network operation and optimization. 

Leseprobe

LTE BACKHAUL

PLANNING AND OPTIMIZATION

Edited by

Esa Markus MetsäläJuha T.T. Salmelin

Nokia Networks, Espoo, Finland

This edition first published 2016© 2016 John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

LTE backhaul : planning and optimization / edited by Esa Markus Metsälä and Juha T.T. Salmelin.  pages cm Includes bibliographical references and index.

 ISBN 978-1-118-92464-8 (hardback)1. Long-Term Evolution (Telecommunications) 2. Telecommunication–Traffic. I. Metsälä, Esa Markus. II. Salmelin, Juha T.T. TK5103.48325.L7345 2015 621.3845′6–dc23

                       2015021968

A catalogue record for this book is available from the British Library.

List of Contributors

Gerald BedürftigNokia NetworksBerlin, Germany

Raimo KangasNokia NetworksTampere, Finland

Jouko KapanenNokia NetworksEspoo, Finland

Raija LiliusNokia NetworksEspoo, Finland

Esa Markus MetsäläNokia NetworksEspoo, Finland

José Manuel Tapia PérezNokia NetworksEspoo, Finland

Juha T.T. SalmelinNokia NetworksEspoo, Finland

Jari SaloNokia NetworksDoha, Qatar

Csaba VulkánNokia NetworksBudapest, Hungary

Gabriel WallerNokia NetworksEspoo, Finland

Foreword

With LTE, the mobile network has evolved into a 150+ Mbps per user high-speed always-on packet network. Next we will see high-speed LTE networks becoming available for even larger populations, and solving capacity and speed bottlenecks that users currently experience. For many of us, mobile broadband is the preferred and primary access to the Internet.

The competition for the hearts and minds of LTE subscribers makes the user experience increasingly critical. Understanding the technology behind the service is the key to business success. Delving into the details of LTE technology soon reveals many items that affect performance, allowing room for optimization—and differentiation—in the market.

In general, operators today have more choice and support than ever in choosing their strategy for LTE planning and optimization tasks, including IP and backhaul tasks. The myriad challenges operators face can be addressed by specific professional services, purchased from an expert organization, or issues can be solved by in-house professionals. Many large networks are operated as a service, and a continuum of possibilities exists, from traditional in-house operation to fully managed service operations, and everything in between.

Whatever the technology and business strategy of the operator, high-bandwidth LTE radio needs to be reflected in the IP backhaul. For the LTE backhaul, a number of new areas call for special attention, namely security, synchronization, availability, end-user QoS and dimensioning, to name a few.

LTE IP planning professionals depend on both LTE and IP knowledge, and greatly benefit from realistic guidance for their projects. This book is of great help when assessing technical and economical alternatives and when creating solid and reliable real-life backhaul designs for LTE success.

Igor LeprinceExecutive Vice President, Global ServicesNokia

Acknowledgments

The editors would first like to acknowledge the contributing authors, who are our colleagues at Nokia Networks: Gerald Bedürftig, Raimo Kangas, Jouko Kapanen, Raija Lilius, José Manuel Tapia Pérez, Jari Salo, Csaba Vulkán and Gabriel Waller. Your knowledge has been the most essential ingredient in this project.

For specific review comments and for bigger and smaller suggestions and contributions we would like to thank: Heikki Almay, Antti Pietiläinen, Jyri Putkonen, Eugen Wallmeier, Raimo Karhula, Pekka Koivistoinen, Zoltán Vincze, Péter Szilágyi, Balázs Héder, Attila Rákos, Gábor Horváth, Lajos Bajzik, Dominik Dulas, Michal Malcher, Puripong Thepchatri, Lasse Oka, Steve Sleiman, Taufik Siswanto, Matti Manninen (Elisa), Timo Liuska (Juniper Networks) and Mika Kivimäki.

Also, we would like to thank the team at John Wiley & Sons for very good cooperation and an easy editing process, especially Mark Hammond, Tiina Wigley, Sandra Grayson, Teresa Netzler, Tim Bettsworth and Victoria Taylor.

We appreciate the patience and support of our families and our authors’ families during the writing period.

We are grateful for comments and suggestions for improvements or changes that could be implemented in forthcoming editions of this book. This feedback can be sent to the editors’ email addresses: [email protected] and [email protected].

Esa Markus Metsälä and Juha T.T. SalmelinEspoo, Finland

List of Abbreviations

2G

second generation (mobile system)

3G

third generation (mobile system)

3GPP

Third Generation Partnership Program

ASN.1

Abstract Syntax Notation One

ABS

almost blank subframe

ACK

acknowledgement signal

AFxx

assured forwarding behavior group xx

AH

Authentication Header

AM

acknowledged mode

AMBR

Aggregate Maximum Bit Rate

AMR

adaptive multi-rate coding

ANR

automatic neighbor relation

AOM

administration of measurements

AP

Application Protocol

APN-AMBR

Access Point Name–Aggregate Maximum Bit Rate

AQM

active queue management

ARP

Address Resolution Protocol

ATM

asynchronous transfer mode

BBF

Broadband Forum

BC

boundary clock

BCMP

Baskett, Chandy, Muntz and Palacios

BE

best effort

BFD

bidirectional forwarding detection

BH

Backhaul, Busy Hour

BITW

bump in the wire

BMAP

batch Markovian arrival processes

BMCA

best master clock algorithm

BSC

base station controller

BSHR

bidirectional self-healing ring

BTS

base station

CA

carrier aggregation, certification authority

CAC

connection admission control

capex

capital expenditure

CBS

committed burst size

CDF

cumulative distribution function

CDMA

Code Division Multiple Access

CE

customer equipment

CET

carrier Ethernet

CIR

committed information rate

CLI

command line interface

CM

configuration management

CMP

Certificate Management Protocol

CoDel

controlled delay

CoMP

coordinated multi-point

CORBA

Common Object Request Broker Architecture

CoS

class of service

C-plane

control plane

CPE

customer premises equipment

CPU

central processing unit

CRC

cyclic redundancy check

CRL

certificate revocation list

CRS

common reference signals

CSFB

Compact Small Form-factor Pluggable

CSV

comma-separated values

CUBIC

TCP with cubic window increases function

CWDM

coarse wavelength division multiplexing

DC

dual connectivity

DCF

discounted cash flows

DCH

dedicated channel

DCN

data communications network

DHCP

Dynamic Host Configuration Protocol

DL

downlink

DNS

domain name system

DNU

do not use

DOCSIS

data over cable service interface specification

DoS

denial of service

DPD

dead peer detection

DSCP

differentiated services code point

DSL

digital subscriber line

DWDM

dense wavelength division multiplexing

DWRR

deficit weighted round robin

EAPS

Ethernet Automatic Protection Switching

EBS

excess burst size

ECMP

equal cost multipath

eCoMP

enhanced CoMP

EDGE

enhanced data rates for GSM evolution

EF

expedited forwarding

eICIC

enhanced inter-cell interference coordination

EIR

excess information rate

E-LAN

Ethernet service, multipoint-to-multipoint

E-line

Ethernet service, point-to-point

EMS

element management system

eNB

evolved NodeB

e2e

end-to-end

EPC

evolved packet core

ERP

Ethernet ring protection

ESM

EPS session management

ESP

Encapsulating Security Payload

E-tree

Ethernet service, point-to-multipoint

E-UTRAN

Evolved Universal Terrestrial Radio Access Network

EXP

experimental bits

FCAPS

fault, configuration, accounting, performance, security

FCFS

first come, first served

FDD

frequency division duplex

FD-LTE

full duplex LTE

FeICIC

further enhanced inter-cell interference coordination

FIFO

first in, first out

FTP

File Transfer Protocol

GbE

gigabit Ethernet

GBR

guaranteed bit rate

GE

gigabit Ethernet

G.Fast

up to Gigabit/s fast short distance digital subscriber line

GLONASS

Global Navigation Satellite System, Russia

GNSS

global navigation satellite system

GPON

gigabit-capable passive optical network

GPRS

general packet radio service

GPS

Global Positioning System

GSM

Global System for Mobile communications

GTP

general packet radio service Tunneling Protocol

GTP-U

general packet radio service Tunneling Protocol user

HARQ

hybrid automatic repeat request

HetNet

heterogeneous networks

HRM

hypothetical reference model

HSPA

high-speed packed access

HSRP

Hot Standby Router Protocol

HTML

Hypertext Markup Language

HTTP

Hypertext Transfer Protocol

ICIC

inter-cell interference coordination

ICMP

Internet Control Message Protocol

IEEE

Institute of Electrical and Electronics Engineers

IETF

Internet Engineering Task Force

IKE

Internet key exchange

IMS

IP Multimedia Subsystem

IMT-A

international mobile telecommunications advanced

impex

implementation expenditure

IP

Internet protocol

IPsec

Internet Protocol Security architecture

IRC

interference rejection combining

IRR

internal rate of return

ISD

inter-site distance

itag

video parameter classification

ITU

International Telecommunication Union

ITU-T

ITU Telecommunication Standardization Sector

IU

indoor unit

KPI

key performance indicator

L1

Layer 1 in Open Systems Interconnection data link layer

L2

Layer 2 in Open Systems Interconnection data link layer

L2 VPN

Layer 2 virtual private network

L3 VPN

Layer 3 virtual private network

LAG

link aggregation group

LAN

local area network

LDF

load distribution factor

LFA

loop-free alternate

LOS

line of sight

LSP

label switched path

LTE

long-term evolution

LTE-A

long term evolution advanced

M/G/R-PS

M/G/R Processor Sharing model

MAC

media access control

MAN

metropolitan area network

MAP

Markovian arrival processes

MBH

mobile backhaul

MBMS

Multimedia Broadcast Multicast Service

MEF

Metro Ethernet Forum

MeNB

master eNB

MGW

media gateway

MIB

management information base

MIMO

multiple input, multiple output

MLO

multilayer optimization

ML-PPP

multilayer point-to-point protocol

MME

mobile management entity

MPEG4

Moving Pictures Experts Group

M-plane

management plane

MPLS

multiprotocol label switching

MPLS TC

multiprotocol label switching traffic class

MPLS-TP

multiprotocol label switching traffic profile

MSP

multiplex section protection

MS-SPRING

multiplex section protection ring

MSTP

Multiple Spanning Tree Protocol

MTBF

mean time between failures

MTTR

mean time to repair

MTU

maximum transfer unit

MVI

multi-vendor interface

MWR

microwave radio

NaaS

network management system as a service

NAS

network application server

NETCONF

Network Configuration Protocol

NGMN

Next Generation Mobile Network

NG-SDH

Next Generation Synchronous Digital Hierarchy

nLOS

near line of sight

NLOS

non line of sight

NMS

network management system

non-GBR

non-guaranteed bit rate

NP

non-protected

NPV

net present value

NTP

Network Time Protocol

O&M

operation and maintenance

OAM

operations administration and maintenance

OC-3

optical carrier level 3

ODU

outdoor unit

OID

object identifier

opex

operational expenditure

OSPF

Open Shortest Path First

OSS

operation support system

OTDOA

observed time difference of arrival

OTT

over the top

OU

outdoor unit

P

protected (IPsec)

PDF

probability distribution function

PDH

plesiochronous digital hierarchy

PDN

public data network

PDP

packet data protocol

PDU

protocol data unit

PE

provider edge

PE–PE

provider edge to provider edge

P-GW

packet data network gateway

PHB

per-hop behaviors

PHY

physical layer

PKI

public key infrastructure

PLMN

public land mobile network

PM

performance monitoring

PON

passive optical network

ppb

parts per billion

PPP

point-to-point protocol

ppm

pulse per minute

pps

pulse per second

PRC

primary reference clock

PS HO

packet service handover

PSK

pre-shared key

PTP

Precision Time Protocol

QCI

quality of service class indicator

QNA

queuing network analyzer

QoE

quality of experience

QoS

quality of service

RA

radio access

RBID

radio bearer identification

RC

resource coordination

RE

range extension

RED

random early detection

RF

radio frequency

RFCs

request for comments

RLC

radio link control

RN

relay node

RNC

radio network controller

ROI

return on investment

RRC

radio resource control

RRH

remote radio head

RRM

radio resource management

RSTP

Rapid Spanning Tree Protocol

RTO

retransmission timeout timer

RTP

Real-time Transport Protocol

RTT

round trip time

RX

receive, receiver

S1

Interface between eNB and MME/S-GW

S1-AP

S1 Application Protocol

S1-MME

interface between eNB and MME

S1-U

interface between eNB and S-GW

SA

security association

SACK

selective acknowledgment

SCEP

Simple Certificate Enrollment Protocol

SCF

Small Cell Forum

SCTP

Stream Control Transmission Protocol

SDH

synchronous digital hierarchy

SEG

security gateway

SeNB

slave eNB

S-GW

serving gateway

SLA

service level agreement

SMS

short message service

SMTP

Simple Mail Transfer Protocol

SNMP

Single Network Management Protocol

SOA

service-oriented architecture

SOAP

Simple Object Oriented Access Protocol

SON

self-organizing network

SONET

synchronous optical network

SP

strict priority scheduling

SP-GW

combined node of S-GW and P-GW

S-plane

synchronization plane

SPQ

strict priority queuing

SRLG

shared risk link group

SRVCC

single radio-voice call continuity

SS7

signaling system 7

SSH

secure shell

SSL

secure sockets layer

SSM

synchronization status messages

STM

synchronous transport module

STP

Spanning Tree Protocol

SyncE

Synchronous Ethernet

TCP

Transmission Control Protocol

TDD

time division duplex

TD-LTE

time division duplex LTE

TDM eICIC

time domain enhanced inter-cell interference coordination

TFRC

Transmission Control Protocol-friendly rate control

TLS

Transport Layer Security protocol

TMN

Telecom Management Network

TTI

transmission time interval

TTL

Time to Live

TWAMP

Two-Way Active Measurement Protocol

TX

transmit, transmitter

UDP

User Datagram Protocol

UE

user equipment

UL

Uplink

U-plane

user plane

USB

universal serial bus

VDSL

very high bit rate digital subscriber line

VLAN

virtual local area network

VLL

virtual leased line

VoIP

voice over Internet protocol

VoLTE

voice over long-term evolution

VPLS

virtual private local area network service

VPN

virtual private network

VPWS

virtual private wire service

VRF

virtual routing and forwarding

VRRP

Virtual Router Redundancy Protocol

WACC

weighted average cost of capital

W-CDMA

Wideband Code Division Multiple Access

WDM

wavelength division multiplexing

WFQ

weighted fair queuing

WRR

weighted round robin

X2-AP

X2 application protocol

X2-U

interface between eNB and eNB

xDSL

“any kind of” digital subscriber line

XG-PON

10 gigabits/passive optical network

XML

Extensible Markup Language

XPIC

cross-polarization interference cancellation

1Introduction

Esa Markus Metsälä and Juha T.T. Salmelin

Nokia Networks, Espoo, Finland

1.1 To the reader

This book intends to offer guidelines and insight for long-term evolution (LTE) backhaul planning and optimization tasks and is aimed at technical professionals working in the field of network planning and operations. With LTE backhaul, several functional areas like synchronization, Quality of Service (QoS) and security, to name a few, require major new analysis, when designing for a high-performing and well-protected network. And in addition, the capacity needs of the LTE and LTE Advanced (LTE-A) radio typically mandate a major upgrade to the currently supported backhaul capacity, which often means introducing new backhaul links and technologies.

As with any network design project, several feasible and technically sound approaches exist. Many of the examples given in this text highlight topics that the authors find especially important. For every design, high-level goals are unique, as are the boundaries set for the project, and the examples should be tailored where necessary to match the individual design target. All of the views presented reflect the authors’ personal opinions and are not necessarily that of their employers.

The book aims to give an objective, standards-based view of the topics covered. Many of the LTE backhaul related aspects are, however, not written as binding standards. As such, there is room for different implementations, dependent on the capabilities of mobile network elements such as evolved NodeB (eNB), security gateways, backhaul elements and related management systems.

A basic command of LTE and Internet Protocol (IP) networking is useful for getting the most out of this book. Mobile backhaul, and its key services and functions, is discussed in greater detail in Metsälä and Salmelin (2012), which can be used as a reading companion.

The book’s chapters approach each major topic of LTE with illustrations, complementing these with both short examples, including questions with model answers, and a few longer case studies.

Network designs are also influenced by non-technical drivers, such as the budget available for the project. Strategic input and comparing alternative designs from the financial side is important, which is why these topics are covered in a separate chapter.

1.2 Content

The book is divided into eight chapters. Chapter 2 is a bird’s-eye view of LTE backhaul: what is it all about? While the book’s focus is on technical matters and planning advice, the financial modeling of LTE backhaul is discussed in Chapter 3. Backhaul dimensioning is a challenge, and the theoretical basis for backhaul dimensioning and end user application behavior is covered in Chapter 4. Chapter 5 covers planning advice in the form of guidelines and examples, while Chapter 6 focuses on two bigger walk-through network design cases. Network management of the backhaul network and its relation to the LTE radio network management is the topic of Chapter 7 and the book is summarized in Chapter 8.

1.3 Scope

The essential scope of the book is the planning of the LTE IP backhaul, with focus on the LTE-specific design requirements and how to meet these requirements using Ethernet, IP and other packet protocols, with security of the backhaul taken into account in all phases of the design.

In order to help dimensioning LTE backhaul, the theoretical basis for analyzing backhaul capacity needs is given. As well, several end user aspects related to Transmission Control Protocol (TCP) behavior over the LTE network are investigated, since these may heavily affect end user perception of the LTE service.

Network management systems are traditionally separate for the backhaul and for the LTE radio network; however, several benefits can be exploited from the integration of these tools, as discussed in a Chapter 7.

Detailed planning of backhaul physical layer technologies—like optical links, wavelength division multiplexing and wireless (microwave) links—would all need a book of their own to be properly covered, and such reference books exist, and those should be used as additional sources of knowledge for detailed planning with those technologies.

The standards for the radio technologies discussed in this book in relation to the evolution of LTE-A are those finalized by the Third Generation Partnership Program (3GPP) at the spring of 2015. Constructing design guidelines for functionalities where standardization is in progress is difficult; however, key LTE advanced functions and their foreseen impact to backhaul are included in section 2.7.

Reference

Metsälä E. and Salmelin J. (eds) (2012)

Mobile Backhaul

. John Wiley & Sons, Ltd, Chichester, UK, doi: 10.1002/9781119941019.

2LTE Backhaul

Gerald Bedürftig1, Jouko Kapanen2, Esa Markus Metsälä2 and Juha T.T. Salmelin

1Nokia Networks, Berlin, Germany

2Nokia Networks, Espoo, Finland

2.1 Introduction

This chapter gives an overview of different aspects of LTE backhaul. An introduction about the different elements of a backhaul network, the different end-to-end (e2e) services and the requirements of the LTE Mobile Access network are given. A short explanation of the different L1 possibilities for the defined network areas is also included. In addition, a prospective of future requirements and an overview of the relevant standards are provided. A general understanding of packet-based backhaul networks is a prerequisite for an understanding of this chapter. More detailed information can be found in Metsälä and Salmelin (2012).

With LTE, the backhaul network will be further extended toward the core network. For second-generation (2G) networks the backhaul consisted of an access part and in most cases one level of aggregation until the base station controller (BSC) was reached. Third generation (3G) further concentrated the radio network controller (RNC) site locations and so additional aggregation layers needed to be considered. With the elimination of the RNC in the LTE architecture, a further concentration of the mobility management entity (MME), serving gateway (S-GW) and packet data network gateway (P-GW)—which in most cases were combined as a serving and packet data network gateway (SP-GW)—was the consequence. In addition to multiple aggregation levels, parts of the IP backbone network may be passed to connect the eNBs with the core network elements. Figure 2.1 gives an overview of the nomenclature and Figure 2.2 shows the different network areas used in this book.

Figure 2.1 Network element symbols

Figure 2.2 Definition of network areas

As seen in Figure 2.2 the network that is the focus of this book will be the network between eNB and the respective core network elements. Backhaul elements are the network elements which are located between eNB and core network elements. Their purpose is simply to provide the relevant e2e services (see Section 2.2) by closing the geographic distance between the mobile nodes in a secure and cost-efficient manner.

In the future, “fronthaul” will be relevant for remote radio heads (RRH) or distributed antenna systems and “small cell” backhaul, which is about the backhauling of high-density base stations in urban areas. Currently, fronthaul still requires dedicated fibers. Other wireless technologies or even fronthaul via a shared switching network are being considered but are not yet ready. Section 2.8 gives a short overview of small cell backhaul planning. Additionally, in the future functionality like caching and distributed security features may become relevant.

2.2 LTE Backhaul Planes

Different kinds of user and control planes (C-planes) have to be transported across the backhaul network.

Figure 2.3 gives an overview of the logical planes which are needed. This section gives a first overview and all different planes will be described in detail in subsequent sections.

Figure 2.3 LTE related e2e planes of backhaul network

Most important are the user traffic S1 user plane (U-plane), which has to be transported from eNB to S-GW and P-GW (or combined SP-GW), and the control traffic S1 C-plane between eNB and MME. In addition, the management traffic which consists of all the data needed for FCAPS (fault, configuration, accounting, performance, security) support of the network elements and the optional X2 traffic consisting of user and control plane between neighboring eNBs has to be considered. The special role of the synchronization plane can be realized in various ways. Finally, additional traffic which is due to active measurements of either probes or inbuilt functionality to monitor the e2e performance of the backhaul network is relevant.

2.2.1 3GPP Planes and Protocol Stacks

This section gives a short overview of the different planes and protocol stacks. Respective security considerations are provided in Section 2.4.6.

2.2.1.1 S1-U Plane

The S1-U plane is used to transport user data between the eNB and the S- and P-GW using a general packet radio service tunneling protocol user (GTP-U). Each S1 bearer consists of a pair of GTP-U tunnels (one for the uplink [UL] and one for the downlink [DL]). The eNB performs mapping between radio bearer IDs (RBID) and GTP-U tunnel endpoints. The GTP-U protocol is defined in TS29.060 (3GPP, 2015a) and its position in the protocol stack is shown in Figure 2.4.

Figure 2.4 LTE U-plane protocol stack

2.2.1.2 S1-MME (S1-C Plane)

The S1-MME plane is used to transfer signaling information between the eNB and MME using S1-AP protocol TS36.413 (3GPP, 2015b). It is used for S1 bearer management, mobility and security handling and for the transport of network application server (NAS) signaling messages between the user equipment (UE) and MME. S1-MME protocol stack is shown in Figure 2.5.

Figure 2.5 LTE C-plane protocol stack

2.2.1.3 X2-U-Plane

The X2-U-plane is used for forwarding user data between the source eNB and target eNB during inter-eNB handovers. A GTP-U tunnel is established across the X2 between the source eNB and the target eNB. Thus the protocol stack is the same as for S1-U (Figure 2.4).

2.2.1.4 X2-C-Plane

The X2-C-plane is used for transferring signaling information between neighboring eNBs using the X2-application protocol (X2-AP) TS36.423 (3GPP, 2015c). This signaling is used for handovers and inter-cell radio resource management (RRM) signaling. An X2-C protocol stack is shown in Figure 2.5. X2-AP connection is established at the time neighbor relations are formed, i.e. at eNB startup or when neighbors are added by manual operation and maintenance (O&M) intervention, or by automatic neighbor relations. Prior to X2-AP setup, a Stream Control Transmission Protocol (SCTP) connection is initialized via the usual SCTP four-way handshake.

It is important to note that all the protocol stacks include IP. There is no other possibility given by the standard.

2.2.2 Synchronization Plane

Synchronization is important to assure proper functionality of the eNBs. In section 2.4.4 it is shown that different radio technologies have different synchronization requirements. Frequency synchronization has to be distinguished from phase and time synchronization. For frequency synchronization it is sufficient to transport information about the frequency across the backhaul network. This can be achieved by physical mechanisms (synchronous Ethernet, synchronous digital hierarchy [SDH]) or by Precision Time Protocol (PTP) based mechanisms. Another opportunity is to decouple this completely from the backhaul network and to use GNSS (global navigation satellite system) solutions. Figure 2.6 shows schematically two frequency-aligned signals.

Figure 2.6 Frequency aligned signals

In addition to frequency, the phase information includes additional information about the point in time a signal occurs. This is shown in Figure 2.7.

Figure 2.7 Phase and frequency aligned signals

The next complexity step would be the time alignment. A simplified picture is shown in Figure 2.8.

Figure 2.8 Time, phase and frequency aligned signals

Chapter 6 of Metsälä and Salmelin (2012) gives a much more detailed overview of the different possibilities to transport synchronization signals across the network and the respective standards and challenges. For a planner it is important to understand the fundamental difference between frequency and time and phase synchronization. Frequency synchronization is rather simple compared to phase and time synchronization, in that it is simple to plan the network and the impact of frequencies being out of specification is not as dramatic as when time synchronization is out of specification. Three possibilities are distinguished to achieve frequency synchronization:

physically based like plesiochronous digital hierarchy (PDH), SDH or synchronous Ethernet

algorithm-based PTP solutions

GNSS-based systems.

At first glance, the reliability of the physically based seems to be higher than for PTP solutions, as they do not depend on network performance. But this benefit can only be achieved if these networks are planned properly and all configuration and topology changes are well considered and updated in the synchronization plan. As many operators do not spend this effort once the network is deployed, their networks have synchronization problems. In many cases this does not affect one but a couple of base stations and these problems are static problems, i.e. they remain and will not be solved automatically. The quality of PTP solutions depends on the network performance. This increases the fluctuation of the signal and on a short timescale the frequency is not as stable as for the other solutions. On the other hand, the benefit is that all base stations are tuned independently and network problems typically only occur for a certain period. The algorithm will tune all the frequencies back to target value once the problem is resolved. Thus the PTP solution is much more robust and the effort which has to be spent to plan and maintain this solution is lower compared to any physical solution (in a volatile network environment with many changes happening in the network). In addition it has to be considered that base stations have very good oscillators. They can stay in holdover for several days without being out of specification.

For network-based phase and time synchronization the following challenges occur:

planning and maintenance challenges like for physically based solutions

network performance dependencies like for frequency PTP solutions

much shorter holdover times for base station oscillators

a significant impact on the overall performance of the base station in case they are out of specification.

It is obvious that the need for time and phase synchronization leads to the most challenging network requirements and, in the event of failure, has the biggest impact on the radio network performance. This combination of challenging requirements and huge impact makes it difficult to deploy the e2e synchronization solution and additional effort has to be spent to plan, implement and maintain the network.

Section 2.4.4 specifies the exact synchronization requirements for different LTE features.

2.2.3 Management Plane

The management plane (M-plane) is the interface between eNB and the O&M system. This interface is not specified in detail by 3GPP. Typically, the transport layer uses TCP. NTP (Network Time Protocol) uses User Datagram Protocol (UDP) instead of TCP. NTP is typically used to define the time of the base station which is used for time stamping alarms, performance counter, certification expiry and other notifications. This has nothing to do with the time synchronization mentioned in Section 2.2.2.

The M-plane consists of all data needed to manage and monitor the status of the network elements. From a planning perspective especially, the performance monitoring (PM) data are relevant. More and more data from network elements are collected to monitor the network status and to initiate proactive analysis. The volume of this traffic has to be estimated and the right point in time has to be defined to collect these data. A stable management connection is essential to avoid any unnecessary site visits and to assure continuous counter collections. The M-plane protocol stack is shown in Figure 2.9.

Figure 2.9 Management plane protocol stack

2.2.4 Active Monitoring Plane

The e2e performance of the backhaul network is important for the e2e quality of experience (QoE) of mobile services. Thus it is very important to monitor different e2e key performance indicators (KPIs). The most cost-efficient solution for this is to initiate active measurement traffic in different class of service (CoS) categories and to analyze the collected data. Although the amount of traffic should be small compared to user and control traffic, it is important to consider this properly. In addition to the measurement traffic itself, the M-plane traffic (especially collection of PM data) of dedicated probes has to be considered. The frequency and phase of active measurements may also have an impact on the performance of PTP algorithms in the network. PTP algorithms do not like signals that are sent with a correlated phase. For example, many dedicated messages which are sent phase synchronous with one pulse per second (pps) may fill certain buffers and this may have an impact on the performance of the PTP messages.

2.2.5 Security Control Plane

Setting up secured communications with Internet Protocol Security architecture (IPsec) tunnels requires control protocols in addition to the tunnels themselves. With public key infrastructure (PKI) architecture, certificates need to be fetched from a certification authority (CA). The protocol may be based on the Certificate Management Protocol (CMP) or Simple Certificate Enrollment Protocol (SCEP).

For IPsec, the IKE (Internet key exchange) protocol has to be used.

2.2.6 Control and User Plane of Additional Proprietary Applications

Vendors are working on additional proprietary functionality in the eNBs, for example the storage of local content for low-latency applications or the caching of frequently used information. These applications cause additional control and user traffic and have to be considered for proper dimensioning. On the other hand, they may also reduce the user traffic, for example in the case of caching.

Example 2.1

Question:

What are the key traffic types in the LTE backhaul?

Answer: U-plane traffic carries the mobile user packets encapsulated in GTP-U tunneling protocol for both the S1 and the X2 interfaces. U-plane bearers at S1 and X2 are managed by related signaling (C-plane) with S1-AP (S1 application protocol) and X2-AP. Both the U-plane and the C-plane may be protected by IPsec.

When synchronization is arranged by backhaul network, for example PTP, these packets need to be delivered to the eNB from the timing server. The packet timing mechanism is not specified by 3GPP so different implementations exist.

Network management traffic (O&M connectivity) allows remote management of the eNBs. As with synchronization, O&M channel implementation may vary but often relies on TCP. O&M traffic clearly needs to be cryptographically protected. Related to management, different measurement protocols typically need to be supported for monitoring and troubleshooting purposes.

To support the identification of nodes, and the security and protection of the network, PKI relies on certificates and on the use of protocols like CMP and SCEP. Additionally, IPsec is mandated by 3GPP for protection of the S1 and X2 interfaces and relies on the use of IKE as the IPsec C-plane protocol. Furthermore, there may exist vendor- or operator-specific applications.

2.3 Radio Features of LTE and LTE-A

2.3.1 LTE

The specifications for the initial LTE system that were defined in 3GPP’s Release 8 were intended to optimize the system for increased IP and data traffic compared to 2G and 3G. New air interface technology with single-carrier bandwidths up to 20 MHz enables data rates up to 50 Mbps in the UL and 150 Mbps in the DL (assuming category 4 UE).

Compared to 3G and high-speed packed access (HSPA) systems, the network architecture is simplified so that no separate radio network controller is needed. Instead most of the functionality is embedded in the base station, which is the only type of network element within the radio network. The base station eNB then interfaces directly with the core network and other eNBs, both of which features are different from 2G and 3G.

U-plane and C-plane functionality are clearly separated in the core network by having MME as the C-plane element and S-GW as the U-plane element. All this supports better scalability of the network: the radio network is flat, consisting only of eNBs, and the U-plane and C-plane capacities in the core network can be scaled up independently.

Scalability is also embedded in the air interface, since now carrier bandwidth may range from 1.4 MHz to 20 MHz.

The circuit switched core network is abandoned and, instead, the core network is completely IP based. And the SS7 (signaling system 7) signaling stack has also been retired. As the LTE radio does not support circuit switched services, the voice services need to be implemented with IP-based voice over long-term evolution (VoLTE) or, alternatively, circuit switched fallback to 2G/3G is possible.

For backhaul, all interfaces are by 3GPP definition IP protocol based and there are no existing alternatives. Because, unlike the controller (like the RNC in 3G), the air interface encryption has been terminated in the eNB and so now part of the mobile system traffic path (between the base station/eNB site and the core) is unsecure and needs some other means of encryption (e.g. IPsec). The X2 interface between eNBs means horizontal traffic streams between eNBs. Due to topological and other restrictions in practice, it is often arranged via a central point higher in the network. Nevertheless, the X2 type of traffic between neighbor base stations is new compared to 3G or 2G.

2.3.2 LTE-A

The drivers of 3GPP for LTE advanced (LTE-A) has been to further develop LTE to fulfill the International Telecommunication Union (ITU) requirements set for international mobile telecommunications advanced (IMT-A):

increased number of simultaneously active users

increased peak data rate

higher spectral efficiency

improved performance at cell edges.

Further drivers on mobile operator point of view may include aspects such as:

increased adoption of mobile broadband, and greater availability and choice in terms of devices

enhanced coverage (spreading across more locations), and increase in usage intensity

machine-to-machine communications.

LTE-A is enabled by new technologies and features, and enhancements to existing technologies, like carrier aggregation, MIMO (multiple input, multiple output), Coordinated Multi-Point (CoMP), Inband Relaying (relay nodes), and HetNets (heterogeneous networks).

2.3.2.1 Carrier Aggregation

Carrier aggregation (CA) allows up to five LTE Release 8 compatible component carriers being combined, each having a bandwidth of 1.4 MHz to 20 MHz and providing a maximum of 100 MHz aggregated bandwidth, and provides almost as high a spectrum efficiency and peak rates as single allocation does. The principle of CA is presented in Figure 2.10.

Figure 2.10 Principle of carrier aggregation

CA can be implemented either intra-band, meaning the component carriers belong to the same operating frequency band, or inter-band, meaning the component carriers belong to different operating frequency bands.