1. OEA000041 LTE SAE System Overview ISSUE 1.pdf

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LTE/SAE System Overview

Training Manual Contents

Issue 01 (2010-05-01) Huawei Proprietary and Confidential

Copyright Huawei Technologies Co., Ltd

i

Contents

1 Network Architecture ........................................................................................................... 1-2

1.1 Evolution of Cellular Networks .................................................................................................................... 1-3

1.1.1 Evolution of Cellular Networks ........................................................................................................... 1-3

1.1.2 3GPP Releases ..................................................................................................................................... 1-7

1.2 EPS Architecture ......................................................................................................................................... 1-13

1.2.1 User Equipment ................................................................................................................................. 1-14

1.2.2 Evolved Node B ................................................................................................................................. 1-16

1.2.3 Mobility Management Entity ............................................................................................................. 1-17

1.2.4 The Serving Gateway (S-GW) ........................................................................................................... 1-18

1.2.5 The PDN Gateway ............................................................................................................................. 1-18

1.3 E-UTRAN Protocol Stack Structure ........................................................................................................... 1-18

1.3.1 Uu Interface ....................................................................................................................................... 1-19

1.3.2 S1 Interface ........................................................................................................................................ 1-21

1.3.3 X2 Interface ....................................................................................................................................... 1-22

2 LTE Air Interface Principles ................................................................................................ 2-1

2.1 Principle of OFDM ....................................................................................................................................... 2-2

2.1.2 OFDM Symbol Mapping ..................................................................................................................... 2-6

2.1.3 Advantage 1 of OFDM: High Spectral Efficiency ............................................................................... 2-7

2.1.4 Advantage 2 of OFDM: Effectively Withstand Multi-Path ............ ............. .............. ............ .............. . 2-8

2.1.5 Advantage 3 of OFDM: Resistant to Frequency Selection Fading ...................................................... 2-9

2.1.6 Disadvantage 1 of OFDM: Vulnerable to Frequency Offset .............. ............. .............. ............ ......... 2-10

2.1.7 Disadvantage 2 of OFDM: High PAPR ............................................................................................. 2-11

2.1.8 OFDM Advantages and Disadvantages.............................................................................................. 2-11

2.2 Multiple Access and Duplex Technologies ................................................................................................. 2-12

2.3 Carrier Frequency and EARFCN ................................................................................................................ 2-22

2.3.1 LTE Release 8 Bands ......................................................................................................................... 2-22

2.4 LTE Frame Structures ................................................................................................................................. 2-24

2.4.1 LTE Frame Structure Type1-FDD ...................................................................................................... 2-25

2.4.2 LTE Frame Structure Type2-TDD...................................................................................................... 2-25

2.4.3 Cyclic Prefix ...................................................................................................................................... 2-27

2.4.4 LTE Resource Block Conception ....................................................................................................... 2-28

2.5 LTE Channel Structures .............................................................................................................................. 2-31


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2.5.1 Logical Channels ............................................................................................................................... 2-31

2.5.2 Transport Channels ............................................................................................................................ 2-33

2.5.3 Physical Channels .............................................................................................................................. 2-34

2.5.4 Radio Channels .................................................................................................................................. 2-34

2.5.5 Mapping Relationship between Physical Channels and Other Channels ....................... .............. ...... 2-35

2.5.6 Application of LTE Physical Channels .............................................................................................. 2-36

2.5.7 Cell Specific Reference Signals ......................................................................................................... 2-37

2.5.8 LTE Physical Signals ......................................................................................................................... 2-39

2.5.9 Downlink Reference Signals .............................................................................................................. 2-39

2.6 Physical Procedures .................................................................................................................................... 2-40

2.6.1 LTE Cell Search Procedure ................................................................................................................ 2-40

2.6.2 Cell Search ......................................................................................................................................... 2-41

2.6.3 PLMN Selection ................................................................................................................................. 2-45

2.6.4 Random Access Procedure Overview ................................................................................................ 2-47

2.7 Multiple Input Multiple Output ................................................................................................................... 2-49

2.7.1 Background of Multi-antenna Technology ......................................................................................... 2-49

2.7.2 The Classification of Multi-antenna Technology ............................................................................... 2-50

2.7.3 MIMO Overview ............................................................................................................................... 2-51

2.7.4 The Advantage of MIMO ................................................................................................................... 2-55

3 eNB Product Overview ......................................................................................................... 3-1

3.1 The Huawei eNB Family ............................................................................................................................... 3-2

3.1.1 BTS3900(A) LTE ................................................................................................................................. 3-2

3.1.2 DBS3900 LTE ...................................................................................................................................... 3-4

3.2 Products and Application Scenarios .............................................................................................................. 3-5

3.2.1 BTS3900(A) LTE ................................................................................................................................. 3-5

3.2.2 DBS3900 LTE ...................................................................................................................................... 3-6

3.3 Operation and Maintenance .......................................................................................................................... 3-6

3.3.1 The Operations and Maintenance System ............................................................................................ 3-6

3.3.2 Benefits ................................................................................................................................................ 3-7

4 Glossary .................................................................................................................................. 4-9


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Figures

Figure 1-1Evolution of Cellular Networks ........................................................................................................ 1-3

Figure 1-2Second Generation Mobile Systems ................................................................................................. 1-4

Figure 1-3Third Generation Mobile Systems .................................................................................................... 1-6

Figure 1-4Fourth Generation Mobile Systems .................................................................................................. 1-7

Figure 1-5 3GPP Releases .................................................................................................................................. 1-7

Figure 1-6HSDPA (Release 5) ........................................................................................................................... 1-9

Figure 1-7HSUPA (Release 6) ........................................................................................................................... 1-9

Figure 1-8HSPA+ (Release 7) ......................................................................................................................... 1-10

Figure 1-9Release 8 HSPA+ and LTE ............................................................................................................. 1-11

Figure 1-10Release 9 and Beyond ................................................................................................................... 1-11

Figure 1-11LTE Reference Architecture .......................................................................................................... 1-13

Figure 1-12EPS Network Architecture-2G/3G Co-existence .......................................................................... 1-14

Figure 1-13User Equipment Functional Elements .......................................................................................... 1-15

Figure 1-14Evolved Node B Functional Elements .......................................................................................... 1-17

Figure 1-15E-UTRAN Interfaces .................................................................................................................... 1-19

Figure 1-16Uu Interface Protocols .................................................................................................................. 1-19

Figure 1-17S1 Interface Protocols ................................................................................................................... 1-21

Figure 1-18X2 Interface Protocols .................................................................................................................. 1-23

Figure 2-1Use of OFDM in LTE ....................................................................................................................... 2-2

Figure 2-2Frequency Division Multiple ............................................................................................................ 2-3

Figure 2-3Time Division Multiple ..................................................................................................................... 2-3

Figure 2-4Code Division Multiple .................................................................................................................... 2-3

Figure 2-5FDM Carriers .................................................................................................................................... 2-4

Figure 2-6OFDM Subcarriers ............................................................................................................................ 2-4

Figure 2-7Inverse Fast Fourier Transform ......................................................................................................... 2-5

Figure 2-8Fast Fourier Transform ..................................................................................................................... 2-5


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Figure 2-9OFDM Symbol Mapping .................................................................................................................. 2-6

Figure 2-10OFDM PAPR (Peak to Average Power Ratio) ....................... .............. ............. ............ .............. .... 2-7

Figure 2-11Multicarrier modulation technology ............................................................................................... 2-7

Figure 2-12Delay Spread ................................................................................................................................... 2-8

Figure 2-13Cyclic Prefix ................................................................................................................................... 2-9

Figure 2-14Resistant to Frequency Selection Fading ........................................................................................ 2-9

Figure 2-15Vulnerable to Frequency Offset .................................................................................................... 2-10

Figure 2-16Multi-carrier system signal process procedure ............................................................................. 2-11

Figure 2-17Radio Interface Techniques ........................................................................................................... 2-12

Figure 2-18Frequency Division Multiple Access ............................................................................................ 2-12

Figure 2-19Time Division Multiple Access ..................................................................................................... 2-13

Figure 2-20Code Division Multiple Access .................................................................................................... 2-13

Figure 2-21Orthogonal Frequency Division Multiple Access ......................................................................... 2-14

Figure 2-22The comparison between DM and DMA ...................................................................................... 2-15

Figure 2-23From FDM/FDMA to OFDM/OFDMA ........................................................................................ 2-16

Figure 2-24SC-FDMA Subcarrier Mapping Concept ...................................................................................... 2-18

Figure 2-25SC-FDMA Signal Generation ....................................................................................................... 2-19

Figure 2-26SC-FDMA and the eNB ................................................................................................................ 2-19

Figure 2-27Frequency Division Duplex .......................................................................................................... 2-20

Figure 2-28Time Division Duplex ................................................................................................................... 2-21

Figure 2-29TDD: The uplink and downlink use different slots. ...................................................................... 2-21

Figure 2-30FDD: The uplink and downlink use different frequencies. ........................ .............. ............ ......... 2-22

Figure 2-31EARFCN Calculation ................................................................................................................... 2-24

Figure 2-32Example Downlink EARFCN Calculation ................................................................................... 2-24

Figure 2-33FDD Radio Frame......................................................................................................................... 2-25

Figure 2-34TDD Radio Frame ........................................................................................................................ 2-26

Figure 2-35Special Subframe .......................................................................................................................... 2-27

Figure 2-36Normal and Extended Cyclic Prefix ............................................................................................. 2-27

Figure 2-37 CP classification ........................................................................................................................... 2-28

Figure 2-38LTE resource block ....................................................................................................................... 2-28

Figure 2-39Resource Block and Resource Element ........................................................................................ 2-30

Figure 2-40Relationship between Channel BW and RB ................................................................................. 2-31

Figure 2-41LTE Channels ............................................................................................................................... 2-31


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Figure 2-42Location of Channels .................................................................................................................... 2-31

Figure 2-43BCCH and PCH Logical Channels ............................................................................................... 2-32

Figure 2-44CCCH and DCCH Signaling ........................................................................................................ 2-32

Figure 2-45Dedicated Traffic Channel ............................................................................................................ 2-33

Figure 2-46LTE Release 8 Transport Channels ............................................................................................... 2-34

Figure 2-47 Radio Channel .............................................................................................................................. 2-35

Figure 2-48Mapping Relationship between Physical Channels and Other Channels ............. ............ ............. 2-35

Figure 2-49Application of LTE Physical Channels ......................................................................................... 2-36

Figure 2-50Reference Signals - One Antenna Port .............. ............. ............ .............. ............. .............. .......... 2-37

Figure 2-51Reference Signal Physical Cell ID Offset ..................................................................................... 2-38

Figure 2-52Reference Signals - Two Antenna Ports (Normal CP) .................. ............. .............. ............ ......... 2-38

Figure 2-53Reference Signals - Four Antenna Ports (Normal CP).................................................................. 2-38

Figure 2-54Downlink Cell ID ......................................................................................................................... 2-39

Figure 2-55 Initial Procedures .......................................................................................................................... 2-40

Figure 2-56PSS and SSS for Cell Search (FDD Mode) .................................................................................. 2-41

Figure 2-57PSS and SSS Location for FDD.................................................................................................... 2-42

Figure 2-58PSS and SSS Location for TDD ................................................................................................... 2-42

Figure 2-59Downlink Cell ID ......................................................................................................................... 2-43

Figure 2-60Physical Cell Identities ................................................................................................................. 2-43

Figure 2-61System information scheduling..................................................................................................... 2-44

Figure 2-62Contents of System Information ................................................................................................... 2-44

Figure 2-63 PLMN Selection ........................................................................................................................... 2-45

Figure 2-64LTE Cell Selection ........................................................................................................................ 2-47

Figure 2-65Overall Random Access Procedure ............................................................................................... 2-48

Figure 2-66Random Access RRC Signaling Procedure .................................................................................. 2-48

Figure 2-67Uplink synchronization ................................................................................................................. 2-49

Figure 2-68The relationship between spectrum efficiency of channel and signal power & signal bandwidth 2-50

Figure 2-69Tx diversity mode ......................................................................................................................... 2-50

Figure 2-70Spatial multiplexing mode ............................................................................................................ 2-51

Figure 2-71Beamforming mode ...................................................................................................................... 2-51

Figure 2-72 MIMO ........................................................................................................................................... 2-52

Figure 2-73 SISO ............................................................................................................................................. 2-52

Figure 2-74 MISO ............................................................................................................................................ 2-52


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Figure 2-75 SIMO ............................................................................................................................................ 2-53

Figure 2-76 MIMO ........................................................................................................................................... 2-53

Figure 2-77SU-MIMO, MU-MIMO and Co-MIMO ....................................................................................... 2-54

Figure 2-78The advantage of MIMO .............................................................................................................. 2-55

Figure 3-1BTS3900(A) LTE Architecture ......................................................................................................... 3-3

Figure 3-2 BBU3900 .......................................................................................................................................... 3-3

Figure 3-3 LRFU ................................................................................................................................................ 3-4

Figure 3-4DBS3900 LTE Architecture .............................................................................................................. 3-4

Figure 3-5 RRU .................................................................................................................................................. 3-5

Figure 3-6O&M System .................................................................................................................................... 3-7


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Tables

Table 1-12G, 2.5G and 2.75G GSM/GPRS Systems ......................................................................................... 1-5

Table 1-2IMT Advanced Features ..................................................................................................................... 1-6

Table 1-3UE Categories ................................................................................................................................... 1-15

Table 2-1LTE Channel and FFT Sizes ............................................................................................................... 2-6

Table 2-2SC-FDMA verses OFDMA .............................................................................................................. 2-20

Table 2-3LTE Release 8 Frequency Bands ...................................................................................................... 2-22

Table 2-4DL/UL Subframe Allocation Item .................................................................................................... 2-26

Table 2-5Special Subframe Allocation Item .................................................................................................... 2-27

Table 2-6Channel bandwidth and RB .............................................................................................................. 2-30

Table 2-7LTE DL/UL MIMO mode ................................................................................................................ 2-54


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1Network Architecture

Objectives

On completion of this section the participants will be able to:

1.1 Describe the evolution of cellular networks.

1.1.2 Summarize the evolution of 3GPP releases, from Release 99 to Release 9 and beyond.

Error! No se encuentra el origen de la referencia.Explain the logical architecture of theE-UTRAN.

1.3 Describe the interfaces and associated protocols within the E-UTRAN.

Error! No se encuentra el origen de la referencia.Explain the logical architecture of the

EPS.


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1.1 Evolution of Cellular Networks

1.1.1 Evolution of Cellular Networks

Cellular mobile networks have been evolving for many years. The initial systems, which are

referred to as First Generation, now had been replaced with Second Generation andThird Generation solutions. However today, 4G or Fourth Generation systems are nowbeing deployed.

Figure 1-1Evolution of Cellular Networks

First Generation Mobile Systems

The 1G (First Generation) mobile systems were not digital, i.e. they utilized analoguemodulation techniques. The main systems included:

AMPS (Advanced Mobile Telephone System) - This first appeared in 1976 in the United

States and was mainly implemented in the Americas, Russia and Asia. Various issues

including weak security features made the system prone to hacking and handset cloning.

TACS (Total Access Communications System) - This was the European version of

AMPS but with slight modifications including the operation on different frequency

bands. It was mainly used in the United Kingdom, as well as parts of Asia.

ETACS ((Extended Total Access Communication System) - This provided an improved

version of TACS. It enabled a greater number of channels and therefore facilitated moreusers.

These analogue systems were all proprietary based FM (Frequency Modulation) systems and

therefore they all lacked security, any meaningful data service and international roaming

capability.

Second Generation Mobile Systems

2G (Second Generation) systems utilize digital multiple access technology, such as TDMA

(Time Division Multiple Access) and CDMA (Code Division Multiple Access). Figure 1-2

illustrates some of the different 2G mobile systems including:

GSM (Global System for Mobile communications) - this is the most successful of all 2G

technologies. It was initially developed by ETSI (European Telecommunications


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Standards Institute) for Europe and designed to operate on the 900MHz and 1800MHz

frequency bands. It now has world-wide support and is available for deployment on

many other frequency bands, such as 850MHz and 1900MHz. A mobile described as tri

band or quad band indicates support for multiple frequency bands on the same device.GSM utilizes TDMA and as such, it employs 8 timeslots on a 200kHz radio carrier.

cdmaOne - this is a CDMA (Code Division Multiple Access) system based on the IS-95

(Interim Standard 95). It uses a spread spectrum technique which incorporates a mixtureof codes and timing to identify cells and channels. The system bandwidth is 1.25MHz.

D-AMPS (Digital - Advanced Mobile Phone System) - this is based on the IS-136

(Interim Standard 136) and is effectively an enhancement to AMPS. Supporting a TDMA

access technique, D-AMPS is primarily used on the North American continent, as well asin New Zealand and parts of the Asia-Pacific region.

Figure 1-2Second Generation Mobile Systems

In addition to being digital, with the associated improvements in capacity and security, these

2G digital systems also offer enhanced services such as SMS (Short Message Service) and

circuit switched data.

2.5G Systems

Most 2G systems have now been evolved. For example, GSM was extended with GPRS

(General Packet Radio System) to support efficient packet data services, as well as increasingthe data rates.

As this feature does not meet 3G requirements, GPRS is therefore often referred to as 2.5G. Acomparison been 2G and 2.5G systems is illustrated in Table 1-1.

2.75G Systems

GSM/GPRS systems also added EDGE (Enhanced Data Rates for Global Evolution). Thisnearly quadruples the throughput of GPRS. The theoretical data rate of 473.6kbit/s enablesservice providers to efficiently offer multimedia services. Like that of GPRS, EDGE is

usually categorized as 2.75G as it does not fulfill all the requirements of a 3G system.


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Table 1-12G, 2.5G and 2.75G GSM/GPRS Systems

System Service Theoretical Data Rate Typical Data Rate

2G GSM Circuit Switched 9.6kbit/s or 14.4kbit/s 9.6kbit/s or 14.4kbit/s

2.5G GPRS Packet Switched 171.2kbit/s 4kbit/s to 50kbit/s

2.75G EDGE Packet Switched 473.6kbit/s 120kbit/s

Third Generation Mobile Systems

3G (Third Generation) systems, which are defined by IMT2000 (International Mobile

Telecommunications - 2000), state that they should be capable of providing highertransmission rates, for example: 2Mbit/s for stationary or nomadic use and 348kbit/s in a

moving vehicle.

The main 3G technologies are illustrated in Figure 1-3.These include:

W-CDMA (Wideband CDMA) - This was developed by the 3GPP (Third GenerationPartnership Project). There are numerous variations on this standard, including

TD-CDMA and TD-SCDMA. W-CDMA is the main evolutionary path from GSM/GPRSnetworks. It is a FDD (Frequency Division Duplex) based system and occupies a 5MHz

carrier. Current deployments are mainly at 2.1GHz, however deployments at lower

frequencies are also being seen, e.g. UMTS1900, UMTS900, UMTS850 etc. W-CDMAsupports voice and multimedia services with an initial theoretical rate of 2Mbit/s

however, most service providers were initially offering 384kbit/s per user. This

technology is continuing to evolve and later 3GPP releases have increased the rates to inexcess of 40Mbit/s.

TD-CDMA (Time Division CDMA) - This is typically referred to as UMTS TDD (TimeDivision Duplex) and is part of the UMTS specifications, however it has only limitedsupport. The system utilizes a combination of CDMA and TDMA to enable efficient

allocation of resources.

TD-SCDMA (Time Division Synchronous CDMA) - This was jointly developed bySiemens and the CATT (China Academy of Telecommunications Technology).TD-SCDMA has links to the UMTS specifications and is often identified as UMTS-TDD

LCR (Low Chip Rate). Like TD-CDMA, it is also best suited to low mobility scenariosin micro or pico cells.

CDMA2000 - This is a multi-carrier technology standard which uses CDMA.

CDMA2000 is actually a set of standards including CDMA2000 EV-DO

(Evolution-Data Optimized) which has various revisions. It is worth noting thatCDMA2000 is backward compatible with cdmaOne.


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Figure 1-3Third Generation Mobile Systems

WiMAX (Worldwide Interoperability for Microwave Access) - This is another wireless

technology which satisfies IMT2000 3G requirements. The air interface is part of the

IEEE (Institute of Electrical and Electronics Engineers) 802.16 standard which originallydefined PTP (Point-To-Point) and PTM (Point-To-Multipoint) systems. This was laterenhanced to provide mobility and greater flexibility. The success of WiMAX is mainlydown to the WiMAX Forum, an organization formed to promote conformity and

interoperability between vendors.

Fourth Generation Mobile Systems

4G (Fourth Generation) cellular wireless systems need to meet the requirements set out by theITU (International Telecommunication Union) as part of IMT Advanced (International Mobile

Telecommunications Advanced). Illustrated in Table 1-2, these features enable IMT Advanced

to address evolving user needs.

Table 1-2IMT Advanced FeaturesKey IMT Advanced Features

A high degree of common functionality worldwide while retaining the flexibility to support

a wide range of services and applications in a cost efficient manner.

Compatibility of services within IMT and with fixed networks.

Capability of interworking with other radio access systems.

High quality mobile services.

User equipment suitable for worldwide use.

User-friendly applications, services and equipment.

Worldwide roaming capability.

Enhanced peak data rates to support advanced services and applications (100Mbit/s for high

and 1Gbit/s for low mobility were identified as targets).

The three main 4G systems include:


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LTE Advanced - LTE (Long Term Evolution) is part of 3GPP family of specifications,

however it does not meet all IMT Advanced features, as such it is sometimes referred to

as 3.99G. In contrast, LTE Advanced is part of a later 3GPP Release and this has been

designed specifically to meet 4G requirements.

WiMAX 802.16m - The IEEE and the WiMAX Forum have identified 802.16m as theiroffering for a 4G system.

UMB (Ultra Mobile Broadband) - This is identified as EV-DO Rev C. It is part of 3GPP2however most vendors and service providers have decided to promote LTE instead.

Figure 1-4Fourth Generation Mobile Systems

1.1.2 3GPP Releases

The development of GSM, GPRS, EDGE, UMTS, HSPA and LTE is in stages known as 3GPP

Releases. Hardware vendors and software developers use these releases as part of their

development roadmap. Figure 1-5 illustrates the main 3GPP Releases that included keyenhancements of the radio interface.

Figure 1-53GPP Releases

GSM

9.6kbit/s

GPRS

171.2kbit/s

EDGE

473.6kbit/s

UMTS

2Mbit/s

HSDPA

14.4Mbit/s

HSUPA

5.76Mbit/s

HSPA+

28.8Mbit/s

42Mbit/s

LTE

+300Mbit/s

LTEAdvanced

3GPP Releases enhance various aspects of the network and not just the radio interface. Forexample, Release 5 started the introduction of the IMS (IP Multimedia Subsystem) in the core

network.


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Pre-Release 99

Pre-Release 99 saw the introduction of GSM, as well as the addition of GPRS. The main

GSM Phases and 3GPP Releases include:

GSM Phase 1.

GSM Phase 2. GSM Phase 2+ (Release 96).

GSM Phase 2+ (Release 97).

GSM Phase 2+ (Release 98).

Release 99

3GPP Release 99 saw the introduction of UMTS, as well as the EDGE enhancement to GPRS.

UMTS contains all the features needed to meet the IMT-2000 requirements as those defined

by the ITU. It is able to support CS (Circuit Switched) voice and video services, as well as PS(Packet Switched) data services over common and dedicated bearers. Initial data rates for

UMTS were 64kbit/s, 128kbit/s and 384kbit/s. Note that the theoretical maximum was

2Mbit/s.

Release 4

Release 4 included enhancements to the core network and in particular the notion of it being

bearer independent. Thus the concept of All IP Networks was included and service

providers were able to deploy Soft Switch based networks, i.e. the MSC (Mobile SwitchingCentre) was replaced by the MSC Server and MGW (Media Gateways). This improved

network utilization in addition to consolidating engineering knowledge and increasing vendor

competition.

Release 5

Release 5 introduces the first major addition to the UMTS air interface by specifying HSDPA(High Speed Downlink Packet Access) in order to improve both capacity and spectral

efficiency. Figure 1-6 illustrates some of the main features associated with Release 5 and

these include:

Adaptive Modulation - In addition to the original UMTS modulation scheme of QPSK(Quadrature Phase Shift Keying), HSDPA also includes support for 16 QAM

(Quadrature Amplitude Modulation).

Flexible Coding - Based on fast feedback from the mobile in the form of a CQI (ChannelQuality Indicator), the UMTS base station, i.e. the Node B, is able to modify the

effective coding rate and thus increase system efficiency.

Fast Scheduling - HSDPA includes a 2ms TTI (Time Transmission Interval) which

enables the Node B scheduler to quickly and efficiently allocate resources to mobiles.

HARQ (Hybrid Automatic Repeat Request) - In the event a packet does not get throughto the UE (User Equipment) successfully, the system employs HARQ. This improves the

retransmission timing, thus requiring less reliance on the RNC (Radio NetworkController).


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Figure 1-6HSDPA (Release 5)

Release 6

Release 6 adds various features, with HSUPA (High Speed Uplink Packet Data) being of mostinterest to RAN development. Even though the term HSUPA is widespread, this 3GPP

enhancement also goes under the term Enhanced Uplink. It is also worth noting thatHSDPA and HSUPA work in tandem and thus the term HSPA (High Speed Packet Access) is

now in common use.

HSUPA, like HSDPA adds functionality to improve packet data. Figure 1-7 illustrates the

three main enhancements which include:

Flexible Coding - HSUPA has the ability to dynamically change the coding and therefore

improve the efficiency of the system.

Fast Power Scheduling - A key fact of HSUPA is that it provides a method to schedulethe power from different mobiles. This scheduling can use either a 2ms or 10ms TTI.

HARQ - Like HSDPA, HSUPA also utilizes HARQ. The main difference is the timing

relationship for retransmissions.

Figure 1-7HSUPA (Release 6)

Enhancements introduced in Release 6 are not limited to HSUPA. For example, GAN

(Generic Access Network) technologies are also included which enables alternative radio

access technologies such as Wi-Fi (Wireless Fidelity) to be used yet still support trueinterworking.

Although no longer the correct terminology, UMA (Unlicensed Mobile Access) is still in common use todescribe the 3GPPs GAN technology.


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Release 7

The main RAN based feature of Release 7 is HSPA+. This, like HSDPA and HSUPA,

provides various enhancements to improve packet switched data delivery. Figure 1-8illustrates the main features which include:

64 QAM - This is available in the DL (Downlink) and enables HSPA+ to operate at atheoretical rate of 21.6Mbit/s.

16 QAM - This is available in the UL (Uplink) and enables the uplink to theoretically

achieve 11.76Mbit/s.

MIMO (Multiple Input Multiple Output) Operation - this is added to HSPA+ Release 7and offers various benefits including the ability to offer a theoretical 28.8Mbits/s in the

downlink.

Figure 1-8HSPA+ (Release 7)

Power Enhancements -Various enhancements such as CPC (Continuous PacketConnectivity) have been included. This includes DTX (Discontinuous Transmission),

DRX (Discontinuous Reception) and HS-SCCH (High Speed - Shared Control Channel)

Less Operation etc. Collectively these improve the mobiles battery consumption.

Less Overhead - The downlink includes an enhancement to the MAC (Medium Access

Control) layer which effectively means that fewer headers are required. This in turnreduces overhead and thus improves the system efficiency.

Release 8

There are many additions to the RAN functionality in Release 8, such as an enhancement to

HSPA+. However the main aspect is the inclusion of LTE (Long Term Evolution). Figure 1-9

illustrates some of the main features for Release 8 HSPA+ and LTE.

Release 8 HSPA+ enables various key enhancements, these include:

64 QAM and MIMO - Release 8 enables the combination of 64 QAM and MIMO, thusquoting a theoretical rate of 42Mbit/s, i.e. 2 x 21.6Mbit/s.

Dual Cell Operation - DC-HSDPA (Dual Cell - HSDPA) is a Release 8 feature which isfurther enhanced in Release 9 and Release 10. It enables a mobile to effectively utilizetwo 5MHz UMTS carriers. Assuming both are using 64 QAM (21.6Mbit/s), the

theoretical maximum is 42Mbps. Note that in Release 8, a mobile is not able to combineMIMO and DC-HSDPA.

Less Uplink Overhead - In a similar way to Release 7 in the downlink, the Release 8

uplink has also been enhanced to reduce overhead.


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Figure 1-9Release 8 HSPA+ and LTE

LTE provides a new radio access technique, as well as enhancements in the E-UTRAN

(Evolved - Universal Terrestrial Radio Access Network). These enhancements are furtherdiscussed as part of this course.

Release 9 and Beyond

Even though LTE is a Release 8 system, it is yet further enhanced in Release 9. There are a

huge number of features in Release 9. One of the most important is the support of additionalfrequency bands.

Figure 1-10Release 9 and Beyond

Release 10 includes the standardization of LTE Advanced, i.e. the 3GPPs 4G offering. As

such, it includes the modification of the LTE system to facilitate 4G services.


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3GPP Evolution: From LTE to LTE-A/B/C

Heterogeneous or HetNet for short stands for the different types of base stations (macro,

micro, pico, relay) that are operating on different technologies (GSM, WCDMA and LTE)that

are used together in the same network to build the good coverage and high capacity that

end-users demand from their operator (contrary to homogeneous networks that are mainlybuilt with one type of base station, often macro).

FusionNet

Huawei in Barcelona at the Mobile World Congress (MWC 2013) demonstrated the nextgeneration LTE-B (R12/R13) network architecture FusionNet. It combines multi-system,

multi-band, multi-layer heterogeneous networks, improved 500% cell edge user

throughput, which really create borderless networks.

The core of FusionNet is bases on LTE-B techniques (such as multi-flow aggregation,

interference coordination, service adaptation, spectrum efficiency optimization, etc.), andwith the existing LTE, LTE-A (such as multi-point coordinate, carrier aggregation),

which realizes multi-system, multi-band, multi-layer network of deep integration, help

operators significantly reduce CAPEX and OPEX, allowing users to enjoy

ultra-broadband, zero-waiting and ubiquitous connectivity.


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LTE Technical Obiectives

1.2 EPS Architecture

In contrast to the 2G and 3G networks defined by the 3GPP, LTE can be simply divided into aflat IP based bearer network and a service enabling network. The former can be further

subdivided into the E-UTRAN (Evolved - Universal Terrestrial Radio Access Network) and

the EPC (Evolved Packet Core) where as support for service delivery lies in the IMS (IP

Multimedia Subsystem). This reference architecture can be seen in Figure 1-11.

Figure 1-11LTE Reference Architecture

Whilst UMTS is based upon W-CDMA technology, the 3GPP developed new specificationsfor the LTE air interface based upon OFDMA (Orthogonal Frequency Division Multiple

Access) in the downlink and SC-FDMA (Single Carrier - Frequency Division Multiple

Access) in the uplink. This new air interface is termed the E-UTRA (Evolved - UniversalTerrestrial Radio Access).


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Figure 1-12EPS Network Architecture-2G/3G Co-existence

In the evolution of core network, packet domain of core network also evolves forward toSAE(System Architecture Evolution, also usually called EPC(Evolved Packet Core). SAE isbased on packet domain, and does not support circuit domain any longer.

1.2.1 User Equipment

Like that of UMTS, the mobile device in LTE is termed the UE (User Equipment) and is

comprised of two distinct elements; the USIM (Universal Subscriber Identity Module) and theME (Mobile Equipment).

The ME supports a number of functional entities including:

RR (Radio Resource) - this supports both the Control Plane and User Plane and in so

doing, is responsible for all low level protocols including RRC (Radio ResourceControl), PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), MAC

(Medium Access Control) and the Phy (Physical) Layer.

EMM (EPS Mobility Management) - is a Control Plane entity which manages themobility management states the UE can exist in; LTE Idle, LTE Active and LTE

Detached. Transactions within these states include procedures such as TAU (Tracking

Area Update) and handovers.

ESM (EPS Session Management) - is a Control Plane activity which manages the

activation, modification and deactivation of EPS bearer contexts. These can either bedefault EPS bearer contexts or dedicated EPS bearer contexts.


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Figure 1-13User Equipment Functional Elements

In terms of the Phy layer, the capabilities of the UE may be defined in terms of the

frequencies and data rates supported. Devices may also be capable of supporting adaptivemodulation including QPSK (Quadrature Phase Shift Keying), 16QAM (16 Quadrature

Amplitude Modulation) and 64QAM (Quadrature Amplitude Modulation).

In terms of the radio spectrum, the UE is able to support several scalable channels including;

1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz and 20MHz whilst operating in FDD (Frequency

Division Duplex) and/or TDD (Time Division Duplex). Furthermore, the UE may alsosupport advanced antenna features such as MIMO (Multiple Input Multiple Output) which is

discussed in at 2.6 .

Table 1-3UE Categories

UE Identities

An LTE capable UE will be allocated / utilize a number of identities during operation within

the network. These include:


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IMSI (International Mobile Subscriber Identity) - this complies with the standard 3GPP

format and is comprised of the MCC (Mobile Country Code), MNC (Mobile Network

Code) and the MSIN (Mobile Subscriber Identity Number). This uniquely identifies a

subscriber from within the family of 3GPP technologies - GSM, GPRS, UMTS etc.

IMEI (International Mobile Equipment Identity) - is used to uniquely identify the ME. Itcan be further subdivided into a TAC (Type Approval Code), FAC (Final Assembly Code)

and SNR (Serial Number).

GUTI (Globally Unique Temporary Identity) - is allocated to the UE by the MME(Mobility Management Entity) and identifies a device to a specific MME. The identity is

comprised of a GUMMEI (Globally Unique MME Identity) and an M-TMSI (MME -

Temporary Mobile Subscriber Identity).

S-TMSI (Serving - Temporary Mobile Subscriber Identity) - is used to protect asubscribers IMSI during NAS (Non Access Stratum) signaling between the UE and

MME as well as identifying the MME from within a MME pool. The S-TMSI is

comprised of the MMEC (MME Code) and the M-TMSI.

IP Address - the UE requires a routable IP address from the PDN (Packet Data Network)

from which it is receiving higher layer services. This may either be an IPv4 or IPv6

address.

1.2.2 Evolved Node B

In addition to the new air interface, a new base station has also be specified by the 3GPP and

is referred to as an eNB (Evolved Node B). These, along with their associated interfaces formthe E-UTRAN and in so doing, are responsible for:

Functions for Radio Resource Management: Radio Bearer Control, Radio Admission

Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both

uplink and downlink (scheduling);

IP header compression and encryption of user data stream;

Selection of an MME at UE attachment when no routing to an MME can be determinedfrom the information provided by the UE;

Routing of User Plane data towards Serving Gateway;

Scheduling and transmission of paging messages (originated from the MME);

Scheduling and transmission of broadcast information (originated from the MME or

O&M);

Measurement and measurement reporting configuration for mobility and scheduling;

Scheduling and transmission of PWS (which includes ETWS and CMAS) messages

(originated from the MME);

CSG handling


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Figure 1-14Evolved Node B Functional Elements

Security in LTE is not solely limited to encryption and integrity protection of information passing across

the air interface but instead, NAS encryption and integrity protection between the UE and MME also takesplace. In addition, IPSec may also be used to protect user data within both the E-UTRAN and EPC.

eNB Identities

In addition to the UE identities already discussed, there are a number of specific identities

associated with the eNB. These include:

TAI (Tracking Area Identity) - is a logical group of neighboring cells defined by theservice provider in which an LTE idle UE is able to move within without needing to

update the network. As such, it is similar to a RAI (Routing Area Identity) used in 2Gand 3G packet switched networks.

ECGI (Evolved Cell Global Identity) - is comprised of the MCC, MNC and ECI

(Evolved Cell Identity), the later being coded by each service provider.

1.2.3 Mobility Management Entity

The MME hosts the following functions (see 3GPP TS 23.401 [17]):

NAS signalling;

NAS signalling security;

AS Security control;

Inter CN node signalling for mobility between 3GPP access networks;

Idle mode UE Reachability (including control and execution of paging retransmission);

Tracking Area list management (for UE in idle and active mode);

PDN GW and Serving GW selection;

MME selection for handovers with MME change;

SGSN selection for handovers to 2G or 3G 3GPP access networks;

Roaming;

Authentication;

Bearer management functions including dedicated bearer establishment;

Support for PWS (which includes ETWS and CMAS) message transmission;


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Optionally performing paging optimisation.

NOTE 1: For macro eNBs, the MME should not filter the PAGING message based on the

CSG IDs.

1.2.4 The Serving Gateway (S-GW)

The Serving Gateway (S-GW) hosts the following functions (see 3GPP TS 23.401 [17]):

The local Mobility Anchor point for inter-eNB handover;

Mobility anchoring for inter-3GPP mobility;

E-UTRAN idle mode downlink packet buffering and initiation of network triggered

service request procedure;

Lawful Interception;

Packet routeing and forwarding;

Transport level packet marking in the uplink and the downlink;

Accounting on user and QCI granularity for inter-operator charging;

UL and DL charging per UE, PDN, and QCI.

1.2.5 The PDN Gateway

The PDN Gateway (P-GW) hosts the following functions (see 3GPP TS 23.401 [17]):

Per-user based packet filtering (by e.g. deep packet inspection);

Lawful Interception;

UE IP address allocation;

Transport level packet marking in the downlink;

UL and DL service level charging, gating and rate enforcement;

DL rate enforcement based on APN-AMBR;

NOTE 2: it is assumed that no other logical E-UTRAN node than the eNB is needed for RRMpurposes. Moreover, due to the different usage of inter-cell RRM functionalities, eachinter-cell RRM functionality should be considered separately in order to assess whether it

should be handled in a centralised manner or in a distributed manner.

1.3 E-UTRAN Protocol Stack Structure

As with all 3GPP technologies, it is the actual interfaces which are defined in terms of the

protocols they support and the associated signaling messages and user traffic that traversethem.


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Figure 1-15E-UTRAN Interfaces

1.3.1 Uu Interface

The Uu Interface supports both a Control Plane and a User plane and spans the link between

the UE and the eNB / HeNB. The principle Control Plane protocol is RRC while the UserPlane is designed to carry IP datagrams. However, both Control and User Planes utilize the

services of PDCP, RLC and MAC.

Figure 1-16Uu Interface Protocols

Radio Resource Control

RRC deals with all the signaling between the UE and the E-UTRAN in addition to

transporting NAS signaling between the UE and the MME. It also provides the mainconfiguration and parameters to the lower layer protocols. For example, the Phy Layer will

receive information from RRC on how to configure certain of its aspects.

A UE A UE has 2 RRC states. There are RRC_IDLE and RRC_CONNECTED.

RRC_IDLE: A UE is in RRC_IDLE state when the UE does not have an RRC connection.

DRX can be used for the UE to save the UE power.

The UE monitors the paging channel.

The UE measures the neighboring cell and reselects a cell.

The UE gets system information.


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The UE updates TAU periodically.

RRC_CONNECTED: A UE is in RRC_CONNECTED state when at least one RRCconnection is established for the UE.

The UE transmits downlink and uplink data.

The UE manages the mobility.

The UE provides channel quality and feedback information.

The UE supports DRX configuration to save the UE power.

Packet Data Convergence Protocol

PDCP operates on both the Control Plane and User Plane. In addition to IP header

compression and sequencing / duplicate packet detection, PDCP is also responsible forsecurity on the air interface. As such, its key responsibilities include:

Encryption - Control Plane and User Plane.

Integrity Checking - Control Plane.

IP Header Compression - User Plane.

Sequencing and Duplicate Detection - User Plane.

Radio Link Control

As the name would suggest, RLC provides radio link control in the UE and eNB and in so

doing, it provides three delivery services to the higher layers. These are:

TM (Transparent Mode) - this provides a connectionless service and is utilized for some

of the air interface channels e.g. broadcast and paging.

UM (Unacknowledged Mode) - like that of TM, this also provides a connectionlessservice but with additional functionality incorporating sequencing, segmentation and

concatenation.

AM (Acknowledged Mode) - this supports ARQ (Automatic Repeat Request) therebyoperating in a connection orientated mode.

Medium Access Control

MAC provides the interface between the E-UTRA protocols and the Phy Layer and supports

the following services:

Mapping - this is the mapping of information between the logical and transport

channels.

Multiplexing - in order to increase system efficiency, information from different RadioBearers is multiplexed into the same TB (Transport Block).

HARQ (Hybrid Automatic Repeat Request) - provides error correction services over theair interface. This requires close interworking with the Physical Layer.

Radio Resource Allocation - this is the scheduling of traffic and signaling to users based

upon QoS.

Physical

The Physical Layer incorporates a number of functions. These include:


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Error Detection.

FEC (Forward Error Correction) Encoding / Decoding.

Rate Matching.

Physical Channel Mapping.

Power Weighting.

RF (Radio Frequency) Modulation and Demodulation.

Frequency and Time Synchronization.

Radio Measurements.

MIMO Processing.

Transmit Diversity.

Beamforming.

RF Processing.

1.3.2 S1 Interface

The S1 Interface can be subdivided into the S1-MME interface supporting Control Plane

signaling between the eNB and the MME and the S1-U Interface supporting User Plane traffic

between the eNB and the S-GW.

Figure 1-17S1 Interface Protocols

S1AP: The S1 Application Protocol is the application layer protocol between eNodeBand MME.

SCTP: The Stream Control Transmission Protocol ensures the delivery of signalingmessages on the S1 interface between the MME and the eNodeB. For details about SCTP,

see RFC2960.

GTP-U: The GPRS Tunneling ProtocolUser plane is used for user data transmissionbetween the eNdoeB and S-GW.

UDP: User Datagram Protocol is used for the user data transmission. For details aboutUDP, see RFC 768.

The data link layer can use layer 2 technologies, such as PPP and Ethernet.


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S1 Application Protocol

The S1AP spans the S1-MME interface and in so doing, supports the following functions:

E-RAB (Evolved - Radio Access Bearer) Management - this incorporates the setting up,

modifying and releasing of the E-RABs by the MME.

Initial Context Transfer - is used to establish an S1UE context in the eNB, setup thedefault IP connectivity and transfer NAS related signaling.

UE Capability Information Indication - is used to inform the MME of the UE Capability

Information.

Mobility - this incorporates mobility features to support a change in eNB or change in

RAT.

Paging.

S1 Interface Management - this incorporates a number of sub functions dealing withresets, load balancing and system setup etc.

NAS Signaling Transport - the transport of NAS related signaling over the S1-MME

Interface.

UE Context Modification and Release - this allows for the modification and release ofthe established UE Context in the eNB and MME respectively.

Location Reporting - this enables the MME to be made aware of the UEs current

location within the network.

1.3.3 X2 Interface

The X2 Interface interconnects two eNBs and in so doing supports both a Control Plane andUser Plane. It also extends the S1 Interface when two or more eNBs lie between the UE and

the EPC. The X2AP (X2 Application Protocol) Control Plane protocol resides on SCTP

(Stream Control Transmission Protocol) where as the IP is transferred over the User Planeusing the services of GTP-U (GPRS Tunneling Protocol - User) and UDP (User Datagram

Protocol).

The X2 interface is divided into the user plane (X2-U) and control plane (X2-C). The X2-U

interface is required to be the same as the S1-U, and the X2-C is required to be the same asS1-C.

The X2 interface data link layer can use layer 2 technologies, such as PPP and Ethernet.

X2 Application Protocol

The X2AP is responsible for the following functions:

Mobility Management - this enables the serving eNB to move the responsibility of aspecified UE to a target eNB. This includes Forwarding the User Plane, Status Transfer

and UE Context Release functions.

Load Management - this function enables eNBs to communicate with each other in order

to report resource status, overload indications and current traffic loading.

Error Reporting - this allows for the reporting of general error situations for whichspecific error reporting mechanism have not been defined.

Setting / Resetting X2 - this provides a means by which the X2 interface can be setup /

reset by exchanging the necessary information between the eNBs.


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Configuration Update - this allows the updating of application level data which is needed

for two eNBs to interoperate over the X2 interface.

Figure 1-18X2 Interface Protocols

Stream Control Transmission Protocol

Defined by the IETF (Internet Engineering Task Force) rather than the 3GPP, SCTP was

developed to overcome the shortfalls in TCP (Transmission Control Protocol) and UDP whentransferring signaling information over an IP bearer. Functions provided by SCTP include:

Reliable Delivery of Higher Layer Payloads.

Sequential Delivery of Higher Layer Payloads.

Improved resilience through Multihoming.

Flow Control.

Improved Security.

SCTP is also found on the S1-MME Interface which links the eNB to the MME.

GPRS Tunneling Protocol - User

GTP-U tunnels are used to carry encapsulated PDU (Protocol Data Unit) and signaling

messages between endpoints or in the case of the X2 interface. Numerous GTP-U tunnels mayexist in order to differentiate between EPS bearer contexts and these are identified through aTEID (Tunnel Endpoint Identifier).

GTP-U is also found on the S1-U Interface which links the eNB to the S-GW and may also beused on the S5 Interface linking the S-GW to the PDN-GW.


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Training Manual 2 LTE Air Interface Principles



2-1

2 LTE Air Interface PrinciplesObjectives

On completion of this section the participants will be able to:

2.1 Describe the principles of OFDM.

0.0.0 Describe the multiple access and duplex technology.

2.4 Describe the carrier frequency and EARFCN

Error! No se encuentra el origen de la referencia.Describe the LTE frame structure

Error! No se encuentra el origen de la referencia.Describe the LTE channel structure

Error! No se encuentra el origen de la referencia.Have a good understanding of theOFDMA and SC-FDMA.

2.6 Describe cell selection procedure and random access procedure.

Error! No se encuentra el origen de la referencia.Describe MIMO.


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2.1 Principle of OFDM

Principles of OFDM

The LTE air interface utilizes two different multiple access techniques, both of which are

based on OFDM (Orthogonal Frequency Division Multiplexing). These are:

OFDMA (Orthogonal Frequency Division Multiple Access) - used on the downlink.

SC-FDMA (Single Carrier - Frequency Division Multiple Access) - used on the uplink.

Figure 2-1Use of OFDM in LTE

OFDM

(OFDMA)

OFDM

(SC-FDMA)

The concept of OFDM is not new and is currently being used on various systems such as

Wi-Fi (Wireless Fidelity) and WiMAX (Worldwide Interoperability for Microwave Access).

Furthermore, it was even considered for UMTS back in 1998. One of the main reasons why itwas not chosen at the time however was the handsets limited processer power and the poor

battery capabilities.

LTE was able to choose an OFDM based access due to the fact mobile handset processing

capabilities and battery performance have both significantly improved over the interveningyears. In addition, there is continual pressure to produce ever more spectrally efficientsystems.

Division Multiplexing Overview

Multiplexed data streams can be used for one or multiple UEs.

FDM (Frequency Division Multiplexing): Available spectrum divides into multi-sub-bands or

channels. Each sub-bands or channel transmits one signal (data streams).

TDM (Time Division Multiplexing): The entire transmission channel time is divided into

several time slots, and these time slots are assigned to each signal source, each of the signal

sources in their own exclusive channel time slot for data transmission.

CDM (Code Division Multiplexing): All sub-channels can use the entire channel for datatransmission at the same time, different codes are used to distinguish various original signals.


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Figure 2-2Frequency Division Multiple

Frequency

Power Time

Data

stream1

Data

stream2

Data

stream3

Data

stream4

Figure 2-3Time Division Multiple

Frequency

PowerTime

Data stream 1

Data stream 2

Data stream 3

Data stream 4

Figure 2-4Code Division Multiple

Frequency

Power Time

Data stream 1

Data stream 2

Data stream 3

Data stream 4

OFDM Overview

OFDM is based on FDM (Frequency Division Multiplexing) and is a method whereby

multiple frequencies are used to simultaneously transmit information. Figure 2-5 illustrates anexample of FDM with four subcarriers. These can be used to carry different information and

to ensure that each subcarrier does not interfere with the adjacent subcarrier, a guard band is

utilized. In addition, each subcarrier has slightly different radio characteristics and this may be

used to provide diversity.


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Figure 2-5FDM Carriers

FDM systems are not that spectrally efficiency (when compared to other systems) since

multiple guard bands are required.

OFDM follows the same concept as FDM but it drastically increases spectral efficiency by

reducing the spacing between the subcarriers. Figure 2-6 illustrates how the subcarriers canoverlap due to their orthogonally with the other subcarriers, i.e. the subcarriers are

mathematically perpendicular to each other. As such, when a subcarrier is at its maximum, the

two adjacent subcarriers are passing through zero. Furthermore, OFDM systems still employguard bands. These are however located at the upper and lower parts of the channel in order to

reduce adjacent channel interference.

Figure 2-6OFDM Subcarriers

The centre subcarrier, known as the DC (Direct Current) subcarrier, is not typically used in OFDM

systems due to its lack of orthogonality.

Fast Fourier TransformsOFDM subcarriers are generated and decoded using mathematical functions called FFT (Fast

Fourier Transform) and IFFT (Inverse Fast Fourier Transform). The IFFT is used in thetransmitter to generate the waveform. Figure 2-7 illustrates how the coded data is first mapped

to parallel streams before being modulated and processed by the IFFT.


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Figure 2-7Inverse Fast Fourier Transform

Modulation procedure of OFDM is realized by IFFT (Inverse Fast Fourier Transform), N is

the sampling period of symbol.

For example: Sampling rate fs =1/Ts =N*f

For bandwidth 20MHz, N=2048, f 15kHz sampling rate 30.72MHz

At the receiver side, this signal is passed to the FFT which analyses the complex/combined

waveform to generate the original streams. Figure 2-8 illustrates the FFT process.

Figure 2-8Fast Fourier Transform

Similar to modulation procedure of OFDM FFT process is used in the demodulation

procedure of OFDM.

LTE FFT Sizes

Fast Fourier Transforms and Inverse Fast Fourier Transforms both have a defining size. For

example, an FFT size of 512 indicates that there are 512 subcarriers. In reality, not all 512

subcarriers can be utilized for data transfer due to the channel guard bands and the fact that a

DC (Direct Current) subcarrier is also required.

Table 2-1 illustrates the channel bandwidth options available to LTE, as well as the FFT sizeand associated sampling rate. Using the sampling rate and the FFT size, the subcarrier spacing

can be calculated, e.g. 7.68MHz/512 = 15kHz.


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Table 2-1LTE Channel and FFT Sizes

ChannelBandwidth

FFT Size SubcarrierBandwidth

Sampling Rate

1.4MHz 128

15kHz

1.92MHz

3MHz 256 3.84MHz

5MHz 512 7.68MHz

10MHz 1024 15.36MHz

15MHz 1536 23.04MHz

20MHz 2048 30.72MHz

The subcarrier spacing of 15kHz is also used to identify the OFDM symbol duration.

2.1.2 OFDM Symbol MappingThe mapping of OFDM symbols to subcarriers is dependent on the system design. The first12 modulated OFDM symbols are mapped to 12 subcarriers, i.e. they are transmitted at the

same time but using different subcarriers. The next 12 subcarriers are then mapped to the next

OFDM symbol period. In addition, a CP (Cyclic Prefix) is added between the symbols.

Figure 2-9OFDM Symbol Mapping

LTE allocates resources in groups of 12 subcarriers. This is referred to as a PRB (Physical Resource

Block).

In the previous example, 12 different modulated OFDM symbols were transmitted

simultaneously. Figure 2-10 illustrates how the combined energy from this will result in eitherconstructive peak (when the symbols are the same) or destructive nulls (when the symbols are

different).


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Figure 2-10OFDM PAPR (Peak to Average Power Ratio)

2.1.3 Advantage 1 of OFDM: High Spectral Efficiency

Subcarriers in the OFDM system are overlapping and orthogonal, which greatly improves thespectral efficiency.

How does OFDM work?

IFFT on the OFDM transmitter side and FFT on the OFDM receiver side reducesystem complexity, enabling OFDM to be widely used.

Why does OFDM not become a practical reality until the latest two decades?

The development of DSP chips turns OFDM to a practical reality.

Figure 2-11 illustrate the traditional FDM multicarrier modulation technology and OFDM

multicarrier modulation technology.

Figure 2-11Multicarrier modulation technology

In traditional FDMA transmission, a channel is divided into multiple independentsub-channels to transmit data streams in parallel, and the sub-channels are separated by a

group of filters on the receiver. This method is simple and direct while the spectral efficiencyis low because guard-bands are required between sub-channels, which are difficult to achieve

by filters. However, subcarriers in the OFDM system are overlapping and orthogonal, which

greatly improves the spectral efficiency compared with common FDA systems, as shown inthe preceding figure.

The orthogonal modulation and demodulation in each sub-channel can be performed usingIDFT and DFT. For systems with large N value, FFT can be used. IFFT and FFT are easy to

perform with the development of large-scale integrated circuit and DSP technologies, asshown in the preceding figure.


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2.1.4 Advantage 2 of OFDM: Effectively Withstand Multi-Path

The OFDM signal provides some protection in the frequency domain due to the orthogonality

of the subcarriers. The main issue to overcome however is delay spread, i.e. multipathinterference.

Figure 2-12 illustrates two of the main multipath effects, namely delay and attenuation.Without the protection interval between symbols, multi path will produce ISI and ICI.

ISI: Inter-symbol Interference, time domain

ICI: Inter-Carrier Interference, frequency domain

Figure 2-12Delay Spread

One OFDM symbol

Time

ISI is typically combated with equalizers. However for the equalizer to be effective, a

known bit pattern or training sequence is required. This reduces the system capacity, as wellas impacting on the processing required within the device. Instead, OFDM systems employ a

CP (Cyclic Prefix).

In OFDM system, the loss of orthogonality among subcarriers causes ICI. ICI is oftenmodeled as Gaussian noise and affects both channel estimation and detection of the OFDMsymbols.

Cyclic Prefix

A Cyclic Prefix is utilized in most OFDM systems to combat multipath delays. It effectively

provides a guard period for each OFDM symbol. Figure 2-13 illustrates the Cyclic Prefix and

identifies its location in the OFDM Symbol. Notice that the Cyclic Prefix is effectively a copyfrom the back of the original symbol which is then placed in front to make the OFDM symbol

(Ts).

Altitude


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Figure 2-13Cyclic Prefix

LTE has two defined Cyclic Prefix sizes, normal and extended. The extended Cyclic Prefix is designed

for larger cells.

The size of the Cyclic Prefix relates to the maximum delay spread the system can tolerate. As

such, systems designed for macro coverage, i.e. large cell radius, should have a large CP. This

does however impact on system capacity as the number of symbols per second is will bereduced.

2.1.5 Advantage 3 of OFDM: Resistant to Frequency SelectionFading

If deep fading occurs in a frequency, modulate the UE to another subcarrier.

Deep fading does not occur simultaneously in all subcarriers due to the frequency selectivity.Therefore, dynamic bit or subcarrier allocation technology can be used to utilize the

sub-channels with high SNR and improve the system performance.

In a multi-user system, a subcarrier that is in poor performance for a user probably is in good

performance for another user. Therefore, a sub-channel is not disabled unless it is in poorperformance for all users, which occurs at a low probability. The single-carrier system

performs adaptive modulation and coding (AMC) based on the average SINR in the entire

system, while the multi-carrier system performs AMC based on the average SINR in different

frequency bands.

Figure 2-14Resistant to Frequency Selection Fading


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2.1.6 Disadvantage 1 of OFDM: Vulnerable to Frequency OffsetOrthogonality is required because spectrums of sub-channels overlap each other. Frequencyoffset of radio signals, such as Doppler Shift, can be caused by radio channel change with

time. In addition, the difference between transmitter carrier frequency and receiver oscillator

frequency can also cause frequency offset, destroying the orthogonality of subcarriers in theOFDM system. As a result, inter-carrier interference (ICI) among sub-channels is generated,

deteriorating the BER of the system. The vulnerability to the frequency offset is the primarydisadvantage of the OFDM system.

Figure 2-15Vulnerable to Frequency Offset

We can use frequency synchronization to solve the frequency offset.


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2.1.7 Disadvantage 2 of OFDM: High PAPR

OFDM systems can suffer from high PAPR (Peak to Average Power Ratio), resulting from the

great number of subcarriers in the same phase overlapping in time domain, thus increasing therequirement to power amplifier.

Figure 2-16Multi-carrier system signal process procedure

Different from single-carrier systems, multi-carrier system outputs combined signals ofmultiple sub-channels. If these signals are in the same phase, the power of combined signals

must be higher than the average power of signals, resulting in a high PAR. To reduce the highPAR, high linearity of the PA in the transmitter is required. If the dynamic range of the PAcannot adjust to the signal change, signals are deformed, changing the spectrum of the

combined signals. As a result, the orthogonality of signals in multiple sub-channels isdestroyed, leading to interference and deteriorated system performance.

We can use high-performance PA in the downlink and SC-FDMA in the uplink to solve theseproblems.

2.1.8 OFDM Advantages and Disadvantages

OFDM Advantages

OFDM systems typically have a number of advantages:

OFDM is almost completely resistant to multi-path interference due to its very long

symbol duration.

Higher spectral efficiency for wideband channels - 5MHz and above.

Flexible spectrum utilization. Relatively simple implementation using FFT and IFFT.

OFDM Disadvantages

OFDM also has some disadvantages:

Frequency errors and phase noise can cause issues.

Doppler shift impacts subcarrier orthogonality.


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