APIS Telecom Training: Long Term Evolution (LTE)

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LTE- Long Term Evolution

1

1 _{LTE/SAE Introduction}

_{LTE/SAE Introduction}

2

2 _{Orthogonal FDM}

_{Orthogonal FDM}

3

3 _{Multi-antenna Systems}

_{Multi-antenna Systems}

4

4 _{E-UTRA Physical Layer}

_{E-UTRA Physical Layer}

5

5 _{E-UTRA Layer 2 and 3}

_{E-UTRA Layer 2 and 3}

6

6 _{The X2- and S1-interface}

_{The X2- and S1-interface}

7

7 _{The Evolved Packet Core}

_{The Evolved Packet Core}

8

8 _{LTE/SAE Signalling}

_{LTE/SAE Signalling}

Procedures

9

10

11

12

13

14

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15

15 _Foldouts

_Foldouts

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Apis Technical Training AB Apis Technical Training AB LTE - LTE/SAE

LTE - LTE/SAE IntroductionIntroduction Copyright

1

1 L

LTE/SAE

TE/SAE Introd

Introduction

uction

1.1

1.1 BBACKGROUNDACKGROUND... 1-21-2

1.2

1.2 EEVOLVEDVOLVED UTRA & UTRA & UTRAN...UTRAN... 1-31-3

1.2.1

1.2.1 Network Architecture...1-3Network Architecture...1-3 1.2.2

1.2.2 RequiremRequirements on ents on E-UTRA/UTE-UTRA/UTRAN...RAN...1-41-4 1.2.3

1.2.3 Overview of Technical Solutions...1-5Overview of Technical Solutions...1-5

1.3

1.3 EEVOLVEDVOLVED PPACKETACKET CCOREORE, , EPC...EPC... 1-71-7

1.3.1

1.3.1 Network Architecture...1-7Network Architecture...1-7 1.3.2

1.3.2 RequireRequirements on ments on the EPCthe EPC ...1-8...1-8

1.4

1.4 EEVOLVEDVOLVED HSPA (HSPA+)HSPA (HSPA+) ... 1-91-9

1.5

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1.1 Background

3GPP

3GPP Long Term Evolution Long Term Evolution (LTE) is the name given to a project within(LTE) is the name given to a project within the Third Generation Partnership Project (3GPP) to improve the UMTS 3G the Third Generation Partnership Project (3GPP) to improve the UMTS 3G mobile system standard to cope with future requirements. Goals include mobile system standard to cope with future requirements. Goals include improving efficiency, lowering costs, reducing complexity, improving improving efficiency, lowering costs, reducing complexity, improving services, making use of new spectrum opportunities and better integration services, making use of new spectrum opportunities and better integration with other open standards (such as WLAN and WiMAX). Thus, the term with other open standards (such as WLAN and WiMAX). Thus, the term ‘LTE’ really means a standardisation

‘LTE’ really means a standardisation project project . The final outcome from this. The final outcome from this project will be a new set of

project will be a new set of standardsstandards defining the functionality anddefining the functionality and requirements of an evolved, packet based, radio access network and a new requirements of an evolved, packet based, radio access network and a new radio access. The new radio access network is referred to as the ‘Evolved radio access. The new radio access network is referred to as the ‘Evolved UTRAN’ (E-UTRAN) and the new radio access is referred to as the UTRAN’ (E-UTRAN) and the new radio access is referred to as the ‘Evolved UTRA’ (E-UTRA). The LTE project belongs to 3GPP Release 8. ‘Evolved UTRA’ (E-UTRA). The LTE project belongs to 3GPP Release 8. The term ‘LTE’ has recently become more or less synonymous to the The term ‘LTE’ has recently become more or less synonymous to the (proper) terms ‘Evolved UTRA’ (the new radio access) and ‘Evolved (proper) terms ‘Evolved UTRA’ (the new radio access) and ‘Evolved UTRAN’ (the new radio access network). With this in mind, t

UTRAN’ (the new radio access network). With this in mind, t he author hashe author has taken the freedom to use the terms ‘LTE’ and ‘E-UTRA’ interchangeably taken the freedom to use the terms ‘LTE’ and ‘E-UTRA’ interchangeably for the new OFDM-based radio interface. The term ‘E-UTRAN’ explicitly for the new OFDM-based radio interface. The term ‘E-UTRAN’ explicitly means the whole radio access network (i.e. it includes the eNBs, the means the whole radio access network (i.e. it includes the eNBs, the X2-interface and the S1-X2-interface).

interface and the S1-interface).

The work on LTE started with a workshop, 2-3 Nov 2004 in Toronto, The work on LTE started with a workshop, 2-3 Nov 2004 in Toronto, Canada. The workshop was open to members and non-members of 3GPP. Canada. The workshop was open to members and non-members of 3GPP. Operators, vendors and research institutes presented contributions with Operators, vendors and research institutes presented contributions with views and proposals on the future evolution of 3G. A set of high level views and proposals on the future evolution of 3G. A set of high level requirements were initially identified:

requirements were initially identified:

•

• Reduced cost per transmitted bitReduced cost per transmitted bit •

• More services at lower cost with better user experienceMore services at lower cost with better user experience •

• Flexibility of use of existing and new frequency bandsFlexibility of use of existing and new frequency bands •

• Simplified architecture, open interfacesSimplified architecture, open interfaces •

• Reasonable terminal power consumption.Reasonable terminal power consumption.

It was also recommended that the E-UTRAN should bring significant It was also recommended that the E-UTRAN should bring significant improvements to justify the standardization effort and that it should avoid improvements to justify the standardization effort and that it should avoid unnecessary options. A feasibility study on the UTRA & UTRAN Long unnecessary options. A feasibility study on the UTRA & UTRAN Long Term Evolution was then started i

Term Evolution was then started in December 2004. The objective was "ton December 2004. The objective was "to develop a framework for the evolution of the 3GPP radio access develop a framework for the evolution of the 3GPP radio access technology towards a high data rate, low latency and packet optimized technology towards a high data rate, low latency and packet optimized radio access technology". The study focused on supporting services radio access technology". The study focused on supporting services exclusively from the Packet Switched (PS)

exclusively from the Packet Switched (PS) domain.domain.

In parallel to, and coordinated with, the LTE project there is also a 3GPP In parallel to, and coordinated with, the LTE project there is also a 3GPP standardisation project relating to the core network. This project is called standardisation project relating to the core network. This project is called System Architecture Evolution

System Architecture Evolution (SAE) and aims at standardising the(SAE) and aims at standardising the Evolved Packet Core (EPC). The SAE project was started in December Evolved Packet Core (EPC). The SAE project was started in December 2004, with the objective to “develop a framework for an evolution or 2004, with the objective to “develop a framework for an evolution or

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migration of the 3GPP system to a higher data rate, lower latency, packet migration of the 3GPP system to a higher data rate, lower latency, packet optimized system that supports multiple RATs”. The EPC will be a fully optimized system that supports multiple RATs”. The EPC will be a fully IP-based core network (‘all-IP’) supporting access not only via GERAN, IP-based core network (‘all-IP’) supporting access not only via GERAN, UTRAN and E-UTRAN but also WiFi, WiMAX and wired technologies UTRAN and E-UTRAN but also WiFi, WiMAX and wired technologies such as xDSL. The SAE project also belongs to 3GPP Release 8.

such as xDSL. The SAE project also belongs to 3GPP Release 8.

A short introduction to the Evolved UTRA/N can be found in section 1.2 A short introduction to the Evolved UTRA/N can be found in section 1.2 in this chapter, and an introduction to the EPC in section 1.3. The Stage 2 in this chapter, and an introduction to the EPC in section 1.3. The Stage 2 set (general architecture, protocol structure and key concepts) of LTE set (general architecture, protocol structure and key concepts) of LTE standardisation documents is, according to 3GPP, to be completed at the standardisation documents is, according to 3GPP, to be completed at the time of writing this document (Oct 2007). The completion date for the time of writing this document (Oct 2007). The completion date for the Stage 3 work (i.e. detailed protocol specifications) is still a bit uncertain, Stage 3 work (i.e. detailed protocol specifications) is still a bit uncertain, but a reasonable estimate is ‘early 2008’. The Stage 2 set of SAE but a reasonable estimate is ‘early 2008’. The Stage 2 set of SAE standardisation documents are (again according to 3GPP) to be completed standardisation documents are (again according to 3GPP) to be completed by March 2008, with Stage 3 following shortly afterwards. One should be by March 2008, with Stage 3 following shortly afterwards. One should be aware that major updates/changes/additions to the E-UTRAN/EPC specs aware that major updates/changes/additions to the E-UTRAN/EPC specs are expected throughout 2008-09. Real-life deployment of LTE/SAE are expected throughout 2008-09. Real-life deployment of LTE/SAE networks should therefore not be

networks should therefore not be expected until 2009-10.expected until 2009-10.

The reader is strongly encouraged to regularly check the 3GPP website The reader is strongly encouraged to regularly check the 3GPP website ((www.3gpp.orgwww.3gpp.org)) for new versions of the standardisation documents referenced at the for new versions of the standardisation documents referenced at the end of each chapter in the current document.

end of each chapter in the current document.

1.2

1.2 Evolved

Evolved UTRA

UTRA &

& UTRAN

UTRAN

1.2.1

1.2.1 Network

Network Architecture

Architecture

Figure 1-1:

Figure 1-1: The Evolved UTRAN The Evolved UTRAN architecturearchitecture The Evolved UTRAN consists of the

The Evolved UTRAN consists of the evolved NodeBevolved NodeB (eNB), providing the(eNB), providing the E-UTRA User Plane (UP) and Control Plane (CP) protocol terminations E-UTRA User Plane (UP) and Control Plane (CP) protocol terminations towards the UE. The eNBs are

towards the UE. The eNBs are interconnected with each other by means of interconnected with each other by means of

X2 X2 X2 X2 X2 X2 S1 S1 Evolved Packet Core Evolved Packet Core Evolved UTRAN Evolved UTRAN eNB eNB eNB eNB eNB eNB MME MME SGW SGW X2 X2 X2 X2 X2 X2 S1 S1 Evolved Packet Core Evolved Packet Core Evolved UTRAN Evolved UTRAN eNB eNB eNB eNB eNB eNB eNB eNB eNB eNB eNB eNB MME MME SGW SGW MME MME SGW SGW

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the X2-interface X2-interface, e.g. for support of , e.g. for support of handovers without data loss. The eNBshandovers without data loss. The eNBs are connected by means of the

are connected by means of the S1-interfaceS1-interface to the EPC. The S1-interfaceto the EPC. The S1-interface supports a many-to-many relation between eNBs and MME/SGWs (see supports a many-to-many relation between eNBs and MME/SGWs (see section 1.3.1). The X2- and S1-interfaces are described in in chapter 6. section 1.3.1). The X2- and S1-interfaces are described in in chapter 6. The eNB can be seen as a combination of the UMTS NodeB and Radio The eNB can be seen as a combination of the UMTS NodeB and Radio Network Controller, hosting functions like dynamic resource allocation Network Controller, hosting functions like dynamic resource allocation (through packet scheduling) and

(through packet scheduling) and radio resource management.radio resource management.

1.2.2

1.2.2 Requirements

Requirements on

on E-UTRA/UTRAN

E-UTRA/UTRAN

At the onset of the LTE project a series of requirement targets relating to At the onset of the LTE project a series of requirement targets relating to performance, complexity and interworking were defined. Some of these performance, complexity and interworking were defined. Some of these are listed below:

are listed below:

•

• Peak data rate Peak data rate: at least 100 Mb/s DL and 50 Mb/s UL (assuming: at least 100 Mb/s DL and 50 Mb/s UL (assuming

20 MHz system bandwidth). 20 MHz system bandwidth).

•

• Control Plane (CP) latencyControl Plane (CP) latency: transition time less than 100 ms from: transition time less than 100 ms from

an idle state to an active state, and less than 50 ms between a an idle state to an active state, and less than 50 ms between a dormant state (such as R6 CELL_PCH) and an active state.

dormant state (such as R6 CELL_PCH) and an active state.

•

• User Plane (UP) latencyUser Plane (UP) latency: less than 5 ms in unloaded condition: less than 5 ms in unloaded condition

(single user with single data stream) for small

(single user with single data stream) for small IP packet.IP packet.

•

• CP capacityCP capacity: at least 200 users per cell should be supported in the: at least 200 users per cell should be supported in the

active state (5 MHz system bandwidth). active state (5 MHz system bandwidth).

•

• Mobility: E-UTRAN should be optimized for low mobile speed (0- Mobility: E-UTRAN should be optimized for low mobile speed

(0-15 km/h) and higher speeds ((0-15-120 km/h) should be supported 15 km/h) and higher speeds (15-120 km/h) should be supported with high performance. Mobility shall be maintained between with high performance. Mobility shall be maintained between 120-350 km/h (up to 500 km/h depending on the frequency band).

350 km/h (up to 500 km/h depending on the frequency band).

•

• CoverageCoverage: the throughput and mobility targets above should be met: the throughput and mobility targets above should be met

for 5 km cells with

for 5 km cells with a slight degradation for 30 km cells. Cells rangea slight degradation for 30 km cells. Cells range up to 100 km should be possible.

up to 100 km should be possible.

•

• Spectrum flexibilitySpectrum flexibility: E-UTRA shall operate in different spectrum: E-UTRA shall operate in different spectrum

allocations of different sizes, including 1.25, 1.6, 2.5, 5, 10, 15 and allocations of different sizes, including 1.25, 1.6, 2.5, 5, 10, 15 and 20 MHz in both UL and DL. Operation in paired (FDD) and 20 MHz in both UL and DL. Operation in paired (FDD) and unpaired (TDD) spectrum shall be supported.

unpaired (TDD) spectrum shall be supported.

•

• Interworking: co-existence in the same geographical area and co- Interworking: existence in the same geographical area and

co-location with GERAN/UTRAN on adjacent channels. E-UTRAN location with GERAN/UTRAN on adjacent channels. E-UTRAN terminals supporting also UTRAN/GERAN operation should be terminals supporting also UTRAN/GERAN operation should be able to support measurement of, and handover from/to, both able to support measurement of, and handover from/to, both UTRAN and GERAN. The interruption time during a handover of UTRAN and GERAN. The interruption time during a handover of real-time services between E-UTRAN and UTRAN/GERAN real-time services between E-UTRAN and UTRAN/GERAN should be less than 300ms.

should be less than 300ms.

•

• Architecture : the E-UTRAN architecture shall be packet based, Architecture: the E-UTRAN architecture shall be packet based,

supporting real-time and conversational class traffic. The supporting real-time and conversational class traffic. The architecture shall minimize the presence of "single points of architecture shall minimize the presence of "single points of failure".

failure".

•

• ComplexityComplexity: minimised number of options and avoidance of : minimised number of options and avoidance of

redundant mandatory features. redundant mandatory features.

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1.2.3

1.2.3 Overview

Overview of

of Technical

Technical Solutions

Solutions

The E-UTRA radio interface makes exclusive use of shared channels for The E-UTRA radio interface makes exclusive use of shared channels for both data and signalling transfer. In this respect the E-UTRA is similar to both data and signalling transfer. In this respect the E-UTRA is similar to the 3GPP R5/R6 High Speed Packet Access, HSPA. The selected radio the 3GPP R5/R6 High Speed Packet Access, HSPA. The selected radio access technology, however, is very different to HSPA. Where HSPA uses access technology, however, is very different to HSPA. Where HSPA uses WCDMA, the E-UTRA uses Orthogonal Frequency Division Multiplexing WCDMA, the E-UTRA uses Orthogonal Frequency Division Multiplexing (OFDM).

(OFDM).

OFDM splits the available system bandwidth into hundreds, or even OFDM splits the available system bandwidth into hundreds, or even thousands, of narrow-band so-called ‘sub-carriers’. This means that a high thousands, of narrow-band so-called ‘sub-carriers’. This means that a high bitrate data stream to a given UE is split by

bitrate data stream to a given UE is split by the eNB into a large number of the eNB into a large number of narrow-band, low bitrate, data streams. The received parallel data streams narrow-band, low bitrate, data streams. The received parallel data streams (sub-carriers) are then ‘multiplexed’ by the UE in order to re-create the (sub-carriers) are then ‘multiplexed’ by the UE in order to re-create the original high bitrate data stream. This has several advantages over original high bitrate data stream. This has several advantages over WCDMA:

WCDMA:

•

• Better spectral efficieny Better spectral efficieny. More information can be sent using the. More information can be sent using the

same system bandwidth as compared to a

same system bandwidth as compared to a single-carrier system.single-carrier system.

•

• Flexible/scalable spectrum allocation Flexible/scalable spectrum allocation. The system bandwidth can. The system bandwidth can

be expanded in increments (by ‘adding’ more sub-carriers) as more be expanded in increments (by ‘adding’ more sub-carriers) as more spectrum becomes available to the operator. For example, the spectrum becomes available to the operator. For example, the initial system roll-out may use a system bandwidth of 1.25 MHz initial system roll-out may use a system bandwidth of 1.25 MHz and at a later stage this may be increased to, say, 2.5MHz (and then and at a later stage this may be increased to, say, 2.5MHz (and then 5MHz, 10MHz and so on).

5MHz, 10MHz and so on).

•

• Better performance under multipath fading conditions Better performance under multipath fading conditions. Multipath. Multipath

effects leads to so-called frequency selective fading, which is much effects leads to so-called frequency selective fading, which is much more damaging to a wideband signal than to a narrowband signal more damaging to a wideband signal than to a narrowband signal (the sub-carrier).

(the sub-carrier).

There are, of course, drawbacks with OFDM as well. One such drawback There are, of course, drawbacks with OFDM as well. One such drawback is that an OFDM signal exhibits a very high peak-to-average power ratio is that an OFDM signal exhibits a very high peak-to-average power ratio (PAPR). This is not really

(PAPR). This is not really a problem on the network side, but leads to verya problem on the network side, but leads to very inefficient use of power amplifiers, and hence high power consumption, in inefficient use of power amplifiers, and hence high power consumption, in a mobile terminal. The E-UTRA system therefore uses a variant of OFDM a mobile terminal. The E-UTRA system therefore uses a variant of OFDM for uplink transmission that reduces PAPR. This variant of OFDM is for uplink transmission that reduces PAPR. This variant of OFDM is called Single-Carrier Frequency Division Multiple Access (SC-FDMA). called Single-Carrier Frequency Division Multiple Access (SC-FDMA). Despite the name, there is very little that differentiates SC-FDMA from Despite the name, there is very little that differentiates SC-FDMA from ‘classic’ OFDM. Chapter 2 contains more information on OFDM.

‘classic’ OFDM. Chapter 2 contains more information on OFDM.

The use of Multiple Input Multiple Output antenna arrays (MIMO) is an The use of Multiple Input Multiple Output antenna arrays (MIMO) is an integral part of the E-UTRA standard. The standard supports up to four integral part of the E-UTRA standard. The standard supports up to four transmit/receive antennas while the expected baseline configuration is two transmit/receive antennas while the expected baseline configuration is two transmit antennas at the eNB and two receive antennas at the UE. In short, transmit antennas at the eNB and two receive antennas at the UE. In short, MIMO can be used in two different ways:

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•

• To transmit more information over the radio interface withoutTo transmit more information over the radio interface without

using more bandwidth than a single antenna system. The number of using more bandwidth than a single antenna system. The number of antennas used increases the system capacity in a linear manner, i.e. antennas used increases the system capacity in a linear manner, i.e. two antennas allows twice the amount of information to be two antennas allows twice the amount of information to be transmitted (or, equivalently, the bitrate is doubled).

transmitted (or, equivalently, the bitrate is doubled).

•

• To transmit the same information, with the same bitrate as a singleTo transmit the same information, with the same bitrate as a single

antenna system, but with less output power (or, equivalently, with antenna system, but with less output power (or, equivalently, with higher reliability).

higher reliability).

An overview of various MIMO techniques and the mechanisms selected An overview of various MIMO techniques and the mechanisms selected for E-UTRA can be found in chapter 3.

for E-UTRA can be found in chapter 3. The E-UTRA physical layer

The E-UTRA physical layer channel processing chain (channel coding andchannel processing chain (channel coding and modulation) is very similar to what is used today for HSPA. It was agreed modulation) is very similar to what is used today for HSPA. It was agreed at an early stage

at an early stage in the standardisation process that Turbo coding should bein the standardisation process that Turbo coding should be used for error correction purposes and that the supported data modulation used for error correction purposes and that the supported data modulation schemes should be QPSK, 16QAM, and 64QAM for

schemes should be QPSK, 16QAM, and 64QAM for downlink and uplink.downlink and uplink.

Figure 1-2: Constellation diagrams for QPSK (left), 16QAM (middle) and 64QAM (right) Figure 1-2: Constellation diagrams for QPSK (left), 16QAM (middle) and 64QAM (right)

The mapping of modulation symbols onto physical channel resources is The mapping of modulation symbols onto physical channel resources is very different compared to HSPA though. The nature of OFDM gives rise very different compared to HSPA though. The nature of OFDM gives rise to the concept of 2-dimensional radio resources. The information to be to the concept of 2-dimensional radio resources. The information to be transmitted over the radio interface is mapped onto a 2-dimensional transmitted over the radio interface is mapped onto a 2-dimensional time-frequency ‘resource grid’. The E-UTRA physical layer is described in all frequency ‘resource grid’. The E-UTRA physical layer is described in all its glorious detail in chapter 4.

its glorious detail in chapter 4.

(A common misunderstanding is that OFDM, by itself, makes very high (A common misunderstanding is that OFDM, by itself, makes very high bit rates possible. This is not true.

bit rates possible. This is not true. Rather, the very high bit rates mentionedRather, the very high bit rates mentioned for E-UTRA are made possible first and foremost by a higher transmission for E-UTRA are made possible first and foremost by a higher transmission bandwidth (up to 20MHz), higher order modulation (64QAM) and the bandwidth (up to 20MHz), higher order modulation (64QAM) and the support for MIMO with up t

support for MIMO with up to four antennas).o four antennas).

The channel and protocol architecture for E-UTRAN layer 2 and layer 3 is The channel and protocol architecture for E-UTRAN layer 2 and layer 3 is quite similar to the one used in UTRAN today. For example, the UE quite similar to the one used in UTRAN today. For example, the UE protocol stack is close to

protocol stack is close to identical and the channel hierarchy is still dividedidentical and the channel hierarchy is still divided

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into logical, transport and physical channels. The E-UTRAN protocol/ into logical, transport and physical channels. The E-UTRAN protocol/ channel architecture is described in chapter 4.

channel architecture is described in chapter 4.

The exact functionality of the layer 2 and layer 3 protocols in E-UTRAN The exact functionality of the layer 2 and layer 3 protocols in E-UTRAN is, at the time of writing, far from decided. An overview of the

is, at the time of writing, far from decided. An overview of the expected expected functionality can be found in chapter 5.

functionality can be found in chapter 5.

1.3

1.3 Evolved

Evolved Packet

Packet Core

Core (EPC)

(EPC)

1.3.1

1.3.1 Network

Network Architecture

Architecture

IMS / Internet /… IMS / Internet /… SGSN SGSN SGW SGW E-UTRAN E-UTRAN PGWPGW Gb/Iu Gb/Iu MME MME GERAN/ GERAN/ UTRAN UTRAN Non-3GPP Non-3GPP access access S1-MME S1-MME S1-U S1-U S2 S2 S5 S5 SGiSGi S11 S11 S4 S4 S3 S3 IMS / Internet /… IMS / Internet /… IMS / Internet /… IMS / Internet /… SGSN SGSN SGW SGW E-UTRAN E-UTRAN E-UTRAN E-UTRAN PGWPGW Gb/Iu Gb/Iu Gb/Iu Gb/Iu MME MME GERAN/ GERAN/ UTRAN UTRAN GERAN/ GERAN/ UTRAN UTRAN Non-3GPP Non-3GPP access access Non-3GPP Non-3GPP access access S1-MME S1-MME S1-MME S1-MME S1-U S1-U S1-U S1-U S2 S2 S2 S2 S5 S5 S5

S5 SGiSGiSGiSGi S11 S11 S11 S11 S4 S4 S3 S3

Figure 1-3: the Evolved Packet Core network architecture Figure 1-3: the Evolved Packet Core network architecture

Figure 1-3 shows the network architecture of the Evolved Packet Core Figure 1-3 shows the network architecture of the Evolved Packet Core (EPC). The EPC consists of three main nodes: the

(EPC). The EPC consists of three main nodes: the Mobility Management Mobility Management Entity

Entity (MME), the(MME), the Serving GatewayServing Gateway (SGW) and the(SGW) and the Packet Data Network Packet Data Network Gateway

Gateway (PGW). The MME may be co-located with the SGW, and the(PGW). The MME may be co-located with the SGW, and the SGW may be co-located with the PGW. Hence, the standard allows a SGW may be co-located with the PGW. Hence, the standard allows a completely collapsed ‘one-node’ core network or a distributed (easily completely collapsed ‘one-node’ core network or a distributed (easily scalable) core network, or any

scalable) core network, or any possible ‘combination’ in-between.possible ‘combination’ in-between. The MME connects to the E-UTRAN via the

The MME connects to the E-UTRAN via the S1-MME interfaceS1-MME interface and isand is present solely in the CP. It is responsible for handling mobility and present solely in the CP. It is responsible for handling mobility and security procedures, such as network Attach, Tracking Area updates security procedures, such as network Attach, Tracking Area updates (similar to Location/Routing Area updates) and authentication. The MME (similar to Location/Routing Area updates) and authentication. The MME also connects to the SGSN via the

also connects to the SGSN via the S3-interfaceS3-interface.. The SGW connects to the E-UTRAN via the

The SGW connects to the E-UTRAN via the S1-U interfaceS1-U interface and is presentand is present solely in the UP. Its prime responsibility is routing and forwarding of user solely in the UP. Its prime responsibility is routing and forwarding of user IP-packets. It acts as a UP anchor when the UE moves between 3GPP IP-packets. It acts as a UP anchor when the UE moves between 3GPP radio access technologies (

radio access technologies (S4-interfaceS4-interface).).

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The PGW connects to the SGW via the S5-interfaceS5-interface and to external packetand to external packet data networks (or IMS) via the

data networks (or IMS) via the SGi-interfaceSGi-interface. It is responsible for the. It is responsible for the enforcing of QoS and charging policies. It also acts as a UP anchor when enforcing of QoS and charging policies. It also acts as a UP anchor when the UE moves between 3GPP and

the UE moves between 3GPP and non-3GPP radio access (non-3GPP radio access (S2-interfaceS2-interface).). It should be noted that additional network nodes/functions, not shown in It should be noted that additional network nodes/functions, not shown in figure 1-3, might be present as well. For example, a Packet Data Gateway figure 1-3, might be present as well. For example, a Packet Data Gateway (PDG) is needed for non-trusted IP access and a Policy and Charging (PDG) is needed for non-trusted IP access and a Policy and Charging Rules Function (PCRF) is required for IMS controlled QoS and charging Rules Function (PCRF) is required for IMS controlled QoS and charging mechanisms. The EPC is described further in chapter 7.

mechanisms. The EPC is described further in chapter 7.

1.3.2

1.3.2 Requirements

Requirements on

on the

the EPC

EPC

A (rather long) list of general requirements has been set up as guidelines A (rather long) list of general requirements has been set up as guidelines for the standardisation work related to the EPC. Some of those are:

for the standardisation work related to the EPC. Some of those are:

•

• 3GPP and non-3GPP access systems shall be supported.3GPP and non-3GPP access systems shall be supported. •

• Scalable system architecture and solutions without compromisingScalable system architecture and solutions without compromising

the system capacity (e.g. by separating CP from UP). the system capacity (e.g. by separating CP from UP).

•

• CP response time shall be such that the UE can move from an idleCP response time shall be such that the UE can move from an idle

state to one where it is

state to one where it is sending/receiving data in less than 200 ms.sending/receiving data in less than 200 ms.

•

• Basic IP connectivity is established during the initial access phaseBasic IP connectivity is established during the initial access phase

(hence, the UE is ‘always-on’). (hence, the UE is ‘always-on’).

•

• The Mobility Management (MM) solution shall be able toThe Mobility Management (MM) solution shall be able to

accommodate terminals with different mobility requirements accommodate terminals with different mobility requirements (fixed, nomadic and mobile terminals).

(fixed, nomadic and mobile terminals).

•

• The MM functionality shall allow the network operator to controlThe MM functionality shall allow the network operator to control

the type of

the type of access system being used by a subscriber.access system being used by a subscriber.

•

• MM procedures shall provide seamless operation of both real-timeMM procedures shall provide seamless operation of both real-time

(e.g. VoIP) and non real-time applications. (e.g. VoIP) and non real-time applications.

•

• In order to maximise users' access opportunities, the architectureIn order to maximise users' access opportunities, the architecture

should allow a UE that is roaming to use a non-3GPP access (e.g. should allow a UE that is roaming to use a non-3GPP access (e.g. WLAN) network with which the VPLMN has a business WLAN) network with which the VPLMN has a business agreement. For example, it should be possible for a user to use a agreement. For example, it should be possible for a user to use a WLAN access network with which only the visited operator has a WLAN access network with which only the visited operator has a direct relationship (not the home operator).

direct relationship (not the home operator).

•

• The evolved system shall support Ipv4 and The evolved system shall support Ipv4 and Ipv6 connectivity.Ipv6 connectivity. •

• Access to the evolved system shall be possible with R99 USIM.Access to the evolved system shall be possible with R99 USIM.

(Please note that this does not explicitly allow access using SIM) (Please note that this does not explicitly allow access using SIM)

•

• The authentication framework should be independent from the The authentication framework should be independent from the usedused

access network technology. access network technology.

•

• Radio interface multicast capability shall be a built-in feature.Radio interface multicast capability shall be a built-in feature. •

• The SAE/LTE system shall support network-sharing functionality.The SAE/LTE system shall support network-sharing functionality. •

• It shall be possible to support service continuity between IMS overIt shall be possible to support service continuity between IMS over

SAE/LTE access and the Circuit Switched

SAE/LTE access and the Circuit Switched (CS) domain.(CS) domain.

•

• It shall be possible for the operator to provide the UE with accessIt shall be possible for the operator to provide the UE with access

network information pertaining to l

network information pertaining to locally supported 3GPP and non-ocally supported 3GPP and non-3GPP access technologies.

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1.4

1.4 Evolved

Evolved HSPA

HSPA (HSPA+)

(HSPA+)

Figure 1-4: Evolved HSPA network architecture Figure 1-4: Evolved HSPA network architecture

A parallel 3GPP R8 project to LTE and SAE is the Evolved High Speed A parallel 3GPP R8 project to LTE and SAE is the Evolved High Speed Packet Access, eHSPA, project (also referred to

Packet Access, eHSPA, project (also referred to as HSPA+). The proposedas HSPA+). The proposed eHSPA features represent a logical evolution from today’s HSDPA and eHSPA features represent a logical evolution from today’s HSDPA and HSUPA systems. Roughly speaking, the eHSPA project focuses on three HSUPA systems. Roughly speaking, the eHSPA project focuses on three areas: areas: cSGSN cSGSN Iur Iur IMS / Internet /… IMS / Internet /… xGGSN xGGSN

IIuu//GGnn GGii

RNC RNC NB NB Iu Iu Gn Gn cSGSN cSGSN Iur Iur IMS / Internet /… IMS / Internet /… xGGSN xGGSN

IIuu//GGnn GGii

RNC RNC NB NB Iu Iu Gn Gn cSGSN cSGSN Iur Iur IMS / Internet /… IMS / Internet /… IMS / Internet /… IMS / Internet /… xGGSN xGGSN Iu/Gn Iu/Gn

IIuu//GGnn GGiiGiGi

RNC RNC NB NB NB NB Iu Iu Iu Iu Gn Gn •

• Optimising HSPA for real-time packet data services, like VoIP. AOptimising HSPA for real-time packet data services, like VoIP. A

large part of achieving this goal relates to a more efficient use of large part of achieving this goal relates to a more efficient use of the HSPA control channels.

the HSPA control channels.

•

• Increasing the system and user throughput. This is achieved by theIncreasing the system and user throughput. This is achieved by the

introduction of higher order modulation (64QAM) and MIMO for introduction of higher order modulation (64QAM) and MIMO for HSPA. The theoretical maximum bit rate is around 40Mb/s for the HSPA. The theoretical maximum bit rate is around 40Mb/s for the DL and around 20Mb/s for the UL.

DL and around 20Mb/s for the UL.

•

• Simplifying the network architecture. The eHSPA NodeB will takeSimplifying the network architecture. The eHSPA NodeB will take

on parts of, or all of, the functionality of the RNC. In addition, the on parts of, or all of, the functionality of the RNC. In addition, the SGSN will be removed from the User Plane path (the so-called SGSN will be removed from the User Plane path (the so-called ‘one-tunnel solution’) allowing IP packets to be routed directly ‘one-tunnel solution’) allowing IP packets to be routed directly between eHSPA NodeB and GGSN. This can be seen in figure 1-4 between eHSPA NodeB and GGSN. This can be seen in figure 1-4 above, where ‘cSGSN’ is the

above, where ‘cSGSN’ is the SGSN Controller SGSN Controller , and ‘xGGSN’ is, and ‘xGGSN’ is the

the enhanced GGSN enhanced GGSN ..

Thus, E-UTRA/E-UTRAN and Evolved HSPA have many concepts in Thus, E-UTRA/E-UTRAN and Evolved HSPA have many concepts in common (collapsed architecture, 64QAM, MIMO). As a matter of fact, the common (collapsed architecture, 64QAM, MIMO). As a matter of fact, the performance (bit rates, spectral efficiency etc) of eHSPA is very close to performance (bit rates, spectral efficiency etc) of eHSPA is very close to the performance of E-UTRA with 5MHz channel bandwidth. This has led the performance of E-UTRA with 5MHz channel bandwidth. This has led to some level of debate whether to refer to eHSPA as a ‘migration path’ or to some level of debate whether to refer to eHSPA as a ‘migration path’ or a ‘complement’ or a

a ‘complement’ or a ‘competing technology’.‘competing technology’.

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1.5 References

23.401

23.401 GPRS GPRS enhancements enhancements for for Long Long Term Term Evolution Evolution (LTE)(LTE) 23.402

23.402 3GPP 3GPP SAE: SAE: Architecture Architecture enhancemeenhancements nts for for non-3GPP non-3GPP accessesaccesses 23.882

23.882 3GPP 3GPP SAE: SAE: Report Report on on technical technical options options and and conclusionsconclusions 25.912

25.912 Feasibility study Feasibility study for for E-UTRA E-UTRA and and E-UTRANE-UTRAN 25.913

25.913 Requirements for Requirements for E-UTRA E-UTRA and and E-UTRANE-UTRAN 25.999

25.999 High High Speed Speed Packet Packet Access Access (HSPA) (HSPA) evolution, evolution, FDDFDD 36.300

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Apis Technical Training AB Apis Technical Training AB LTE - OFDM

LTE - OFDM Copyright

2 OFDM

2.1

2.1 OFDM BOFDM BASICSASICS ... 2-22-2

2.1.1

2.1.1 IntroductionIntroduction ...2-2...2-2 2.1.2

2.1.2 Sub-carrieSub-carriers and rs and MultiplexingMultiplexing ...2-3...2-3 2.1.3

2.1.3 Orthogonality...2-4Orthogonality...2-4 2.1.4

2.1.4 Cyclic Prefixes...2-6Cyclic Prefixes...2-6

2.2

2.2 OFDM SOFDM SIGNALIGNALGGENERATIONENERATION ... 2-72-7

2.3

2.3 SC-FDSC-FDMA...MA... 2-82-8 2.4

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2.1

2.1 OFDM

OFDM Basics

Basics

(The theory behind OFDM is very mathematical in its nature. The following is just a brief (The theory behind OFDM is very mathematical in its nature. The following is just a brief overview in ‘layman’s terms’ to

overview in ‘layman’s terms’ to convey the basic characteristics of OFDM. For a convey the basic characteristics of OFDM. For a deeper deeper understanding of OFDM it is recommended to consult a textbook on the subject)

understanding of OFDM it is recommended to consult a textbook on the subject)

2.1.1 Introduction

Orthogonal Frequency Division Multiplexing (OFDM) is a digital Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme that uses a large number of closely-spaced carrier modulation scheme that uses a large number of closely-spaced orthogonal

orthogonal sub-carrierssub-carriers. Each sub-carrier is modulated with a. Each sub-carrier is modulated with a conventional modulation scheme (such as 16QAM) at a low symbol rate, conventional modulation scheme (such as 16QAM) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. The primary advantage of OFDM over schemes in the same bandwidth. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions single-carrier schemes is its ability to cope with severe channel conditions without complex equalization filters. Low symbol rate makes the use of a without complex equalization filters. Low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle guard interval between symbols affordable, making it possible to handle time-spreading and inter-symbol interference (ISI).

time-spreading and inter-symbol interference (ISI).

OFDM has only become widely used during the last decade or so, but the OFDM has only become widely used during the last decade or so, but the technology as such is about 50 years old (it was first used around 1957 in technology as such is about 50 years old (it was first used around 1957 in an experimental communications system developed for the US Navy). an experimental communications system developed for the US Navy). During the 70’s and 80’s several important theoretical contributions from During the 70’s and 80’s several important theoretical contributions from various sources made it possible to implement more efficient and robust various sources made it possible to implement more efficient and robust OFDM-based systems. Today, OFDM has proved itself as the preferred OFDM-based systems. Today, OFDM has proved itself as the preferred radio access technology in a wide variety of communication systems. radio access technology in a wide variety of communication systems. Some examples of OFDM use: IEEE 802.11a/g (WLAN/WiFi), IEEE Some examples of OFDM use: IEEE 802.11a/g (WLAN/WiFi), IEEE 802.16 (WiMAX), Digital Audio Broadcasting (DAB), Digital Video 802.16 (WiMAX), Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB-T and DVB-H) and Asynchronous Digital Subscriber Broadcasting (DVB-T and DVB-H) and Asynchronous Digital Subscriber Line (ADSL).

Line (ADSL).

Some advantages of OFDM: Some advantages of OFDM:

•

• Allows adaptation to severe channel conditions without veryAllows adaptation to severe channel conditions without very

complex equalization methods complex equalization methods

•

• Robust against narrow-band co-channel interferenceRobust against narrow-band co-channel interference •

• Robust against Inter-Symbol Interference (ISI) Robust against Inter-Symbol Interference (ISI) and fading causedand fading caused

by multipath propagation by multipath propagation

•

• High spectral efficiencyHigh spectral efficiency •

• Efficient implementation using FFTEfficient implementation using FFT •

• Low sensitivity to time synchronization errorsLow sensitivity to time synchronization errors •

• Facilitates Single Frequency Networks (iFacilitates Single Frequency Networks (i.e. synchronised broadcast.e. synchronised broadcast

from several transmitters). from several transmitters). Some disadvantages of OFDM: Some disadvantages of OFDM:

•

• Sensitive to Doppler shiftSensitive to Doppler shift •

• Sensitive to frequency synchronization problemsSensitive to frequency synchronization problems •

• High peak-to-average power ratio (PAPR), requiring High peak-to-average power ratio (PAPR), requiring moremore

expensive transmitter circuitry and lowering power

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2.1.2

2.1.2 Sub-carriers

Sub-carriers and

and Multiplexing

Multiplexing

As we have already mentioned in chapter 1, OFDM spreads the data to be As we have already mentioned in chapter 1, OFDM spreads the data to be transmitted over a large number of sub-carriers- typically more than a transmitted over a large number of sub-carriers- typically more than a thousand. The data rate to be conveyed by each of these sub-carriers is thousand. The data rate to be conveyed by each of these sub-carriers is correspondingly reduced

correspondingly reduced, thus transforming a , thus transforming a single high bitrate channel tosingle high bitrate channel to many low bitrate

many low bitrate channels.channels.

It follows that the modulation (OFDM) symbol length is in turn extended, It follows that the modulation (OFDM) symbol length is in turn extended, which dramatically reduces the system’s sensitivity to inter-symbol which dramatically reduces the system’s sensitivity to inter-symbol interference due to multipath effects (i.e. different versions, or ‘echoes’, of interference due to multipath effects (i.e. different versions, or ‘echoes’, of the same signal travelling different paths over the radio interface and the same signal travelling different paths over the radio interface and arriving at the receiver at different points in time, causing interference). arriving at the receiver at different points in time, causing interference). This is true as long as the maximum delay of the ‘echoes’ is smaller than This is true as long as the maximum delay of the ‘echoes’ is smaller than the OFDM symbol time duration.

the OFDM symbol time duration.

Symbol n Symbol n S Syymmbbool l nn--33 SSyymmbbool l nn--22 n n--11 nn++11 SSyymmbboollnn S Syymmbbool l nn--11 SSyymmbbool l nn n n--11 nn++11 Main Path: Main Path: Delayed Path: Delayed Path: LONG delay LONG delay (or short symbols) (or short symbols)

SHORT delay SHORT delay (or long symbols) (or long symbols)

B

Bootth h ccaauusse e IISSII CCaauussees s IISSII AAdddds s coconnssttrruuccttiivveellyy or destructively or destructively Symbol n Symbol n S Syymmbbool l nn--33 SSyymmbbool l nn--22 n n--11 nn++11 SSyymmbboollnn S Syymmbbool l nn--11 SSyymmbbool l nn n n--11 nn++11 Main Path: Main Path: Delayed Path: Delayed Path: LONG delay LONG delay (or short symbols) (or short symbols)

B

Bootth h ccaauusse e IISSII CCaauussees s IISSII AAdddds s coconnssttrruuccttiivveellyy or destructively or destructively Symbol n Symbol n S Syymmbbool l nn--33 SSyymmbbool l nn--22 n n--11 nn++11 SSyymmbboollnn S Syymmbbool l nn--11 SSyymmbbool l nn n n--11 nn++11 Main Path: Main Path: Delayed Path: Delayed Path: LONG delay LONG delay (or short symbols) (or short symbols)

B

Bootth h ccaauusse e IISSII CCaauussees s IISSII AAdddds s coconnssttrruuccttiivveellyy or destructively or destructively

Figure 2-1: multipath delays versus symbol length Figure 2-1: multipath delays versus symbol length

With hundreds or thousands of sub-carriers available it becomes quite With hundreds or thousands of sub-carriers available it becomes quite straightforward how to multiplex users on the radio interface. Simply straightforward how to multiplex users on the radio interface. Simply allocate different sets of sub-carriers to different users (this is the ‘FDM’ allocate different sets of sub-carriers to different users (this is the ‘FDM’ in OFDM). More complex multiplexing schemes can be implemented by in OFDM). More complex multiplexing schemes can be implemented by allowing users to share the available sub-carriers both in the frequency allowing users to share the available sub-carriers both in the frequency domain (FDM) and the time domain (TDM).

domain (FDM) and the time domain (TDM).

ff F F D D M M : : S S u u b b - - c c a a r r r r i i e e r r m m

u u l l t t i i p p l l e e x x i i n n g g tt TDM:

TDM: radio frame multiplexingradio frame multiplexing

Sub-carrier 1 Sub-carrier 1 Sub-carrier n Sub-carrier n

R

Raaddiio o ffrraamme e 11 RRaaddiio o ffrraamme e mm

ff F F D D M M : : S S u u b b - - c c a a r r r r i i e e r r m m

u u l l t t i i p p l l e e x x i i n n g g tt TDM:

TDM: radio frame multiplexingradio frame multiplexing

Sub-carrier 1 Sub-carrier 1 Sub-carrier n Sub-carrier n

R

Raaddiio o ffrraamme e 11 RRaaddiio o ffrraamme e mm

Figure 2-2: OFDM multiplexing using both FDM and TDM Figure 2-2: OFDM multiplexing using both FDM and TDM

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Furthermore, sub-carrier frequency hopping schemes may be applied to Furthermore, sub-carrier frequency hopping schemes may be applied to reduce fading effects that are frequency selective. The transmitter (base reduce fading effects that are frequency selective. The transmitter (base station) can also order the receivers (mobile stations) to send feedback station) can also order the receivers (mobile stations) to send feedback information in the form of channel quality reports. This allows dynamic information in the form of channel quality reports. This allows dynamic channel dependent scheduling in the base station, making sure that each channel dependent scheduling in the base station, making sure that each mobile station is always allocated a subset of sub-carriers where it mobile station is always allocated a subset of sub-carriers where it experiences the least amount of interference. A graphical representation of experiences the least amount of interference. A graphical representation of frequency selective fading effects can be seen in

frequency selective fading effects can be seen in figure 2-3 below.figure 2-3 below.

Figure 2-3: frequency selective fading effect on OFDM sub-carriers Figure 2-3: frequency selective fading effect on OFDM sub-carriers

E-UTRA combines OFDM with FDM and TDM multiplexing schemes as E-UTRA combines OFDM with FDM and TDM multiplexing schemes as well as

well as sub-carrier frequency hopping and channel sub-carrier frequency hopping and channel dependent scheduling.dependent scheduling.

2.1.3 Orthogonality

In traditional FDM different users are allocated different frequencies, or In traditional FDM different users are allocated different frequencies, or channels, for their transmission (e.g. analog 1G systems such as NMT). To channels, for their transmission (e.g. analog 1G systems such as NMT). To avoid interference between these channels the FDM frequencies must be avoid interference between these channels the FDM frequencies must be spaced apart- there must be a guard band between them. This leads to spaced apart- there must be a guard band between them. This leads to waste of the

waste of the available frequency spectrum.available frequency spectrum.

Figure 2-4: FDM versus OFDM (spectrum efficiency) Figure 2-4: FDM versus OFDM (spectrum efficiency)

In OFDM, the frequencies of the individual sub-carriers are chosen in such In OFDM, the frequencies of the individual sub-carriers are chosen in such a way that they do not interfere with each other- they are orthogonal (this a way that they do not interfere with each other- they are orthogonal (this is the ‘O’ in OFDM). The demodulator for one sub-carrier does not 'see' is the ‘O’ in OFDM). The demodulator for one sub-carrier does not 'see'

FDM

FDM:: guard band between carriersguard band between carriers

Saving bandwidth Saving bandwidth OFDM

OFDM:: carriers can be packed tightercarriers can be packed tighter

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the modulation of the others, so there is no crosstalk between sub-carriers the modulation of the others, so there is no crosstalk between sub-carriers even though their spectra overlap. This allows us to ‘pack’ the sub-carriers even though their spectra overlap. This allows us to ‘pack’ the sub-carriers much more densely than in a traditional FDM system, thus increasing much more densely than in a traditional FDM system, thus increasing spectrum efficiency.

spectrum efficiency.

All the sub-carriers allocated to a given user are transmitted in parallel. All the sub-carriers allocated to a given user are transmitted in parallel. Fortunately, the apparently very complex processes of modulating (and Fortunately, the apparently very complex processes of modulating (and demodulating) thousands of sub-carriers simultaneously are equivalent to demodulating) thousands of sub-carriers simultaneously are equivalent to Discrete Fourier Transform (DFT) operations for which efficient Fast Discrete Fourier Transform (DFT) operations for which efficient Fast Fourier Transform (FFT) algorithms exist, allowing affordable Fourier Transform (FFT) algorithms exist, allowing affordable mass-produced transceivers.

produced transceivers.

To ensure orthogonality the sub-carriers must have a common, precisely To ensure orthogonality the sub-carriers must have a common, precisely chosen frequency spacing (‘carrier spacing’). This frequency spacing is chosen frequency spacing (‘carrier spacing’). This frequency spacing is exactly the inverse of the

exactly the inverse of the OFDM symbol duration, called theOFDM symbol duration, called the active symbolactive symbol period

period , over which the receiver will demodulate the signal. In the case of , over which the receiver will demodulate the signal. In the case of E-UTRA the carrier spacing is 15kHz (7.5kHz for MBMS dedicated cells). E-UTRA the carrier spacing is 15kHz (7.5kHz for MBMS dedicated cells).

Received signals are Received signals are evaluated at their maximum evaluated at their maximum

Figure 2-5: orthogonal OFDM sub-carriers (frequency domain) Figure 2-5: orthogonal OFDM sub-carriers (frequency domain)

Figure 2-5 shows a few sub-carriers represented in the frequency domain Figure 2-5 shows a few sub-carriers represented in the frequency domain (compare figure 2-3). The receiver will demodulate (or sample) each (compare figure 2-3). The receiver will demodulate (or sample) each sub-carrier precisely where it has it maximum value. Due to the carrier precisely where it has it maximum value. Due to the ‘precisely-chosen frequency spacing’ all other sub-carrier have the value zero at this chosen frequency spacing’ all other sub-carrier have the value zero at this precise frequency, despite that they overlap, thus not creating any precise frequency, despite that they overlap, thus not creating any interference at all.

interference at all.

However, this nice relationship between sub-carriers can be destroyed, However, this nice relationship between sub-carriers can be destroyed, resulting in loss of orthogonality and severe bit error rates as a result. The resulting in loss of orthogonality and severe bit error rates as a result. The bit errors can be rectified to some extent with the use of error correcting bit errors can be rectified to some extent with the use of error correcting codes, so called Forward Error Correction (FEC). A combination of FEC codes, so called Forward Error Correction (FEC). A combination of FEC and OFDM is called Coded OFDM (COFDM). In E-UTRA, OFDM is and OFDM is called Coded OFDM (COFDM). In E-UTRA, OFDM is combined with Turbo coding.

combined with Turbo coding.

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Such loss of orthogonality can be caused by frequency synchronisation Such loss of orthogonality can be caused by frequency synchronisation errors due to slight differences in the local oscillators, used for frequency errors due to slight differences in the local oscillators, used for frequency generation, in the transmitter and the receiver. Another cause is Doppler generation, in the transmitter and the receiver. Another cause is Doppler effects arising from the relative motion between the transmitter and the effects arising from the relative motion between the transmitter and the receiver- an effect that must be taken seriously in any mobile system!

receiver- an effect that must be taken seriously in any mobile system!

2.1.4

2.1.4 Cyclic

Cyclic Prefixes

Prefixes

As mentioned earlier, OFDM is robust against multipath fading due to the As mentioned earlier, OFDM is robust against multipath fading due to the long OFDM symbol duration. However, there will always be

long OFDM symbol duration. However, there will always be somesome inter-

inter-symbol interference due to multipath echoes, even for OFDM. A further symbol interference due to multipath echoes, even for OFDM. A further refinement therefore adds the concept of a

refinement therefore adds the concept of a guard intervalguard interval. Each OFDM. Each OFDM symbol is transmitted for a total symbol period that is longer than the symbol is transmitted for a total symbol period that is longer than the active symbol period by a period called the guard interval or guard period. active symbol period by a period called the guard interval or guard period.

n-1 n-1 n n--11 nn++11 Main Path: Main Path: Delayed Path: Delayed Path: Guard Guard period period Useful Useful part part Symbol n Symbol n CP CP ISI only ISI only during CP during CP n n n-1 n-1 n n--11 nn++11 Main Path: Main Path: Delayed Path: Delayed Path: Guard Guard period period Useful Useful part part Symbol n Symbol n CP CP ISI only ISI only during CP during CP n n n-1 n-1 n n--11 nn++11 Main Path: Main Path: Delayed Path: Delayed Path: Guard Guard period period Useful Useful part part Symbol n Symbol n CP CP Symbol nSymbol n CP CP ISI only ISI only during CP during CP n n

Figure 2-6: guard interval with cyclic prefix Figure 2-6: guard interval with cyclic prefix

This means that the receiver will experience neither symbol nor This means that the receiver will experience neither symbol nor inter-carrier interference provided that any echoes present in the signal have a carrier interference provided that any echoes present in the signal have a delay that does not exceed the guard interval. Naturally, the

delay that does not exceed the guard interval. Naturally, the addition of theaddition of the guard interval reduces the data capacity by an amount dependent on its guard interval reduces the data capacity by an amount dependent on its length. Different systems use different (relative) lengths of the guard length. Different systems use different (relative) lengths of the guard interval, common values being 5-25% of the OFDM symbol length.

interval, common values being 5-25% of the OFDM symbol length.

There are several ways to ‘fill’ the guard interval with information (to There are several ways to ‘fill’ the guard interval with information (to avoid turning the transmitter on and off abruptly). A common mechanism avoid turning the transmitter on and off abruptly). A common mechanism is the use of a so-called

is the use of a so-called cyclic prefixcyclic prefix. A cyclic prefix (CP) is created. A cyclic prefix (CP) is created simply by selecting the last part of an OFDM symbol, make a copy of it simply by selecting the last part of an OFDM symbol, make a copy of it and place the copy in front of the symbol (hence the term ‘prefix’). The and place the copy in front of the symbol (hence the term ‘prefix’). The concept of a guard interval is illustrated in

concept of a guard interval is illustrated in figure 2-6 above.figure 2-6 above.

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By doing so, a continuous signal is created (easier to implement) and any By doing so, a continuous signal is created (easier to implement) and any multipath delayed echoes will only cause interference in the CP part of the multipath delayed echoes will only cause interference in the CP part of the received OFDM symbol. The receiver treats the CP portion of the OFDM received OFDM symbol. The receiver treats the CP portion of the OFDM symbol as ‘rubbish’ and removes it prior to demodulating the information. symbol as ‘rubbish’ and removes it prior to demodulating the information. E-UTRA defines a ‘normal’ length and an ‘extended’ length of the CP, to E-UTRA defines a ‘normal’ length and an ‘extended’ length of the CP, to cater for the different requirements of small versus large cells. There are cater for the different requirements of small versus large cells. There are also different CP lengths defined for MBMS transmission, when multiple also different CP lengths defined for MBMS transmission, when multiple synchronised eNBs act as a Single Frequency Network (SFN).

synchronised eNBs act as a Single Frequency Network (SFN).

2.2

2.2 OFDM

OFDM Signal

Signal Generation

Generation

S S P P II F F F F T T Add Add CP CP RFRF … … Coding Coding Modulation Modulation ffoo S S P P II F F F F T T Add Add CP CP RFRF … … Coding Coding Modulation Modulation ffoo S S P P II F F F F T T Add Add CP CP RFRF … … Coding Coding Modulation Modulation ffoo

Figure 2-7: the OFDM transmitter using IFFT Figure 2-7: the OFDM transmitter using IFFT

There are several ways to realize an OFDM transmit-receive chain. For There are several ways to realize an OFDM transmit-receive chain. For example, the addition of a cyclic prefix is not mandatory and filtering/ example, the addition of a cyclic prefix is not mandatory and filtering/ equalization of the baseband signal (the ‘RF’ box in fig 2-7)

equalization of the baseband signal (the ‘RF’ box in fig 2-7) can be done incan be done in many different ways. Thus, figure 2-7 below does not represent

many different ways. Thus, figure 2-7 below does not represent thethe way toway to create an OFDM signal.

create an OFDM signal. Coding and Modulation

Coding and Modulation: this step is any conventional Forward Error: this step is any conventional Forward Error Correction (FEC) mechanism, such as convolutional coding, and any Correction (FEC) mechanism, such as convolutional coding, and any conventional modulation scheme, such as QPSK or 64QAM.

conventional modulation scheme, such as QPSK or 64QAM. Serial-to-Parallel

Serial-to-Parallel : a group of modulation symbols are ‘fed’ to the Inverse: a group of modulation symbols are ‘fed’ to the Inverse Fast Fourier Transform (IFFT) in parallel. The number of modulation Fast Fourier Transform (IFFT) in parallel. The number of modulation symbols fed to IFFT

symbols fed to IFFT equals the number of assigned sub-carriers.equals the number of assigned sub-carriers. Inverse Fast Fourier Transform

Inverse Fast Fourier Transform: each modulation symbol is used for: each modulation symbol is used for modulating one sub-carrier, in effect acting as a complex wight setting the modulating one sub-carrier, in effect acting as a complex wight setting the amplitude and phase of the sub-carrier. These modulated sub-carriers are amplitude and phase of the sub-carrier. These modulated sub-carriers are then summed together, creating one OFDM symbol.

then summed together, creating one OFDM symbol. Cyclic Prefix

Cyclic Prefix: the last portion of the OFDM symbol is copied and: the last portion of the OFDM symbol is copied and appended at the ‘front’ of the symbol. This creates a guard interval with appended at the ‘front’ of the symbol. This creates a guard interval with well-defined content.

well-defined content.