eng LTE radio network design ISSUE for huke

Figure 1-1 LTE Reference Architecture Whilst UMTS is based upon WCDMA technology, the 3GPP developed new specifications for the LTE air interface based upon OFDMA Orthogonal Frequency D

Contents

1 LTE Architecture 1-11.1 EPS Architecture 1-21.1.1 User Equipment 1-21.1.2 Evolved Node B 1-41.1.3 Mobility Management Entity 1-51.1.4 Serving Gateway 1-61.1.5 Packet Data Network - Gateway 1-71.2 E-UTRAN Architecture and Interfaces 1-81.2.1 Uu Interface 1-81.2.2 X2 Interface 1-91.2.3 X2 Interface - X2 Application Protocol 1-91.2.4 X2 Interface - Stream Control Transmission Protocol 1-91.2.5 X2 Interface - GPRS Tunneling Protocol - User 1-101.2.6 S1 Interface 1-101.2.7 S1 Interface - S1 Application Protocol 1-101.2.8 S1 Interface - SCTP and GTP-U 1-111.3 UE States and Areas 1-111.3.1 RRC State Interaction 1-121.3.2 Interaction with CDMA2000 States 1-131.3.3 Tracking Areas 1-14

2 LTE Air Interface 2-12.1 LTE Access Techniques 2-22.1.1 Principles of OFDM 2-22.1.2 Frequency Division Multiplexing 2-32.1.3 OFDM Subcarriers 2-32.1.4 Fast Fourier Transforms 2-42.1.5 LTE FFT Sizes 2-42.1.6 OFDM Symbol Mapping 2-52.1.7 Time Domain Interference 2-62.1.8 General OFDMA Structure 2-82.1.9 Physical Resource Blocks and Resource Elements 2-92.1.10 SC-FDMA Signal Generation 2-10

Contents

LTE Radio Network Design

Training Manual 2.2 Channel Coding in LTE 2-132.2.1 Channel Coding 2-132.2.2 Modulation and Coding Scheme 2-142.3 LTE Channel Structure 2-172.3.1 Logical Channels 2-172.3.2 Transport Channels 2-192.3.3 Physical Channels 2-192.3.4 Radio Channels 2-202.3.5 Channel Mapping 2-202.4 LTE Data Rates 2-222.4.1 Physical Data Rates 2-232.4.2 Downlink Overheads 2-252.4.3 Uplink Overhead 2-282.4.4 Total Physical Overhead 2-332.5 UE Categories 2-34

3 LTE Traffic 3-13.1 Traffic Types Carried by LTE Networks 3-23.2 Transport Layer Protocols 3-23.2.1 User Datagram Protocol 3-33.2.2 Transmission Control Protocol 3-33.3 Protocols used in Support of Various Traffic Types 3-53.3.1 Real Time Services 3-53.3.2 Web Browsing 3-73.3.3 File Transfer 3-73.4 Issues Surrounding Voice over LTE 3-93.4.1 PDCP ROHC 3-9

4 Radio Planning Process 4-14.1 Radio Planning Process 4-24.1.1 Pre-Planning 4-24.1.2 Detailed Planning 4-34.1.3 Optimization 4-64.2 Frequency Deployment Options 4-64.2.1 LTE Bands 4-64.2.2 Spectrum Refarming 4-84.2.3 Advanced Wireless Services 4-84.2.4 700MHz Deployment 4-8

5 LTE Link Budget 5-15.1 Cell Coverage and Range 5-25.2 Link Budget 5-25.2.1 Tx Parameters 5-25.2.2 Rx Parameters 5-3

5.2.3 Rx Sensitivity 5-4 5.2.4 Propagation Margins 5-4 5.2.5 Maximum Allowable Path Loss 5-4

6 Coverage and Capacity Planning 6-1 6.1 Coverage Planning 6-2 6.1.1 Radio Propagation 6-2 6.1.2 Radio Channel 6-2 6.1.3 Propagation Models 6-4 6.1.4 Cell Range and Coverage 6-5 6.2 Capacity Planning 6-6 6.2.1 Cell/ Site Capacity 6-6 6.3 Optimization 6-7 6.3.1 Pre-Launch Optimization 6-7 6.3.2 Post-Launch Optimization 6-7

7 Huawei LTE Tools 7-1 7.1 Huawei Tools 7-2 7.1.1 U-Net - Professional Radio Network Planning Tool 7-2 7.1.2 Probe & Assistant - Drive Testing & Data Analysis Tool 7-3 7.1.3 Nastar - Network Performance Analysis Tool 7-3 7.2 GENEX U-Net for LTE 7-4 7.2.1 Product Overview 7-4 7.2.2 U-Net LTE Planning Functions 7-4 7.2.3 Simulation 7-8 7.2.4 Neighbor Cell and PCI Planning 7-9

8 Glossary 8-1

Contents

LTE Radio Network Design

Training Manual

Figures

Figure 1-1 LTE Reference Architecture 1-2 Figure 1-2 User Equipment Functional Elements 1-3 Figure 1-3 Evolved Node B Functional Elements 1-4 Figure 1-4 MME Functional Elements 1-6 Figure 1-5 S-GW Functional Elements 1-7 Figure 1-6 PDN-GW Functional Elements 1-7 Figure 1-7 E-UTRAN Interfaces 1-8 Figure 1-8 Uu Interface Protocols 1-8 Figure 1-9 X2 Interface Protocols 1-9 Figure 1-10 S1 Interface Protocols 1-10 Figure 1-11 RRC States 1-12 Figure 1-12 E-UTRA RRC State Interaction 1-13 Figure 1-13 Mobility Procedures between E-UTRA and CDMA2000 1-13 Figure 1-14 Tracking Areas 1-14 Figure 2-1 Orthogonal Frequency Division Multiple Access 2-2 Figure 2-2 Use of OFDM in LTE 2-2 Figure 2-3 FDM Carriers 2-3 Figure 2-4 OFDM Subcarriers 2-3 Figure 2-5 Inverse Fast Fourier Transform 2-4 Figure 2-6 Fast Fourier Transform 2-4 Figure 2-7 OFDM Symbol Mapping 2-5 Figure 2-8 OFDM PAPR (Peak to Average Power Ratio) 2-6 Figure 2-9 Delay Spread 2-6 Figure 2-10 Inter Symbol Interference 2-7 Figure 2-11 Cyclic Prefix 2-8 Figure 2-12 OFDMA in LTE 2-9

Figure 3-7 Web Browsing Using HTTP 3-7 Figure 3-8 TCP Connections Required for FTP 3-8 Figure 3-9 FTP Data Connection Establishment 3-9 Figure 3-10 Overheads Associated with a Voice Packet 3-9 Figure 3-11 ROHC Feedback 3-10 Figure 4-1 Radio Planning Process 4-2 Figure 4-2 Pre-Planning Dimensioning 4-3 Figure 4-3 Model Tuning 4-4 Figure 4-4 Site Selection 4-5 Figure 4-5 Cell and Site Coverage Planning 4-5 Figure 5-1 Path Loss and Cell Range 5-2 Figure 6-1 Radio Channel Propagation 6-2 Figure 6-2 Impact of Shadowing and Multipath 6-3 Figure 6-3 LTE Site Dimensioning 6-6 Figure 7-1 LTE Tools 7-2 Figure 7-2 U-Net LTE Planning Procedure 7-4 Figure 7-3 RF Results 7-5 Figure 7-4 U-Net Traffic Parameters 7-6 Figure 7-5 Example U-Net Coverage Predictions 7-7 Figure 7-6 U-Net Monte Carlo Statistics 7-8 Figure 7-7 PCI Planning 7-10

Figures

LTE Radio Network Design

Training Manual

Tables

Table 2-1 LTE Channel and FFT Sizes 2-5 Table 2-2 Downlink PRB Parameters 2-10 Table 2-3 Transport Channel Coding Options 2-14 Table 2-4 Control Information Coding Options 2-14 Table 2-5 Modulation and TBS index table for PDSCH 2-14 Table 2-6 LTE Channel and FFT Sizes 2-23 Table 2-7 LTE FDD Downlink Peak Rates (FDD using Normal CP) 2-23 Table 2-8 LTE FDD Uplink Peak Rates (FDD using Normal CP) 2-24 Table 2-9 PUCCH Overhead 2-30 Table 2-10 PRACH Configuration Index 2-31 Table 2-11 Downlink Physical Channel Overhead 2-33 Table 2-12 Uplink Physical Channel Overhead 2-33 Table 2-13 UE Categories 2-34 Table 3-1 3-2 Table 3-2 Port Allocations 3-3 Table 4-1 Business Model Inputs 4-3 Table 4-2 LTE Release 8 FDD Frequency Bands 4-7 Table 4-3 LTE Release 8TDD Frequency Bands 4-7 Table 5-1 LTE Downlink and Uplink Link Budget 5-3 Table 6-1 Example of Cost 231 Hata Cell Ranges 6-5

Tables

LTE Radio Network Design

Training Manual

1 LTE Architecture

Objectives

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

1.1 Describe the structure of the Evolved Packet System

1.2 List the nodes and interfaces that make up the Evolved UTRAN

1.3 Explain the LTE UE states and area concepts

Network) and the EPC (Evolved Packet Core) whereas support for service delivery lies in the IMS (IP Multimedia Subsystem) This reference architecture can be seen in Figure 1-1

Figure 1-1 LTE Reference Architecture

Whilst UMTS is based upon WCDMA technology, the 3GPP developed new specifications for 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 - Universal Terrestrial Radio Access)

1.1.1 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 the

ME (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 Resource Control), 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 the mobility 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 be default EPS bearer contexts or dedicated EPS bearer contexts

Figure 1-2 User Equipment Functional Elements

UserPlane

EPS Session ManagementBearer Activation

Bearer ModificationBearer Deactivation

Radio ResourceRRC, PDCP, RLC, MAC &

PHY Layer Protocols

EPS Mobility Management

RegistrationTracking Area Update

Handover

In terms of the Physical 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 adaptive modulation including QPSK (Quadrature Phase Shift Keying), 16QAM (16 Quadrature Amplitude Modulation) and 64QAM (Quadrature Amplitude Modulation)

IMEI (International Mobile Equipment Identity) - is used to uniquely identify the ME It can 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 a subscriber’s IMSI during NAS (Non Access Stratum) signaling between the UE and

1 LTE Architecture

LTE Radio Network Design

Training Manual 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

1.1.2 Evolved Node B

In addition to the new air interface, a new base station has also been specified by the 3GPP and is referred to as an eNB (Evolved Node B) These, along with their associated interfaces form the E-UTRAN and in so doing, are responsible for:

RRM (Radio Resource Management) - this involves the allocation to the UE of the physical resources on the uplink and downlink, access control and mobility control

Data Compression - is performed in both the eNB and the UE in order to maximize the amount of user data that can be transferred on the allocated resource This process is undertaken by PDCP

Data Protection - is performed at the eNB and the UE in order to encrypt and integrity protect RRC signaling and encrypt user data on the air interface

Routing - this involves the forwarding of Control Plane signaling to the MME and User Plane traffic to the S-GW (Serving - Gateway)

Packet Classification - this involves the “marking” of uplink packets based upon subscription information or local service provider policy

Figure 1-3 Evolved 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 takes place In addition, IPSec may also be used to protect user data within both the E-UTRAN and EPC

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

ECGI (E-UTRAN Cell Global Identifier) - is comprised of the MCC, MNC and ECI (Evolved Cell Identity), the latter being coded by each service provider

Femto Cells

In order to improve both network coverage and capacity, the 3GPP have developed a new type

of base station to operate within the home or small business environment Termed the HeNB (Home Evolved Node B), this network element forms part of the E-UTRAN and in so doing supports the standard E-UTRAN interfaces However, it must be stated that HeNBs do not support the X2 interface

The architecture may include a HeNB-GW (Home Evolved Node B - Gateway) which resides between the HeNB in the E-UTRAN and the MME / S-GW in the EPC in order to scale and support large numbers of base station deployments

1.1.3 Mobility Management Entity

The MME is the Control Plane entity within the EPC and as such is responsible for the following functions:

NAS Signaling and Security - this incorporates both EMM (EPS Mobility Management) and ESM (EPS Session Management) and thus includes procedures such as Tracking Area Updates and EPS Bearer Management The MME is also responsible for NAS security

S-GW and PDN-GW Selection - upon receipt of a request from the UE to allocate a bearer resource, the MME will select the most appropriate S-GW and PDN-GW This selection criterion is based on the location of the UE in addition to current load conditions within the network

Tracking Area List Management and Paging - whilst in the LTE Idle state, the UE is tracked by the MME to the granularity of a Tracking Area Whilst UEs remain within the Tracking Areas provided to them in the form of a Tracking Area List, there is no

requirement for them to notify the MME The MME is also responsible for initiating the paging procedure

Inter MME Mobility - if a handover involves changing the point of attachment within the EPC, it may be necessary to involve an inter MME handover In this situation, the serving MME will select a target MME with which to conduct this process

Authentication - this involves interworking with the subscriber’s HSS (Home Subscriber Server) in order to obtain AAA (Access Authorization and Accounting) information with which to authenticate the subscriber Like that of other 3GPP systems, authentication is based on AKA (Authentication and Key Agreement)

Downlink Packet Buffering - when traffic arrives for a UE at the S-GW, it may need to

be buffered in order to allow time for the MME to page the UE and for it to enter the LTE Active state

Packet Routing and Forwarding - traffic must be routed to the correct eNB on the downlink and the specified PDN-GW on the uplink

Lawful Interception - this incorporates the monitoring of VoIP (Voice over IP) and other packet services

GTP/PMIP Support - if PMIP (Proxy Mobile IP) is used on the S5/S8 Interfaces, the S-GW must support MAG (Mobile Access Gateway) functionality Furthermore, support for GTP/PMIP chaining may also be required

Figure 1-5 S-GW Functional Elements

1.1.5 Packet Data Network - Gateway

The PDN-GW is the network element which terminates the SGi Interface towards the PDN (Packet Data Network) If a UE is accessing multiple PDNs, there may be a requirement for multiple PDN-GWs to be involved Functions associated with the PDN-GW include:

Packet Filtering - this incorporates the deep packet inspection of IP datagrams arriving from the PDN in order to determine which TFT (Traffic Flow Template) they are to be associated with

Lawful Interception - as with the S-GW, the PDN-GW may also monitor traffic as it passes across it

IP Address Allocation - IP addresses may be allocated to the UE by the PDN-GW This is included as part of the initial bearer establishment phase or when UEs roam between different access technologies

Transport Level Packet Marking - this involves the marking of uplink and downlink packets with the appropriate tag e.g DSCP (Differentiated Services Code Point) based

on the QCI (QoS Class Identifier) of the associated EPS bearer

Accounting - through interaction with a PCRF (Policy Rules and Charging Function), the PDN-GW will monitor traffic volumes and types

Figure 1-6 PDN-GW Functional Elements

1 LTE Architecture

LTE Radio Network Design

Training Manual 1.2 E-UTRAN Architecture and Interfaces

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 traverse them Figure 1-7 illustrates the main interfaces in the E-UTRAN

Figure 1-7 E-UTRAN Interfaces

1.2.1 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 in the Access Stratum and EMM (EPS Mobility Management)/ ESM (EPS Session Management) in the Non Access Stratum In contrast, the User Plane is designed to carry IP datagrams However, both Control and User Planes utilize the services of the lower layers, namely PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control) and MAC (Medium Access Control),

as well as the PHY (Physical Layer)

Figure 1-8 Uu Interface Protocols

1.2.2 X2 Interface

As previously mentioned, the X2 interface interconnects two eNBs and in so doing supports both a Control Plane and User Plane The principle Control Plane protocol is X2AP (X2 Application Protocol) This resides on SCTP (Stream Control Transmission Protocol) whereas the User Plane IP is transferred using the services of GTP-U (GPRS Tunneling Protocol - User) and UDP (User Datagram Protocol)

Figure 1-9 illustrates the X2 User Plane and Control Plane protocols

Figure 1-9 X2 Interface Protocols

1.2.3 X2 Interface - X2 Application Protocol

The X2AP is responsible for the following functions:

Mobility Management - this enables the serving eNB to move the responsibility of a specified 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 which specific error reporting mechanisms 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

Configuration Update - this allows the updating of application level data which is needed for two eNBs to interoperate over the X2 interface

1.2.4 X2 Interface - 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 when transferring signaling information over an IP bearer Functions provided by SCTP include:

Reliable Delivery of Higher Layer Payloads

Sequential Delivery of Higher Layer Payloads

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

1.2.5 X2 Interface - GPRS Tunneling Protocol - User

GTP-U tunnels are used to carry encapsulated PDU (Protocol Data Unit) and in-band signaling messages between endpoints Numerous GTP-U tunnels may exist in order to differentiate between EPS bearer contexts and these are identified through a pair of TEID (Tunnel Endpoint Identifier)

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

1.2.6 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 1-10 S1 Interface Protocols

eNB

IPLayer 2Layer 1

SCTPS1AP

IPLayer 2Layer 1

UDPGTP-U

S1-MME

S1-U

S-GW

1.2.7 S1 Interface - S1 Application Protocol

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

E-RAB (E-UTRAN - Radio Access Bearer) Management - this incorporates the setting

up, modifying and releasing of the E-RABs by the MME

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

UE Capability Information Indication - this 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

1.2.8 S1 Interface - SCTP and GTP-U

The S1-MME and S1-U lower layer protocols are similar to the X2 interface As such, they also utilize the services of SCTP (discussed in Section 1.2.4 ) and GTP-U (discussed in Section 1.2.5 )

1.3 UE States and Areas

There are three LTE mobility states, namely: LTE Idle, LTE Active and LTE Detached The initial EMM Attach procedure enables a UE to transition into the LTE Active State from the LTE Detached State

In LTE, RRC has two main states, namely:

RRC Idle - this provides services to support DRX (Discontinuous Reception), broadcast

of SI (System Information) to enable access, cell reselection and paging information

RRC Connected - in this state the UE has state information stored in the eNB and has an RRC connection, i.e SRB (Signaling Radio Bearer) The eNB can track the UE to the cell level and RRC provides services to support cell measurements in order to facilitate network controlled handovers

Figure 1-11 illustrates the different LTE states, as well as some of the key functions performed

by RRC in these states

In addition to having a GUTI (Globally Unique Temporary Identity) and S-TMSI (Serving - Temporary Mobile Subscriber Identity), whilst in the RRC Connected mode, the UE is also allocated an E-UTRAN identifier(s) The most common is the C-RNTI (Cell - Radio Network Temporary Identity), however other forms of RNTI (Radio Network Temporary Identity) also exist

LTE IdleRRC Idle

PLMN SelectionBroadcast of System Information

Cell Selection

DRX configured by NASBroadcast of System Information

PagingCell Reselection Mobility

GUTI AllocatedLocated in Tracking Area(s)

No RRC Context Stored in the eNB

RRC Connection (SRB)RRC Context in eNB

UE Known in a CellSend and/or Receive Data to/from UE

Network Controlled MobilityMeasurement Control

UE Monitors Scheduling Control Channel

UE Reports Channel Quality

UE can send Feedback Information

DRX can be Configured

1.3.1 RRC State Interaction

In addition to RRC Idle and RRC Connected there are various transitions to and from UTRA (Universal Terrestrial Radio Access) and GERAN (GSM/EDGE Radio Access Network) States Figure 1-12 illustrates the main states and inter-RAT mobility procedures

In contrast to the GERAN and UTRA states, the E-UTRA (Evolved - Universal Terrestrial Radio Access) state is simplified This is mainly due to the fact that it is an optimized packet system

Figure 1-12 E-UTRA RRC State Interaction

Handover

Connection Establishment/Release

Reselection

Handover

CCO, ReselectionReselection

Connection Establishment/

ReleaseConnection

Establishment/

Release

CCO, ReselectionReselection

CCO with NACC

Cell_DCH

Cell_FACH

Cell_PCHURA_PCH

UTRA_Idle

E-UTRARRC Connected

E-UTRARRC Idle

GSM Idle/GPRSPacket Idle

GPRS PacketTransfer ModeGSM Connected

1.3.2 Interaction with CDMA2000 States

In addition to interworking with UMTS and GERAN, the LTE system is also able to interwork with CDMA2000 1xRTT CS (Circuit Switched) and HRPD (High Rate Packet Data) based systems Figure 1-13 illustrates the main mobility transitions for CDMA2000

interworking

Figure 1-13 Mobility Procedures between E-UTRA and CDMA2000

on crossing Tracking Area boundaries or on the expiry of the Tracking Area Periodic Timer, namely the T3412 timer By default this is set to 54 minutes in the 3GPP specifications

Figure 1-14 Tracking Areas

2 LTE Air Interface

Objectives

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

2.1 Explain the principles of OFDMA and SC-FDMA

2.2 Explain the coding and modulation adaptation used in LTE

2.3 List the LTE logical, transport and physical channels

2.4 Explain how the LTE downlink and uplink data rates are achieved

2.5 List the LTE UE category capabilities

2 LTE Air Interface

LTE Radio Network Design

Training Manual 2.1 LTE Access Techniques

OFDMA (Orthogonal Frequency Division Multiple Access) is the latest addition to cellular systems It provides a multiple access technique based on OFDM (Orthogonal Frequency Division Multiplexing) Figure 2-1 illustrates the basic view of OFDMA It can be seen that the bandwidth is broken down to smaller units known as “subcarriers” These are grouped together and allocated as a resource to a device It can also be seen that a device can be allocated different resources in both the time and frequency domain

Figure 2-1 Orthogonal Frequency Division Multiple Access

2.1.1 Principles of OFDM

The LTE air interface utilizes two different multiple access techniques both based on OFDM (Orthogonal Frequency Division Multiplexing):

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

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

Figure 2-2 Use of OFDM in LTE

eNB

UE

OFDM(OFDMA)

OFDM(SC-FDMA)

The concept of OFDM is not new and is currently being used on various systems such as Wi-Fi and WiMAX In addition, it was even considered for UMTS back in 1998 One of the main reasons why it was not chosen at the time was the handset’s limited processing power and poor battery capabilities

LTE was able to choose OFDM based access due to the fact mobile handset processing capabilities and battery performance have both improved In addition, there is continual pressure to produce more spectrally efficient systems

2.1.2 Frequency Division Multiplexing

OFDM is based on FDM (Frequency Division Multiplexing) and is a method whereby multiple frequencies are used to simultaneously transmit information Figure 2-3 illustrates an example 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

mathematically perpendicular to each other As such, when a subcarrier is at its maximum the two adjacent subcarriers are passing through zero In addition, OFDM systems still employ guard bands These are located at the upper and lower parts of the channel and reduce adjacent channel interference

Figure 2-4 OFDM Subcarriers

2 LTE Air Interface

LTE Radio Network Design

Training Manual

The centre subcarrier, known as the DC (Direct Current) subcarrier, is not typically used in OFDM system due to its lack of orthogonality

2.1.4 Fast Fourier Transforms

OFDM subcarriers are generated and decoded using mathematical functions called FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) The IFFT is used in the transmitter to generate the waveform Figure 2-5 illustrates how the coded data is first mapped

to parallel streams before being modulated and processed by the IFFT

Figure 2-5 Inverse Fast Fourier Transform

Coded

SerialtoParallel

SubcarrierModulation

RF

Inverse Fast FourierTransform

Complex Waveform

At the receiver side, this signal is passed to the FFT which analyses the complex/combined waveform into the original streams Figure 2-6 illustrates the FFT process

Figure 2-6 Fast Fourier Transform

2.1.5 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 due to the channel guard bands and the fact that a DC (Direct Current) subcarrier is also required

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

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

Table 2-1 LTE Channel and FFT Sizes

Channel Bandwidth

FFT Size Subcarrier

Bandwidth

Sampling Rate 1.4MHz 128

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

2.1.6 OFDM Symbol Mapping

The mapping of OFDM symbols to subcarriers is dependent on the system design Figure 2-7 illustrates an example of OFDM mapping The first 12 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 mapped to the next OFDM symbol period In addition, a CP (Cyclic Prefix) is added between the symbols

Figure 2-7 OFDM Symbol Mapping

LTE allocates resources in groups of 12 subcarriers This is known as a PRB (Physical Resource Block)

In the previous example 12 different modulated OFDM symbols are transmitted simultaneously Figure 2-8 illustrates how the combined energy from this will result in either constructive peaks (when the symbols are the same) or destructive nulls (when the symbols are different) This means that OFDM systems have a high PAPR (Peak to Average Power Ratio)

2 LTE Air Interface

LTE Radio Network Design

Training Manual

Figure 2-8 OFDM PAPR (Peak to Average Power Ratio)

2.1.7 Time Domain Interference

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

of the subcarriers The main issue is with delay spread, i.e multipath interference

Figure 2-9 illustrates two of the main multipath effects, namely delay and attenuation The delayed signal can manifest itself as ISI (Inter Symbol Interference), whereby one symbol impacts the next This is illustrated in Figure 2-10

Figure 2-9 Delay Spread

ISI (Inter Symbol Interference) is typically reduced with “equalizers” However, for the equalizer to be effective a known bit pattern or “training sequence” is required However, this reduces the system capacity, as well as impacts processing on a device Instead, OFDM systems employ a CP (Cyclic Prefix)

Figure 2-10 Inter Symbol Interference

1stReceived

Signal

InterferenceCaused

Cyclic Prefix

A CP (Cyclic Prefix) is utilized in most OFDM systems to combat multipath delays It effectively provides a guard period for each OFDM symbol Figure 2-11 illustrates the Cyclic Prefix and its location in the OFDM Symbol Notice that the Cyclic Prefix is effectively a copy taken from the back of the original symbol which is then placed in front of the symbol to make the OFDM symbol (Ts)

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 cells, should have a large CP This does however impact the system capacity since the number of symbols per second is reduced

2 LTE Air Interface

LTE Radio Network Design

Training Manual

Figure 2-11 Cyclic Prefix

Symbol Period T(s)T(g)

CPCPCP

CPCPCP

CPCPCP

CPCPCPFrequency

TimeSymbol Period T(s)

Bit Period T(b)Cyclic Prefix

LTE has two defined Cyclic Prefix sizes, normal and extended The extended Cyclic Prefix is designed for larger cells

2.1.8 General OFDMA Structure

The E-UTRA downlink is based on OFDMA As such, it enables multiple devices to receive information at the same time but on different parts of the radio channel In most OFDMA systems this is referred to as a “Subchannel”, i.e a collection of subcarriers However, in E-UTRA, the term subchannel is replaced with the term PRB (Physical Resource Block) Figure 2-12 illustrates the concept of OFDMA, whereby different users are allocated one or more resource blocks in the time and frequency domain, thus enabling efficient scheduling of the available resources

Figure 2-12 OFDMA in LTE

Frequency

Channel Bandwidth E.g 3MHz

Time

Device is allocated one

or more PRB (Physical Resource Blocks)

PRB consists of 12 subcarriers for 0.5msOFDMA

It is also worth noting that a device is typically allocated 1ms of time, i.e a subframe, and not

an individual PRB

2.1.9 Physical Resource Blocks and Resource Elements

A PRB (Physical Resource Block) consists of 12 consecutive subcarriers and lasts for one slot, i.e 0.5ms Figure 2-13 illustrates the size of a PRB

The NRB

DL parameter is used to define the number of RB (Resource Blocks) used in the DL (Downlink) This is dependent on the channel bandwidth In contrast, NRB

UL

is used to identify the number of resource blocks in the uplink Each RB (Resource Block) consists of

NSC RB subcarriers, which for standard operation is set to 12 In addition, another configuration

is available when using MBSFN and a 7.5kHz subcarrier spacing

The PRB is used to identify an allocation It typically includes 6 or 7 symbols, depending on whether an extended or normal cyclic prefix is configured

The term RE (Resource Element) is used to describe one subcarrier lasting one symbol This can then be assigned to carry modulated information, reference information or nothing

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Physical Resource Block

Resource ElementSubframe

NSymbDL

The different configurations for the downlink E-UTRA PRB are illustrated in Table 2-2

Table 2-2 Downlink PRB Parameters

Normal Cyclic Prefix ∆f = 15kHz

12

7 Extended Cyclic

Prefix

The uplink PRB configuration is similar; however the 7.5kHz option is not available

2.1.10 SC-FDMA Signal Generation

The uplink in LTE, as previously mentioned, is based on SC-FDMA (Single Carrier - Frequency Division Multiple Access) This was chosen for its low PAPR (Peak to Average Power Ratio) and flexibility which reduced complexity in the handset and improved power performance and battery life SC-FDMA tries to combine the best characteristics of single carrier systems like low peak-to-average power ratio, with the advantages of multi carrier OFDM and as such, is well suited to the LTE uplink requirements

The basic transmitter and receiver architecture is very similar (nearly identical) to OFDM, and

it offers the same degree of multipath protection Importantly, because the underlying waveform is essentially single carrier, the PAPR is lower It is quite difficult to visually represent SC-FDMA in the time and frequency domain This section aims to illustrate the concept Figure 2-14 illustrates the basic structure of the SC-FDMA process

Figure 2-14 SC-FDMA Subcarrier Mapping Concept

Subcarrier Mapping

Symbols

0000

000

CP Insertion

In Figure 2-14 the SC-FDMA signal generation process starts by creating a time domain waveform of the data symbols to be transmitted This is then converted into the frequency domain, using a DFT (Discrete Fourier Transform) DFT length and sampling rate are chosen

so that the signal is fully represented, as well as being spaced 15kHz apart Each bin (subcarrier) will have its own fixed amplitude and phase for the duration of the SC-FDMA symbol Next the signal is shifted to the desired place in the channel bandwidth using the zero insertion concept, i.e subcarrier mapping Finally, the signal is converted to a single carrier waveform using an IDFT (Inverse Discrete Fourier Transform) and other functions Finally a cyclic prefix can be added Note that additional functions such as S-P (Serial to Parallel) and P-S (Parallel to Serial) converters are also required as part of a detailed functional description Figure 2-15 illustrates the concept of the DFT, such that a group of N symbols map to N subcarriers However depending on the combination of N symbols into the DFT the output will vary As such, the actual amplitude and phase of the N subcarriers is like a “code word” For example the first combination represents the first set of symbols Since the second set of symbols is different the amplitude and phase of the N subcarriers would then be different

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Figure 2-15 SC-FDMA Signal Generation

DFT

N symbols sequence produces N subcarriers

Different input sequence produces different output

First N Symbols

DFT Output

Modulated and Coded Symbols

DFTSecond N Symbols

The process at the eNB receiver takes the N subcarriers and reverses the process This is achieved using an IDFT (Inverse Discrete Fourier Transform) which effectively reproduces the original N symbols

Figure 2-16 illustrates the basic view of how the subcarriers received at the eNB are converted back into the original signals

Note that the SC-FDMA symbols have a constant amplitude and phase and like ODFMA, a

CP (Cyclic Prefix) is still required

Figure 2-16 SC-FDMA and the eNB

2.2 Channel Coding in LTE

The term “channel coding” can be used to describe the overall coding for the LTE channel It can also be used to describe one of the individual stages

LTE channel coding is typically focused on a TB (Transport Block) This is a block of information which is provided by the upper layer, i.e MAC (Medium Access Control) Figure 2-17 summarizes the typical processes performed by the PHY (Physical Layer), these include:

CRC (Cyclic Redundancy Check) attachment for the Transport Block

Code block segmentation and CRC attachment

Channel Coding

Rate Matching

Code Block Concatenation

Figure 2-17 Summary of LTE Transport Channel Processing

Transport Block CRC Attachment

Code Block CRC Attachment and

Segmentation

Channel Coding

Rate Matching

Code Block Concatenation

Additional Layer 1 Processes

PHY Layer

The coding stages in Figure 2-17 are indicative of the LTE DL-SCH (Downlink Shared Channel) and the PCH (Paging Channel) Other channels, such as the UL-SCH (Uplink Shared Channel), BCH (Broadcast Channel) etc are different but they can still utilize similar processes, e.g they all have a “channel coding” stage

2.2.1 Channel Coding

Channel coding in LTE facilitates FEC (Forward Error Correction) across the air interface There are four main types:

Repetition Coding

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Table 2-3 Transport Channel Coding Options

DL-SCH

UL-SCH PCH MCH

Table 2-4 Control Information Coding Options

Tail Biting Convolutional Coding 1/3

2.2.2 Modulation and Coding Scheme

One of the key parameters in the DCI messages is the MCS Index Parameter Table 2-5 illustrates the mapping of the MCS index to the modulation and TBS (Transport Block Set) Index

Table 2-5 Modulation and TBS index table for PDSCH

MCS Index

MCS

I

Modulation Order

m

Q

TBS Index

TBS

I

MCS Index

MCS

I

Modulation Order

m

Q

TBS Index