LTE Overview

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LTE

Overview

Reference

Guide For LSO Course 60070459

AT&T Custom eLearning by Award Solutions

Contains AT&T Proprietary and Confidential Information

LTE Overview Reference Guide


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Table of Contents

1.0 LTE INTRODUCTION ........................................................................................................ 1

2.0 LTE BUILDING BLOCKS .................................................................................................. 1

LTE Air Interface ................................................................................................................................................. 2 Evolved UTRAN Architecture .............................................................................................................................. 2 Evolved Core Architecture .................................................................................................................................. 3 LTE Targets ......................................................................................................................................................... 3

3.0 NETWORK ARCHITECTURE ............................................................................................ 4

Evolved Packet Core Architecture ....................................................................................................................... 4 eUTRAN (Evolved UTRAN) .................................................................................................................................. 5 S1 Control Plane Interface (S1-MME) ................................................................................................................. 6 S1 User Plane Interface (S1-U) ........................................................................................................................... 7 X2 Control Plane Interface (X2-C) ....................................................................................................................... 7 X2 User Plane Interface (X2-U) ........................................................................................................................... 8 LTE Air Interface (LTE-Uu) Control Plane and User Plane ................................................................................... 9

4.0 LTE AIR INTERFACE ...................................................................................................... 10

Evolution of OFDM ........................................................................................................................................... 10 What is OFDM? ................................................................................................................................................ 11 Role of Fourier Transform in OFDM ................................................................................................................. 11 IFFT in OFDM .................................................................................................................................................... 12 FFT in OFDM ..................................................................................................................................................... 13 What is OFDMA? .............................................................................................................................................. 13 Challenges for OFDMA ..................................................................................................................................... 14 What is SOFDMA? ............................................................................................................................................ 15 Cyclic Prefix....................................................................................................................................................... 16 PAPR in OFDMA ................................................................................................................................................ 17 DFT-spread OFDMA .......................................................................................................................................... 18

5.0 FRAME STRUCTURE ..................................................................................................... 19

Frame Structure Type 1 .................................................................................................................................... 19 OFDM Symbol ................................................................................................................................................... 19 Cyclic Prefix....................................................................................................................................................... 20 OFDM Symbols per Slot .................................................................................................................................... 20 Resource Blocks ................................................................................................................................................ 21

6.0 PHYSICAL CHANNELS .................................................................................................. 22

Downlink Physical Channels ............................................................................................................................. 22 Uplink Physical Channels ................................................................................................................................. 23

7.0 LTE UE OPERATIONS .................................................................................................... 24

Acquisition Procedure ....................................................................................................................................... 24 Timing Acquisition ............................................................................................................................................ 25 Broadcast Information ..................................................................................................................................... 26 UE Access Procedure ........................................................................................................................................ 26 System Acquisition ........................................................................................................................................... 27 Registration Procedure ..................................................................................................................................... 28

8.0 BEARER SERVICE ......................................................................................................... 29



QOS Functional Elements ................................................................................................................................. 30 EPS Bearer ........................................................................................................................................................ 31

9.0 CALL SESSION SETUP .................................................................................................. 32

10.0 DOWNLINK (DL)/UPLINK (UL) TRAFFIC ...................................................................... 33

Downlink Traffic ............................................................................................................................................... 33 Uplink Traffic .................................................................................................................................................... 34

11.0 LTE MOBILITY................................................................................................................ 35

Overview of Mobility in LTE .............................................................................................................................. 35 LTE Cell Selection/Reselection .......................................................................................................................... 36 Why Tracking Area Update? ............................................................................................................................ 37 Intra-MME/Serving Gateway Mobility ............................................................................................................. 38 Handover Preparation ...................................................................................................................................... 39 Handover Execution ......................................................................................................................................... 40 Handover Completion ....................................................................................................................................... 41

12.0 LTE DEPLOYMENT ......................................................................................................... 42

Evolution from GSM to LTE ............................................................................................................................... 42 LTE Deployment with GSM/GPRS/EDGE ........................................................................................................... 43 LTE Deployment with UMTS/HSPA ................................................................................................................... 43 Evolution from CDMA to LTE ............................................................................................................................ 44

13.0 ACRONYMS ................................................................................................................... 46


© 2010 Award Solutions, Inc. www.awardsolutions.com +1-877-47-AWARD 1

1.0 LTE Introduction

Long Term Evolution or LTE is a Fourth Generation (4G) Wireless technology used for broadband multimedia

applications.

This course will cover the following key objectives:

LTE building blocks and network architecture

Orthogonal Frequency Division Multiple Access and Single Carrier–FDMA principles

LTE OFDMA frame structure and physical channels in UL and DL

The life of a mobile in LTE

End-to-End data sessions setup

Mobility and handover in LTE

Evolution and deployment scenarios to LTE

2.0 LTE Building Blocks

LTE introduces significant changes in three major areas: the air interface, the radio network and the core

network. Key air interface changes include the introduction of Orthogonal Frequency Division Multiplexing

(OFDM) technology along with the fully-integrated use of multiple-antenna techniques such as Multiple Input

Multiple Output (MIMO) antenna techniques. As a result of these key changes, LTE air interface supports very

high data rates.

LTE changes the hierarchical architecture of the UMTS or 1xEV-DO network and converts the radio network into

an IP-based distributed network. IP-based backhaul networks will reduce backhaul cost. A fully packetized IP-

based core network will replace the separate circuit-switched and packet-switched core networks, which

provides scalability and reduce cost. The new services can be introduced via the IP Multimedia Subsystem

(IMS).



LTE Air Interface

LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) technology for downlink transmission, and

Single Carrier Frequency Division Multiple Access (SC-FDMA) technology for uplink transmission. LTE supports

both TDD (Time Division Duplex) and FDD (Frequency Division Duplex) modes of operation.

Evolved UTRAN Architecture

The 3GPP standards call LTE‟s radio access network the Evolved Universal Terrestrial Radio Access Network (E-

UTRAN). In order to reduce the latency experienced by packets, LTE reduces the UTRAN network to a single

node type called an evolved NodeB (eNB). The eNB combines the functions of the Radio Network Controller

(RNC) and the Node B, reducing the number of nodes in the network.



Evolved Core Architecture

The core network in LTE is based on the 3GPP System Architecture Evolution (SAE) model. The LTE core

network consists of only the packet-switched domain, which is called the Evolved Packet Core (EPC). There is

no separate circuit-switched core network. The key components of the EPC are the Mobility Management Entity

(MME), the Serving Gateway (S-GW), the Packet Data Network Gateway (P-GW) and the Home Subscriber

Server (HSS).

LTE Targets

LTE networks are aiming at several targets. LTE supports data rates of more than 300 Mbps in the downlink

and more than 75 Mbps in the uplink. LTE reduces packet latency by reducing the number of nodes in the

network. It uses OFDM-based technology along with multiple-antenna technology (MIMO), resulting in higher

spectral efficiency and increased capacity. By improving data rates, LTE reduces latency and offers a rich

multimedia user experience.

In summary, LTE aims to offer a rich user experience that includes voice, data, and video in a mobile

environment – anywhere, anytime with lower deployment costs.



3.0 Network Architecture

LTE Network Architecture is composed of two components

The Evolved Packet Core (EPC)

Evolved UTRAN (eUTRAN) supporting air interface requirements

Evolved Packet Core Architecture

Evolved Packet Core refers to LTE-related core network evolution. LTE architecture is based on the System

Architecture Evolution (SAE) model defined by the 3G Partnership Project (3GPP).

EPC consists of the following network elements:

The Mobility Management Entity (MME), which, as the name indicates, is primarily responsible for

managing the UE‟s mobility-related context. The MME is also responsible for selection of the PDN

Gateway, triggering and enabling authentication, and saving the subscriber profile downloaded from

the HSS.

The Serving Gateway is responsible for anchoring the user plane for inter-eNB handover and inter-

3GPP mobility.

The PDN Gateway is responsible for IP-address allocation to the UE. The PDN GW is also the policy

enforcement point to enforce Quality of Service (QoS)-specific rules on traffic packets.

The HSS is a user database that contains subscription-related information and performs

authentication and authorization of the user.



The LTE network components work together to deliver end-to-end high-speed packet-data services. LTE has

defined interfaces that enable communication between network elements such as the E-UTRAN, Serving

Gateway, PDN Gateway, MME, HSS, Serving GPRS Support Note (SGSN) and Policy and Charging Rules

Function (PCRF).

eUTRAN (Evolved UTRAN)

The E-UTRAN consists of only eNBs. This is a big change from previous UMTS releases since there is no Radio

Network Controller (RNC). The eNB must support all the standard Node B and RNC functions such as:

Radio interface physical-layer OFDMA functions

Coding

Modulation

Scheduling for downlink and uplink

Radio resource management

Call admission control

Handover support



Ciphering and IP header compression

The eNB supports two new interfaces - S1 and X2 - in LTE. The S1 interface is located between the eNB and

the MME and S-GW nodes (jointly referred to as MME/S-GW) in the EPC. The S1 interface supports the attach

operation that results in the authentication and IP-address allocation for UEs. The new X2 interface is

necessitated by the removal of the RNC. When a UE moves from one cell in an eNB to another cell in a

different eNB, mobility-related signaling and traffic exchange is supported by the X2 interface.

S1 Control Plane Interface (S1-MME)

S1 functions are split into control-plane (S1-MME) and user-plane (S1-U) functions. The S1-MME interface is

responsible for delivering signaling protocols between the eNB and the MME. It uses Stream Control

Transmission Protocol (SCTP) to transfer several independent streams of messages in parallel over IP and

provides guaranteed data delivery. The application signaling protocol is S1-AP (Application Protocol). The S1-

MME interface is responsible for bearer setup/release procedures, the handover signaling procedure, the

paging procedure and the NAS (Non Access Stratum) transport procedure.



S1 User Plane Interface (S1-U)

The S1-U interface is located between the eNB and S-GW. The S1-U interface supports the transfer of user

traffic. The GPRS user-plane tunneling protocol, or GTP-U, is used across the S1-U interface to move user

traffic. GTP-U, in turn, uses UDP over IP to transport the traffic.

X2 Control Plane Interface (X2-C)

The X2 interface is the interface between the two eNBs. The X2 interface functions are also split into control-

plane (X2-C) and user-plane (X2-U) functions. The X2 control-plane protocol functions include:

Handover signaling

Setting up and managing user-plane tunnels

Multi-cell radio resource management

Measurement reporting



The transport network layer is built using SCTP over IP. The set of signaling messages used across the X2

Control plane is defined as X2-AP (X2 Application Protocol) in LTE.

X2 User Plane Interface (X2-U)

Let„s examine the X2 user-plane protocol stack. It is the same as the S1 user-plane protocol. The X2 user-plane

protocol tunnels end-user packets between the eNBs. The S1 user plane and X2 user plane use the same user-

plane protocol to minimize protocol processing for the eNB at the time of data forwarding.



LTE Air Interface (LTE-Uu) Control Plane and User Plane

Let‟s now examine the LTE-Uu control-plane protocol stack.

The physical layer in LTE supports the following functions:

Hybrid Automatic Repeat Request (Hybrid ARQ) with soft combining

OFDMA-based physical layer

Uplink power control

Multi-stream transmission and reception (MIMO)

The MAC sublayer performs the following functions:

Scheduling

Error correction through HARQ

Priority handling across UEs as well as across different logical channels of a UE

Multiplexing/demultiplexing of different RLC radio bearers into/from the physical layer on transport

channels

The RLC sublayer supports functions such as:

In-sequence delivery of RLC Protocol Data Units (PDUs)

Transfer of upper layer PDUs

Error correction through Automatic Repeat Request (ARQ)

Flow control and concatenation or re-assembly of packets

The Packet Data Convergence Protocol (PDCP) sublayer performs:

Header compression

Ciphering

The RRC sublayer is between the UE and the eNB. It performs functions such as:

Broadcasting system information

Paging

Connection management

Radio bearer control

Mobility functions

UE measurement reporting and control



4.0 LTE Air Interface

Evolution of OFDM

Orthogonal Frequency Division Multiplexing (OFDM) is a technology used for high-speed data communication

systems. It evolved from two important techniques, Frequency Division Multiplexing (FDM) and Multicarrier

Multiplexing (MCM).

The FDM technique has been widely used in communication systems. It divides the available bandwidth into

many subcarriers and allows multiple users to access a system simultaneously. Each user transmits their data

on a different subcarrier. To avoid interference, guard bands are assigned between subcarriers. Since guard

bands do not transmit any information, they introduce spectrum inefficiency.

With multicarrier multiplexing, a user can split the data into multiple substreams and transmit them in parallel.

In multicarrier FDM, the user data is converted from serial to parallel. Then, the parallel data substreams are

sent over multiple subcarriers. At the receiver, the parallel data is combined back into a serial data stream. A

higher data rate can be achieved by using multicarrier multiplexing.



What is OFDM?

OFDM adds the orthogonal feature into multicarrier FDM. Orthogonal means “do not cause interference with

each other.” In OFDM, the subcarriers are designed to be orthogonal. This allows subcarriers to overlap and

saves bandwidth. Therefore, OFDM obtains both higher data rates and good spectrum efficiency.

Role of Fourier Transform in OFDM

OFDM divides a high-speed data stream into multiple slow substreams. The substreams are transmitted over

multiple subcarriers in parallel. As shown in the figure, we know the system needs multiple pairs of

transmitters and receivers, one pair per subcarrier. This results in high system costs. If we could add a module

A at the transmitter side for integrating the parallel signals and a module B at the receiver side for distributing

the signals back into parallel, then only one transmitter/receiver pair is needed.

Fourier Transform (FT) and Inverse Fourier Transform (IFT) fulfill the functions of module B and A respectively.

This enables the use of one transmitter and one receiver, and reduces system costs. The lower cost makes the



wide application of OFDM systems possible. With the development of Digital Signal Processing (DSP)

technology, Fourier Transform can be practically implemented in an OFDM system.

IFFT in OFDM

Let‟s look at how a simple OFDM transmitter works. A high-speed data stream is divided into multiple

substreams by the process of serial to parallel conversion. Each substream is carried by a subcarrier.

Subcarriers are like an array of piano keys; each operates in a different frequency and has a different strength.

The frequency values are related to each other such that the subcarriers are orthogonal to each other. The

subcarriers carry information and pass through the IFFT module. IFFT transforms the frequency components

into a time domain signal. The time domain signal is sent over the channel. The key benefit of IFFT is that it

requires only one transmitter to transmit multiple substreams of data over the air.



FFT in OFDM

How does a simple OFDM receiver work? Only one receiver is needed to receive the varying time signal. The

operations at the OFDM receiver side is exactly reverse to those operations performed at the transmitter side.

The varying time signal passes through the Fast Fourier Transform (FFT) module. FFT transforms the time

signal into a set of frequency components. Each frequency component carries the information of a data

substream. These data substreams are combined by a parallel to serial conversion process to get back the

high-speed data stream.

What is OFDMA?



OFDMA allows multiple users to access subcarriers simultaneously. In this example, three users share four

subcarriers. At each symbol time, all users can have access. The assignment of subcarriers for a user can be

changed at every symbol time.

OFDMA provides more flexibility for system design. Different combinations of the number of carriers and

symbol times can be allocated.

Challenges for OFDMA

In OFDMA, the number of subcarriers is usually kept the same within the available spectrum. The constant

number of subcarriers results in variable subcarrier spacing in different systems. This makes handover

between systems difficult. Also, each system needs a special design and the system cost is high.



What is SOFDMA?

Scalable OFDMA (SOFDMA) solves these problems by keeping subcarrier spacing constant. In other words, the

number of subcarriers proportionally increases or decreases with changes in available bandwidth. For

example, if a 5 MHz bandwidth is divided into 512 subcarriers, a 10 MHz bandwidth will be split into 1024

subcarriers.

Since the subcarrier spacing is the same for systems using SOFDMA, a mobile can handover between systems

smoothly. Also, with constant subcarrier spacing, one design is suitable for many systems and can be reused.

This lowers design and product costs. SOFDMA keeps the subcarrier spacing constant and varying number of

subcarriers with varying channel bandwidth. This ensures simplicity in system design and receiver complexity.



Cyclic Prefix

Intersymbol Interference (ISI) is a form of signal distortion, in which one symbol interferes with subsequent

symbols. It is caused by multipath propagation. OFDM uses cyclic prefix to overcome ISI.

Extending OFDM symbols into periodical symbols can help the spectral analysis at the receiver maintain the

orthogonality of subcarriers. This means redundant information is sent out to make sure analysis can be

conducted on the undistorted information. We call this cyclic extension. It can be implemented by copying a

portion of the original symbol from the end and attaching it to the front, or copying the front and attaching it to

the end. When it is attached to the front, it is called cyclic prefix.

OFDM uses guard time to overcome the effects of Intersymbol Interference (ISI). Cyclic extension can be put

into the guard time interval. We call this cyclic prefix. With cyclic prefix, the delayed version of a previous

symbol cannot shift onto the current symbol, so ISI is eliminated.

Extension:



PAPR in OFDMA

Peak-to-Average Power Ratio (PAPR) is a typical problem in multicarrier modulation. PAPR is due to the

summation of a large number of independent data symbols for transmission.

In OFDM systems, the time domain signal transmitted over the air is the weighted sum of multiple subcarriers.

As the number of subcarriers becomes larger, a small percentage of the time domain samples have high

magnitudes (i.e., peak values). These peak values can be much larger than the average value that leads to

high PAPR.

Since the practical power amplifier is linear in a certain range, the nonlinear distortion occurs at the peak

values. Extending the linear range of a power amplifier significantly increases the cost and reduces power

efficiency. Hence, solving the PAPR issue is a critical requirement for mobile stations in OFDMA systems.



DFT-spread OFDMA

In LTE, DFT is used to spread the high speed stream over multiple subcarriers, thereby distributing the energy

of the input signal over a larger spectrum. This results in lower PAPR. The DFT-spread OFDMA signal looks like

a single carrier modulated waveform, hence the name SC-FDMA.

So, how does DFT-spread OFDMA work?

The high-speed input data stream is first converted into blocks of N bits, each using a serial to parallel

converter. DFT spreads each input modulation symbol over N output symbols. Each of the N output symbols

from the DFT is then fed as input into an M-point IFFT function. Typically, N (in N-point DFT) is much smaller

than M (in M-point IFFT). The mapping of DFT output symbols to IFFT input is based on an LTE-specific

subcarrier mapping function. The net effect of applying frequency domain samples from N-point DFT as input

to M-point IFFT (where M > N) results in a single-carrier time domain signal. DFT-spread OFDMA is also called

Single Carrier Frequency Division Multiple Access (SC-FDMA). This sequence of processing results in very low

Peak-to-Average Power Ratio (PAPR). Due to lower Peak-to-Average Power Ratio, SC-FDMA is used in the uplink.



5.0 Frame Structure

Frame Structure Type 1

Frame structure Type 1 is applicable for FDD-based transmissions. Let‟s analyze the Type 1 frame structure in

more detail. One LTE radio frame duration is 10 ms. Each frame is divided into 10 subframes of 1 ms each.

Each subframe is further subdivided into two slots of 0.5 ms each. To summarize, each frame has 20 slots.

OFDM Symbol

In LTE, each 0.5 ms slot is divided into many OFDM symbols. Each OFDM symbol consists of two parts: Cyclic

Prefix (CP) followed by Useful Symbol Time (Tu). A CP is created by copying the tail-end of the symbol to the



beginning of the symbol period, thus creating a guard time. The duration of CP depends on the multipath and

delay-spread characteristics of the radio environment.

Cyclic Prefix

LTE specifies two possible values for CP duration. The first one, called Normal CP, is defined for radio

environments with low multipath delay spread. Typical urban environment LTE deployments use Normal CP

value, which is about 5 microseconds. The second one, Extended CP, is used in radio environments with a high

delay-spread value. LTE defines Extended CP value to be approximately 17 microseconds.

OFDM Symbols per Slot

Each 10 ms frame in LTE is made up of 20 0.5 ms slots. The number of OFDM symbols per slot will vary based

on the Tu and the length of CP. For non-multicast applications, LTE defines Tu as 66.67 microseconds. When



normal CP is used, each slot can transmit seven OFDM symbols. When Extended CP is used, each slot carries

six OFDM symbols.

Resource Blocks

A UE requires radio resources to allow it to receive information from the eNB in the DL direction and transmit

information to the eNB in the UL direction. The scheduler at the eNB allocates the radio resources in terms of

resource blocks to a UE.

LTE defines a resource block as a group of 12 subcarriers assigned for one slot. A network may assign one or

more resource blocks to a UE. For example, User A may be allocated a group of resource blocks for one or

more time slots (seven symbols per time slot). In the diagram, each block is a resource block, i.e., a group of

12 subcarriers. User B may be allocated a different group of resource blocks at the same time as User A.

The LTE approach provides the flexibility to allocate a single resource block or a group of resource blocks to a

user. LTE also allows different numbers of OFDM symbols to be allocated as part of burst allocation to a user.

Using these options, LTE systems may allocate resources to address QoS requirements for different types of

services.



6.0 Physical Channels

Downlink Physical Channels

Now, we look at the LTE channel structure. LTE defines two reference signals to help the UE acquire the system

and to achieve synchronization with the system. These reference signals are called Primary and Secondary

Synchronization signals. When a UE successfully acquires and locks on to these two signals, the UE has slot

and frame-level synchronization with the system.

LTE is defined for a wide range of channel bandwidths starting from as low as 1.4 MHz to as high as 20 MHz.

The network transmits a small amount of critical system information, such as channel bandwidth used by the

LTE system, on the Physical Broadcast Channel. LTE systems transmit a downlink reference signal. The UE

uses the reference signal for coherent demodulation, channel estimation and handover functions.

The Physical Downlink Shared Channel (PDSCH) is the main channel that carries user traffic, signaling

messages, paging, and system configuration messages. The Physical Downlink Control Channel (PDCCH) is

used to convey uplink and downlink.



Uplink Physical Channels

In the uplink, the Physical Random Access Channel (PRACH) is a collision-based access channel that enables

multiple UEs to make initial requests. The Physical Uplink Control Channel (PUCCH) is used to carry ACK/NACK

(Acknowledgment/Negative Acknowledgment) messages and Channel Quality Indicators (CQIs) for downlink

transmission. Also, this channel carries a scheduling request for uplink transmission. The Physical Uplink

Shared Channel (PUSCH) carries uplink shared-data transmission and signaling messages.



7.0 LTE UE Operations

Acquisition Procedure

Let‟s review the life of a UE as it powers on, looks for the LTE network, registers with the selected network, and

exchanges data with the network.

1. When the UE is powered up, it uses programmed information or the most recently used network

information to decide which frequency to use to start looking for an LTE network. After tuning its

receiver to that frequency, the UE then searches for LTE cell transmissions on that frequency.

2. Once the UE finds LTE cell transmissions, it starts listening to the system information that is broadcast

in that cell. During this time, the UE is in “Idle” mode where it just listens to the system.

3. After the UE learns the LTE system information, including system access parameters, the UE tries to

access the network using random access channel procedures. Since multiple UEs may try to access

the network at the same time using the same resources, collisions may occur. So random access

procedures are defined to minimize and resolve collisions.

4. Typically, the UE now registers its presence with the network. The network authenticates the UE and

allocates an IP address. Finally, the network may allocate radio resources to the UE to enable efficient

user traffic exchange.

Let‟s peek into the mind of a UE when it is powered up and it starts looking for an available LTE network to

acquire service. At this point, it needs to answer the following three key questions to ensure proper acquisition

of the LTE network.

1. Which band and channel frequency do I scan to look for an LTE system?

The UE will first retrieve configured information about the frequency band (for example, 2.1 GHz) and

channel frequency to start searching for an LTE system.

2. What is the channel bandwidth to scan during this stage?

The UE now needs to figure out how wide the bandwidth of the LTE system it found is.

3. LTE is defined to be scalable. It supports various channel bandwidths ranging from 1.4 MHz to 20

MHz. At this stage in the system acquisition process, the UE may not know the channel bandwidth

used by the system.



To facilitate faster acquisition, LTE defines the initial acquisition procedure based on the smallest

channel bandwidth supported, which is 1.4 MHz.

4. How do I synchronize with the system in order to properly receive system transmissions?

After frequency-based acquisition, the UE needs to synchronize with the system. Once timing

synchronization is achieved, the UE can achieve critical system information including the channel

bandwidth used by the LTE system

Timing Acquisition

LTE defines a frame size of 10 ms. Each frame is subdivided into ten subframes of 1 ms duration. Each

subframe is further subdivided into two half millisecond slots. In other words, each frame consists of 20 slots.

Each slot contains six or seven OFDMA symbols depending on the cyclic prefix used. In this example, we

assume the use of Normal CP. When Normal CP is used, a slot carries seven OFDMA symbols. LTE defines two

physical-layer reference signals to help UEs achieve time synchronization with the network. Both primary and

secondary synchronization signals are transmitted on slots 0 and 10 of every frame. Synchronization signals

are transmitted only on the center 62 subcarriers of the radio channel independent of the channel bandwidth.

The primary synchronization signal is always transmitted on the last OFDMA symbol duration of the slot. The

primary synchronization signal in each cell transmits one of three possible Zadoff-Chu sequences. The entire

sequence is repeated in slot 0 of each frame and slot 10 of each frame, which helps the UE achieve slot level

synchronization with the network.

The secondary synchronization signal is always transmitted on the second to last OFDMA symbol of slots 0 and

10 of each frame. The secondary synchronization signal is made up of two binary sequences. When the UE

acquires these two sequences, it can reliably derive the start and end of 10 ms frames, i.e., the UE achieves

frame synchronization with the help of secondary synchronization signals. LTE systems use one of 168

possible sequences on the secondary synchronization signal. There are 504 possible combinations using three

different sequences in the primary sync signal and 168 possible sequences in the secondary sync signal. This

allows the UE to uniquely identify a cell in a given area.



Broadcast Information

LTE systems broadcast critical system information such as LTE channel bandwidth using the Physical

Broadcast Channel. This helps UEs during the acquisition stage. After acquiring critical system information, the

UE moves on to listen to the Physical Downlink Shared Channel (PDSCH).

UE Access Procedure

UE now contends with other UE‟s to access the system. The UE sends this preamble at a lower power level

initially and ramps the power level up on each subsequent transmission. The UE continues to send this

preamble until one of the following events occurs:

1. Maximum number of attempts has been reached

2. A response has been received on the Downlink Shared Channel



The high-level view of the access procedure:

1. The UE sends a preamble using the sequence already explained. When the first preamble does not get

a response, the UE increases the transmit power by the configured power step value and sends the

preamble again. Note that each successive preamble transmission has a random back-off component

in addition to a fixed time-out value. This approach minimizes the chance of the same two UEs

colliding again and again. Uplink preambles are sent on the PRACH.

2. Let‟s assume that the eNB received the last preamble transmission properly. The eNB will compare

the symbol arrival time from the access preamble with that of other UEs already receiving traffic on the

shared channel. The eNB determines the timing adjustment this UE should make to avoid collisions on

uplink transmission and sends that value as a timing adjustment value. The eNB also sends an uplink

transmission allocation to the UE along with the timing adjustment. The eNB sends the timing

adjustment and UL grant using the PDCCH and the PDSCH.

3. The UE uses the uplink allocation received to send its first message to the eNB. This message is sent

on the PUSCH.

System Acquisition

Here are the steps involved in establishing a data call when Sue‟s mobile is powered on:

1. System Acquisition

2. Radio Signaling Procedure

3. Registration Procedure

The first step is system acquisition. The UE scans for a downlink channel and establishes synchronization with

the eNB. Then, the UE listens to system information blocks to learn cell, network, access, and neighbor

configuration data.

As soon as the UE acquires an LTE system and learns configuration information, the UE will try and register

with the network. The UE will first establish a logical radio signaling connection. To do so, it will start by using

the Random Access Channel (RACH). The eNB will provide timing adjustment and uplink allocation to the UE.

The UE will use the allocated uplink resources to initiate Radio Resource Control (RRC) logical signaling

connection setup. The Attach procedure, which is used for UE registration with the MME, follows next.



Registration Procedure

With the RRC connection established, the UE performs a Registration procedure. The Registration procedure

starts with the UE sending an Attach message to the MME where the UE requests to register with the LTE

network. The MME initiates the authentication procedure with the UE. When authentication is successfully

completed, the security mode is enabled to protect the information sent over the air. The MME selects the PDN

Gateway to setup a default bearer between the Serving Gateway and the PDN Gateway. The PDN Gateway

eventually allocates an IP address for the UE. The UE can initiate packet data services using this default IP

address (e.g., email).

A default radio bearer and default S1 and S5 bearers are then set up. Now, the UE may trigger a request for

other service-specific bearers or use the default bearer for IP services.

.



8.0 Bearer Service

LTE is designed to support various applications with a wide range of data rates, error rates and latency

requirements. A user may run applications such as VoIP, web browsing and streaming on his UE. These

applications require QoS support from the network. For example, VoIP is a constant data rate, low latency

application. Web browsing, on the other hand, is bursty in nature but can tolerate delays better than VoIP

applications. Streaming is somewhere in between, it needs higher data rates and low latency but can tolerate

errors to a certain extent.

From an application perspective, QoS can be supported by allocating resources to transport packets from a UE

to an application server and back. In LTE systems, an end-to-end bearer is defined using two components. First

is the Evolved Packet System (EPS) bearer, which is defined between the UE and the PDN Gateway. Second is

the external bearer, which is defined between the PDN Gateway and the external application server such as a

web server. For example, a VoIP application is set up between the UE and a VoIP server. Let‟s look at a

streaming video application as an example. When a streaming video application is set up, QoS needs are

understood and an LTE bearer is configured to meet the data rate, latency and error rate requirements.

However, LTE does not control the external bearer setup. IP-based QoS mechanisms including Service Level

Agreements (SLA) will be used to set up QoS on the external bearer. Similarly for each application, an EPS

bearer and an external bearer will be constructed.



QOS Functional Elements

There are two aspects of enforcing QoS in LTE. The first occurs during the service set up. LTE defines Allocation

and Retention Priority (ARP), which is used to enforce relative priorities between users and preemption

decisions. The second aspect is to enforce QoS on individual traffic packets after service set up is completed.

When the UE requests a service with certain QoS requirements, these requirements are eventually handed

down to the P-GW by the Policy Charging Rule Function (PCRF). The P-GW then communicates with admission

control at the MME/S-GW. Admission control at the MME then verifies the subscriber profile and may change

targeted QoS levels for the service. This updated QoS requirement is then forwarded to the admission control

function at the eNB. The eNB verifies its resource availability and load levels to see if the new requirements will

be supported by setting a new radio bearer. If yes, the eNB will configure the new radio bearer and inform the

UE. It will also trigger the schedulers to include the new radio bearers in scheduling computations. Finally, the

supported QoS information makes its way back to the P-GW so that all nodes are in sync with the QoS to be

supported for this service.



EPS Bearer

A UE requests a service with certain QoS requirements. This may be triggered by the PCRF indicating QoS

needs to the P-GW. The PDN Gateway sets up a GTP tunnel between the PDN Gateway and the Serving

Gateway. This is called the S5 access bearer. The admission control entity at the Gateways decides on the

resource to be allocated for the service and sets up a GTP tunnel between the Serving Gateway and the eNB.

This is called the S1 access bearer. The admission control at eNB decides on the processing techniques and

allocates the radio resource blocks to the UE for a particular service. The radio bearer, S1 bearer and S5

bearer together form the EPS bearer to deliver QoS for the requested service.



9.0 Call Session Setup

We have looked at the default EPS bearer setup and IP address allocation sequence. Next, we walk through a

new service addition. For example, Sue initiates a video streaming application. The high-level sequence of

steps to set up bearers for this new service is as follows:

1. Sue‟s streaming application uses the default bearer to send the service request to the streaming

video server. QoS needs for this service as detected by the server or other entities in the network is

eventually communicated to the PCRF.

2. The PCRF conveys the QoS requirements for this service to the P-GW. The P-GW updates its Traffic

Flow Template (TFT) to enable QoS-specific processing for packets coming from the server to the UE

for this service.

3. The P-GW initiates a GTP-U tunnel setup with an S-GW for transporting packets related to this service.

The S-GW will perform the admission control function on this request. Based on the current load, the

subscriber profile and the request, the S-GW may reduce the offered QoS for the service.

4. It is now the MME‟s turn to initiate a GTP-U tunnel set up between the S-GW and the eNB. On receiving

this request, the eNB will apply its admission control function to check if the current conditions,

including traffic load, existing set of services and their QoS requirements, can be supported. Similar to

the S-GW, the eNB may also reduce the supported QoS before proceeding to the next step.

5. Next, the eNB sets up a radio bearer (RB) between the eNB and the UE for the current service request.

The RB will support the QoS decision made by the eNB in the previous step.

6. On successful completion of the RB setup, the eNB will complete the S1 GTP-U tunnel setup by

sending an S1 bearer setup response message to the MME. This allows the eNB to send the updated

QoS used for the RB back to the S-GW.

7. The MME completes the GTP-U tunnel setup between the S-GW and the P-GW by sending the S5

bearer setup response message. As part of the response, the MME forwards the updated QoS

received from the eNB to the P-GW. Now the P-GW knows the QoS parameters it should use for

incoming packet treatment for this service.

These steps are repeated for every new service instance. Now streaming video packets start flowing between

the server and the UE. The LTE network delivers the QoS by appropriate treatment of packets at the P-GW, the

S-GW and the eNB using the configured EPS bearer.



10.0 Downlink (DL)/Uplink (UL) Traffic

How does Sue receive streaming data?

Sue is receiving packets from the video streaming server. Her data arrives at the eNB after traversing the LTE

core network. Sue‟s data is currently buffered in the eNB and now it needs to be transmitted over the air.

Downlink Traffic

1. The UE reports the CQI to the eNB periodically. The eNB uses this feedback to decide the number of

resources, modulation and protection schemes such that the resource allocation is optimized. The UE

continuously monitors the DL reference signals or the overall signal strength to report a particular

value of the CQI.



2. The scheduler at the eNB uses feedback information along with other inputs, like the UE‟s buffer

status, to decide the DL resource allocation. The scheduler decides the number of users to receive the

data in the subframe as well as their resources.

3. Once the scheduler decides the users and their resources, the selected users are informed about their

resources on the PDCCH channel to ensure that the selected users will only decode the information on

the shared channel. The scheduled (or selected) users will receive their high-speed data on the

PDSCH. Note that different users will receive at different data rates depending upon the scheduled

resources.

4. The UE now performs the HARQ process. In HARQ, the UE tries to decode the transmitted information

by verifying the checksum. If the checksum verification is successful, the UE transmits an ACK

(Acknowledge message) on the PUCCH, else the UE transmits a NACK (negative Acknowledge

message). The eNB retransmits additional redundant bits when a NACK is received.

Uplink Traffic

1. On the Uplink, the resources are again scheduled by the eNB. The UE requests uplink resources from

the eNB by transmitting a Buffer Status Report (BSR). The buffer status report informs the data bits

buffered for transmission, available power and the data buffer growth trend at the UE.

2. Like the DL scheduler, the UL scheduler at the eNB collects the BSR information from the UE, along

with the current UL load condition, recent allocation history and the BSR of other UEs requesting

resources to decide on the UL resources for the selected UE.

3. The eNB transmits Uplink allocation informing the UE about the resources allocation and the power for

UL transmission. The UE performs data rate selection based on the UL resource allocation, the UE

capability and the buffer status.

4. The UE now forms the data packet for transmission and transmits the high-speed data using the

allocated UL resources and transmission characteristics on PUSCH.

5. The HARQ process, similar to that on the DL, is now initiated by the eNB.



11.0 LTE Mobility

Overview of Mobility in LTE

In LTE, a UE can be in one of two mobility states, idle or active. A UE is said to be in the idle mode when the UE

is not actively exchanging traffic. A UE is said to be in active mode when it is actively exchanging traffic.

When a UE is in idle mode, it executes one of three mobility related procedures: cell selection, cell reselection

and Tracking Area Update (TAU). In the active mode, LTE needs to support two possibilities. One is the case in

which a UE moves from one cell to another cell with no change in MME (i.e., intra-MME mobility). The second

case is when a UE moves to a cell that requires a change in MME (i.e., inter-MME mobility). We will walk

through the scenarios for both idle and active modes in this module.



LTE Cell Selection/Reselection

Cell Selection: When the UE is first powered up, the UE decides, based on stored information, with which

technology band and frequency it should start. If the UE is configured to look for an LTE network, it tries to

acquire an LTE cell. The UE acquires an LTE cell by scanning for LTE reference signals. The UE then verifies

that the network it has acquired is one of the valid networks per its configuration. The UE then receives system

broadcast information, which includes cell selection/reselection criteria and neighbor cell information.

Cell Reselection: Cell reselection is the procedure to address UE mobility across cells when no active traffic

exchange is taking place between the UE and the network. Cell reselection is controlled by the network through

configuration data sent on System Information Blocks (SIBs). In the idle state, the UE measures the signal

quality of neighbor cells. The type of measurement and the criteria used to decide when to move to another

cell is sent by the network as part of SIBs.

In this scenario, the UE is camping on cell 1. Cell 1 has sent system information about neighboring cells (e.g.,

cell 2 and cell 3) and cell reselection criteria. The UE measures the reference signals from cells 1, 2 and 3 on a

periodic basis as configured. Let‟s assume that cell 3 meets the reselection criteria. Now, the UE will start

camping on cell 3.



Why Tracking Area Update?

Now that we understand how the UE decides which cell to camp on, there are a couple more questions that we

need to answer. How does the network track a UE that is in its idle state? How does the network know where

to page a UE if data arrives for that UE?

The network needs to know the location of the UE to page it. LTE tracks the UE by requiring the UE to register

its location with the network. If the UE is asked to update its location whenever it changes its cell, it may result

in a lot of uplink signaling just for the location updates. To minimize the location updates and keep paging

effective, we can track a UE at a higher granularity.

LTE defines Tracking Areas (TAs) for location tracking. A Tracking Area is a group of cells. Each cell will

broadcast the Tracking Areas that it supports. If a UE moves from a cell in one TA to a cell in a different TA,

then the UE will execute the Tracking Area Update procedure. When data arrives for a UE at the Serving

Gateway, the UE will be paged on the last TA per its last TA update procedure.



Intra-MME/Serving Gateway Mobility

LTE supports two kinds of mobility – Intra-MME/S-GW Handover and Inter-MME/S-GW Handover. In the Intra-

MME/S-GW handover scenario, a UE moves from one cell to another cell without a change in the serving cell.

In case of Inter-MME/S-GW handover, UE moves from one cell to another cell with a change in MME.

Let‟s focus on mobility when a UE is actively exchanging traffic with the network. The UE is currently located in

cell C1. The UE has established the radio, S1 and S5 bearers as shown in the picture.

As the UE moves from cell C1 in the direction of cell C2, the UE measures the neighboring cell C2‟s reference

signal. When cell C2‟s reference signal quality meets the trigger criteria for a handover from cell C1, the

handover procedure is initiated. In this scenario, we are assuming that cells C1 and C2 are both connected to

the same S-GW. Intra-MME/S-GW handover procedure consists of three steps:

Handover preparation

Handover execution

Handover completion



Handover Preparation

The UE has established the radio bearer over the air, the S1 bearer between the eNB and the S-GW, and the

S5 bearer between the S-GW and the P-GW. Cell C1 has already configured the UE on measurement

requirements and triggers. In this scenario, the current serving cell is C1 and the intended target cell for

handover is C2.

Handover preparation is the first step. In this step, the UE reports handover trigger conditions by typically

sending a measurement report to the serving cell - C1. C1 then initiates a handover by sending a handover

request to the target cell - C2. C2 will go through its admission control functions. If successful, C2 will allocate

radio resources and convey the information to C1. C1 will convey radio resources allocated by C2 to the UE.



Handover Execution

Once the preparation of the handover phase is completed, the handover execution phase starts. After C1

conveys the handover command to the UE, C1 will establish a data pipe to C2 and start forwarding the UE‟s

data packets waiting for transmission in its buffer. This approach helps minimize packet loss during handover

between cells. In the meantime, the UE synchronizes with C2 and establishes a new radio bearer with C2. At

this time, C2 may start scheduling traffic packets for delivery to the UE. However, the S-GW is still forwarding

packets for the UE to C1, which, in turn, will forward them to C2.



Handover Completion

Now we move to the final phase of handover, the completion phase. At the start of this phase, user data is

forwarded from the S-GW to C1. C1 forwards data from its buffer to C2. C2 will schedule delivery using the

established radio bearer to the UE. When the radio bearer synchronization and establishment is completed

successfully with the UE, C2 will trigger handover messaging with the MME. This will result in the set up of the

S1 bearer between C2 and the S-GW. The S-GW will start forwarding packets for the UE using the new S1

bearer to C2. Once C1 forwards all the data packets from its buffer for the UE, it will release the data pipes

between C1 and C2 as well as the S1 bearer between C1 and S-GW. This completes the handover.

At the end of the handover, data flows from the S-GW to C2 and then on to the UE.



12.0 LTE Deployment

Evolution from GSM to LTE

An operator may have successfully deployed a GSM/GPRS/EDGE network. Based on market demand for voice

and packet data services and availability of spectrum, the operator may choose to upgrade the 2G

GSM/GPRS/EDGE network to a 3G UMTS network by overlaying UMTS on GSM/GPRS/EDGE. And, if there is a

demand for high speed packet data, the operator may deploy HSPA (i.e., HSDPA and HSUPA). As the subscriber

demand for packet data and multimedia services continue to grow, the operator may decide to deploy LTE to

meet customer demands and needs. This is the natural evolutionary path as defined by 3GPP.

Alternatively, let us say a network operator has deployed GSM-based networks. Due to lack of a strong

business case or lack of resources such as spectrum, an operator may choose not to upgrade to UMTS and

HSPA systems. However, when the need arises for deploying high-speed packet data and multimedia services,

the operator may choose to deploy LTE as an overlay to the GSM/GPRS/EDGE networks, directly bypassing

UMTS deployment. 3GPP has defined LTE to support such a direct evolutionary path so as to benefit the

operator community. A 2G GSM/GPRS/EDGE network operator can decide to deploy 3G UMTS depending on

the availability of spectrum and the subscriber demand for voice and packet data. The 3G network will be an

on the upgraded GSM/GPRS/EDGE network. If the demand for higher speed data increases, the network

operator may choose to deploy HSPA (HSDPA and HSUPA). In rural areas, both voice and data services can still

be provided using the GSM/GPRS/EDGE network and for the urban areas will have both UMTS and 2G

networks. Finally, the operator may decide to go for LTE to cater the growing demand for higher data and

multimedia services from the customers. This is natural evolution to 3GPP LTE.



LTE Deployment with GSM/GPRS/EDGE

Here is an evolution scenario where a network operator evolves his network from GSM/GPRS/EDGE to LTE.

Let‟s assume that the operator is a large operator covering both urban and rural areas. When the demand for

new services such as high-speed packet data is seen, it is typically the urban areas that need to be addressed

first. The operator will start by deploying LTE in the urban areas. In a large network this will result in LTE

coverage appearing as islands in the middle of the largest GSM coverage area. The size and location of LTE

coverage areas will be driven by market demand. In the rural areas, which do not have a demand for high data

rate services, the operator can hand-down LTE users to the GSM/GPRS/EDGE network.

LTE Deployment with UMTS/HSPA

Now let us look at an evolution scenario where a network operator follows the 3GPP standard evolutionary

path and grows the network from GSM/GPRS/EDGE to UMTS/HSDPA/HSUPA to LTE.

The operator provides wireless voice and data service coverage to the entire service area with the

GSM/GPRS/EDGE network. In the areas where there is moderate demand for data services, the network



operator may overlay UMTS/HSDPA/HSUPA over the GSM/GPRS/EDGE network to support optimized data

services. Typically, urban areas are the first ones with the need for new services such as high data-rate

services. While the operator may not have deployed UMTS/HSPA services as a complete overlay to the GSM-

based network, ever increasing demand in urban areas may be met by deploying small islands of LTE in

appropriate areas. The size and location of smaller LTE coverage areas will be driven by market demand. In

areas which do not have a demand for high data rate services, the operator can hand-down users to

UMTS/HSPA and/or the GSM/GPRS/EDGE network as dictated by the deployed network.

Evolution from CDMA to LTE

How does an EV-DO network evolve to an LTE Network? We‟ll begin with a legacy EV-DO Network.

The Access Terminal (AT) communicates with the Internet by establishing a connection through the EV-DO

Radio Access Network (EV-DO RAN). The connection then goes through the EV-DO Packet Switched Core

Network (EV-DO PS-CN). The Radio Network Controller (RNC) provides access to the Packet Data Service Node

(PDSN) in the core network while the Home Agent (HA) connects the core network to the Internet.



As described in the 3GPP standards, we now see the interworking between an EV-DO network and LTE. Note

that access is still provided by the EV-DO CDMA-based network. However, new interfaces (S101, S103 and

S2a) are defined to enable the EV-DO network to connect to the LTE packet core network; the Evolved Packet

Core (EPC). The S101 interface connects the EV-DO-based RNC to the LTE-based MME for signaling and

session establishment. The S103 interface connects the PDSN to the S-GW and serves a data forwarding role.

The S2a interface connects the PDSN to the PDN-GW for transport of data. The user is now using EV-DO for the

air interface, but using the LTE-EPC to transport data to and from the Internet.

A total migration from EV-DO to LTE occurs by replacing the EV-DO RAN with the Evolved Universal Terrestrial

Radio Access Network (E-UTRAN). As stated before, E-UTRAN is simpler than the EV-DO RAN in that the eNB

assumes the responsibility of both the Base Station (BS) and the RNC. Control and signaling uses the S1-MME

link between the eNB and the MME, while data uses the S1-U link between the eNB and the S-GW. As

networks evolve from EV-DO-based to LTE-based, there will likely be pockets of LTE-capable cell sites within an

existing EV-DO network. Dual mode devices will enable the subscriber to hand off back and forth between the

legacy EV-DO network and the LTE network using Mobile-IP.



13.0 Acronyms

2 2G Second Generation Wireless Systems

3 3G Third Generation Wireless Systems

3GPP 3rd Generation Partnership Project

4 4G Fourth Generation Wireless Systems

A AAA Authentication, Authorization and Accounting

ACK Acknowledgement

ARQ Automatic Repeat-reQuest

AT Access Terminal

B BSR Buffer Status Report

C CDMA Code Division Multiple Access

CP Cyclic Prefix

CQI Channel Quality Indicator

D DFT Discrete Fourier Transform

DL Downlink

DL-SCH Downlink Shared Channel

DSP Digital Signal Processing

E EDGE Enhanced Data for GSM Evolution

eNB Evolved NodeB

EPC Evolved Packet Core

EPS Evolved Packet System

eUTRAN Evolved UTRAN

EVDO Evolution Data Optimized

EVDO RNC EVDO Radio Network Controller

F FDD Frequency Division Duplexing

G GPRS General Packet Radio Service

GSM Global System for Mobile Communication

GTP-U GPRS Tunneling Protocol - User plane

H HARQ Hybrid Automatic Repeat-reQuest

HSDPA High Speed Downlink Packet Access

HSPA/HSPA+ High Speed Packet Access

HSS Home Subscriber Server

HSUPA High Speed Uplink Packet Access

I IMS IP Multimedia Subsystem

IP Internet Protocol

ISI Intersymbol Interference

L LTE Long Term Evolution

M MIMO Multiple Input Multiple Output

MME Mobility Management Entity

N NACK Negative Acknowledgement

O OFDM Orthogonal Frequency Division Multiplexing

OFDMA Orthogonal Frequency Division Multiple

Access

P PAPR Peak-to-Average Power Ratio

PBCH Physical Broadcast Channel

PCH Paging Channel

PCRF Policy and Charging Rules Function

PDCCH Physical Downlink Control Channel

PDN-GW Packet Data Network Gateway

PDSCH Physical Downlink Shared Channel

PDSN Packet Data Serving Node

P-GW Packet Gateway

PRACH Physical Random Access Channel

PUCCH Physical Uplink Control Channel

PUSCH Physical Uplink Shared Channel

Q QoS Quality Of Service

R RACH Random Access Channel

S SC-FDMA Single Carrier-Frequency Division Multiple

Access

S-GW Serving Gateway

SIB System Information Block

SO-FDMA Scalable Orthogonal-Frequency Division

Multiple Access

T TAU Tracking Area Update

TDD Time Division Duplexing

TFT Traffic Flow Template



U UE User Equipment

UL Uplink

UL-SCH Uplink Shared Channel

UMTS Universal Mobile Telecommuniations System

UTRAN Universal Terrestrial Radio Access Network

V VOIP Voice Over IP

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LTE Overview

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