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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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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
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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
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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).
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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.
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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.
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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.
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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
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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.
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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
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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.
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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
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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.
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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
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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.
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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?
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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.
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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.
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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:
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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
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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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