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LTE Overview
LTE stands for Long Term Evolution and it was started as a project in 2004 by
telecommunication body known as the Third Generation Partnership Project
(3GPP). SAE (System Architecture Evolution) is the corresponding evolution of
the GPRS/3G packet core network evolution. The term LTE is typically used to
represent both LTE and SAE.

LTE evolved from an earlier 3GPP system known as the Universal Mobile
Telecommunication System (UMTS), which in turn evolved from the Global
System for Mobile Communications (GSM). Even related specifications were
formally known as the evolved UMTS terrestrial radio access (E-UTRA) and
evolved UMTS terrestrial radio access network (E-UTRAN). First version of LTE
was documented in Release 8 of the 3GPP specifications.

A rapid increase of mobile data usage and emergence of new applications such
as MMOG (Multimedia Online Gaming), mobile TV, Web 2.0, streaming contents
have motivated the 3rd Generation Partnership Project (3GPP) to work on the
Long-Term Evolution (LTE) on the way towards fourth-generation mobile.

The main goal of LTE is to provide a high data rate, low latency and packet
optimized radioaccess technology supporting flexible bandwidth deployments.
Same time its network architecture has been designed with the goal to support
packet-switched traffic with seamless mobility and great quality of service.
LTE Evolution

Year Event

Mar 2000 Release 99 - UMTS/WCDMA

Mar 2002 Rel 5 - HSDPA

Mar 2005 Rel 6 - HSUPA

Year 2007 Rel 7 - DL MIMO, IMS (IP Multimedia Subsystem)

November 2004 Work started on LTE specification

January 2008 Spec finalized and approved with Release 8

2010 Targeted first deployment

Facts about LTE

LTE is the successor technology not only of UMTS but also of CDMA
2000.
LTE is important because it will bring up to 50 times performance
improvement and much better spectral efficiency to cellular networks.

LTE introduced to get higher data rates, 300Mbps peak downlink and 75
Mbps peak uplink. In a 20MHz carrier, data rates beyond 300Mbps can
be achieved under very good signal conditions.
LTE is an ideal technology to support high date rates for the services
such as voice over IP (VOIP), streaming multimedia, videoconferencing
or even a high-speed cellular modem.

LTE uses both Time Division Duplex (TDD) and Frequency Division
Duplex (FDD) mode. In FDD uplink and downlink transmission used
different frequency, while in TDD both uplink and downlink use the same
carrier and are separated in Time.

LTE supports flexible carrier bandwidths, from 1.4 MHz up to 20 MHz as

well as both FDD and TDD. LTE designed with a scalable carrier
bandwidth from 1.4 MHz up to 20 MHz which bandwidth is used depends
 on the frequency band and the amount of spectrum available with a
network operator.
All LTE devices have to support (MIMO) Multiple Input Multiple Output
transmissions, which allow the base station to transmit several data
streams over the same carrier simultaneously.

All interfaces between network nodes in LTE are now IP based, including
the backhaul connection to the radio base stations. This is great
simplification compared to earlier technologies that were initially based
on E1/T1, ATM and frame relay links, with most of them being
narrowband and expensive.

Quality of Service (QoS) mechanism have been standardized on all

interfaces to ensure that the requirement of voice calls for a constant
delay and bandwidth, can still be met when capacity limits are reached.
Works with GSM/EDGE/UMTS systems utilizing existing 2G and 3G
spectrum and new spectrum. Supports hand-over and roaming to
existing mobile networks.

Advantages of LTE

High throughput: High data rates can be achieved in both downlink as

well as uplink. This causes high throughput.

Low latency: Time required to connect to the network is in range of a

few hundred milliseconds and power saving states can now be entered
and exited very quickly.
FDD and TDD in the same platform: Frequency Division Duplex
(FDD) and Time Division Duplex (TDD), both schemes can be used on
same platform.

Superior end-user experience: Optimized signaling for connection

establishment and other air interface and mobility management
procedures have further improved the user experience. Reduced latency
(to 10 ms) for better user experience.

Seamless Connection: LTE will also support seamless connection to

existing networks such as GSM, CDMA and WCDMA.
Plug and play: The user does not have to manually install drivers for
the device. Instead system automatically recognizes the device, loads
 new drivers for the hardware if needed, and begins to work with the
newly connected device.
Simple architecture: Because of Simple architecture low operating
expenditure (OPEX).

LTE - QoS
LTE architecture supports hard QoS, with end-to-end quality of service and
guaranteed bit rate (GBR) for radio bearers. Just as Ethernet and the internet
have different types of QoS, for example, various levels of QoS can be applied
to LTE traffic for different applications. Because the LTE MAC is fully scheduled,
QoS is a natural fit.

Evolved Packet System (EPS) bearers provide one-to-one correspondence with

RLC radio bearers and provide support for Traffic Flow Templates (TFT). There
are four types of EPS bearers:

GBR Bearer resources permanently allocated by admission control

Non-GBR Bearer no admission control

Dedicated Bearer associated with specific TFT (GBR or non-GBR)

Default Bearer Non GBR, catch-all for unassigned traffic

LTE Basic Parameters

This section will summarize the Basic parameters of the LTE:

Parameters Description

UMTS FDD bands and TDD bands defined in

Frequency range
36.101(v860) Table 5.5.1, given below

Duplexing FDD, TDD, half-duplex FDD

Channel coding Turbo code

Mobility 350 km/h

Channel Bandwidth
(MHz) 1.4
 3
5

10
15
20

Transmission Bandwidth 15
Configuration NRB : (1 25
resource block = 180kHz
50
in 1ms TTI )
75

100

UL: QPSK, 16QAM, 64QAM(optional)

Modulation Schemes
DL: QPSK, 16QAM, 64QAM

UL: SC-FDMA (Single Carrier Frequency Division

Multiple Access) supports 50Mbps+ (20MHz
spectrum)
Multiple Access Schemes
DL: OFDM (Orthogonal Frequency Division
Multiple Access) supports 100Mbps+ (20MHz
spectrum)

UL: Multi-user collaborative MIMO

Multi-Antenna Technology DL: TxAA, spatial multiplexing, CDD ,max 4x4
array

UL: 75Mbps(20MHz bandwidth)

DL: 150Mbps(UE Category 4, 2x2 MIMO, 20MHz
Peak data rate in LTE bandwidth)
DL: 300Mbps(UE category 5, 4x4 MIMO, 20MHz
bandwidth)

MIMO
UL: 1 x 2, 1 x 4
(Multiple Input Multiple
DL: 2 x 2, 4 x 2, 4 x 4
Output)
 Coverage 5 - 100km with slight degradation after 30km

E2E QOS allowing prioritization of different class

QoS
of service

Latency End-user latency < 10mS

E-UTRA Operating Bands

Following is the table for E-UTRA operating bands taken from LTE Sepecification
36.101(v860) Table 5.5.1:

LTE Network Architecture

The high-level network architecture of LTE is comprised of following three main
components:

The User Equipment (UE).

The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
 The Evolved Packet Core (EPC).

The evolved packet core communicates with packet data networks in the
outside world such as the internet, private corporate networks or the IP
multimedia subsystem. The interfaces between the different parts of the
system are denoted Uu, S1 and SGi as shown below:

The User Equipment (UE)

The internal architecture of the user equipment for LTE is identical to the one
used by UMTS and GSM which is actually a Mobile Equipment (ME). The mobile
equipment comprised of the following important modules:

Mobile Termination (MT) : This handles all the communication

functions.
Terminal Equipment (TE) : This terminates the data streams.
Universal Integrated Circuit Card (UICC) : This is also known as the
SIM card for LTE equipments. It runs an application known as the
Universal Subscriber Identity Module (USIM).

A USIM stores user-specific data very similar to 3G SIM card. This keeps
information about the user's phone number, home network identity and
security keys etc.

The E-UTRAN (The access network)

The architecture of evolved UMTS Terrestrial Radio Access Network (E-UTRAN)
has been illustrated below.
The E-UTRAN handles the radio communications between the mobile and the
evolved packet core and just has one component, the evolved base stations,
called eNodeB or eNB. Each eNB is a base station that controls the mobiles in
one or more cells. The base station that is communicating with a mobile is
known as its serving eNB.

LTE Mobile communicates with just one base station and one cell at a time and
there are following two main functions supported by eNB:

The eNB sends and receives radio transmissions to all the mobiles using
the analogue and digital signal processing functions of the LTE air
interface.
The eNB controls the low-level operation of all its mobiles, by sending
them signalling messages such as handover commands.

Each eNB connects with the EPC by means of the S1 interface and it can also
be connected to nearby base stations by the X2 interface, which is mainly used
for signalling and packet forwarding during handover.

A home eNB (HeNB) is a base station that has been purchased by a user to
provide femtocell coverage within the home. A home eNB belongs to a closed
subscriber group (CSG) and can only be accessed by mobiles with a USIM that
also belongs to the closed subscriber group.

The Evolved Packet Core (EPC) (The core network)

The architecture of Evolved Packet Core (EPC) has been illustrated below.
There are few more components which have not been shown in the diagram to
keep it simple. These components are like the Earthquake and Tsunami
Warning System (ETWS), the Equipment Identity Register (EIR) and Policy
Control and Charging Rules Function (PCRF).

Below is a brief description of each of the components shown in the above

architecture:

The Home Subscriber Server (HSS) component has been carried forward
from UMTS and GSM and is a central database that contains information
about all the network operator's subscribers.
The Packet Data Network (PDN) Gateway (P-GW) communicates with
the outside world ie. packet data networks PDN, using SGi interface.
Each packet data network is identified by an access point name (APN).
The PDN gateway has the same role as the GPRS support node (GGSN)
and the serving GPRS support node (SGSN) with UMTS and GSM.
The serving gateway (S-GW) acts as a router, and forwards data
between the base station and the PDN gateway.

The mobility management entity (MME) controls the high-level operation

of the mobile by means of signalling messages and Home Subscriber
Server (HSS).
The Policy Control and Charging Rules Function (PCRF) is a component
which is not shown in the above diagram but it is responsible for policy
 control decision-making, as well as for controlling the flow-based
charging functionalities in the Policy Control Enforcement Function
(PCEF), which resides in the P-GW.

The interface between the serving and PDN gateways is known as S5/S8. This
has two slightly different implementations, namely S5 if the two devices are in
the same network, and S8 if they are in different networks.

Functional split between the E-UTRAN and the EPC

Following diagram shows the functional split between the E-UTRAN and the EPC
for an LTE network:

2G/3G Versus LTE

Following table compares various important Network Elements & Signaling
protocols used in 2G/3G abd LTE.

2G/3G LTE
 GERAN and UTRAN E-UTRAN

SGSN/PDSN-FA S-GW

GGSN/PDSN-HA PDN-GW

HLR/AAA HSS

VLR MME

SS7-MAP/ANSI-41/RADIUS Diameter

DiameterGTPc-v0 and v1 GTPc-v2

MIP PMIP

LTE Roaming Architecture

A network run by one operator in one country is known as a Public Land Mobile
Network (PLMN) and when a subscribed user uses his operator's PLMN then it is
said Home-PLMN but roaming allows users to move outside their home network
and using the resources from other operator's network. This other network is
called Visited-PLMN.

A roaming user is connected to the E-UTRAN, MME and S-GW of the visited LTE
network. However, LTE/SAE allows the P-GW of either the visited or the home
network to be used, as shown in below:
The home network's P-GW allows the user to access the home operator's
services even while in a visited network. A P-GW in the visited network allows a
"local breakout" to the Internet in the visited network.

The interface between the serving and PDN gateways is known as S5/S8. This
has two slightly different implementations, namely S5 if the two devices are in
the same network, and S8 if they are in different networks. For mobiles that
are not roaming, the serving and PDN gateways can be integrated into a single
device, so that the S5/S8 interface vanishes altogether.

LTE Roaming Charging

The complexities of the new charging mechanisms required to support 4G
roaming are much more abundant than in a 3G environment. Few words about
both pre-paid and post-paid charging for LTE roaming is given below:

Prepaid Charging - The CAMEL standard, which enables prepaid

services in 3G, is not supported in LTE; therefore, prepaid customer
information must be routed back to the home network as opposed to
being handled by the local visited network. As a result, operators must
rely on new accounting flows to access prepaid customer data, such as
through their P-Gateways in both IMS and non-IMS environments or via
their CSCF in an IMS environment.
Postpaid Charging - Postpaid data-usage charging works the same in
LTE as it does in 3G, using versions TAP 3.11 or 3.12. With local
breakout of IMS services, TAP 3.12 is required.

Operators do not have the same amount of visibility into subscriber activities as
they do in home-routing scenarios in case of local breakout scenarios because
subscriber-data sessions are kept within the visited network; therefore, in
order for the home operator to capture real-time information on both pre- and
postpaid customers, it must establish a Diameter interface between charging
systems and the visited network's P-Gateway.

In case of local breakout of ims services scenario, the visited network creates
call detail records (CDRs) from the S-Gateway(s), however, these CDRs do not
contain all of the information required to create a TAP 3.12 mobile session or
messaging event record for the service usage. As a result, operators must
correlate the core data network CDRs with the IMS CDRs to create TAP records.
 LTE Numbering & Addressing
An LTE network area is divided into three different types of geographical areas
explained below:

S.N. Area and Description

The MME pool areas

This is an area through which the mobile can move without a change
1
of serving MME. Every MME pool area is controlled by one or more
MMEs on the network.

The S-GW service areas

This is an area served by one or more serving gateways S-GW,
2
through which the mobile can move without a change of serving
gateway.

The Tracking areas

The MME pool areas and the S-GW service areas are both made from
smaller, non-overlapping units known as tracking areas (TAs). They
3
are similar to the location and routing areas from UMTS and GSM and
will be used to track the locations of mobiles that are on standby
mode.

Thus an LTE network will comprise of many MME pool areas, many S-GW
service areas and lots of tracking areas.

The Network IDs

The network itself will be identified using Public Land Mobile Network Identity
(PLMN-ID) which will have a three digit mobile country code (MCC) and a two
or three digit mobile network code (MNC). For example, the Mobile Country
Code for the UK is 234, while Vodafone's UK network uses a Mobile Network
Code of 15.

The MME IDs

Each MME has three main identities. An MME code (MMEC) uniquely identifies
the MME within all the pool areas. A group of MMEs is assigned an MME Group
Identity (MMEGI) which works along with MMEC to make MME identifier
(MMEI). A MMEI uniquely identifies the MME within a particular network.

If we combile PLMN-ID with the MMEI then we arrive at a Globally Unique MME
Identifier (GUMMEI), which identifies an MME anywhere in the world:

The Tracking Area IDs

Each tracking area has two main identities. The tracking area code (TAC)
identifies a tracking area within a particular network and if we combining this
with the PLMN-ID then we arrive at a Globally Unique Tracking Area Identity
(TAI).

The Cell IDs

Each cell in the network has three types of identity. The E-UTRAN cell identity
(ECI) identifies a cell within a particular network, while the E-UTRAN cell global
identifier (ECGI) identifies a cell anywhere in the world.

The physical cell identity, which is a number from 0 to 503 and it distinguishes
a cell from its immediate neighbours.

The Mobile Equipment ID

The international mobile equipment identity (IMEI) is a unique identity for the
mobile equipment and the International Mobile Subscriber Identity (IMSI) is a
unique identity for the UICC and the USIM.

The M temporary mobile subscriber identity (M-TMSI) identifies a mobile to its

serving MME. Adding the MME code in M-TMSI results in a S temporary mobile
subscriber identity (S-TMSI), which identifies the mobile within an MME pool
area.

Finally adding the MME group identity and the PLMN identity with S-TMSI
results in the Globally Unique Temporary Identity (GUTI).

LTE Radio Protocol Architecture

The radio protocol architecture for LTE can be separated into control plane
architecture and user plane architecture as shown below:

At user plane side, the application creates data packets that are processed by
protocols such as TCP, UDP and IP, while in the control plane, the radio
resource control (RRC) protocol writes the signalling messages that are
exchanged between the base station and the mobile. In both cases, the
information is processed by the packet data convergence protocol (PDCP), the
radio link control (RLC) protocol and the medium access control (MAC)
protocol, before being passed to the physical layer for transmission.

User Plane
The user plane protocol stack between the e-Node B and UE consists of the
following sub-layers:

PDCP (Packet Data Convergence Protocol)

RLC (radio Link Control)
Medium Access Control (MAC)

On the user plane, packets in the core network (EPC) are encapsulated in a
specific EPC protocol and tunneled between the P-GW and the eNodeB.
Different tunneling protocols are used depending on the interface. GPRS
Tunneling Protocol (GTP) is used on the S1 interface between the eNodeB and
S-GW and on the S5/S8 interface between the S-GW and P-GW.

Packets received by a layer are called Service Data Unit (SDU) while the packet
output of a layer is referred to by Protocol Data Unit (PDU) and IP packets at
user plane flow from top to bottom layers.

Control Plane
The control plane includes additionally the Radio Resource Control layer (RRC)
which is responsible for configuring the lower layers.
The Control Plane handles radio-specific functionality which depends on the
state of the user equipment which includes two states: idle or connected.

Mode Description

The user equipment camps on a cell after a cell selection or

reselection process where factors like radio link quality, cell
status and radio access technology are considered. The UE also
Idle
monitors a paging channel to detect incoming calls and acquire
system information. In this mode, control plane protocols
include cell selection and reselection procedures.

The UE supplies the E-UTRAN with downlink channel quality and

neighbour cell information to enable the E-UTRAN to select the
Connected
most suitable cell for the UE. In this case, control plane protocol
includes the Radio Link Control (RRC) protocol.

The protocol stack for the control plane between the UE and MME is shown
below. The grey region of the stack indicates the access stratum (AS)
protocols. The lower layers perform the same functions as for the user plane
with the exception that there is no header compression function for the control
plane.

LTE Protocol Stack Layers

Let's have a close look at all the layers available in E-UTRAN Protocol Stack
which we have seen in previous chapter. Below is a more ellaborated diagram
of E-UTRAN Protocol Stack:
Physical Layer (Layer 1)
Physical Layer carries all information from the MAC transport channels over the
air interface. Takes care of the link adaptation (AMC), power control, cell search
(for initial synchronization and handover purposes) and other measurements
(inside the LTE system and between systems) for the RRC layer.

Medium Access Layer (MAC)

MAC layer is responsible for Mapping between logical channels and transport
channels, Multiplexing of MAC SDUs from one or different logical channels onto
transport blocks (TB) to be delivered to the physical layer on transport
channels, de multiplexing of MAC SDUs from one or different logical channels
from transport blocks (TB) delivered from the physical layer on transport
channels, Scheduling information reporting, Error correction through HARQ,
Priority handling between UEs by means of dynamic scheduling, Priority
handling between logical channels of one UE, Logical Channel prioritization.

Radio Link Control (RLC)

RLC operates in 3 modes of operation: Transparent Mode (TM),
Unacknowledged Mode (UM), and Acknowledged Mode (AM).

RLC Layer is responsible for transfer of upper layer PDUs, error correction
through ARQ (Only for AM data transfer), Concatenation, segmentation and
reassembly of RLC SDUs (Only for UM and AM data transfer).

RLC is also responsible for re-segmentation of RLC data PDUs (Only for AM
data transfer), reordering of RLC data PDUs (Only for UM and AM data
transfer), duplicate detection (Only for UM and AM data transfer), RLC SDU
discard (Only for UM and AM data transfer), RLC re-establishment, and protocol
error detection (Only for AM data transfer).

Radio Resource Control (RRC)

The main services and functions of the RRC sublayer include broadcast of
System Information related to the non-access stratum (NAS), broadcast of
System Information related to the access stratum (AS), Paging, establishment,
maintenance and release of an RRC connection between the UE and E-UTRAN,
Security functions including key management, establishment, configuration,
maintenance and release of point to point Radio Bearers.

Packet Data Convergence Control (PDCP)

PDCP Layer is responsible for Header compression and decompression of IP
data, Transfer of data (user plane or control plane), Maintenance of PDCP
Sequence Numbers (SNs), In-sequence delivery of upper layer PDUs at re-
establishment of lower layers, Duplicate elimination of lower layer SDUs at re-
establishment of lower layers for radio bearers mapped on RLC AM, Ciphering
and deciphering of user plane data and control plane data, Integrity protection
and integrity verification of control plane data, Timer based discard, duplicate
discarding, PDCP is used for SRBs and DRBs mapped on DCCH and DTCH type
of logical channels.

Non Access Stratum (NAS) Protocols

The non-access stratum (NAS) protocols form the highest stratum of the
control plane between the user equipment (UE) and MME.

NAS protocols support the mobility of the UE and the session management
procedures to establish and maintain IP connectivity between the UE and a
PDN GW.

LTE Layers Data Flow

Below is a logical digram of E-UTRAN Protocol layers with a depiction of data
flow through various layers:

Packets received by a layer are called Service Data Unit (SDU) while the packet
output of a layer is referred to by Protocol Data Unit (PDU). Let's see the flow
of data from top to bottom:

IP Layer submits PDCP SDUs (IP Packets) to the PDCP layer. PDCP layer
does header compression and adds PDCP header to these PDCP SDUs.
PDCP Layer submits PDCP PDUs (RLC SDUs) to RLC layer.
PDCP Header Compression : PDCP removes IP header (Minimum 20
bytes) from PDU, and adds Token of 1-4 bytes. Which provides a
tremendous savings in the amount of header that would otherwise have
to go over the air.
 RLC layer does segmentation of these SDUS to make the RLC PDUs. RLC
adds header based on RLC mode of operation. RLC submits these RLC
PDUs (MAC SDUs) to the MAC layer.
RLC Segmentation : If an RLC SDU is large, or the available radio data
rate is low (resulting in small transport blocks), the RLC SDU may be
split among several RLC PDUs. If the RLC SDU is small, or the available
radio data rate is high, several RLC SDUs may be packed into a single
PDU.
MAC layer adds header and does padding to fit this MAC SDU in TTI.
MAC layer submits MAC PDU to physical layer for transmitting it onto
physical channels.
Physical channel transmits this data into slots of sub frame.

LTE Communication Channels

The information flows between the different protocols are known as channels
and signals. LTE uses several different types of logical, transport and physical
channel, which are distinguished by the kind of information they carry and by
the way in which the information is processed.

Logical Channels : Define whattype of information is transmitted over

the air, e.g. traffic channels, control channels, system broadcast, etc.
Data and signalling messages are carried on logical channels between
the RLC and MAC protocols.
Transport Channels : Define howis something transmitted over the
air, e.g. what are encoding, interleaving options used to transmit data.
Data and signalling messages are carried on transport channels between
the MAC and the physical layer.
Physical Channels : Define whereis something transmitted over the
air, e.g. first N symbols in the DL frame. Data and signalling messages
 are carried on physical channels between the different levels of the
physical layer.

Logical Channels
Logical channels define what type of data is transferred. These channels define
the data-transfer services offered by the MAC layer. Data and signalling
messages are carried on logical channels between the RLC and MAC protocols.

Logical channels can be divided into control channels and traffic channels.
Control Channel can be either common channel or dedicated channel. A
common channel means common to all users in a cell (Point to multipoint)
while dedicated channels means channels can be used only by one user (Point
to Point).

Logical channels are distinguished by the information they carry and can be
classified in two ways. Firstly, logical traffic channels carry data in the user
plane, while logical control channels carry signalling messages in the control
plane. Following table lists the logical channels that are used by LTE:

Control Traffic
Channel Name Acronym
channel channel

Broadcast Control Channel BCCH X

Paging Control Channel PCCH X

Common Control Channel CCCH X

Dedicated Control Channel DCCH X

Multicast Control Channel MCCH X

Dedicated Traffic Channel DTCH X

Multicast Traffic Channel MTCH X

Transport Channels
Transport channels define how and with what type of characteristics the data is
transferred by the physical layer. Data and signalling messages are carried on
transport channels between the MAC and the physical layer.

Transport Channels are distinguished by the ways in which the transport

channel processor manipulates them. Following table lists the transport
channels that are used by LTE:

Channel Name Acronym Downlink Uplink

Broadcast Channel BCH X

Downlink Shared Channel DL-SCH X

Paging Channel PCH X

Multicast Channel MCH X

Uplink Shared Channel UL-SCH X

Random Access Channel RACH X

Physical Channels
Data and signalling messages are carried on physical channels between the
different levels of the physical layer and accordingly they are divided into two
parts:

Physical Data Channels

Physical Control Channels

Physical data channels

Physical data channels are distinguished by the ways in which the physical
channel processor manipulates them, and by the ways in which they are
mapped onto the symbols and sub-carriers used by Orthogonal frequency-
division multiplexing (OFDMA). Following table lists the physical data
channels that are used by LTE:

Channel Name Acronym Downlink Uplink

Physical downlink shared PDSCH X

 channel

Physical broadcast channel PBCH X

Physical multicast channel PMCH X

Physical uplink shared

PUSCH X
channel

Physical random access

PRACH X
channel

The transport channel processor composes several types of control

information, to support the low-level operation of the physical layer. These are
listed in the below table:

Field Name Acronym Downlink Uplink

Downlink control information DCI X

Control format indicator CFI X

Hybrid ARQ indicator HI X

Uplink control information UCI X

Physical Control Channels

The transport channel processor also creates control information that supports
the low-level operation of the physical layer and sends this information to the
physical channel processor in the form of physical control channels.

The information travels as far as the transport channel processor in the

receiver, but is completely invisible to higher layers. Similarly, the physical
channel processor creates physical signals, which support the lowest-level
aspects of the system.

Physical Control Channels are listed in the below table:

Channel Name Acronym Downlink Uplink

Physical control format

PCFICH X
indicator channel
 Physical hybrid ARQ indicator
PHICH X
channel

Physical downlink control

PDCCH X
channel

Relay physical downlink

R-PDCCH X
control channel

Physical uplink control

PUCCH X
channel

The base station also transmits two other physical signals, which help the
mobile acquire the base station after it first switches on. These are known as
the primary synchronization signal (PSS) and the secondary synchronization
signal (SSS).

LTE OFDM Technology

To overcome the effect of multi path fading problem available in UMTS, LTE
uses Orthogonal Frequency Division Multiplexing (OFDM) for the downlink -
that is, from the base station to the terminal to transmit the data over many
narrow band careers of 180 KHz each instead of spreading one signal over the
complete 5MHz career bandwidth ie. OFDM uses a large number of narrow sub-
carriers for multi-carrier transmission to carry data.

Orthogonal frequency-division multiplexing (OFDM), is a frequency-division

multiplexing (FDM) scheme used as a digital multi-carrier modulation method.

OFDM meets the LTE requirement for spectrum flexibility and enables cost-
efficient solutions for very wide carriers with high peak rates. The basic LTE
downlink physical resource can be seen as a time-frequency grid, as illustrated
in Figure below:

The OFDM symbols are grouped into resource blocks. The resource blocks have
a total size of 180kHz in the frequency domain and 0.5ms in the time domain.
Each 1ms Transmission Time Interval (TTI) consists of two slots (Tslot).
Each user is allocated a number of so-called resource blocks in the
time.frequency grid. The more resource blocks a user gets, and the higher the
modulation used in the resource elements, the higher the bit-rate. Which
resource blocks and how many the user gets at a given point in time depend on
advanced scheduling mechanisms in the frequency and time dimensions.

The scheduling mechanisms in LTE are similar to those used in HSPA, and
enable optimal performance for different services in different radio
environments.

Advantages of OFDM
 The primary advantage of OFDM over single-carrier schemes is its ability
to cope with severe channel conditions (for example, attenuation of high
frequencies in a long copper wire, narrowband interference and
frequency-selective fading due to multipath) without complex
equalization filters.
Channel equalization is simplified because OFDM may be viewed as
using many slowly-modulated narrowband signals rather than one
rapidly-modulated wideband signal.

The low symbol rate makes the use of a guard interval between symbols
affordable, making it possible to eliminate inter symbol interference
(ISI).

This mechanism also facilitates the design of single frequency networks

(SFNs), where several adjacent transmitters send the same signal
simultaneously at the same frequency, as the signals from multiple
distant transmitters may be combined constructively, rather than
interfering as would typically occur in a traditional single-carrier system.

Drawbacks of OFDM

High peak-to-average ratio

Sensitive to frequency offset, hence to Doppler-shift as well

SC-FDMA Technology
LTE uses a pre-coded version of OFDM called Single Carrier Frequency Division
Multiple Access (SC-FDMA) in the uplink. This is to compensate for a drawback
with normal OFDM, which has a very high Peak to Average Power Ratio (PAPR).

High PAPR requires expensive and inefficient power amplifiers with high
requirements on linearity, which increases the cost of the terminal and drains
the battery faster.

SC-FDMA solves this problem by grouping together the resource blocks in such
a way that reduces the need for linearity, and so power consumption, in the
power amplifier. A low PAPR also improves coverage and the cell-edge
performance.
 LTE Glossary

Term Description

3GPP 3rd Generation Partnership Project

3GPP2 3rd Generation Partnership Project 2

ARIB Association of Radio Industries and Businesses

ATIS Alliance for Telecommunication Industry Solutions

AWS Advanced Wireless Services

CAPEX Capital Expenditure

CCSA China Communications Standards Association

CDMA Code Division Multiple Access

CDMA2000 Code Division Multiple Access 2000

DAB Digital Audio Broadcast

DSL Digital Subscriber Line

DVB Digital Video Broadcast

eHSPA evolved High Speed Packet Access

ETSI European Telecommunications Standards Institute

FDD Frequency Division Duplex

FWT Fixed Wireless Terminal

GSM Global System for Mobile communication

HSPA High Speed Packet Access

HSS Home Subscriber Server

IEEE Institute of Electrical and Electronics Engineers

IPTV Internet Protocol Television

LTE Long Term Evolution

MBMS Multimedia Broadcast Multicast Service

MIMO Multiple Input Multiple Output

MME Mobility Management Entity

NGMN Next Generation Mobile Networks

OFDM Orthogonal Frequency Division Multiplexing

OPEX Operational Expenditure

PAPR Peak to Average Power Ratio

PCI Peripheral Component Interconnect

PCRF Policing and Charging Rules Function

PDSN Packet Data Serving Node

PS Packet Switched

QoS Quality of Service

RAN Radio Access Network

SAE System Architecture Evolution

SC-FDMA Single Carrier Frequency Division Multiple Access

SGSN Serving GPRS Support Node

TDD Time Division Duplex

TTA Telecommunications Technology Association

TTC Telecommunication Technology Committee

TTI Transmission Time Interval

UTRA Universal Terrestrial Radio Access

UTRAN Universal Terrestrial Radio Access Network

WCDMA Wideband Code Division Multiple Access

WLAN Wireless Local Area Network

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