Introduction to LTE

No more Codes
Key technologies….
For Downlink : OFDM and MIMO

For Uplink : SC - FDMA

IMT – Advanced Requirements
 Support for at least 40 MHz Bandwidth

 Peak Spectral Efficiencies :
 DL : 15 bits/s/Hz (600 Mbps)
 UL : 6.75 bits/s/Hz(270 Mbps)

 Control Plane Latency < 100ms
 User Plane Latency < 10ms

Releases of 3GPP Specifications

Rel. 8 LTE EPC/SAE

Location Multi-
Rel.9 Services
MBMS Standard BS

Rel.10 LTE - A
Carrier
Aggregation
Relays

Enhanced Intra Band
Rel.11 Carrier
Aggregation
Carrier
Aggregation

System Architecture Evolution (SAE)

From 3G to 4G…

 UTRAN in 3G,
E-UTRAN in 4G

 CN in 3G, EPC in
4G

 NodeB in
3G, E-NodeB in
4G

 No RNC as in 3G

 RNC tasks perform
by eNodeB and
EPC

LTE/SAE Network
Internet
Architecture
P-GW
HSS
MME S-GW EPC
S5
S6a
MME/S-GW MME/S-GW

S1 S1 S1 S1

X2
E-UTRAN
X2 X2
eNodeB eNodeB

eNodeB

Evolved UTRAN (E-UTRAN)
 eNodeB :

 Directly connected to the Core via S1 interface
 No RNC as in WCDMA
 eNodeBs interconnected via X2 interface
 Handovers are handled by eNodeBs it self, communicating via X2
interface
 This is an intelligent Node

Evolved Packet Core (EPC)
Supports only packet switched domain only
 Mobility Management Entity (MME) :

 Control Plane Node of the EPC
 handling connection/release of bearers to a terminal
 handling of IDLE to ACTIVE Transition
 handling of security keys

 Serving Gateway(S-GW) :

 User plane node which connects EPC to E-UTRAN
 Acts as a mobility anchor when Terminals move between eNodeBs
 Mobility Anchor for other 3GPP technologies (GSM,HSPA)
 Collecting information for charging purposes

 Packet Data Network Gateway (P-GW) :
 Connects EPC to the Internet
 Allocation of the IP address for a specific terminal
 QoS handling

 Home Subscriber Service (HSS) :
 A database containing subscriber information

What is Orthogonal Frequency
Division Multiplexing (OFDM) ?

ISI – Inter Symbol Interference

Time domain :

Data Rate ISI

Time Spreading (Freq. Selective
Fading)

• When an impulse is transmitted , how
does the average power received by Power Delay Profile
Mobile
vary as a function of time delay ζ ?

 Freq. Selective Fading : Ts < ζ0

 Non Freq. Selective Fading : Ts > ζ0

Power Delay Profile Spaced Freq.
Correlation function

FT

Inside Coherence BW channel passes all freq. components with
equal gain and linear phase
 Freq. Selective Fading : W > f0
 Non Freq. Selective Fading : W < f0

• Symbol rate not increased in order to achieve high data
rates.
• Instead of that Available BW breaks in to many narrower
subcarriers and modulate generated symbols to these
subcarriers.
• These subcarriers then combine linearly and transmit
(OFDM symbol).

OFDM Modulation OFDM demodulation

Single carrier transmission Vs OFDM
Transmission

: Single Carrier
1 0 1 Transmission

1

: OFDM Transmission
0

1

t

Sub carrier Pulse shape and Spectrum

Subcarrier BW < Coherance BW

Why “Orthogonal” ?

Subcarriers “Orthogonal” in the time domain

In OFDM, Subcarriers are overlapped in Frequency
domain while maintaining orthogonality in time domain

Overlapping subcarriers in Freq.
domain

Overlapping Subcarriers Spectral Efficiency

OFDM Symbol
• Generated by Multiplexing several overlapping
subcarriers and a Cyclic Prefix (CP).

CP Modulated Subcarriers

• Cyclic Prefix added to the beginning of the OFDM
symbol in order to eliminate IBI
• At the Receiver CP is removed and only the information
bearing part is further processed .

OFDM as a Multiple Access Scheme
(OFDMA)

 OFDMA : In each OFDM symbol interval, Different
subsets of the overall set of available subcarriers are
used for transmission to different terminals.

What is Multiple-Input Multiple-
Output (MIMO) ?

Main Transmission Techniques

Spatial Diversity : Signal copies are transmitted at
multiple antennas or received at more than one antenna

.

 Spatial Multiplexing : Transmit independent and
separately encoded data streams over different antennas

Why MIMO?
 Significant increase in Spectral efficiency and data
rates - Spatial Multiplexing
 High QoS - Spatial diversity
 Wide Coverage - Spatial diversity

Received signal y at the receiver when signal x is transmitted,

What is Single Carrier FDMA
(SC – FDMA)?

SC – FDMA (DFTS-OFDM)
Why not Multi Carrier OFDM in Uplink ?
 One of the main drawbacks in OFDM : Large instantaneous
power variations in the Transmitting signal
 This leads to High Peak-to-Average-Power Ratio (PAPR) in
the Power Amplifier.

Power Amplifier Efficiency

Power Amplifier Cost

Hence Multicarrier OFDM is not a Viable solution for Low
power Mobiles

 In OFDM, each subcarrier carries information relating
to one specific Symbol
 In SC-FDMA, each subcarrier contains information of
All Transmitted symbols.
 Hence no need of transmitting with High Power. Signal
energy is distributed among sub carriers.

User Multiplexing in SC-FDMA
 Localized Transmission :  Distributed Transmission :

User 1 User 2 User 3 User 1 User 2 User 3

LTE Physical Layer

Overall RAN Protocol Architecture

Available DL BW and Physical Resource Blocks
(PRBs)

Bandwidth (MHz) 1.25 2.5 5.0 10.0 15.0 20.0

Subcarrier BW (kHz) 15

PRB BW (kHz) 180

No. of available RBs 6 12 25 50 75 100

Generic Frame Structure
1 Frame (10 ms)

1 Slot (0.5 ms)

0 1 2 n 18 19

1 Sub Frame (1 ms)

0 1 2 3 4 5 6

7 OFDM symbols

Resource Grid 7 OFDM symbols

Time
F
R
E
q
R
E
S
O
U
R
C R
E E
S
B O
L U
O R
C C
K E

G
R
I
D
Resource Element

 Physical Resource Block (PRB) allocation is done by the
scheduling function in eNodeB

 PRB is the smallest element of resource allocation
assigned by the base station scheduler.

LTE Radio Access : An Overview

 Channel dependent Scheduling and Rate adaptation :
 Depending on the channel conditions, time – frequency resources
are allocated to users by the scheduler
 Scheduling decisions taken once every 1ms with frequency
domain granularity of 180 kHz.
 Scheduler allocates resources depending on the Channel State
Information(CSI) provided by the UE

 Inter Cell interference Coordination (ICIC) :
 In LTE, Frequency Reuse Factor equals to one (full spectrum
availability at each Cell)
 This leads to high performance degradation specially the Users in
cell edge.
 ICIC reduce ICI at cell edge applying certain restrictions on
resource assignment.
Adaptive Fractional Frequency Reuse
Coordination:
3

1

2

Inner Region

Outer Region

 Multicast / Broadcast Single frequency Network
(MBSFN)

 As Identical information is transmitted from transmitters (time
aligned), UEs in Cell edge can utilize received power of several
surrounding cells to detect / decode broadcasted data.

Special Features in LTE – A (Rel.10)
Carrier Aggregation :

Relaying:

Extended Multi Antenna Transmission :

 DL Spatial Multiplexing has been expanded to support up to 8
transmission Layers.

Heterogeneous Deployments :
Ex : Pico Cell placed inside a Macro Cell

References :

 . “4G LTE/LTE-Advanced for Mobile Broadband” by Erik
Dhalman, Stefan Parkvall, Johan Skold

 “Overview of the 3GPP Long Term Evolution Physical Layer ”
by Jim Zyren, Dr.Wes McCoy

 “Wireless Communication” by Andrea Goldsmith

THANK YOU!

Nadisanka Rupasinghe
Engineer – Network Optimization

Introduction to LTE

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