Introduction to LTE

By:-

Nadisanka Rupasinghe,

Engineer – Planning and Optimization,

Etisalat Lanka (Private) Limited

For Downlink : OFDM and MIMO

For Uplink : SC - FDMA

Key technologies….

No more Codes

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

Rel.9 Location Services

MBMS Multi-

Standard BS

Rel.10 LTE - A Carrier

Aggregation Relays

Rel.11 Enhanced

Carrier Aggregation

Intra Band Carrier

Aggregation

LTE/SAE Network

Architecture

X2 X2

X2

S1 S1 S1 S1

eNodeB

eNodeB

eNodeB

MME/S-GW MME/S-GW

E-UTRAN

Internet

P-GW

EPC S5

HSS

S6a

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 UTRAN (E-UTRAN)

Evolved Packet Core (EPC)

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) ?

OFDM Why ?

ISI – Inter Symbol Interference

Data Rate ISI

Time domain :

Time Spreading (Freq. Selective

Fading)

• When an impulse is transmitted , how

does the average power received by Mobile

vary as a function of time delay ζ ?

Power Delay Profile

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).

1 0 1

1

0

1

: Single Carrier

Transmission

: OFDM Transmission

t

Single carrier transmission Vs OFDM

Transmission

Sub carrier Pulse shape and Spectrum

Subcarrier BW < Coherance BW

Why “Orthogonal” ?

Two modulated OFDM subcarriers 𝑥𝑘1 and 𝑥𝑘2 are mutually

orthogonal over the time interval m𝑇𝑢 ≤ t < (m+1)𝑇𝑢

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

• Generated by Multiplexing several overlapping

subcarriers and a Cyclic Prefix (CP).

• Cyclic Prefix added to the beginning of the OFDM

symbol in order to eliminate ISI

• At the Receiver CP is removed and only the information

bearing part is further processed .

OFDM Symbol

CP Modulated Subcarriers

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) ?

𝑇1

𝑇2

𝑇𝑛

𝑅1

𝑅2

𝑅𝑛

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

𝑇1 𝑅1

𝑅2 𝑇2

Why MIMO?

Significant increase in Spectral efficiency and data

rates - Spatial Multiplexing

High QoS - Spatial diversity

Wide Coverage - Spatial diversity

SISO Channel Capacity :

𝐶 = 𝐵 log2(1 + 𝑆𝐼𝑁𝑅)

MIMO Channel Capacity (MIMO system with M×N antenna

configuration) :

𝑪 = 𝑩 𝒍𝒐𝒈𝟐(𝟏 + 𝑴𝑵 × 𝑺𝑰𝑵𝑹)

B : Channel Bandwidth

SINR : Signal to Interference plus Noise ratio

𝑇1 𝑅1

𝑅2 𝑇2

𝑦1

𝑦2 =

ℎ11 ℎ12

ℎ21 ℎ22

𝑥1

𝑥2

Channel impulse responses (ℎ𝑖𝑗) are determined by transmitting

reference signals from each transmitting antenna.

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

What is Single Carrier FDMA

(SC – FDMA)?

SC – FDMA (DFTS-OFDM)

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

Why not Multi Carrier OFDM in Uplink ?

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

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

Available DL BW and Physical Resource Blocks (PRBs)

1 Frame (10 ms)

1 Slot (0.5 ms)

1 Sub Frame (1

ms)

7 OFDM symbols

Generic Frame Structure

7 OFDM symbols Resource Grid

𝑁𝑆𝐶𝑅𝐵

𝑁𝑅𝐵𝐷𝐿

R

E

S

O

U

R

C

E

B

L

O

C

K

R

E

S

O

U

R

C

E

G

R

I

D

Resource Element

Time F

R

E

q

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) :

1

3

2

Inner Region

Outer Region

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:

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 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!