Introduction to LTE

Introduction to

For Downlink : OFDM and MIMO

For Uplink : SC - FDMA

Key technologies….

No more Codes

Topics to discuss… System Architecture Evolution (SAE)

What is OFDM ?

What is MIMO ?

What is SC-FDMA ?

LTE Physical Layer

LTE Radio Access – An Overview

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

MBMSMulti-

Standard BS

Rel.10 LTE - A Carrier Aggregation Relays

Rel.11Enhanced

Carrier Aggregation

Intra Band 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 Architecture

X2 X2

X2

S1S1 S1 S1

eNodeB

eNodeB

eNodeB

MME/S-GW MME/S-GW

E-UTRAN

Internet

P-GW

EPCS5

HSS

S6a

MME S-GW

S11

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)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) ?

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).

OFDM Modulation OFDM demodulation

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 and are mutually orthogonal over the time interval m ≤ t < (m+1)

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 IBI

• 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) ?

𝑇 1

𝑇 2

𝑇 𝑛

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

𝑇 1𝑅1

𝑅2𝑇 2

Why MIMO?

Significant increase in Spectral efficiency and data rates - Spatial Multiplexing

High QoS - Spatial diversityWide Coverage - Spatial diversity

SISO Channel Capacity :

MIMO Channel Capacity (MIMO system with M×N antenna configuration) :

B : Channel BandwidthSINR : Signal to Interference plus Noise ratio

𝑇 1𝑅1

𝑅2𝑇 2

=

Channel impulse responses (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

Overall RAN Protocol Architecture

LTE Physical Layer Processing

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)

0 1 2 n 18 19

1 Slot (0.5 ms)

1 Sub Frame (1 ms)

0 1 3 6542

7 OFDM symbols

Generic Frame Structure

7 OFDM symbolsResource Grid

𝑁 𝑆𝐶𝑅𝐵

𝑁𝑅𝐵𝐷𝐿

RESOURCE

BLOCK

RESOURCE

GRID

Resource Element

TimeFREq

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 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 RupasingheEngineer – Network Optimization