LTE – Long Term Evolution

Why LTE ?

Network Topology

Overall RAN Architecture

IP Packet PDCP RLC MAC

PHY

Logical

Tra

nsp

ort

PDCP : Packet Data Convergence Protocol RLC : Radio Link Control MAC : Medium Access Control PHY : Physical Layer

MAC Layer

Physical LayerTra

nsp

ort

ch

an

nels

Log

ical

ch

an

nels

Ph

ysical

ch

an

nels

Channel Mapping

Physical Layer

DL UL

PDSCH PUSCH

PBCH PUCCH

PMCH PRACH

PDCCH

PHICH

PCFICH

Physical Channels :

1 Frame (10 ms)

1 Slot (0.5 ms)

1 Sub Frame (1 ms)

7 OFDM symbols

Generic Frame Structure

7 OFDM symbolsResource Grid

𝑁 𝑆𝐶𝑅𝐵

𝑁𝑅𝐵𝐷𝐿

RESOURCE

BLOCK

RESOURCE

GRID

Resource Element

TimeFREq

OFDM

Why ?

ISI – Inter Symbol Interference

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

Cyclic Prefix added to the beginning of the OFDM symbol in order to eliminate ISI

ICI – Inter Carrier Interference has already been removed by selecting subcarriers with Ts > ζ0