SAE architecture Interworking with 2G/3G Networks

MME UPE SAEGW

Operator IP services

(including IMS, PSS, ...)

Evolved Packet Core

S11

S2

S3 S4

S7

S6

SGiS1

Gb

IuRx+

LTE Uu

X2

E-UTRAN

PDNSAE GW

S5eNB

cell

eNB

cell

GERAN

UTRAN

HSS

GPRSCore

PCRF

Um

Uu

LTE Uu

LTE uses OFDM for the Downlink Link.

LTE uses SC-FDMA for the Uplink.

LTE Air Interface

UE

eNB

cell

OFDMA D/L Tx

SC- FDMA U/L Tx

OFDM Basic Concepts

OFDM is a multicarrier system.

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data.

Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no mutual interference.

Ch.1

Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10

Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10

Ch.1

Conventional multicarrier techniques

Orthogonal multicarrier techniques OFDM

50% bandwidth saving

frequency

frequency

A

B

Traditional FDM Signal and OFDM

The OFDMA refers indeed to the Orthogonal FDMA as the parameters for the sub-carrier are chosen to that neighboring sub-carriers have zero value the desired sampling point for any sub-carrier.

Total transmission bandwidth

Maintaining sub-carriers orthogonal

Zero value for other sub-carriers.

Sampling point for a single sub-carrier.

15 kHz

Multi‐Path Propagation and ISI

The cancellation of inter‐symbol interference(ISI) makes hardware design of the receivers more complex.

In WCDMA for instance the RAKE receiver requires a huge  amount of DSP capacity.

One of the goals of future radio systems is to simplify receiver   design.

Multi‐Path Propagation and ISI

Cyclic Extension (CE)

Cyclic extension is the part of the symbol to be transmitted , it is copied from the end and is attached at the beginning of the symbol.

OFDM symbol before CE addition

CE

OFDM symbol duration after CE addition

CE is also known as Cyclic Prefix (CP) or Guard Period (GP)

usable symbol

InterferenceUser SubCarriers

Allocation

SubCarriers

Interference

SubCarriers

NulledSubCarriers

Total Frequency band

Narrowband Interference Rejection

Easy to Avoid/Reject Narrowband Dominant Interference.

Less Interfered Part of the Carrier Can Still Be Used.

Interference Avoidance

FFT-based OFDM System

Serial-to-ParallelConverter

SignalMapper

IFFTParallel-to-SerialConverter

GuardIntervalInsertion

Serial Data Input

x bits0d

1d

1nd

0s

1s

1ns

D/A &Low passFilter

Up-Converter

Down-Converter

A/D GuardIntervalRemoval

Serial-to-ParallelConverter

FFTOne-tapEqualizer

SignalDemapper

Parallel-to-SerialConverter

Serial Data Output

0dx bits

1d

1ˆ

nd

0s1s

1ˆ ns

Channel

)(ts

Time

Frequency

Subchannels

Fast Fourier Transform

Guard Intervals

Symbols

LTE Frame Structure

1 Frame of 10 ms duration , 20 slots

1 Sub frame of 1ms , 2 slots , 1 TTI

1 Slot of 0.5 ms , 7 or 8 OFDMA symbols

What is Resource Element (RE)?

A Resource Element (RE) is one OFDM symbol (in the time

domain) of 1 subcarriers (in the frequency domain). It is the

minimum unit of data allocation and power control.

However resource allocation is always done as a group of

REs called Resource Block.

OFDMA time - frequency grid

a subcarrier

an OFDMA symbol

time

frequency

One RE

What is Resource Block (RB)?

A Resource Block (RB) is defined as set of 7 consecutive

OFDM symbols in the time domain and set of 12 consecutive

subcarriers in the frequency domain.

The resource block in the specifications refers to 0.5 ms slot,

but the resource allocation is done anyway with the 1 ms

resolution in the time domain (per sub-frame or 1 TTI).

It is the task of the scheduler to assign resource blocks to

physical channels belonging to different users .

What is Resource Block (RB)?

Time t

Fre

quen

cy

f

1 symbol

15 KHz

7 symbols

12 carriers180 KHz

OFDMA- time frequency multiplexing

LTE downlink frequency profile

Channel BW 1.4 3 5 10 15 20

Transmission BW 1.08 2.7 4.5 9 13.5 18

No. of RBs 6 15 25 50 75 100

FFT size 128 256 512 1024 1536 2048

Single-carrier FDMA

LTE uses OFDMA for downlink but not for uplink.

Instead of OFDMA ,LTE uses SC-FDMA in uplink.

Why ?

Why SC FDMA in Uplink ?

The OFDM transmission consists of several parallel sub- carriers in frequency domain. In the time domain this corresponds to multiple sinusoidal waves with different frequency components and occupying the system bandwidth with steps of 15KHz.

The combined signal envelope vary strongly.

This causes challenges to the amplifier design.

……

The transmitted OFDM signal should be seen as a sum of sinusoid . This is not suited for a highly linear, power efficient terminal amplifier. The envelope needs to be with as low Peak-to-Average Ratio (PAR) as possible.

Power Amplifier ,problem for you

IFFT

Frequency domain

QAM modulated inputs

Time domain signal

(sum of sinusoids)

FFT

Frequency domain

QAM modulated outputs

Why SC FDMA in Uplink ?

Why SC FDMA in Uplink ?

Time domain signal (sum of sinusoids)

sinusoidal wave

max.

avg.

max.

avg.

Back off

Back off

i/p

i/p

o/p

o/p

For high PA efficiency back off must be minimum.

High back off need high power consumption

Why SC FDMA in Uplink ?

Power consumption in user equipment (UE) terminals is limited by battery.

OFDM requires large dynamic range due to high peak to average power ratio(PAPR).

Liner power amplifiers with wide dynamic range have bad efficiency.

SC-FDMA

DFT spread OFDM (DFTS-OFDM) is a transmission scheme that can combine the desired properties for uplink transmission i.e. :-

Small variation in the instantaneous power of the transmitted signal (single carrier property).

Possibility for low complexity high quality equalization in the frequency domain.

Possibility for FDMA with flexible bandwidth assignment.

SC-FDMA can be referred as DFTS- OFDMA

Basic principle of DFTS-OFDMA

S- to- P converter

Size-M DFT

Size-N IDFT

M

00

0

Serial Bit

stream

Ptos Serial

Bit stream

N > M

Unused inputs of IDFT is set to zero

S C constellation

mappi ng

S-to-P converter

Sub carrier

mapping

N- point IDFT

M- point DFT

P-to-S converter

CP insertion

RF

S C detector and

constellation demappi ng

P-to-S converter

Frequency domain

equalizer

N- point DFT

M- point IDFT

S-to-P converter

CP removal

RF

SC FDMA Functional blocks

SC-FDMA Transmitter

SC-FDMA receiver

Bit stream

Bit stream

M < N

How does a SC FDMA signal look like

SC-OFDMA signal is similar to OFDMA signal, but…

In OFDMA, each sub-carrier only carries information related to one specific symbol.

In SC-OFDMA, each sub-carrier contains information of all transmitted symbols.

OFDMA vs SC FDMA

OFDMAData symbol occupy 15 kHz for one

OFDM symbol period

SC-FDMAData symbol occupy N x15 kHz for 1 / N

SC-FDMA symbol period

frequency

User multiplexing in frequency domain• Smallest uplink bandwidth 180 kHz.• Largest 20 MHz (terminal are required to able to receive & transmit up to

20 MHz, depending on the frequency band though.)

Terminal 1 Transmitter

IFFT

FFT

Terminal 2 Transmitter

FFT

IFFT

frequency BTS Receiver

frequency

Uplink Multiple Access – SC-FDMA