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DEEKSHA TECH GROUP

Module 5

OFDMA and SC FDMA

Agenda Understand LTE Duplexing

Single Transmitter FDMA Principle Multi carrier principle

OFDMA and SC FDMA Principle Multipath Propagation Cyclic Prefix OFDMA and SC FDMA

Transmitter Receiver

OFDM and SC FDMA Key ParametersResource Block

Duplexing and Multiple Access

Legacy- Single Transmitter

FDMA Principle

LTE: Multi-Carrier Principle

The Rectangular Pulse

Advantages: Simple to implement: there is no

complex filter system required to detect such pulses and to generate them.

The pulse has a clearly defined duration. This is a major advantage in case of multi-path propagation environments as it simplifies handling of inter-symbol interference.

Disadvantage: It allocates a quite huge spectrum However the spectral power density has

null points exactly at multiples of the frequency fs = 1/Ts.

This will be important in OFDM.

time

ampl

itude

Ts

fs 1

Ts

Time Domain

frequency f/fs

spec

tral

pow

er d

ensi

ty

Frequency Domain

fs

FourierTransform

Inverse FourierTransform

OFDMA Principle

Transmits hundreds or even thousands of separately modulated radio signals using orthogonal subcarriers spread across a wideband channel

Orthogonality:

The peak (centre frequency) of one subcarrier …

…intercepts the ‘nulls’ of the neighbouring subcarriers

15 kHz in LTE: fixed

Total transmission bandwidth

OFDM Basics

Data is sent in parallel across the set of subcarriers, each subcarrier only transports a part of the whole transmission

The throughput is the sum of the data rates of each individual (or used) subcarriers while the power is distributed to all used subcarriers

FFT ( Fast Fourier Transform) is used to create the orthogonal subcarriers. The number of subcarriers is determined by the FFT size ( by the bandwidth)

Power

Frequency

Bandwidth

OFDM Signal

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OFDM: Nutshell

Frequency-Time Representation

FFT/IFFT

It can be shown that the OFDM signal may be obtained by transforming L data symbols by the IFFT, where L is the number of subcarriers.

Therefore, OFDM transmitter and receiver are implemented using IFFT and FFT respectively.

The size of the FFT should be chosen carefully as a balance between protection against multipath (i.e. ISI), temporal variations (i.e. ICI), and design cost/complexity.

LTE FFT period is 66.67 usec, corresponding to the 15 KHz subcarrier separation.

IFFT FFT

d1d2

dL

d1d2

dL

Time-domain(to be transmitted)

Motivation for OFDMA

Good performance in frequency selective fading channels

Low complexity of base-band receiver Good spectral properties and handling of multiple

bandwidths Link adaptation Frequency domain scheduling Compatibility with advanced receiver and antenna

technologies.

Challenges

1) ISI

Solution: CP

2) Multi-Carrier Modulation

The center frequencies must be spaced so that interference between different carriers, known as Adjacent Carrier Interference ACI, is minimized; but not too much spaced as the total bandwidth will be wasted.

Each carrier uses an upper and lower guard band to protect itself from its adjacent carriers. Nevertheless, there will always be some interference between the adjacent carriers.

frequency

∆fsubcarrier

f0 f1 f2 fN-1fN-2

∆fsub-used

ACI = Adjacent Carrier Interference

Solution: OFDM Multi-Carrier

OFDM allows a tight packing of small carrier – called the subcarriers - into a given frequency band.

No ACI (Adjacent Carrier Interference) in OFDM due to the orthogonal subcarriers !

Pow

er D

ensi

ty

Pow

er D

ensi

ty

Frequency (f/fs) Frequency (f/fs)

Saved Bandwidth

3)Inter-Carrier Interference (ICI)

The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.

If the receiver’s frequency slips some fractions from the subcarriers center frequencies, then we encounter not only interference between adjacent carriers, but in principle between all carriers.

This is known as Inter-Carrier Interference (ICI) and sometimes also referred to as Leakage Effect in the theory of discrete Fourier transform.

One possible cause that introduces frequency errors is a fast moving Transmitter or Receiver (Doppler effect).

f0 f1 f2 f3 f4

∆P

I3

I1I4I0

ICI =

Inte

r-Ca

rrie

r Int

erfe

renc

e

Frequency Drift

Two effects begin to work: Subcarrier has no longer its

power density maximum- so loose of signal energy.

The rest of subcarriers have no longer a null point here. So we get some noise from the other subcarrier.

LowPassLowPass

cos(2πfct)

-sin(2πfct)

I

Q

ModulationMapper

ModulationMapper

IFFTIFFT

s0

ModulationMapper

ModulationMapper

s1

ModulationMapper

ModulationMapper

sN-1

b10 ,b11,…

Serial toParallel

Converter(Bit

Distrib.)

Serial toParallel

Converter(Bit

Distrib.)

b20 ,b21,…

bN-1 0 …

BinaryCodedData

.

.

.

D

A

D

Ax0, x1, …, xN-1 IQSplitIQ

Split

LowPassLowPass

D

A

D

A

RF

freq.f1 f2f0 fN-1

…

s0

s1 sN-1

s2

Freq

uenc

y D

omai

n

timet1 t2t0 tN-1 …x0 x1

xN-1

x2

TimeDomain

CP/G

uard

Gen

erati

onCP

/Gua

rdG

ener

ation

I

Q

Time Domain Signal

Frequency Domain Signal:(Collection of Sinusoids)

Each entry to the IFFT module corresponds to a different sub-carrier

Each sub-carrier is modulated independently by Modulation Schemes:

BPSK,QPSK, 16QAM, 64QAM

OFDM Transmitter

reference(pilot)

Chan

nel C

orre

ction

Chan

nel C

orre

ction

Dem

odul

ator

Dem

odul

ator

Bit MappingBit Mapping

j

I

Q

A

D

A

D

ChannelEstimationChannel

Estimation

RF

Low

Noi

se A

mp.

+ Ba

ndpa

ssLo

w N

oise

Am

p.+

Band

pass

A

D

A

D

AGCAutomatic

Gain Control

AGCAutomatic

Gain Control

De-rotator

sign

al st

reng

th

LNA gain

Frequency And Timing SyncFrequency And Timing Sync

sign

al a

utoc

orre

ation

phas

e co

rrec

tion

timee

adju

st

.

.

.

s’0

s’1

s’N-1

chan

nel

resp

onse

s0

Bit MappingBit Mappings1

Bit MappingBit MappingsN-1

.

.

.

.

.

.

.

.

.

B10 ,B11,…

B20 ,B21,…

BN-1 0 …

Bit D

istrib

ution

Bit D

istrib

ution

Soft BitCodedData

freq.f1 f2f0 fN-1

…

s0s1 sN-1

s2

Frequency Domain

Time Domain

timet1 t2t0 tN-1

…y0 y1

yN-1

x2

QPSK

Im

Re

10

11

00

01

sk

d11

d10

OFDM Receiver

Win

dow

ing

+FF

T

Freq

uenc

y D

omai

n

OFDM Key Parameters

2) Subcarrier Spacing (Δf = 15 KHz) → The Symbol time isTsymbol = 1/ Δf = 66,7μs

Δf

TSYMBOL

TCP SYMBOL

TCP

TS

Frequency

Time

Powerdensity

Amplitude

1) Variable Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz1) Variable Bandwidth options: 1.4, 3, 5, 10, 15 and 20 MHz

Frequency

3) The number of Subcarriers Nc

If BW = 20MHz → Transmission BW = 20MHz – 2MHz = 18 MHz→ the number of subcarriers Nc = 18MHz/15KHz = 1200 subcarriers

TransmissionBandwidth [RB]

Transmission Bandwidth Configuration [RB]

Channel Bandwidth [MHz]

Resource block

Channel edge

Channel edge

DC carrier (downlink only)Active Resource Blocks

OFDM Key Parameters

4) IFFT size Nifft

For a bandwidth BW = 20 MHz Nc = 1200 subcarriers not a power of 2

→ The next power of 2 is 2048 → the rest 2048 -1200 848 padded with zeros

5. Sampling rate fs

This parameter indicates what is the sampling frequency:→ fs = Nfft x ΔfExample: for a bandwidth BW = 5 MHz (with 10% guard band)The number of subcarriers Nc = 4.5 MHz/ 15 KHz = 300 300 is not a power of 2 → next power of 2 is 512 → Nfft = 512Fs = 512 x 15 KHz = 7,68 MHz → fs = 2 x 3,84 MHz which is the chip rate in UMTS!!

The sampling rate is a multiple of the chip rate from UMTS/ HSPA. This was acomplished because the subcarriers spacing is 15 KHz. This means UMTS and LTE have the same clock timing!

OFDM Key Parameters

Bandwidth(NC×Δf)

1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Subcarrier Fixed to 15 kHz Spacing (Δf)

Symbol Tsymbol = 1/Δf = 1/15kHz = 66.67μsduration

Sampling rate, fS (MHz)

1.92 3.84 7.68 15.36 23.04 30.72

DataSubcarriers (NC)

72 180 300 600 900 1200

NIFFT (IFFT Length)

128 320 512 1024 1536 2048

Number of Resource Blocks

6 15 25 50 75 100

Symbols/slot Normal CP=7; extended CP=6

CP length Normal CP=4.69/5.12μsec., Extended CP= 16.67μsec

OFDM Recap

OFDMA Challenges

1) Tolerance to frequency offset

(Inter carrier Interference-ICI)

2) High Peak-to-Average Power Ratio (PAPR)

Frequency

ICI

SC FDMA

SC-FDMA Single Carrier Frequency Division Multiple

Access is another variant of OFDMA used to reduce the PAPR for lower RF hardware requirements.

SC-FDMA is a new hybrid modulation scheme that cleverly combines the low PAR of single-carrier systems with the multipath resistance and flexible subcarrier frequency allocation offered by OFDM.

This mechanism can reduce the PAPR of 6..9 dB compared to normal OFDMA.

SC-FDMA is one option in WiMAX (802.16d) and it is the method selected for EUTRAN in the uplink direction.

SC-FDM

A

OFD

MA

SC-FDMA and OFDMA OFDMA transmits data in parallel across multiple subcarriers SC-FDMA transmits data in series employing multiple subcarriers In the example: OFDMA: 6 modulation symbols ( 01,10,11,01,10 and 10) are

transmitted per OFDMA symbol, one on each subcarrier SC-FDMA: 6 modulation symbols are transmitted per SC-FDMA

symbol using all subcarriers. The duration of each modulation symbol is 1/6th of the modulation symbol in OFDMA

OFDMA SC-FDMA

OFDMA vs SC-FDMA: QPSK

From: TS 36.211.

OFDM SC-FDMA

Difference in transmission: for SC-FDMA there is an extra block on the transmission chain: the FFT block

which should “spread” the input modulation symbols over all the allocated subcarriers

SC-FDMA and OFDMA

SC-FDMA Principles PAPR is the same as that used for the input modulation symbols

This could be achieved by transmitting N modulation symbols in series at N times the rate.

One can see that the SC-FDMA symbol which is having 66.66µs is containing N “sub-symbols”

N = 6 in the example shown In Time domain only one modulation symbol

is transmitted at a time.

The number of subcarriers which could be allocated for transmission should be multiple of 2,3 and/or 5

This limitation is imposed by the input of the FFT block which is before the IFFT. This enables efficient implementation of the FFT.

Note that also the number of Resource Blocks should be multiple of 2,3 or/and 5

The FFT output size is always smaller than the IFFT input size

FFT

IFFT

…

.

.

.

Subcarriers allocated for one

UE

Subcarriers allocated to

other users or set to zero

This is because total cell’s uplink capacity will always be greater than bandwidth allocated to any one UE

Other UEs will be assigned other groups of subcarriers to use across the uplink channel bandwidth.

No two UEs will be assigned the same 180KHz block to use simultaneously.

As not all sub-carriers are used by the mobile station, many of them are set to zero in the diagram

Note that if the output size of the FFT is equal to the size of the IFFT input then the overall effect is null since the two operations (FFT and IFFT are complementary)

SC-FDMA Principles

Adjusting the data rate in SC-FDMA

Halved SC-FDMA “sub-symbol”

duration

Initial bandwidth

SC-FDMA “sub-

symbol” duration

Doubled bandwidth

If the data rate increases more bandwidth is needed to transmit more modulation symbols (when data rate is doubled the resource allocation in the frequency domain is also doubled). The individual transmission is now shorter in time but wider in the frequency domain.

For double data rate the amount of inputs in transmitter doubles and the “sub-symbol” duration (Time) is halved. Note that the SC-FDMA is still 67 µs

Double the data rate

SC-FDMA symbol 67µs

SC-FDMA symbol 67µs

SC-FDMA Principles

In the example 6 modulation symbols are sent initially and 12 modulations for double data rate

SC-FDMA: Multiplexing

One user always continuous in frequencySmallest uplink bandwidth, 12 subcarriers: 180 kHz (same for OFDMA in downlink)

Largest uplink bandwidth: 20 MHz (same for OFDMA in downlink)

In time domain the granularity for resource allocation is 1 ms for one user (same for OFDMA in downlink)

User 2 f

User 1 f

f

Receiver

User 1 User 2

Bandwidth Distribution

Carrier Bandwidth

(MHz)

Number of Sub-

Carriers

1.4 72

3 198

5 330

10 660

15 990

20 1320

Resource: Element, Block, Grid

[source: 3GPP TR 25.814]

LTE Reference Signals (R)are Interspersed Among Resource Elements

The Usage of RE

Resource elements

reserved for

reference symbols

Control Channel

Region (1-3 OFDM symbols)

One subframe (1ms)

12 s

ub

carr

iers

Fre

qu

ency

Time Data Region

Duplexing – FDD/TDD

FDD

..

..

..

..

Downlink Uplink

Frequency band 1

Frequency band 2

.. ..Single frequency band

TDD

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Plain OFDM

time

subc

arrie

r

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Time Division Multiple Accesson OFDM

time

subc

arrie

r...

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1

1

1

1

1

1

2

2

2

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2

OFDMA® is registered trademark of Runcom Technologies Ltd.

1 1 1

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2 2

2...

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Plain Orthogonal FrequencyMultiple Access

OFDMA®

time

...

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1 1 1 1

2 22

2 2 2

1

3 33 3 3

1 1 1 1su

bcar

rier

1

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Orthogonal FrequencyMultiple Access

OFDMA®

time

...

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1

1

1 1

2

22

2 2

3 33 3 3

1

subc

arrie

r

1

1 1 1

111

3 3 3

33 3 3 3

3

Resource Block (RB)1 2 3 common info(may be addressed via HL)

UE 1 UE 2 UE 3

Different Methods for OFDMA

“Thank you”