Ofdm vs Ofdma

7/25/2019 Ofdm vs Ofdma

http://slidepdf.com/reader/full/ofdm-vs-ofdma 1/21

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beyond4g.orgbeyond4g.org

Understanding WiMAX PHY

BP Tiwari



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Beyond 4GBeyond 4G

WIMAX SeriesWIMAX Series –– Understanding PHYUnderstanding PHY

Agenda

OFDMA

WiMAX Frame

Sub channels and Permutations

Smart Antenna Systems

Summary of WiMAX PHY features



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Beyond 4GBeyond 4G


Lets understand fading in wirelesscommunications

Delay Spread)

Frequency selective fading (

Delay Spread)

Multiple copies of signal arrive at

the receiver, offset in timeTime Selective Fading ( Doppler )

M UL T I P AT H

F ADI N G C HANN

E L S

Fading (Delay Spread)



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Beyond 4GBeyond 4G


WiMAX Addresses Fading

Multiple copies of signal arrive at

the receiver, offset in time

Fading (Delay Spread) Frequency selective fading (Delay Spread)

OFDMA

Diversity Schemes

Multiple Antenna Systems

Cyclic Prefix ( ISI )

Forward Error Correction



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Beyond 4GBeyond 4G


Concept of OFDMSubcarrier

OFDM is robust to fading

High spectral efficiency

Eliminate Inter-symbolInterference (ISI) by theintroduction of the cyclicprefix (CP)

Sensitivity to frequencyoffset

Higher Peak to averagepower ratio(PAPR)

In OFDM availablebandwidth is dividedinto N (large numberof) orthogonal sub-carriers

Each sub-carrier isindividually modulated

(QAM) at 1/N totaldata rate

FFT implementation(fast and efficient)



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Beyond 4GBeyond 4G


OFDM vs. OFDMA Subcarrier

– Type: Data, Pilot, null(guard and DC)•

– Number: 128, 512, 1024,2048 (at least one)

Subchannel

– A set of sub carriers forms asubchannel

– The sub carriers may andmay not be adjacent

– Distributed allocation(diversity)

– Contiguous allocation(selectivity)

OFDM

OFDMA

SC

Subcarrier

Subchannels



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Beyond 4GBeyond 4G


OFDMA Advantages

• Users/bursts are scheduledacross both frequency(subchannels)and time (symbols)

• Per burst modulation, codingand TX power

• Higher granularity in resourceallocation

• More degrees of freedom inscheduling

• Improved fairness and QoS

D D

C C

D D

C C

B B

A A

B B

E E

TIME

S U B C H A N N E L

•Orthogonality of users are maintainedusing large symbol duration and cyclicprefix

•Users concentrate transmit power overpartial bandwidth

•High granularity in resource allocation

& power control

•Inherent resistance to multi-path(OFDM)

•Flexible Sub-channelization

•Distributed (pseudo-random)permutations –frequency diversity

•Contiguous (clustered)permutations -> selectivity

•Scalability of Channel Sizes



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Beyond 4GBeyond 4G


OFDM Symbols and Cyclic Prefix

Use Cycle Prefix (CP) as

guard interval Prepend segment of

OFDM symbol –>overhead

CP duration > max delay

spread Eliminates ISI

Retains orthogonallybetween sub-carriers (ICI)

Pilots used for channel

estimation and tracking Interleaving (bits

distributed across tones)and error correctingfacilitates recovery underfrequency selective fading

or narrow-bandinterference



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Beyond 4GBeyond 4G


OFDMA Frame Structure

P

R

E

A

M

BL

E

DL-

MAP

UL-

MAP

DL BURST 2

DL BURST 1

DL BURST 3

DL BURST 4

S

U

B

C

H

A

N

N

E

L

S

S1 S2 SnDOWNLINK

CD

M

A

R

A

N

G

I

N

G

P

e

ri

o

d

i

C

R

a

n

g

.

ACKCH

’s

CQICH

s

UL BURST 1

UL BURST 2

UL BURST 3

UPLINKTTG RTG

Uplink and Downlink isdivided into time zones

Different Permutationfor downlink and uplink

– DL PUSC,AMC etc

– UL PUSC,AMC– Activation STC/MIMO

CDMA Codes

– Special allocation inuplink for ranging andBW Request

Fast Feedback/CQICHs

– SS measured CINR



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Beyond 4GBeyond 4G


Localized vs Distributed Subchannels



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Beyond 4GBeyond 4G


Subchannels in 16e

UL PUSC

– 1 tile = 4 subcarriers x 3 OFDM symbols

– 1 subchannel = 6 tiles (frequency domain)

– 1 slot =1 subchannel x 3 OFDM symbols

– 1 slot = 72 subcarriers

= 48 data subcarriers + 24 pilots

DL/UL AMC2X3

– 1 bin = 9 subcarriers x 1 OFDM symbols

– 1 subchannel = 2 bins (frequency domain)

– 1 slot = 1 subchannel x 3 OFDM symbols



DL PUSC – 1 cluster = 14 adjacent subcarriers

x 2 OFDM symbolsx 2 OFDM symbols

– 1 subchannel = 28 subcarriers

– 1 slot = 1 subchannel x 2 OFDM symbols





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Beyond 4GBeyond 4G


Permutation schemes in DL : PUSC

2 S y m

2 S y m

2 S y

m

2 S y m

2 S y m

Cluster



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Beyond 4GBeyond 4G


Permutation schemes in UL : PUSC

3 S Y MB OL S

3 S y m

3 S y m

3 S y m

3 S y m

Pilot

Tile Structure

• 4 Sub carrier X 3 Symbol

• 4 Pilot + 8 Data



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Beyond 4GBeyond 4G


PHY Data Rates

Without MIMO and overheads



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Beyond 4GBeyond 4G


Advanced Antenna Systems : STBC

STC gains vary from 5-10 db depending on channels condition



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Beyond 4GBeyond 4G


MIMO B : Spatial Multiplexing

SM can double throughput in theory by require very highCINR conditions of uncorrelated signal to achieve fulladvantages. 30% sector improvement in loaded conditions.



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Beyond 4GBeyond 4G


Beam forming

Beamforming can provide additional 5-6 db gains in downlink

improving overall network quality. Improves sector capacity by 20 to 25%

4Rx in uplink provides additional 3 db gain



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Beyond 4GBeyond 4G


UL Collaborative MIMO

Collaborative Spatial Multiplexing fortwo MSs with single transmitantenna

Two MS transmit on the sameresource (sub channels)

For orthogonal zing

– Orthogonal pilot patterns (patternA, pattern B)

Multiple data detection

– Using multiple receive antennas

– Interference suppression: MMSE,

ZF– Nonlinear interference

cancellation: SIC, PIC



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Beyond 4GBeyond 4G


MIMO Summary

BF helps in improving capacityand improve network quality.

20 to 25%over MIMO

5-6 db inDownlink

DL-BeamForming

MIMO A in low SINR conditionimproves channel fading.Switching between MIMO A and Bis automatic and controlled byBTS.

5-10*MIMO A

Improves cell radius by 3db in

uplink.

Nil3 db over 2nd

orderdiversity

4th order

Diversity

Average improvement of 30% in

full load , customers higher CINRor uncelebrated signals will takeadvantage of MIMO B

30% in

average

NilMIMO B

CommentsGain in

Capacity

Gain in Link

Budget

Parameters

*STC + 2 Rx in receiver with possibility MRC combining



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Beyond 4GBeyond 4G


Mobile WiMAX PHY Summary

Downlink : 2x2 , STC , SMUplink : 1X2 , Virtual SM

MIMO

AMC or PUSC

UL channel sounding

BeamForming

Downlink :QPSK,16QAM,64QAM

Uplink : QPSK, 16QAM

Modulation

Ranging , CQICH’s , ACKCHUplink Control channels

PUSC,AMC,FUSCSub-carrier allocations

CC, CTC with HARQError correction andHARQ

5 ms , 1/8Frame size and cyclicprefix

TDD , 5 and 10 MHz , 20 (optional)Scalable ChannelBandwidth

SupportFeature



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Beyond 4GBeyond 4G


Thanks

Documents

Ofdm vs Ofdma