Ofdm vs Ofdma

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Understanding WiMAX PHY

BP Tiwari

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Agenda

OFDMA

WiMAX Frame

Sub channels and Permutations

Smart Antenna Systems

Summary of WiMAX PHY features

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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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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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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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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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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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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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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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Localized vs Distributed Subchannels

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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

 – 1 slot = 54 subcarriers

= 48 data subcarriers + 6 pilots

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

 – 1 slot = 56 subcarriers

= 48 data subcarriers + 8 pilots

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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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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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PHY Data Rates

Without MIMO and overheads

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Advanced Antenna Systems : STBC

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

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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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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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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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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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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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Thanks