1

Multi-User Diversity in Single-Radio OFDMA Ad Hoc Networks Based on

Gibbs SamplingMarzieh Veyseh

J.J. Garcia-Luna-AcevesHamid R. Sadjadpour

MotivationConcurrency

• Frequency

• Code

• Space

2

fc1

Channel 1Channel 1

fc2 fc3 fc4 fc5

Channel 1Channel 1

Channel 3Channel 3

fcfc

Channel 1Channel 1 Code 1Code 1

Code 3Code 3Code 1Code 1Code 2Code 2

Code 3Code 3

fcfc

fcfc

fcfc

MotivationAdaptivity• Time

• Frequency, through different modulations…

Diversity

To use fading in our advantage and to improve rate…• MIMO• OFDAM-Multiuser diversity

3

t1 t2 t3 t4 t5 t3

t1

• Proposed for infrastructure-based OFDMA networks• We think that utilizing OFDMA in ad hoc networks gives us the ability to achieve our goals:

1. Concurrency: multiple nodes use different portions of BW2. Diversity: multi-user diversity3. Adaptivity: subchannels can have different sizes

OFDMA

4

fcfc

Subchannel iSubchannel i

Subchannel 1Subchannel 1

Subchannel 2Subchannel 2

Subchannel 3Subchannel 3

Subchannel 4Subchannel 4

OFDMA Synchronization

5

C-Tx

C-Tx

• In an ad hoc multi-transmitter scenario to avoid loss of orthogonality at a common neighbor to multiple transmitters, a quasi-synchronous network is required.

• Quasi-synchronous : transmitter start sending data at the same time• Multi-transmission synchronization achieved via control message

exchanges

Previous work

• Not much work on protocols for OFDMA for ad hoc networks

• CTRMA: In our previous work, we assigned non-overlapping channels unique within the two hop neighborhood based on some priority order. [OFDMA Based Multiparty Medium Access Control in Wireless Ad Hoc Networks, ICC, 09])

• CBD: We added diversity when distributing the assigned subchannels among neighbors[Cross-Layer Channel Allocation Protocol for OFDMA Ad Hoc Networks, Globecom 10]

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Gibbs SamplingGoal: • Maximize concurrency

– Design a MAC that assigns subchannels to each directional link on-demand and based on the present interference, and node’s needs

• Maximize diversity– Subchannel selection should be done based on minimizing fading– Gibbs sampling previously used to distribute channels among multiple 802.11 Aps : min direct interference

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GSA

C-Tx_1C-Tx_1

C-Tx_2C-Tx_2

C-RxC-Rx

C-RxC-Rx

aa

bb

Gibbs Sampling

f (X)e E(X )

e E(Y )

Y SV

8

Sy

yE

xE

v v

v

e

exf )(

)(

)(

• Graph , and state X…want to find),( EVG ))(min(: XEX

vVv xPxE

,

)()(

GSAGibbs Subchannel Assignment (GSA)

• We propose to use Gibbsian method by defining a new energy model and a MAC protocol that works to select subchannels (Schannel) for each communicating link.

9

C-Tx (a)C-Tx (a)C-Tx (c)C-Tx (c)

C-RxC-Rx

bbvv

uu

)()(

1)( kPi

kCSIkE v

vv

Subchannel kSubchannel k

Kk

kE

kE

vv

v

e

ekf

'

' )(

)(

)(

))((arg kfMaxulinkchoose uUu

MDMA

Freq

Control

FreeTx Schannel_1

FreeTx Schannel_2

C-Tx

a

bd

RTM-TRTM-T

time

c

Freq

Control

FreeTx Schannel_1

FreeTx Schannel_2

C-Tx

a

bd

RTM-TRTM-T

Pilot

timePilot

cMDMAMDMA

Freq

Control

FreeTx Schannel_1

FreeTx Schannel_2

C-Tx

a

bd

RTM-TRTM-T

a b c d

CTR

Pilot

timePilot

c

vSINR(k)>Thr

e

MDMA

Freq

Control

FreeTx Schannel_1

FreeTx Schannel_2

C-Tx

a

bd

RTM-TRTM-T

a b c d

CTR

Data: C-Tx->aPilot

time

Broadcast

Broadcast Data: C-Tx->bPilot

cMDMA

C-Rx

a

b

c

dRTM-RRTM-R

Freq

time

Control

FreeRx Schannel_1

FreeRx Schannel_2

MDMA

C-Rx

a

b

c

dRTM-RRTM-R

Freq

time

a b c d

CTT

a b c d

CTT

Control

FreeRx Schannel_1

FreeRx Schannel_2

MDMA

C-Rx

a

b

c

dRTM-RRTM-R

Freq

time

STTSTT

a b c d

CTT

a b c d

CTT

Control

FreeRx Schannel_1

FreeRx Schannel_2

MDMA

C-Rx

a

b

c

dRTM-RRTM-R

Freq

time

STTSTT

a b c d

CTT Data: a->C-Rx

a b c d

CTT Data: b->C-Rx

Control

FreeRx Schannel_1

FreeRx Schannel_2

MDMA

Range =

18

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aaaa vvvv

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3

1 '

'''

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)0Pr()()1log(

)Pr()()1log(

kX

X

a

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PX

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P

C

Analysis

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1

"''

'

)Pr(k

dba CkkCkCC

62)(2

)(4

er

r

Average =4

19

Analysis

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uuuu

aaaa vvvv

da CkCC "

1.3

Analysis Result

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Analytically GSA is outperforming ideal scheduling

2.6 times

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Simulations

C-Tx

Assumptions

Subcarriers 2048

Subchannel 32 (wimax)

Randomly selected neighbors(K)

2 to 5

Channel, Rayleigh, Portable

20-taps, defined in

EN-300-744

SNR 10

SEEDS 10 random

b

a

c u

Matlab Simulations

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Simulations

C

Assumptions

Subcarriers 2048

Subchannel 32 (wimax)

Randomly selected neighbors(K)

2 to 5

Channel, Rayleigh, Portable

20-taps, defined in

EN-300-744

SNR 10

SEEDS 10 random

A

A

A B

B

B

Matlab Simulations

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MDMA

Comparison

McH Orthogonal channel assignment, adaptive rate selection, CH=3

CBD Orthogonal channel assignment, adaptive subchannel assignment,

GSA Adaptive subchannel assignment

GSA increases concurrency comparing to CBD

63%

2.3 times

Matlab Simulations

Model Developed in QUALNET

Number of nodes

50 stationary

SNR 10

Packet length

512 B

topology 10 random

Average number of

node’s neighbors

10

Bit rate 11Mbit/sec

Percentage of the

communicating neighbors 24

MDMA

NK

1.6 times

Qualnet Simulations

• Previous cross-layer MAC fail to effectively utilize OFDMA• We improved our previous works by

• Utilizing Gibbs method • Increasing two-hop neighbor concurrency • Attaining diversity

• Our analysis and simulations proved that MDMA’s performance with the same BW is • 1.6 times better than CBD• 2.2 times better than CTRMA• 2.4 times better than traditional multi-channel networks

• Future work– Mobility– 802.11n OFDMA

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ConclusionConclusion

Questions