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Chorus: Collision Resolution for

Efficient Wireless Broadcast

Xinyu Zhang, Kang G. Shin

University of Michigan

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Outline

Introduction Design Summary

Chorus(broadcast)

PHY layer

MAC layer

Analysis & evaluation

PHY PER

network

simulationmotivation principles

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Motivation: CSMA/CA limitation

Traditional CSMA/CA (Collision Avoidance):

Principle: listen before talking --- akin to human world

Collision: packets overlap at receiver

Limitation:

Listen without interpretation

Collision avoidance in all cases --- too conservative

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Rationale(1/3): CSMA/CR principle

CSMA/CR (CSMA with collision resolution):

CSMA/CR Principle:

Collision caused by packets carrying the same data can be resolved!

A new MAC/PHY paradigm

Overcome the limitation of CSMA/CA

A

B

D

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Rationale(2/3): CSMA/CR advantage

Improving broadcast efficiency

(b) Chorus, a CSMA/CR based broadcast protocol

Taking advantage of spatial reuse and transmit diversity

A

B

C

D

E

S

(a) Traditional CSMA/CA based broadcast

A

B

C

D

E

S

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Chorus: collision resolution based broadcast

PHY

MAC

Resolve collisions via signal processing

Encourage resolvable collisions via intelligent sensing and scheduling

CSMA/CR

BroadcastA broadcast protocol with

asymptotic latency)(r

Chorus

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Chorus: PHY layer

Resolve the collided packet by iterative decoding

S --- the received symbol.A’ --- estimated based on A.C = S – A’

P1

P1

A

A'

B

B'

C

C'

S=A' + C

D E

D' E' Y' Z'

Y ZA

B

D

Decode two versions of the packet: from preamble and postamble, respectively

Multipacket collision resolution:

A

A'

B CHead packet P1

Tail packet P2

D E

A'' B'' C''

A''' B'''

packet P3

packet P4

D''

Head and tail packet: iterative collision resolution

Other packets: hard decoding

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CSMA/CR: MAC layer

Cognitive sensing and scheduling

Basic rules in SEND:

If the channel is busy, and the packet in the air is exactly one of the packets in the transmit queue, then start transmitting the pending packet.

Otherwise, degenerate to 802.11

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Chorus: CSMA/CR-based broadcast

S

Extension to 802.11 broadcast mode

Anonymous and decentralized

Performance analysis

Asymptotic broadcast delay (unit disk graph model):

D

Lr.

D

hLr

.Lowerbound: Upperbound:

header lengthpkt length

network radius data rate

)(r

Best known result for CSMA/CA broadcast: )log( nr

Asymptotic throughput:

3

D

)(3 hL

LD

Lowerbound: Upperbound:

11

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Achievable SNR:

Achievable PER:

PHY layer performance analysis

21 PERPERPER

Error propagation effect (based on a Markov chain model):

},{max 21 SNRSNRSNR

While resolving a given collision, the error propagation probability decays exponentially with the error length.

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Implement Chorus in ns-2

• Simulated application and MAC layers

• Analytical model for PHY-layer packet reception

Benchmark protocol: double coverage broadcast (DCB)

* W. Lou, J. Wu, “Toward Broadcast Reliability in Mobile Ad Hoc Networks with Double Coverage,” IEEE Trans. on Mobile Computing, vol. 6, no. 2, 2007

• Forwarding set selection: remove redundant transmissions

• Each node covered by two forwarders (retransmission improves reliability)

Chorus: Network-level simulation

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PDR and delay in lossy networks

: reception probability at transmission range

Chorus is more resilient to packet losses.

Scalability:

Chorus is less affected by network size.

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Achievable throughput:

Chorus can support much higher throughput.

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Multiple broadcast sessions:

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Conclusion

Chorus(broadcast)

PHY

MACCSMA/CR

transmit diversity

spatial reuse

Chorus: achieve optimal broadcast performance via a software radio based MAC/PHY.

Thank you!

Error propagation effect: a Markov chain model

Probability that error propagation stops, i.e., the next bit is correct even when the current bit is erroneous.

:bcP

:eP BER of clean symbols

15.0 bcPCan be bounded:

Max error length:GF

LG

data length

offset between collided pkts

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Steady state error length distribution:

bc

Gbc

e PP

P)1(1

1

10

GiPP ibcei ,,2,1,)1( 1

0

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Impact of packet size:

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Related Work [1/2]

Broadcast for 802.11 based wireless ad hoc networks

Most focused on forwarding node selection to prevent broadcast storming

* W. Lou, J. Wu, “Toward Broadcast Reliability in Mobile Ad Hoc Networks with Double Coverage,” IEEE Trans. on Mobile Computing, vol. 6, no. 2, 2007

* R. Gandhi, S. Parthasarathy, A. Mishr, Minimizing Broadcast Latency and Redundancy in Ad Hoc Networks, ACM MobiHoc’03

* S.-H. Huang, P.-J. Wan, X. Jia, H. Du, W. Shang, Minimum-Latency Broadcast Scheduling in Wireless Ad Hoc Networks, IEEE INFOCOM’07

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Related Work [2/2]

ZigZag decoding

Interference cancellation

* S. Gollakotam, D. Katabi. ZigZag Decoding: Combating Hidden Terminals in Wireless Networks, in Proc. of ACM SIGCOMM, 2008.

* D. Halperin, et. al. Taking the Sting out of Carrier Sense: Interference Cancellation for Wireless LANs, in Proc. of ACM MobiCom, 2008

A MAC/PHY layer technique. Only works when one packet has much higher SNR than the other.

Similar decoding algorithm. Rely on MAC layer retransmission to obtain multiple collided version of the same packets

PHY/MAC layer technique to combat hidden terminals

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