Chorus: Collision Resolution for
Efficient Wireless Broadcast
Xinyu Zhang, Kang G. Shin
University of Michigan
1
Outline
Chorus
(broadcast)
MAC layer
PHY layer
Introduction
motivation
principles
Design
Analysis &
evaluation
PHY PER
Summary
simulation
network
2
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
3
Rationale(1/3): CSMA/CR principle
CSMA/CR (CSMA with collision resolution):
A new MAC/PHY paradigm
Overcome the limitation of CSMA/CA
CSMA/CR Principle:
Collision caused by packets carrying
the same data can be resolved!
A
D
B
4
Rationale(2/3): CSMA/CR advantage
Improving broadcast efficiency
Taking advantage of spatial reuse and transmit diversity
C
C
A
A
S
D
S
D
B
B
E
(a) Traditional CSMA/CA
based broadcast
E
(b) Chorus, a CSMA/CR
based broadcast protocol
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Chorus: collision resolution based broadcast
Chorus
A broadcast protocol with
(r ) asymptotic latency
Broadcast
CSMA/CR
Encourage resolvable collisions via
intelligent sensing and scheduling
MAC
Resolve collisions via signal
processing
PHY
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Chorus: PHY layer
Resolve the collided packet by iterative decoding
P1
A
A B
C D E
Y
Z
D
B
P1
A' B' C' D' E'
Y'
Z'
S=A' + C
S --- the received symbol.
A’ --- estimated based on A.
C = S – A’
Decode two versions of the packet: from preamble and
postamble, respectively
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Multipacket collision resolution:
Head packet A B C D E
P1
packet P3 A'' B'' C'' D''
packet P4
A''' B'''
Tail packet P2
A'
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
Extension to 802.11 broadcast
mode
Anonymous and decentralized
S
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Performance analysis
Asymptotic broadcast delay (unit disk graph model): (r )
pkt length
Lowerbound:
r.
L
D
header length
Lh
D
network radius
data rate
Upperbound: r.
Best known result for CSMA/CA broadcast: ( r log n)
Asymptotic throughput:
LD
Lowerbound:
3( L  h )
Upperbound:
D
3
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PHY layer performance analysis
Achievable SNR:
SNR  max {SNR1, SNR2}
Achievable PER:
PER  PER1  PER2
Error propagation effect (based on a Markov chain model):
While resolving a given collision, the error propagation
probability decays exponentially with the error length.
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Chorus: Network-level simulation
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)
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PDR and delay in lossy networks
 : reception probability at transmission range
Chorus is more resilient to packet losses.
14
Scalability:
Chorus is less affected by network size.
15
Achievable throughput:
Chorus can support much higher throughput.
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Multiple broadcast sessions:
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Conclusion
Chorus
(broadcast)
transmit
diversity
MAC
CSMA/CR
PHY
spatial
reuse
Chorus: achieve optimal broadcast performance
via a software radio based MAC/PHY.
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Thank you!
Error propagation effect: a Markov chain model
G : Max error length
L
G
F
data length
offset between collided pkts
Pe : BER of clean symbols
Pbc : Probability that error propagation stops, i.e., the next
bit is correct even when the current bit is erroneous.
Can be bounded: 0.5  Pbc  1
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Steady state error length distribution:
1
0 
1  (1  Pbc )G
1  Pe
Pbc
 i   0  Pe  (1  Pbc )i1, i  1, 2,  , G
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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
* S. Gollakotam, D. Katabi. ZigZag Decoding: Combating Hidden Terminals in Wireless
Networks, in Proc. of ACM SIGCOMM, 2008.
PHY/MAC layer technique to combat hidden terminals
Similar decoding algorithm. Rely on MAC layer
retransmission to obtain multiple collided version of the
same packets
Interference cancellation
* 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.
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