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Chorus: Collision Resolution for Efficient Wireless Broadcast

Chorus: Collision Resolution for Efficient Wireless Broadcast. Xinyu Zhang, Kang G. Shin. University of Michigan. PHY layer. MAC layer. Outline. Design. Analysis & evaluation. Introduction. Summary. Chorus (broadcast). simulation. PHY PER . principles. motivation. network.

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Chorus: Collision Resolution for Efficient Wireless Broadcast

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  1. Chorus: Collision Resolution for Efficient Wireless Broadcast Xinyu Zhang, Kang G. Shin University of Michigan

  2. PHY layer • MAC layer Outline • Design • Analysis & evaluation • Introduction • Summary • Chorus(broadcast) simulation PHY PER principles motivation network

  3. 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

  4. 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 D B

  5. Rationale(2/3): CSMA/CR advantage S D A B E C S A B C D E Improving broadcast efficiency Taking advantage of spatial reuse and transmit diversity (b) Chorus, a CSMA/CR based broadcast protocol (a) Traditional CSMA/CA based broadcast

  6. PHY • MAC Chorus: collision resolution based broadcast Chorus A broadcast protocol with asymptotic latency • Broadcast CSMA/CR Encourage resolvable collisions via intelligent sensing and scheduling Resolve collisions via signal processing

  7. Chorus: PHY layer Resolve the collided packet by iterative decoding A C D Y B E Z A D C' D' E' A' B' Z' Y' B S=A' + C S --- the received symbol.A’ --- estimated based on A.C = S – A’ P1 Decode two versions of the packet: from preamble and postamble, respectively P1

  8. Multipacket collision resolution: Head packet P1 A C E B D C'' A'' B'' D'' packet P3 A''' B''' packet P4 A' Tail packet P2 Head and tail packet: iterative collision resolution Other packets: hard decoding

  9. 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

  10. Chorus: CSMA/CR-based broadcast Extension to 802.11 broadcast mode Anonymous and decentralized S

  11. Performance analysis Asymptotic broadcast delay (unit disk graph model): pkt length header length Upperbound: Lowerbound: network radius data rate Best known result for CSMA/CA broadcast: Asymptotic throughput: Upperbound: Lowerbound:

  12. PHY layer performance analysis Achievable SNR: Achievable PER: Error propagation effect (based on a Markov chain model): While resolving a given collision, the error propagation probability decays exponentially with the error length.

  13. 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)

  14. PDR and delay in lossy networks reception probability at transmission range Chorus is more resilient to packet losses.

  15. Scalability: Chorus is less affected by network size.

  16. Achievable throughput: Chorus can support much higher throughput.

  17. Multiple broadcast sessions:

  18. PHY • MAC Conclusion transmit diversity • Chorus(broadcast) CSMA/CR spatial reuse Chorus: achieve optimal broadcast performance via a software radio based MAC/PHY.

  19. Thank you!

  20. Error propagation effect: a Markov chain model data length Max error length offset between collided pkts BER of clean symbols Probability that error propagation stops, i.e., the next bit is correct even when the current bit is erroneous. Can be bounded:

  21. Steady state error length distribution:

  22. Impact of packet size:

  23. 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

  24. 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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