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

2. Outline 2 Introduction Design Summary Chorus (broadcast) PHY layerMAC layer Analysis & evaluation PHY PER network simulation motivationprinciples

3. Motivation: CSMA/CA limitation Traditional CSMA/CA (Collision Avoidance): Principle: listen before talking --- akin to human world 3 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): 4 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

5. 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 5 A B C D E S (a) Traditional CSMA/CA based broadcast A B C D E S

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

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

8. Multipacket collision resolution: A A' B C Head 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 8

9. CSMA/CR: MAC layer Cognitive sensing and scheduling 9 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 S Extension to 802.11 broadcast mode Anonymous and decentralized 10

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

12. Achievable SNR: Achievable PER: PHY layer performance analysis 12 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. 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 13

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

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

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

17. Multiple broadcast sessions: 17

18. Conclusion 18 Chorus (broadcast) PHYMAC CSMA/CR transmit diversity 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 Probability that error propagation stops, i.e., the next bit is correct even when the current bit is erroneous. BER of clean symbols Can be bounded: Max error length data length offset between collided pkts 20

21. Steady state error length distribution: 21

22. Impact of packet size: 22

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 23

24. 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 24