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Email: canzian@dei.unipd.it, zanella@dei.unipd.it, zorzi@dei.unipd.it

IEEE Globecom 2010 Workshop on Pervasive Group Communications ( PerGroup 2010). Overlapped NACKs: Improving Multicast Performance in Multi-access Wireless Networks. Authors : Luca   Canzian , Andrea   Zanella , Michele   Zorzi.

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Email: canzian@dei.unipd.it, zanella@dei.unipd.it, zorzi@dei.unipd.it

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  1. IEEE Globecom 2010 Workshop on Pervasive Group Communications (PerGroup 2010) Overlapped NACKs: Improving Multicast Performance in Multi-access Wireless Networks Authors : Luca  Canzian, Andrea  Zanella, Michele  Zorzi Email: canzian@dei.unipd.it, zanella@dei.unipd.it, zorzi@dei.unipd.it

  2. Outline • Basic problem • Our approach • Case study: Bluetooth • Performance comparison • Conclusion

  3. Outline • Basic problem • Our approach • Case study: Bluetooth • Performance comparison • Conclusion

  4. Multicast communication Wireless network • Multicast: only 1 transmission!!! • -> resource saving (time/frequency, power) • Increasing rate • More bandwidth for other devices/applications

  5. ACK based reliability • Devices send ACKs if the packet is correctly received • Reliability • Scalability • Coordination mechanism ACK ACK ACK ACK

  6. NACK based reliability • Devices send NACKs if the packet is NOT correctly received • Scalability • Reliability • Coordination mechanism NACK NACK

  7. Outline • Basic problem • Our approach • Case study: Bluetooth • Performance comparison • Conclusion

  8. Overlapping NACKs • NACKs sent in the same resource!! (-> collisions) • Scalability • No coordination mechanism • ? Reliability NACK NACK 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Nack slave 1 Nack slave 2 1 1 1 1 1 1 1 Resulting Nack

  9. DPSK example Q Q After 1 symbol period I I Q 120° 120° I 240° -120°

  10. Double-NACK structure First NACK Phase displacement Second NACK SLAVE 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 SLAVE 2 1 1 1 1 1 1 1 1 0 1 0 1 1 1 1 1 1 1 Q Q Q I I I Q 10 11 I 00 01 DQPSK phase-shifts

  11. Outline • Basic problem • Our approach • Case study: Bluetooth • Performance comparison • Conclusion

  12. Bluetooth technology • Ad-hoc Personal Area Network (PAN) • Direct communication only between master and slaves • Time-Division Duplexing (TDD) communication • Packets last 1, 3 or 5 slots • GFSK (1 Mbps), π/4-DPSK (2 Mbps), 8DPSK (3 Mbps) • No multicast (multi-unicast) !!! • -> SIG proposal

  13. ACK and NACK based protocols SIG (Special Interest Group) proposal Our proposal Overlapped NACKs

  14. Outline • Basic problem • Our approach • Case study: Bluetooth • Performance comparison • Conclusion

  15. Throughput vs slaves

  16. Throughput vs BER & 3-DH5

  17. Reliability Average PER / Worst case PER On average 1 packet every 625000 is lost !!!

  18. Outline • Basic problem • Our approach • Case study: Bluetooth • Performance comparison • Conclusion

  19. Conclusion • New simple NACK based protocol for reliable multicast communications • NACK structure to be robust to collisions • Comparison with an ACK based protocol for Bluetooth • Results show: • Little reliability loss • High throughput gain

  20. IEEE Globecom 2010 Workshop on Pervasive Group Communications (PerGroup 2010) Overlapped NACKs: Improving Multicast Performance in Multi-access Wireless Networks Authors : Luca  Canzian, Andrea  Zanella, Michele  Zorzi Email: canzian@dei.unipd.it, zanella@dei.unipd.it, zorzi@dei.unipd.it

  21. Time synchronization 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Q Q Q I I I OVERLAPPING INTERVAL

  22. Time synchronization 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Q Q Q I I I OVERLAPPING INTERVAL

  23. Time synchronization 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 0 1 1 1 1 1 1 1 Q Q Q I I I Q 10 OVERLAPPING INTERVAL 1 OVERLAPPING INTERVAL 2 11 I 00 01 DQPSK phase-shifts

  24. Frequency offset Equal amplitude case Red turns of 0° Blue turns of 60° Green turns of 30° Green turns of 210°!!! -> discontinuity of 180° • Conclusion: • Most time high SNR and constant phase drift • -> it can be compensated • Some time SNR drastically decreases and phase discontinuity • -> this must be taken into account to set correlator threshold

  25. Frequency offset 1 signal much stronger than the other Red turns of 0° Blue turns of 60° Max phase error Green ~ Red + noise Max amplitude error Conclusion: SNR of the received signal decreases a little

  26. Markov model Renewal Process: b: average number of bits transmitted in a cycle d: average duration of a cycle

  27. Markov model Probability that the master does not recognize the overlapping of i double NACKs (missed detection probabilities) Probability to be absorbed in Li

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