1. PPR: Partial Packet Recovery for Wireless Networks Kyle Jamieson and Hari Balakrishnan MIT Computer Science and Artificial Intelligence Laboratory

2. The problem Lots of packets lost to collisions and noise in wireless networks Can’t recover non-colliding bits today! Bits in a packet don’t share fate Non-colliding bits (P1) (P2) Time

3. INTRODUCTION SoftPHY (PHY interface ) provides PHY- independent hints to link-layer Postamble scheme to recover data PP-ARQ

4. Block diagram of the PPR system

5. Receiver

6. HDD? SDD? For better performance at low SINR, a decoder can use soft-decision(SDD). However, SDD will still produce incorrect codewords at very low SINR. Error were mostly attributable to collisions. HDD implementation was conceptually simpler.

9. Postamble packet decoding

10. Postamble decoding Start –of-frame delimiter

11. Steps 1.Roll back as many symbols as are in the packet trailer 2. Decode and parse the trailer to find the start of the entire packet, and the sender and receiver identities 3.Roll back in time as many symbols as are in the entire packet, to decode as much of the packet as possible

12. PP-ARQ:PPR+RETRANSMISSIONS PP-ARQ key idea: resend only incorrect bits Receive Packet

14. The streaming ACK PP-ARQ protocol The sender transmit the full packet with checksum. The receiver decodes the packet (possibly partially), and compute the best feedback set in its reverse-link acknowledgement packet. The sender retransmit only the contents of the runs the receiver requests and checksums

15. 25 senders 6 receivers

16. PP-ARQ performance comparison Packet CRC (no postamble) Fragmented CRC (no postamble) – Tuned against traces for optimal fragment size Preamble Checksum Preamble Checksum

18. 50 bytes

23. PP-ARQ retransmissions are short

24. Conclusion Mechanisms for recovering correct bits from parts of packets – SoftPHY interface (PHY-independent) – Postamble decoding PP-ARQ improves throughput 2.3–2.8  over the status quo