PPR: Partial Packet Recovery for Wireless Networks

Unknown author (2007-02-02)

Bit errors occur over wireless channels when the signal isn't strongenough to overcome the effects of interference and noise. Currentwireless protocols may use forward error correction (FEC) to correct forsome (small) number of bit errors, but generally retransmit the wholepacket if the FEC is insufficient. We observe that current wirelessmesh network protocols retransmit a number of packets and that most ofthese retransmissions end up sending bits that have already beenreceived multiple times, wasting network capacity. To overcome thisinefficiency, we develop, implement, and evaluate a partial packetrecovery (PPR) system.PPR incorporates three new ideas: (1) SoftPHY, an expandedphysical layer (PHY) interface to provide hints to the higher layersabout how ``close'' the actual received symbol was to the one decoded,(2) a postamble scheme to recover data even when a packet'spreamble is corrupted and not decodable at the receiver, and (3) PP-ARQ, an asynchronous link-layer retransmission protocol that allowsa receiver to compactly encode and request for retransmission only thoseportions of a packet that are likely in error.Our experimental results from a 27-node 802.15.4 testbed that includesTelos motes with 2.4 GHz Chipcon radios and GNU Radio nodes implementingthe Zigbee standard (802.15.4) show that PPR increases the framedelivery rate by a factor of 2x under moderate load, and7x under heavy load when many links have marginal quality.