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PPR: Partial Packet Recovery for Wireless Networks
Kyle Jamieson and Hari Balakrishnan
MIT Computer Science and Artificial Intelligence Lab
{jamieson, hari}@csail.mit.edu
Abstract
Bit errors occur over wireless channels when the signal isn’t strong enough to
overcome the effects of interference and noise. Current wireless protocols may use
forward error correction (FEC) to correct for some (small) number of bit errors,
but generally retransmit the whole packet if the FEC is insufficient. We observe
that current wireless mesh network protocols retransmit a number of packets and
that most of these retransmissions end up sending bits that have already been received
multiple times, wasting network capacity. To overcome this inefficiency, we
develop, implement, and evaluate a partial packet recovery (PPR) system.
PPR incorporates three new ideas: (1) SoftPHY, an expanded physical layer
(PHY) interface to provide hints to the higher layers about how “close” the actual
received symbol was to the one decoded, (2) a postamble scheme to recover data
even when a packet’s preamble is corrupted and not decodable at the receiver,
and (3) PP-ARQ, an asynchronous link-layer retransmission protocol that allows a
receiver to compactly encode and request for retransmission only those portions of
a packet that are likely in error.
Our experimental results from a 27-node 802.15.4 testbed that includes Telos
motes with 2.4 GHz Chipcon radios and GNU Radio nodes implementing the
Zigbee standard (802.15.4) show that PPR increases the frame delivery rate by a
factor of 2× under moderate load, and 7× under heavy load when many links have
marginal quality.
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