The SoftPHY abstraction: from packets to symbols in wireless network design

At ever-increasing rates, we are using wireless systems to communicate with others and retrieve content of interest to us. Current wireless technologies such as WiFi or Zigbee use forward error correction to drive bit error rates down when there are few interfering transmissions. However, as more of us use wireless networks to retrieve increasingly rich content, interference increases in unpredictable ways. This results in errored bits, degraded throughput, and eventually, an unusable network. We observe that this is the result of higher layers working at the packet granularity, whereas they would benefit from a shift in perspective from whole packets to individual symbols. From real-world experiments on a 31-node testbed of Zigbee and software-defined radios, we find that often, not all of the bits in corrupted packets share fate. Thus, today's wireless protocols retransmit packets where only a small number of the constituent bits in a packet are in error, wasting network resources. In this dissertation, we will describe a physical layer that passes information about its confidence in each decoded symbol up to higher layers. These SoftPHY hints have many applications, one of which, more efficient link-layer retransmissions, we will describe in detail. PP-ARQ is a link-layer reliable retransmission protocol that allows a receiver to compactly encode a request for retransmission of only the bits in a packet that are likely in error. Our experimental results show that PP-ARQ increases aggregate network throughput by a factor of approximately 2× under various conditions. Finally, we will place our contributions in the context of related work and discuss other uses of SoftPHY throughout the wireless networking stack. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)