Events - Colloquia & Seminars

CCIS Colloquium Spring 2008

A Shift from Packets to Symbols in Wireless Systems

Speaker: Kyle Jamieson

Affiliation : MIT Computer Science and Artificial Intelligence Laboratory

Date: Thursday, March 13, 2008

Talk: 10:30 a.m., 366 WVH

Abstract

At an increasing rate, 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. I will argue 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 Zigbee/software radio testbed, I 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. I 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 I will describe in detail. PP-ARQ is a link‐layer ARQ protocol that allows a receiver to compactly encode a request for retransmission of only the bits in a packet that are likely in error. My experimental results show that PP‐ARQ increases aggregate network throughput by a factor of approximately 2× under various conditions. Finally, I will place PP‐ARQ in context in terms of other systems work I have undertaken to adapt to the harsh wireless channel, and discuss other uses of SoftPHY.