Opportunistic media access for multirate ad hoc networks

TLDR

IEEE 802.11 supports multiple data rates, and various auto‑rate adaptation mechanisms have been proposed to exploit this capability by matching transmission rates to channel conditions. This paper introduces the Opportunistic Auto Rate (OAR) protocol to better exploit periods of high‑quality channel conditions. OAR opportunistically sends multiple back‑to‑back packets during good channel periods, is implementable atop any existing auto‑rate scheme in a nearly IEEE 802.11‑compliant manner, and its throughput gains are analytically characterized and validated through extensive ns‑2 simulations varying node velocity, channel conditions, and topology. Because channel coherence times typically exceed multiple packet durations, OAR delivers significant throughput gains over state‑of‑the‑art auto‑rate mechanisms while, over longer timescales, it guarantees equal channel‑access time‑shares for all nodes as in single‑rate IEEE 802.11.

Abstract

The IEEE 802.11 wireless media access standard supports multiple data rates at the physical layer. Moreover, various auto rate adaptation mechanisms at the medium access layer have been proposed to utilize this multi-rate capability by automatically adapting the transmission rate to best match the channel conditions. In this paper, we introduce the Opportunistic Auto Rate (OAR) protocol to better exploit durations of high-quality channels conditions. The key mechanism of the OAR protocol is to opportunistically send multiple back-to-back data packets whenever the channel quality is good. As channel coherence times typically exceed multiple packet transmission times for both mobile and non-mobile users, OAR achieves significant throughput gains as compared to state-of-the-art auto-rate adaptation mechanisms. Moreover, over longer time scales, OAR ensures that all nodes are granted channel access for the same time-shares as achieved by single-rate IEEE 802.11. We describe mechanisms to implement OAR on top of any existing auto-rate adaptation scheme in a nearly IEEE 802.11 compliant manner. We also analytically study OAR and characterize the gains in throughput as a function of the channel conditions. Finally, we perform an extensive set of ns-2 simulations to study the impact of such factors as node velocity, channel conditions, and topology on the throughput of OAR.