GRaTIS: Free Bits in the Network

Recent work has examined techniques to estimate the “best” modulation rate for data networks such as 802.11a/g. While accurate rate estimation yields better rate-selection decisions and increased throughput, those methods must still choose between a handful of modulation rates. Each modulation rate is effective in a range of actual signal-to-noise ratios (SNRs) but the limited number of practical rates means that transmitters are often forced to “step down” to a lower data rate despite having a higher SNR than the minimum required for that lower rate. In this paper we describe, evaluate and implement a practical multiuser communication scheme that exploits these discrete “steps” in modulation rates to transmit two packets in the time normally needed to transmit a single packet, increasing aggregate throughput precisely when it is most needed—when the network is busy and suffers from rate unfairness. Because the method transmits a group of packets simultaneously, we call this scheme Group Rate Transmission with Intertwined Symbols, or GRaTIS. In addition to up to 120% improvement in network throughput achieved by GRaTIS, the technique is backward compatible with 802.11 and doesn’t require complex DSP algorithms as required by competing methods.