| The 802.11 PHYs provide multiple transmission rates by employing different modulation and channel coding schemes. However, in the MAC layer, the link rate adaptation algorithm is left open.
There are mainly two kinds of rate adaptation schemes for IEEE 802.11 at present. One is based on SNR (Signal to Noise Ratio), such as RBAR protocol. However, this type of approaches usually requires extra implementation effort including a feedback channel and is not compatible with the current 802.11 standard.
The other is based on a history of Acknowledgment (ACK) frame reception. In ARF, if two consecutive ACK frames are not received, the subsequent transmissions are made at the next lower rate and a timer starts. The rate is raised again if ten consecutive ACK frames are successfully received or the timer expires. Prior researches indicate that the success threshold of ARF, which is ten consecutive successful transmissions, is very sensitive to the changing rate of channel parameters. Fast-changing channel requires a small value of success threshold, so that the transmission parameters can keep up with the channel variations. On the other hand, for slowly changing channels, large value is preferred to avoid ineffective switching to a higher rate. Moreover, in ARF, there is only one the success threshold for all the rates.
In this paper, we find the optimum success threshold depends on data rates, packet length and channel parameters. Therefore, we propose an Adaptive Multirate Auto Rate Fallback (AMARF) protocol. The key idea is to give each data rate a unique success threshold, and the success thresholds can be changed dynamically in an adaptive manner according to the running conditions, such as packet length and channel parameters.
Our in-depth simulation shows that AMARF yields significantly higher throughput than other existing schemes including Auto Rate Fallback (ARF) scheme and its variants, in various running conditions.
|