# Evaluation of Rate Adaptation Algorithms in IEEE 802.11 Networks
## 2 Related Work
### Implicit Feedback  
Implicit Feedback is a sender-driven rate adaptation scheme and it is usually based on Packet Error Rate (PER). The main idea is for the sender to select an appropriate data rate that is based on the PER observed on his side. This mainly requires ACK to be used to enable the sender to calculate PER. STRALE [14] jointly adapts PHY rate and A-MPDU length. After receiving a BlockAck, it calculates the optimal A-MPDU length for the highest throughput of the last A-MPDU transmission. Subsequently, the next A-MPDU length of the next transmission is determined using the Exponential \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Name** & **Category** & **Metrics** & **Pros** & **Cons** & **Modify Standard** \\ \hline Ideal & Explicit Feedback & SINR & Instantaneous feedback and rate decision & Uses perfect out-of-band mechanism for feedback & Yes \\ \hline OFRA & Explicit Feedback & SINR & Feedback is sent only when a new rate is selected & Additional frame is sent for feedback at the lowest rate & Yes \\ \hline SIRA & Explicit Feedback & SINR & Uses 2 rates for a single A-MPDU transmission to adapt with channel changes & In a fast-changing channel we may need more than 2 rates for a single A-MPDU transmission to adapt to channel changes & No \\ \hline Strale & Implicit Feedback & Throughput & Prefers to use an optimal A-MPDU length instead decreasing the rate & Does not take interference into consideration & No \\ \hline Minstrel-HT & Implicit Feedback & Throughput & It can select a high rate with a significant FLR as long as it can achieve the best throughput & In some conditions it fails to enhance the throughput [13] & No \\ \hline MiRA & Implicit Feedback & Throughput & Collision-based decision making and probing interval adaptation & Introduces overhead when using the RTS/CTS mechanism & No \\ \hline RAMAS & Implicit Feedback & PLR & It is efficient in MIMO where it selects between different values of transmission metrics & Slow in adapting the number of streams and premature MCS adaptation & No \\ \hline Damyus & Implicit Feedback & PLR & Performs well in dense environments in infrastructure mode & Is not applicable in Ad Hoc mode & No \\ \hline MutFed & Hybrid & SINR,FLR & Distinguishes the cause of frame loss combined with SINR decision & In a dense environment many frame losses cause the algorithm to send at lower rates & Yes \\ \hline EasiRA & Hybrid & ESS,PLR & Combines external sensors and ESS to detect collisions and uses this information with the PLR for rate selection & It tries to identify collisions using external information, such as sensor-hints and ESS in all the devices & No \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of existing rate adaptation algorithms. Weighted Moving Average (EWMA). The difference between the latest and the newly calculated A-MPDU length is compared to a certain threshold. If it is greater than the threshold, STRALE takes the decision on Modulation and Coding Scheme (MCS) and A-MPDU length after checking if the throughput with one lower MCS index using the latest A-MPDU length is better than staying at the same MCS index with the newly calculated A-MPDU length. The main drawback of Strale is that it does not take interference into consideration. Minstrel-HT [15] relies on three parameters: channel width, guard interval, and number of streams to create group rates. Each group contains eight different data rates represented by the MCS index. Minstrel uses probing to determine the best data rate. This process consists of two periods: sampling and non-sampling periods. During the sampling period, Minstrel-HT selects a random data rate from all of the available data rates in each group. If a data rate results in a higher throughput than the previous data rate, then it is used for subsequent MPDU transmissions. Otherwise, Minstrel-HT keeps using the previous data rate. Throughput is calculated based on Frame Loss Rate (FLR) while taking EWMA into account.