# Implementation of a Fast Link Rate Adaptation Algorithm for WLAN Systems
## 2 Link Adaptation Algorithm Basics
On the other hand, preamble-based link adaptation (PBLA), categorized as PHY-measurement-based link adaptation, utilizes the preambles in the received frames such as ACK frames, beacon frames, and so forth. Rate adaptation based on the received packet SNR measurement may be highly erroneous under certain scenarios like mobile multipath channels. Thus, from the SNR concerning the received channel, mutual information (MI) is calculated per modulation scheme and subsequently the mean MI or MMI is obtained by taking the average of MI over the channel subcarriers in the OFDM [26]. Then, the packet error rate (PER) is obtained from the MMI and finally the throughput is calculated from the PER. Through this procedure the throughput is obtained for each of the MCS and among all the MCS values, the MCS with the maximum throughput is selected. As mentioned in Section 1, the MMI-based link quality metric is known to be more accurate than the exponential effective SNR. More specifically, conversion of the SNR to the MI and conversion of the MMI to the PER are explained as follows. MI is the probability that one bit is transmitted successfully, namely, the expectation value of the number of bits that are delivered successfully. MMI is the mean MI over all the subcarriers. PER is the probability that the packet is transmitted unsuccessfully and hence the lower the PER the better the likelihood of successful transmission. \(SNR_{k,i}\) that is the SNR of the \(i\)-th subcarrier of the \(k\)-th packet is computed with Equation (1). \[SNR_{k,i}=\frac{\left|H_{k,i}\right|^{2}}{N_{0}} \tag{1}\]  
where \(H_{k,i}\) is the channel coefficient of the \(i\)-th subcarrier of the \(k\)-th packet and \(N_{0}\) is the noise power spectral density (PSD). MI vs. SNR has the same shape for the same modulation scheme, as plotted in Figure 5. MI per symbol can be calculated from the SNR by using cubic polynomials, which is specified in Appendix A.  
Figure 4: Automatic rate fallback (ARF) simulation results. Figure 6 is obtained from Monte Carlo simulations under the additive white Gaussian noise (AWGN) environment, from which the PER is acquired and the throughput can be expressed in terms of the PER as in Equation (2). \[TP_{\text{\small{MCS}}}=(1-PER_{\text{\small{MCS}}})R_{\text{\small{MCS}}} \tag{2}\]  
where \(TP_{\text{\small{MCS}}}\) is the expected value of the data rate or throughput for each MCS value in Table 1, \(PER_{\text{\small{MCS}}}\) is the expected value of the PER, and \(R_{\text{\small{MCS}}}\) is the peak data rate or throughput listed in Table 1. Under fading channel environments, the \(PER_{\text{\small{MCS}}}\) varies as the channel condition varies, and hence \(TP_{\text{\small{MCS}}}\) also varies. Figure 5: Signal-to-noise ratio (SNR)-to-mutual information (MI) conversion for MCS 0–7. Figure 6: Mean mutual information (MMI)-to- packet error rate (PER) conversion for MCS 0–7. In this paper, an adaptive preamble-based link adaptation (adaptive PBLA or APBLA) algorithm is proposed, which is outlined in the following. If a difference exists between the MMI-to-PER table (Figure 6) in PBLA and the actual RX performance (owing to the mismatch between uplink SNR and downlink SNR), then performance degradation will occur. The right half of Figure 7 shows the MMI-to-PER table used in PBLA while the left half of Figure 7 shows the result after the curves for MCS values are shifted randomly and independently of each other, in order to model the actual RX performance. If PBLA is adopted for link adaptation, the right half of Figure 7 is put to use to determine the MCS value. If the actual RX is assumed to have the MMI-to-PER relationship shown on the left half of Figure 7, then, in this case, PBLA will be unable to carry out correct link adaptation. Besides, if the channel coefficients experience separate scaling procedures, the channel quality will be falsely predicted and hence PBLA will scarcely select the MCS value with the maximum throughput.