# Implementation of a Fast Link Rate Adaptation Algorithm for WLAN Systems
## 5 Performance Measurements and Wireless Tests of the Link Adaptation
### Wireless Tests  
Figure 22 shows the parameters used in the wireless tests (T0-T5) of the link adaptation firmware which includes the three algorithms, ARF, APBLA, and ARF + APBLA (i.e., initially ARF to compensate for the SNR mismatch and later converted to APBLA). An initial value of \(-20\) dB is given as the SNR offset to compensate for the SNR mismatch due to the RF gain difference between the uplink and the downlink, in view of the channel coefficients that the firmware receives. The update step (NACK) column denotes to which extent the SNR offset is updated whenever a NACK is assumed to have occurred. On the other hand, whenever an ACK has occurred, the update step is set to a positive value with a magnitude of one tenth the update step value for a NACK. The coarse and fine update step values in case of NACK and ACK were explained previously in Section 3.3. The SNR offset and the update step values are needed in APBLA and not in ARF.  
The SNR offset variation with time when APBLA is applied in the T3 test environment is plotted in Figure 23. For the first 2k packets NACKs occur heavily and hence the SNR offset drops rapidly from \(-20\) dB to \(-40\) dB. Then as the SNR offset approaches near the expected value, one of the two means, coarse tuning and fine tuning, is selected and the update step is altered accordingly. Most of the wireless tests have the SNR offset reach the \(-45\) dB to \(-40\) dB range. The throughput measurement thereafter is conducted in the 2k-th to 10k-th packet range in which the SNR offset is relatively stabilized.  
Figure 21: Throughput gain of APBLA vs. ARF as a function of Doppler frequency over various packet transmission intervals, assuming a 1-tap channel (**left**) and a 3-tap channel (**right**).  
Figure 24 displays how the SNR offset varies with time in the T5 test environment, in which ARF is initially applied to amend the SNR offset and subsequently the algorithm is converted to APBLA. The first 5k packets are taken to be applied to ARF in this test. Compared with APBLA in Figure 23, ARF + APBLA has its SNR offset converge to the expected range more quickly.  
Figure 23: SNR offset variation over time in case of APBLA only (test T3).  
Figure 22: Firmware and parameters used for the wireless tests.  
Measurement results are analyzed by exploiting the channel information from the wireless tests. In Figure 25, the channel quality (left) and MCS variations (right) with time are plotted in the T1 test environment (ARF only). The MCS selected by ARF goes up and down repeatedly and quickly and hence it is inferred that the width of the channel variation will be large, which may also be identified in the left half of Figure 25 that shows the channel quality variation over time.  
Figure 24: SNR offset variation with time for ARF + APBLA (test T5).  
Figure 25: T1 channel quality and ARF MCS variations as a function of time.  
In Figure 26, the channel quality (left) and MCS variations (right) with time are plotted in the T5 test environment (ARF + APBLA) where initially ARF is applied to amend the SNR offset and subsequently APBLA substitutes ARF. It is shown that during the ARF period, a host of NACKs occur and ARF lowers the MCS index at a fast pace. The first 5k packets are used to amend the SNR offset and then APBLA supplants ARF, leading to stabilized MCS selection. Then, as the channel quality improves, higher MCS indices are selected. The channel quality fluctuation in this test case is not uniform, which adversely impacts the MCS selection by APBLA.