# Implementation of a Fast Link Rate Adaptation Algorithm for WLAN Systems
## 5 Performance Measurements and Wireless Tests of the Link Adaptation  
### Simulation  
To identify the conditions for which the throughput gain of APBLA over ARF is guaranteed, simulation in a MATLAB environment is carried out according to packet transmission intervals and Doppler frequencies. The simulation environment to identify the throughput gain interval of APBLA relative to ARF is set up as drawn in Figure 16. Channel coefficients are first generated in MATLAB and sent to the TX as is whereas they  
Figure 15: Execution time and ratio of each function before and after optimization. are sent to the RX with an SNR error. The reason for this is to express the SNR difference that stems from the RF gain mismatch between the uplink and the downlink channels. Through the MMI table error the disagreement is considered between the RX performance assumed by APBLA and the actual performance. In summary, by means of the SNR error and the MMI table error, the simulation environment is made closely akin to the actual test environment. In case of ARF, link adaptation is applied every packet whereas in case of APBLA, link adaptation is applied every 1 ms when the packet transmission interval is less than 1 ms and it is applied every packet when the interval is longer than 1 ms. For example, if the transmission interval is 0.1 ARFms, ARF is applied every packet but APBLA is applied once every ten applications of ARF, in view of the operating speed of ARF and APBLA. During the time when APBLA is not applied, the MCS value of the most recently applied point in time is taken and sustained until the next applied time point. Figure 17 plots the simulation results of the gain of APBLA against ARF. The \(x\)-axis is the normalized Doppler frequency. For instance, if the packet transmission interval is 1 ms and Doppler frequency is 1 Hz, the normalized Doppler frequency is 1 m, which is defined the product of the packet transmission interval and Doppler frequency. The \(y\)-axis is the throughput that is measured from ACKs stochastically produced with respect to the calculated PER in the RX model. The blue line is for ARF, the grey line for ILA, and the red line for APBLA. Here, ILA means the link adaptation model where the most ideal MCS is selected according to the received channel information. As the channel information is acquired from the preamble of the previous packet, 1 packet delay is reflected in any events. Three-tap Rayleigh fading is assumed as to MCS 0-7 in IEEE 802.11n. Also, the MMI table errors corresponding to the left half of Figure 7 are considered. From Figure 17, it is identified that when the normalized Doppler frequency increases, the throughput for ARF is heavily aggravated. On the other hand, the throughput for APBLA only modestly deteriorates as the normalized Doppler frequency grows. When the throughput gain of APBLA over ARF is maximized or equivalent when the normalized Doppler frequency is 50 m (say, when the packet transmission interval is 10 ms for a 5 Hz Doppler frequency), the ARF throughput is 24 Mbps and the APBLA throughput is 31 Mbps, yielding a throughput gain of around 30% for APBLA against ARF. Figure 16: Simulation setup to identify the throughput gain of APBLA vs. ARF. To identify the conditions to obtain the throughput gain of APBLA over ARF in terms of Doppler frequency and the transmission interval, simulation is carried out with the MMI table errors maximized as in the left half of Figure 18 and also with the worst case of no compensation assumed (in practice, the MMI table errors will be smaller, which can be partly compensated for by the SNR offsets, and hence the actual throughput gain is predicted to be larger). Except for the MMI table errors considered, the simulation environment is identical to that for obtaining Figure 17. One-tap and three-tap Rayleigh fading is assumed. Figure 18 represents the MMI-to-throughput tables with (left) and without (right) the MMI table errors.