# Implementation of a Fast Link Rate Adaptation Algorithm for WLAN Systems
## 6 Discussion and Conclusions  
Link adaptation or rate adaptation is still under active research in various fields. An enhanced outer-loop link adaptation algorithm based on cyclic redundancy code and CSI is proposed [36], coding and modulation formats are adjusted according to the state of the optical link [37], rate is adapted in spatial modulation [38], adaptive modulation and coding is applied in a cognitive radio [39], link is adaptively adjusted in mobile satellite links [40], and link is adapted in 5G cellular networks and LTE Advanced [41; 42; 43]. In [44], MIMO mode, channel bonding, and frame aggregation level are adjusted together with modulation coding scheme in a holistic manner. As was demonstrated from the wireless tests in Section 5, the proposed firmware achieves fast link rate adaptation, when compared with ARF and ideal link adaptation, and is amenable to potential upgrades and changes in a flexible and swift manner. The throughput gain of the proposed algorithm over ARF is 46% (15%) for a 1-tap (3-tap) channel over 10 m-250 m (16 m-160 m) normalized Doppler frequencies. For a 3-tap channel and 30 m-50 m normalized Doppler frequencies, the throughput of the proposed algorithm is about 31 Mbps, all but the same as that of ideal link adaptation, whereas the throughput of ARF is about 24 Mbps, leading to a 30% throughput gain of the proposed algorithm over ARF. Table 2 lists the simulated results of APBLA, PBLA, and ARF, together with theoretical maximum rates, in the presence of MMI-to-PER mapping table errors. The theoretical maximum rate means the data rate of ILA, namely, the throughput achieved when the optimal MCS is always selected with respect to a given channel and hence no LA can achieve a better throughput than this throughput. PBLA is similar to FLA in [26] in its operating principle and since the mapping table is fixed, the error is not overcome and the throughput shows a large degradation, which is much inferior to that with ARF. Slow fading (at 1 m of normalized Doppler) is the optimal environment for ARF whereas fast  
Figure 26: T5 channel quality and (ARF + APBLA) MCS variations as a function of time. fading (at over 5 m) is the optimal environment for APBLA, which is consistent with the remarks in other literatures. APBLA always accomplishes more than 94% of the theoretical maximum rate, irrespective of Doppler, on condition that the ACK offset is set appropriately. If the offset is set overly small, LA is unable to follow the channel variation or unable to compensate for the mapping table error whereas if set overly large, deviation from the mapping table will be severe. As is shown, the ACK offset is generally set small for slow fading and large for fast fading. LA techniques to date are compared with one another in Table 3, in terms of performance and complexity. The LA inputs may be ACK/NACK or CSI but in some cases the cyclic redundancy code (CRC) or the log-likelihood ratio (LLR). Lots of means exist to quantify the channel quality, called link quality metrics (LQMs), but among them, MI calculated from subcarrier SNRs is generally known to be the most accurate LQM especially in coded MIMO-OFDM. The mapping table used by the LQM needs some compensation to reflect the discrepancy between two individual receivers (since different receivers will exhibit different PERs under the same MMI) or between uplink and downlink SNRs. If the mapping is adaptive rather than fixed, then the corresponding LA will be more robust to the mapping table error. Moreover, the LA with adaptive mapping can keep track of the channel variation more favorably. Most of the techniques in Table 3 are based on fixed mapping, leading to considerable performance degradations, similar to the degradation with PBLA in Table 2. Since ARF in [5] and SampleRate in [10] do not compute a separate LQM, the computational complexity is considerably low but both of them are vulnerable to fast fading owing to the fact that the optimal MCS is found by means of trial and error on the ACK/NACK basis.