We developed a high speed wireless LAN prototype in 5 GHz frequency band. Maximum transmission rate of the developed WLAN system is 324 Mbit/sec using 6 multi-channels of the IEEE802.11a. We propose a SINR estimation scheme using the correlation of long training fields, and implemented it in closed-loop link adaptation. We compared the proposed link adaptation schemes with the legacy ACK-based link adaptation schemes by computerized simulation. It was found that the performances of legacy ACK-based link adaptation schemes depended largely on the parameter choices. The proposed link adaptation scheme achieved higher throughput than the legacy ACK-based link adaptation schemes. We also found that the proposed link adaptation scheme selected the optimal MCS on the developed WLAN system in both AWGN and fading conditions.