说明：  large. The second technique is the time–frequency domain equalization with channel shortening [29]. A time-domain equalizer is inserted to reduce the MIMO channels to the ones with the channel length shorter than or equal to the CP length, and then, a one-tap frequency-domain equalizer is applied to each subcarrier. When the MIMO channels are shortened by the time-domain equalizer, residual ICI and ISI are introduced. They cannot be eliminated by the subsequent frequency-domain equalizer and, thus, limit the performance.