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Time-domain channel shortening and equalization of OFDM over doubly-selective channels

We discuss time-domain equalization of OFDM over doubly-selective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results in inter-block interference (IBI). IBI, in conjunction with the Doppler effect, destroys the orthogonality between subcarriers and, hence, results in intercarrier interference (ICI). The time-domain equalizer (TEQ) is assumed to be a time-varying finite impulse response (TV FIR). The purpose of the TEQ is to convert the doubly-selective channel into a purely frequency-selective channel whose delay spread fits within the CP. In other words, the purpose of the TEQ is to restore orthogonality between subcarriers in the OFDM system.