Time-Domain Channel Shortening and Equalization of OFDM over Doubly-Selective Channels

In this paper, we discuss time-domain equalization of OFDM over doublyselective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results into interblock interference (IBI). IBI in conjunction with the Doppler e#ect destroys the orthogonality between subcarriers and hence, results in intercarrier interference (ICI). The time-domain equalizer (TEQ) is assumed to be a timevarying 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