appropriate choice. Hence, through MMSE crite- rion, the transmit data stream d can be estimated as (6.20) It is worth noting that the is block circulant and thus can be block diagonalized using block DFT and IDFT matrices and efficiently inverted. Alterna- tively, soft equalization techniques such as turbo equalization that was used in [4] can be utilized for data detection. 6.5 Simplified Modem Structure The SFFT-1/SFFT and 2D windowing operations in OTFS modem make it more complex than a simple OFDM system to implement. To resolve this issue, this section presents derivations that lead to simple OTFS transmitter and receiver structures. It is worth noting that through these derivations, OTFS modem becomes even simpler than OFDM, when using similar parameters to 3GPP LTE systems. An in-depth computational complexity analysis of the modem structure that is developed in this section is presented in Section 6.6. The class of separable window functions that are channel independent is considered in this chapter. This provides control over the Doppler and delay spread of the LTV channel separately. To understand the effect of windowing, the discussion here starts from using 2D rectangular window functions at both transmitter and receiver sides. Usually, the channel length L is much smaller than the number of frequency domain subcarriers M, i.e. L << M. Hence, the 2D delay-Doppler channel impulse response is sparse along the delay dimension and has the length of L while being zero elsewhere in delay dimension. In contrast, the chan- nel is not very sparse in the Doppler dimension, especially, as mobility and thus, the channel variations increase. Windowing along the frequency bins with a channel-independent win- dow function other than the rectangular window translates into circular convolution in the delay dimension. This slightly increases the delay spread of the channel that is due to the filter transient response. Consequently, channel independent filtering in the delay dimension at both transmitter and/or receive