[4] and its OFDM counter- part, the demodulator structure in Figure 6.3 is 6.5 and 2.75 times simpler, respectively. To further emphasize, we note that in a practical system, the number of subcarriers M is much larger than the number of time symbols N. Hence, lower computational complexity of OTFS 6 Orthogonal Time-Frequency Space Modulation: Principles and Implementation modem in Section 6.5 compared with that of OFDM relies on the fact that OTFS requires N-point IFFT/FFT operations while OFDM demands M-point IFFT/FFT operations. Recent Results and Potential Research Directions The emergence of the diverse set of services and applications in the forthcoming 5G and beyond 5G mobile networks such as enhanced mobile broadband (eMBB), massive machine-type communications (mMTC), ultra-reliable low-latency communications (URLLC), high-speed and/or high frequency applications calls for development of a flexible air interface in different levels and the associated modulation scheme [9]. This air interface technology has to overcome the fading effects of frequency- and/or time-selective channels in many diverse scenarios such as high or low Doppler, delay spread, and carrier frequency. To deal with these scenarios, different sets of numerologies for subcarrier spacing/symbol duration of OFDM are designated for the air interface of 5G New Radio (5G NR). However, such an approach to tackle the time variations of wireless channels in high Doppler scenarios or high frequency wireless systems requires advanced channel tracking and estimation often with large training and signal overheads. For instance, to deal with the channel delay spread when the symbol duration is shortened, a CP length of 25% instead of 7% of the symbol duration, as a guard interval, is introduced in the recent 3GPP 5G NR standard [10]. This leads to bandwidth efficiency loss. To tackle the inherent wireless channel-fading problems, i.e. multipath and Doppler effects, this chapter focused on a new generation of modulation technologies, namely, OTFS modula