Orthogonal Time–Frequency Space Modulation: Principles and Implementation

Abstract

Time‐varying (TV) channels as well as signaling overheads are among the main issues for emerging applications such as vehicle‐to‐vehicle (V2V), vehicle‐to‐infrastructure (V2I), vehicle‐to‐cloud (V2C), and vehicle‐to‐everything (V2X) communications in future networks. As an air interface, orthogonal frequency division multiplexing (OFDM) modulation has been enjoying its dominance in many applications and standards during the last couple of decades. However, while it achieves a performance near the capacity limits in linear time‐invariant channels, it leads to a poor performance in doubly dispersive channels. Orthogonal time–frequency space (OTFS) modulation is a two‐dimensional signaling technique that has recently emerged in the literature to tackle TV wireless channels. OTFS deploys the delay‐Doppler plane to multiplex transmit data where the time variations of the TV channel are integrated and hence the equivalent channel relating the input and output of the system boils down to a time‐invariant one. This signaling technique can be implemented on the top of a given multicarrier waveform with the addition of precoding and post‐processing units to the modulator and demodulator, respectively. This chapter covers the basics of OTFS and presents its discrete‐time formulation. An in‐depth analysis of the channel impact on transmit data symbols is also provided. This brings deeper insights into OTFS. The derived formulation in this chapter reveals that the precoding and post‐processing units can be combined with OFDM modulator and demodulator, respectively, which leads to a simplified modem structure. Through computational complexity analysis, this chapter shows that in realistic scenarios, OTFS modem becomes simpler than OFDM to implement.