OTFS: A New Modulation Scheme for High‑Mobility Use Cases

Abstract

Among the several emerging use case families in 5G, highmobility
use case family is a technologically challenging one. It is
expected that there will be a growing demand for mobile services in
vehicles, high-speed trains, and even aircraft. The degree of mobility
support required (i.e., speed) will depend upon the specific use case
(e.g., 500 km/h in bullet trains and 1000 km/h in airplanes). Mobility-ondemand, ranging from very high mobility to low or no mobility, need to
be supported. The currently used waveforms fail to perform well in highmobility scenarios where the Doppler shifts witnessed are quite high
(e.g., several kHz of Doppler). Orthogonal time–frequency space (OTFS)
is a recently proposed radio access technology waveform suited very
well for high-mobility environments. It is a two-dimensional modulation
scheme in which information symbols are multiplexed in the delay–Doppler
domain. We present an overview of delay–Doppler representation
of wireless channels and introduce OTFS modulation along with OTFS
basis functions. We illustrate the slow variability and sparse nature of the
delay–Doppler channel using an urban multi-lane scenario. Focusing on
MIMO-OTFS systems, we present signal detection and channel estimation
schemes and their performance. MIMO-OTFS is shown to achieve
significantly better performance compared to MIMO-OFDM in high-Doppler
environments operating in 4 GHz and 28 GHz frequency bands.