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

Abstract

Among the several emerging use case families in 5G, high-mobility 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-on-demand, ranging from very high mobility to low or no mobility, need to be supported. The currently used waveforms fail to perform well in high-mobility 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.