Comparison of OTFS and OFDM in Ray Launched sub-6 GHz and mmWave Line-of-Sight Mobility Channels

Orthogonal Time Frequency Space (OTFS) is a recently proposed modulation scheme for doubly-dispersive channels in which symbol multiplexing and processing is performed in the Doppler-delay domain, rather than conventional time-frequency domain. In this paper, the performance of OTFS is compared to orthogonal frequency division multiplexing (OFDM) for line-of-sight mobility automotive channels. Ray launching is used to simulate the channel for two different dynamic 3D vehicle to infrastructure transmission environments, using a Kirchhoff model for diffuse scattering from rough surfaces. Bit level simulations for transmission from a transmitter moving at speeds of 13 m/s and 31 m/s are then carried out, for both OFDM and OTFS. We find that with short length block codes OTFS outperforms OFDM in all simulated scenarios, reducing the block error rate by more than 50% on average. Unlike previous work, simulations are performed in the time domain using practical rectangular pulse shapes, rather than theoretical `ideal pulses'. We provide an analysis of these pulses, and derive relevant expressions for the doubly dispersive channel in terms of the multipath delays and Doppler shifts.