On Orthogonal Time Frequency Space Modulation for Wireless Communications

Abstract

Future wireless communication systems are envisioned to support diverse requirements that include high mobility application scenarios such as high-speed trains, and vehicle-to-vehicle and vehicle-toinfrastructure communications. The dynamic nature of wireless channels in such scenarios makes them doubly-dispersive in nature. Orthogonal time frequency space (OTFS) modulation is a recent two-dimensional (2D) modulation technique specially suited for doubly-dispersive wireless channels. A fundamental feature of OTFS modulation is that the information symbols in OTFS modulation are multiplexed in delay-Doppler domain rather than in time-frequency domain as done in conventional multicarrier modulation techniques. An advantage of signaling in the delay-Doppler domain is that a channel rapidly varying in time manifests as a slowly varying sparse channel when viewed in the delay-Doppler domain, which simplifies channel estimation in rapidly time varying wireless channels. In this thesis, we focus on various fundamental and key aspects of OTFS modulation, which include asymptotic diversity analysis, peak-to-average power ratio analysis, design of low-complexity equalizers, OTFS based multiple access systems, and the performance of OTFS in millimeter wave (28 GHz and 60 GHz) channels in the presence of oscillator phase noise. First, we provide a formal analysis of the asymptotic diversity order achieved by OTFS modulation in doubly-dispersive channels. Our analysis and simulations show that the asymptotic diversity order of OTFS modulation with maximum likelihood detection is one. We propose a phase rotation scheme for OTFS that achieves full diversity in the delay-Doppler domain. We extend the diversity analysis and the proposed phase rotation scheme to OTFS in multiple-input-multiple-output (MIMO) setting as well. We also propose the use of space-time coding to achieve full diversity in both spatial and delay-Doppler domains...