On Orthogonal Time Frequency Space Modulation for Wireless Communications
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. We present an analysis of the peak-to-average-power ratio (PAPR) performance of OTFS modulation. We derive an upper bound on the maximum PAPR in OTFS and analytically characterize the complementary cumulative distribution function of the PAPR of OTFS. Design of low-complexity equalizers is an important requirement for communication in fading channels. We propose low-complexity linear equalizers for OTFS signal detection in doublydispersive channels in both SISO and MIMO settings. The proposed equalizers exploit the block circulant nature of the OTFS channel matrix and achieve exact solutions at a significantly lower complexity compared to that of the conventional approach. We finally consider OTFS based multiple access (OTFS-MA), where delay-Doppler bins serve as the resource blocks for multiple access, in contrast to conventional multiple access schemes where resource blocks are defined in the TF plane. We carry out a comprehensive investigation of key issues in OTFS-MA, such as signal detection , channel estimation , and PAPR characteristics on the multiuser uplink, and compare them with those of OFDMA and SC-FDMA. Finally, we address the problem of high oscillator phase noise in millimeterwave communication systems. We investigate the effect of phase noise on the performance of OTFS modulation in mmWave communications and show that the OTFS is robust to oscillator phase noise.