Doppler-resilient waveform and receiver design for high-mobility communications

Abstract

High-mobility communications face challenges due to doubly selective fading channels caused by both rapid movement and multipath effects. Although orthogonal frequency division multiplexing (OFDM) is widely adopted in communication standards, its performance degrades in time varying channels. While orthogonal time frequency space (OTFS) modulation provides robustness against Doppler spread, it suffers from high pilot overhead. To address these issues, a novel modulation technique called affine frequency division multiplexing (AFDM), based on the discrete affine Fourier transform, has emerged, achieving performance comparable to OTFS with reduced pilot overhead. Moreover, chirped discrete Fourier transform spread OFDM (DFT-s-OFDM) demonstrates enhanced Doppler resilience compared to conventional DFT-s-OFDM. In this dissertation, we investigate a number of modulation waveforms, such as OFDM, OTFS, AFDM, DFT-s-OFDM, and chirped DFT-s-OFDM, followed by the study of equalization and detection techniques, including maximum likelihood (ML), minimum mean square error (MMSE), and message passing (MP). Under the same spectral efficiency, chirped DFT-s-OFDM exhibits performance similar to AFDM and with lower peak-to-average power ratio (PAPR). In terms of equalizers, MP equalizer with lowest complexity generally outperforms MMSE equalizer in most scenarios and also achieves performance close to that of ML equalizer in some scenarios.