Performance comparison of OTFS vs OFDM wireless signals in high-mobility environments

Abstract

This thesis investigates the suitability of Orthogonal Time Frequency Space (OTFS) as a waveform candidate for 5G-Advanced and beyond, by benchmarking it against conventional Orthogonal Frequency Division Multiplexing (OFDM) under high-mobility doubly selective channels. While OFDM remains the baseline in current cellular systems, its sensitivity to Doppler-induced inter-carrier interference motivates the study of alternatives that are inherently more robust in time-varying propagation conditions. A MATLAB-based simulation framework is developed to ensure a fair, like-for-like comparison between both schemes. Multiple mobility scenarios are considered (from 0 up to 540 km/h) at a 3.5 GHz carrier, using the same time-varying multipath channel realization per frame for both links. OFDM is implemented with comb pilots, common phase error compensation and a one-tap MMSE equalizer in the time-frequency domain, whereas OTFS employs a pilot-only preamble for delay-Doppler channel estimation and a 2D MMSE equalizer. The study evaluates uncoded and coded performance using two forward error correction options-NR-LDPC (5G Toolbox) and Turbo codes (Communications Toolbox)-and reports pre-decoding BER, transport-block BER and BLER. In addition, effective spectral efficiency is computed by accounting for coding rate, overhead (cyclic prefix, pilots/preamble) and residual block errors, enabling a joint reliability-throughput assessment. Results show that at low to moderate mobility (up to approximately 120 km/h) OFDM and OTFS exhibit similar reliability, with only minor differences at high SNR. As mobility increases, the performance gap widens and OTFS becomes increasingly advantageous, particularly in high-speed and/or high-SNR regimes where OFDM degradation due to Doppler effects is more pronounced. Building on this observation, the thesis also explores a dynamic waveform selection approach, where the transmitter chooses OFDM or OTFS based on measured channel conditions (e.g., SNR and mobility proxies such as Doppler spread or coherence time) to maximize effective throughput under a BLER constraint. Overall, the study supports OTFS as a compelling option for high-mobility scenarios, while confirming OFDM as a competitive solution at conventional speeds. Future work includes extending the framework to higher-order modulations, improved channel estimation and receiver architectures, and a more systematic optimization of switching thresholds for adaptive OFDM/OTFS operation.

4 - Educació de Qualitat