Performance Analysis of OTFS versus OFDM Waveforms for High-Mobility 6G Velocity Sensing

High-mobility 6G sensing places where strict requirements on waveform robustness are required because the large Doppler shifts can degrade velocity estimation. This paper presents a refined simulation-based comparison of Orthogonal Time Frequency Space (OTFS) and Orthogonal Frequency Division Multiplexing (OFDM) for monostatic velocity sensing at 28 GHz. The evaluation uses a three-way receiver design: the first is OTFS with a single delay–Doppler pilot. The second is a strong full-grid OFDM radar baseline with known time–frequency symbols and range–Doppler matched processing. Third is a communication-oriented sparse-pilot OFDM receiver that reflects practical joint communication and sensing (JCAS) pilot budgets. All branches share identical carrier frequency, subcarrier spacing, grid size, cyclic-prefix length, power normalization, and Monte Carlo settings (250 trials per point). Velocity accuracy is reported through root-mean-square error (RMSE), success rate within ±10 km/h, multipath ablations, a velocity sweep, and quantitative response-concentration metrics. The results show that OTFS with one delay–Doppler pilot achieves near-constant RMSE of about 0.43 m/s and 100% success rate across −10 dB to 25 dB SNR at 120 km/h. A full-grid OFDM radar baseline attains essentially the same accuracy, demonstrating that OFDM is not intrinsically incapable of high-mobility velocity estimation when dense known pilots are available. So sparse-pilot OFDM collapses under the same Doppler conditions (RMSE ≈ 79 m/s and zero success rate) while OTFS preserves compact energy concentration in the delay–Doppler plane. These findings justify OTFS for pilot-efficient high-mobility sensing and clarify that OFDM under-performance in high-mobility sensing can be primarily baseline- and overhead-dependent rather than an absolute waveform verdict.