Fractional Delay and Fractional Doppler Estimation and Mitigation Framework in OTFS Systems

Orthogonal time-frequency space (OTFS) modulation is fast becoming a popular modulation scheme for high-mobility wireless communication due to its ability to render time-varying channels invariant and sparse. However, due to the limited time duration and frequency bandwidth of the transmitted OTFS signal, the occurrence of fractional delay and Doppler shifts becomes inevitable, causing both inter-delay interference (IDeI) and inter-Doppler interference (IDI). In this paper, an impulse-based channel estimation method is used, which takes advantage of the fact that the pilot impulse is a pulse tone (pulsone). A pulsone maintains its shape despite time delay and Doppler shift operations, as well as being self-dual between time and frequency domains. Exploiting these two features, a low-complexity algorithm is proposed to estimate the fractional delay and fractional Doppler channel. We derive the Cramer-Rao lower bound (CRLB) of the estimated fractional channel and show that the developed estimator is efficient. Then, we provide the computational complexity analysis of the proposed algorithm. After that, we show and stress that the proposed channel estimation scheme is better in terms of the channel normalized mean squared error (NMSE) and the bit-error rate (BER). Lastly, the simulation results are confirmed via real experimental results.