Low Overhead Channel Estimation in MIMO OTFS Wireless Communication Systems

Orthogonal Time Frequency Space (OTFS) modulation has recently garnered attention due to its robustness in high-mobility wireless communication environments. In OTFS, the data symbols are mapped to the Doppler-Delay (DD) domain. In this paper, we address bandwidth-efficient estimation of channel state information (CSI) for MIMO OTFS systems. Existing channel estimation techniques either require non-overlapped DD-domain pilots and associated guard regions across multiple antennas, sacrificing significant communication rate as the number of transmit antennas increases, or sophisticated algorithms to handle overlapped pilots, escalating the cost and complexity of receivers. We introduce a novel pilot-aided channel estimation method that enjoys low overhead while achieving high performance. Our approach embeds pilots within each OTFS burst in the Time-Frequency (TF) domain. We propose a novel use of TF and DD guard bins, aiming to preserve waveform orthogonality on the pilot bins and DD data integrity, respectively. The receiver first obtains low-complexity coarse estimates of the channel parameters. Leveraging the orthogonality, a virtual array (VA) is constructed. This enables the formulation of a sparse signal recovery (SSR) problem, in which the coarse estimates are used to build a low-dimensional dictionary matrix. The SSR solution yields high-resolution estimates of channel parameters. Simulation results show that the proposed approach achieves good performance with only a small number of pilots and guard bins. Furthermore, the required overhead is independent of the number of transmit antennas, ensuring good scalability of the proposed method for large MIMO arrays. The proposed approach considers practical rectangular transmit pulse-shaping and receiver matched filtering, and also accounts for fractional Doppler effects.