In view of the high computational complexity of traditional linear equalization algorithms in Orthogonal Time Frequency Space (OTFS) systems, a minimum mean square error (MMSE) channel equalization algorithm based on Matrix Chunking Lower and Upper Triangular Decomposition (CLU) is proposed. The proposed algorithm derives the structural properties of the chunked MMSE equalization matrix by leveraging the block diagonal structure of the Cyclic Prefix OTFS (CP-OTFS) time-domain channel matrix and the quasi-band structure of its constituent block matrices. On this basis, triangular decomposition combined with forward and backward substitution is used to avoid matrix inversion. This approach significantly reduces the complexity of the MMSE algorithm without sacrificing its performance.

Orthogonal time frequency space (OTFS) can resist the Doppler effect and guarantee reliable communication in high-speed scenarios. However, the Doppler rate induced by the relative acceleration between the transmitter and receiver degrades the performance of the OTFS. So far, the impact of the Doppler rate on OTFS systems has not been addressed. In this paper, we first introduce the Doppler rate in the OTFS system and derive the delay-Doppler domain input-output relation. In addition, the impact of the Doppler rate on the effective delay-Doppler domain channel is characterized by utilizing the first mean value theorem for definite integrals to avoid complicated integrals. To mitigate the effect of the Doppler rate, a large-scale antenna array is arranged at the receiver to separate each path of the multi-path channel through a high-resolution spatial matched filter beamformer. Next, the Doppler rate estimation scheme for an arbitrary order Doppler rate is proposed based on the successive interference cancellation pattern and the maximization of the spectrum of the ratio of high-order moments between the received samples in the identified branch and the transmitted samples. Finally, the estimation accuracy of the Doppler rate and the error performance of the proposed transceiver are validated by the numerical results.