Spectral-Efficient Reference Signal Design and Iterative Channel Estimation for OTFS Modulation

The orthogonal time frequency space (OTFS) modulation system has demonstrated superior performance compared to conventional orthogonal frequency division multiplexing systems, especially in time-frequency doubly dispersive channels. However, achieving robust OTFS demodulation requires a well-designed reference signal and channel estimation algorithm. In this paper, we propose an iterative Doppler shift estimation algorithm based on the weighted least squares method for the OTFS system with fractional Doppler shifts. This algorithm fully exploits the block sparsity of the discrete channel impulse response in the delay-Doppler (DD) domain, achieving performance close to the Cramer-Rao lower bound. Additionally, we present a reference signal design in the DD domain that achieves higher spectral efficiency compared to conventional designs. Building upon an analysis of interference between data symbols and pilot symbols, we introduce an iterative interference suppression algorithm to further enhance the channel parameter estimation. Through theoretical analysis and numerical simulations, we validate the convergence of the algorithms and conduct simulations on the normalized mean squared error of channel estimation and the bit error ratio of demodulation, affirming its performance advantages.