Variable Resolution Coarse-to-Fine Channel Estimation Method in Fractional Doppler Scenarios for ZP-OTFS

Orthogonal time frequency space (OTFS) modulation has emerged as a promising waveform for next-generation wireless communication due to its resilience against Doppler effects. However, accurate channel estimation in OTFS systems remains challenging, particularly in fractional Doppler scenarios where the inter-Doppler-interference (IDI) significantly degrades the estimation performance. To address this, we propose a novel variable resolution coarse-to-fine (VRCF) channel estimation method. Specifically, the proposed approach first performs the coarse estimation layer at a low resolution, followed by the fine estimation layer with progressively increasing resolution. In the coarse estimation layer, the primary Doppler components are first identified by the integer estimation module and then fine-tuned by the fractional estimation module, effectively reducing the computational complexity while maintaining the estimation accuracy. In the fine estimation layer, the interference cancellation process is integrated into the fixed and variable resolution modules. The former primarily mitigates the severe IDI effects, while the latter further suppresses the residual IDI, thereby improving channel estimation accuracy. Simulation results indicate that the proposed method achieves equivalent performance to existing algorithms at lower pilot power, with only a slight increase in complexity.