In this paper, we consider the problem of estimating the delay-Doppler (DD) domain input-output (I/O) relation in Zak-OTFS modulation, which is needed for signal detection. Two approaches, namely, model-dependent and model-free approaches, can be employed for this purpose. The model-dependent approach requires explicit estimation of the physical channel parameters (path delays, Dopplers, and gains) to obtain the I/O relation. Such an explicit estimation is not required in the model-free approach, where the I/O relation can be estimated by reading off the samples in the fundamental DD period of the received pilot frame. Model-free approach has the advantage of acquiring fractional DD channels with simplicity. However, the read-off in the model-free approach provides an estimate of the effective channel only over a limited region in the DD plane but it does not provide an estimate for the region outside, and this can affect the estimation performance depending on the pulse shaping characteristics of the DD pulse shaping filter used. A poorly localized DD pulse shape leads to an increased degradation in performance. Motivated by this, in this paper, we propose a novel, yet simple, I/O relation estimation scheme that alleviates the above issue in the model-free approach. We achieve this by obtaining a coarse estimate of the effective channel outside the model-free estimation region using a novel model-dependent scheme and using this estimate along with the model-free estimate to obtain an improved estimate of the overall I/O relation. We devise the proposed estimation scheme for both exclusive and embedded pilot frames. Our simulation results using Vehicular-A, TDL-A and TDL-C channel models with fractional DDs show that the proposed hybrid estimation approach achieves superior performance compared to the pure model-free approach.

翻译：本文研究Zak-OTFS调制中时延-多普勒域输入输出关系的估计问题，该关系为信号检测所必需。为此可采用两种方法：模型依赖方法与模型无关方法。模型依赖方法需通过显式估计物理信道参数（路径时延、多普勒频移与增益）来获取输入输出关系；而模型无关方法则无需此类显式估计，仅需通过读取接收导频帧在基本时延-多普勒周期内的采样值即可估计输入输出关系。模型无关方法的优势在于能够以简洁方式获取分数时延-多普勒信道。然而，该方法通过读取采样仅能估计时延-多普勒平面有限区域内的有效信道，无法估计区域外信道，其估计性能受所采用时延-多普勒脉冲成形滤波器的脉冲成形特性影响。当时延-多普勒脉冲波形局部化特性较差时，会导致性能显著下降。受此启发，本文提出一种新颖而简洁的输入输出关系估计方案，以缓解模型无关方法的上述缺陷。该方案通过创新的模型依赖方法获取模型无关估计区域外有效信道的粗估计，并将此估计与模型无关估计相结合，从而获得整体输入输出关系的改进估计。我们针对独立导频帧与嵌入式导频帧分别设计了所提估计方案。基于Vehicular-A、TDL-A与TDL-C信道模型（含分数时延-多普勒）的仿真结果表明，所提出的混合估计方法相较于纯模型无关方法具有更优越的性能。