Orthogonal time frequency space (OTFS) has been widely acknowledged as a promising wireless technology for challenging transmission scenarios, including high-mobility channels. In this paper, we investigate the pilot design for the multi-user OTFS system based on the a priori statistical channel state information (CSI), where the practical threshold-based estimation scheme is adopted. Specifically, we first derive the a posteriori Cramer-Rao bound (PCRB) based on a priori channel information for each user. According to our derivation, the PCRB only relates to the user's pilot signal-to-noise ratio (SNR) and the range of delay and Doppler shifts under the practical power-delay and power-Doppler profiles. Then, a pilot scheme is proposed to minimize the average PCRB of different users, where a closed-form global optimal pilot power allocation is derived. Our numerical results verify the multi-user PCRB analysis. Also, we demonstrate an around 3 dB improvement in the average normalized-mean-square error (NMSE) by using the proposed pilot design in comparison to the conventional embedded pilot design under the same total pilot power.

翻译：正交时频空间（OTFS）技术已被广泛认为是在高移动性信道等具有挑战性的传输场景中极具前景的无线技术。本文基于先验统计信道状态信息（CSI），研究了多用户OTFS系统的导频设计问题，并采用实用的阈值估计方案。具体而言，我们首先基于每个用户的先验信道信息推导其后验克拉美罗界（PCRB）。根据推导结果，PCRB仅与用户的导频信噪比（SNR）以及实际功率时延和功率多普勒谱下的时延与多普勒频移范围相关。随后，我们提出一种导频方案以最小化不同用户的平均PCRB，并推导出全局最优的闭式导频功率分配方案。数值结果验证了多用户PCRB分析的正确性。同时，在相同总导频功率条件下，与传统嵌入式导频设计相比，采用所提导频设计可使平均归一化均方误差（NMSE）提升约3 dB。