In low latency applications and in general, for overspread channels, channel delay spread is a large percentage of the transmission frame duration. In this paper, we consider OTFS in an overspread channel exhibiting a delay spread that exceeds the block duration in a frame, where traditional channel estimation (CE) fails. We propose a two-stage CE method based on a delay-Doppler (DD) training frame, consisting of a dual chirp converted from time domain and a higher power pilot. The first stage employs a DD domain embedded pilot CE to estimate the aliased delays (due to modulo operation) and Doppler shifts, followed by identifying all the underspread paths not coinciding with any overspread path. The second stage utilizes time domain dual chirp correlation to estimate the actual delays and Doppler shifts of the remaining paths. This stage also resolves ambiguity in estimating delays and Doppler shifts for paths sharing same aliased delay. Furthermore, we present a modified low-complexity maximum ratio combining (MRC) detection algorithm for OTFS in overspread channels. Finally, we evaluate performance of OTFS using the proposed CE and the modified MRC detection in terms of normalized mean square error (NMSE) and bit error rate (BER).

翻译：在低延迟应用及一般过扩展信道场景中，信道时延扩展占传输帧持续时间的很大比例。本文研究过扩展信道中时延扩展超过帧内块持续时间的OTFS系统，此类场景下传统信道估计方法失效。我们提出一种基于时延-多普勒训练帧的两阶段信道估计方法，该训练帧由时域双啁啾转换信号与高功率导频构成。第一阶段采用时延-多普勒域嵌入式导频信道估计捕获由取模运算引起的混叠时延与多普勒频移，进而识别所有不与任何过扩展路径重叠的欠扩展路径。第二阶段利用时域双啁啾相关技术估计剩余路径的实际时延与多普勒频移，该阶段同时解决了共享相同混叠时延路径的时延-多普勒估计模糊问题。此外，我们提出改进的低复杂度最大比合并检测算法适用于过扩展信道中的OTFS系统。最后，通过归一化均方误差与误比特率指标评估了采用所提信道估计方法与改进最大比合并检测的OTFS系统性能。