The OTFS (Orthogonal Time Frequency Space) is widely acknowledged for its ability to combat Doppler spread in time-varying channels. In this paper, another advantage of OTFS over OFDM (Orthogonal Frequency Division Multiplexing) will be demonstrated: much reduced channel training overhead. Specifically, the sparsity of the channel in delay-Doppler (D-D) domain implies strong correlation of channel gains in time-frequency (T-F) domain, which can be harnessed to reduce channel training overhead through interpolation. An immediate question is how much training overhead is needed in doubly-dispersive channels? A conventional belief is that the overhead is only dependent on the product of delay and Doppler spreads, but we will show that it's also dependent on the T-F window size. The finite T-F window leads to infinite spreading in D-D domain, and aliasing will be inevitable after sampling in T-F domain. Two direct consequences of the aliasing are increased channel training overhead and interference. Another factor contributing to channel estimation error is the inter-symbol-carrier-interference (ISCI), resulting from the uncertainty principle. Both aliasing and ISCI are considered in channel modelling, a low-complexity algorithm is proposed for channel estimation and interpolation through FFT. A large T-F window is necessary for reduced channel training overhead and aliasing, but increases processing delay. Fortunately, we show that the proposed algorithm can be implemented in a pipeline fashion. Further more, we showed that data-aided channel tracking is possible in D-D domain to further reduce the channel estimation frequency, i.e., channel extrapolation. The impacts of aliasing and ISCI on channel interpolation error are analyzed.
Characterizing ISCI in Multi-carrier ISAC Systems over Doubly Dispersive Channel: Joint Sensing and Communication Performance Analysis
The Hong Kong University of Science and Technology (Guangzhou)（香港科技大学（广州））；HKUST Fok Ying Tung Research Institute（香港科技大学霍英东研究院）；The Hong Kong University of Science and Technology（香港科技大学）
Doppler Shift Keying Modulation for Uplink Multiple Access over Doubly-Dispersive Channels
Tsinghua University（清华大学）；Osaka Metropolitan University（大阪公立大学）；University of New South Wales（新南威尔士大学）；Tsinghua Shenzhen International Graduate School（清华深圳国际研究生院）
Bistatic Integrated Sensing and Communications with Flexible Intelligent Metasurfaces
School of Computer Science and Engineering, Constructor University, Bremen, Germany（德国不来梅康斯坦茨大学计算机科学与工程学部）；School of Electrical and Electronics Engineering, Nanyang Technological University, Singapore（新加坡南洋理工大学电气与电子工程学院）
Joint Synchronization and Radar Parameter Estimation for OFDM-based DISAC Systems
School of Computer Science and Engineering, Constructor University（康斯坦茨大学计算机科学与工程学院）；Department of Electrical and Computer Engineering, University of California San Diego（加州大学圣地亚哥分校电气与计算机工程学院）
