In this paper, we rethink delay Doppler channels (also called doubly selective channels). We prove that no modulation schemes (including the current active VOFDM/OTFS) can compensate a non-trivial Doppler spread well. We then discuss some of the existing methods to deal with time-varying channels, in particular time-frequency (TF) coding in an OFDM system. TF coding is equivalent to space-time coding in the math part. We also summarize state of the art on space-time coding that was an active research topic over a decade ago.

翻译：本文重新审视时延多普勒信道（亦称双选择性信道）。我们证明任何调制方案（包括当前活跃的VOFDM/OTFS）均无法有效补偿非平凡的多普勒扩展。随后，我们讨论了处理时变信道的若干现有方法，特别聚焦于OFDM系统中的时间-频率编码。在数学层面，时间-频率编码等价于空时编码。本文同时综述了约十年前曾为研究热点的空时编码技术的最新进展。