Orthogonal Time Frequency Space (OTFS) is a framework for communications and active sensing that processes signals in the delay-Doppler (DD) domain. It is informed by 6G propagation environments, where Doppler spreads measured in kHz make it more and more difficult to estimate channels, and the standard model-dependent approach to wireless communication is starting to break down. We consider Zak-OTFS where inverse Zak transform converts information symbols mounted on DD domain pulses to the time domain for transmission. Zak-OTFS modulation is parameterized by a delay period $\tau_{p}$ and a Doppler period $\nu_{p}$, where the product $\tau_{p}\nu_{p}=1$. When the channel spread is less than the delay period, and the Doppler spread is less than the Doppler period, the Zak-OTFS input-output relation can be predicted from the response to a single pilot symbol. The highly reliable channel estimates concentrate around the pilot location, and we configure low-density parity-check (LDPC) codes that take advantage of this prior information about reliability. It is advantageous to allocate information symbols to more reliable bins in the DD domain. We report simulation results for a Veh-A channel model where it is not possible to resolve all the paths, showing that LDPC coding extends the range of Doppler spreads for which reliable model-free communication is possible. We show that LDPC coding reduces sensitivity to the choice of transmit filter, making bandwidth expansion less necessary. Finally, we compare BER performance of Zak-OTFS to that of a multicarrier approximation (MC-OTFS), showing LDPC coding amplifies the gains previously reported for uncoded transmission.

翻译：正交时频空（OTFS）是一种在延迟-多普勒（DD）域处理信号的通信与主动感知架构。该架构源于6G传播环境——当多普勒扩展达到千赫兹量级时，信道估计变得愈发困难，传统依赖模型的无线通信方式开始失效。我们研究Zak-OTFS方法，该方法通过逆Zak变换将加载于DD域脉冲的调制符号转换至时域进行传输。Zak-OTFS调制由延迟周期$\tau_{p}$和多普勒周期$\nu_{p}$参数化，且满足乘积条件$\tau_{p}\nu_{p}=1$。当信道扩展小于延迟周期且多普勒扩展小于多普勒周期时，Zak-OTFS的输入输出关系可通过单导频符号的响应进行预测。高度可靠的信道估计集中在导频位置附近，据此我们设计了利用该先验可靠性信息的低密度奇偶校验（LDPC）码。将信息符号分配至DD域中可靠性更高的资源格具有明显优势。针对无法解析全部路径的Veh-A信道模型，仿真结果表明：LDPC编码扩展了可靠无模型通信可支持的多普勒扩展范围。研究表明，LDPC编码可降低对发射滤波器选择的敏感性，从而减少带宽扩展需求。最后，通过对比Zak-OTFS与多载波近似方案（MC-OTFS）的误码率性能，证实LDPC编码能放大此前非编码传输研究中已报道的性能增益。