前言：和前面的类似，这部分依旧比较偏概念性的描述而缺少实际的公式和例子说明，是对延迟-多普勒信道符号耦合特性的描述

1.Definition定义

The wireless channel is governed by simple physics. It is composed of a collection of specular reflectors, some of which are static and some of which are moving. The transmitted waveform propagates through the medium and bounces off each reflector. The signal that arrives at the receiver is a superposition of the direct signal and the reflected echoes. Each of the reflected echoes arrives at the receiver at a delayed time (multipath effect) and possibly also shifted in frequency (Doppler effect) due to the relative velocity between the reflector and the transmitter/receiver. The channel physics is mathematically modeled through the delay-Doppler impulse response where each tap represents a cluster of reflectors with specific delay and Doppler characteristics, as shown in Figure 4. Our goal in this section is to describe the channel-symbol coupling (CSC for short) between the wireless channel and the OTFS carrier waveform given by a localized pulse in the delay-Doppler representation. As a motivation, we first discuss the channelsymbol coupling of the TDM and FDM pulses.

这个定义揭示的是信道和传输符号耦合的实质。即符号经过由多径产生的时延以及多普勒产生的频移的各个符号的叠加。

2.Example例子

2.1Channel symbol coupling of the TDM pulses

Transmitting a localized TDM pulse in the time representation gives rise at the receiver to a configuration of echoes which appear at specific time displacements which corresponds to the multipath delays imposed by the various reflectors. The phase and amplitude of
each echo depend on the initial position of the transmitted pulse and might change significantly among different coherence time intervals – a phenomenon referred to as time selectivity. There are two mechanisms involved. The phase of the echo changes due to the Doppler effect and the amplitude of the echo changes due to destructive superposition of numerous reflectors sharing the same delay but differing in Doppler, resulting from the inability of the TDM pulse to separate reflectors along Doppler. In Figure 9, counting the TDM echoes from left to right we see that the first and third echoes are due to static reflectors hence are time invariant, the fourth echo is due to moving reflector thus is time varying and the second echo is due to superposition of two reflectors, one of which is moving thus is fading.

解释一下这段话的含义：

• 每个回波的相位和振幅取决于发射脉冲的初始位置，并可能在不同的相干时间间隔之间发生显著变化，这种现象称为时间选择性。
• 回波的相位由于多普勒效应而改变，而回波的振幅由于具有相同延迟但不同多普勒的多个反射器的破坏性叠加而改变，这是由于TDM脉冲无法沿多普勒分离反射器造成的
• 说明一下例子：
  从时间维度来看
  • 一和三是静态的反射器且只有一个，所以是时不变的
  • 四因为车在移动所以是时变的
  • 二是两个的叠加，一个动一个不动，所以是衰落

2.2Channel symbol coupling of the TDM pulses

2.3Channel symbol coupling of the OTFS pulses