这两句该咋理解？

The impact of multipath on the received signal depends on whether the spread of time delays associated with the LOS and different multipath components is large or small relative to the inverse signal bandwidth. If this channel delay spread is small then the LOS and all multipath components are typically nonresolvable, leading to the narrowband fading model described in the next section. If the delay spread is large then the LOS and all multipath components are typically resolvable into some number of discrete components, leading to the wideband
fading model of Section 3.3.
窄带衰落和宽带衰落怎么是这样定义的么？

不是窄(宽)带衰落而是窄(宽)带信号
上面不是说的很清楚么,取决与信道的时延扩展厉害程度
而视信号是narrowband or wideband