Page "Multipath propagation" Paragraph 14

At the receiver, due to the presence of the multiple electromagnetic paths, more than one pulse will be received ( we suppose here that the channel has infinite bandwidth, thus the pulse shape is not modified at all ), and each one of them will arrive at different times.

In fact, since the electromagnetic signals travel at the speed of light, and since every path has a geometrical length possibly different from that of the other ones, there are different air travelling times ( consider that, in free space, the light takes 3 μs to cross a 1 km span ).

Thus, the received signal will be expressed by