What factors affect the propagation delay at wireless communication?
Even if transmitter and receiver are fixed, you have unknown reflecting objects, resulting in unknown multi-path propagation. The resulting propagation delay can be modeled as a random variable. If there are moving objects in the environment (such as people moving in a room), then the propagation is not only unknown but also time-varying. The delay spread of a channel describes the difference between the maximum and the minimum propagation delay in the given environment.