The different rays arriving at a receiver location create constructive and destructive interference patterns. This is known as the multipath effect. This together with the shadowing effects caused by building obstructions lead to channel fading. In many wireless applications, the total received power by the receiver is all that matters. In some others, the angle of arrival of the rays as well as their polarization are of immense interest. A fully polarimetric, coherent ray tracer like [[EM.Cube]]'s Shooting-and-Bouncing-Rays (SBR) solver lets you compute and resolve all the rays received by a receiver including their power levels, time delays and angles of arrival.
=== Free Space Line-of-Sight vs. Multipath Propagation Channel ===
In a free-space line-of-sight (LOS) communication system, the signal propagates directly from the transmitter to the receiver without encountering any obstacles (scatterers).  Click here to learn more about the theory of a [[Free-Space Propagation Channel]]. Electromagnetic waves propagate in the form of spherical waves with a functional dependence of e<sup>j(ω</sup><sup>t-k<sub>0</sub>R)</sup>/R, where R is the distance between the transmitter and receiver, <math>\omega = 2\pi f</math>, f is the signal frequency, <math>k_0 = \frac{\omega}{c} = \frac{2\pi}{\lambda}</math>, c is the speed of light, and λ<sub>0</sub> is the free-space wavelength at the operational frequency. By the time the signal arrives at the location of the receiver, it undergoes two changes. It is attenuated and its power drops by a factor of 1/R<sup>2</sup>, and additionally, it experiences a phase shift of <math>\frac{2\pi R}{\lambda_0}</math>, which is equivalent to a time delay of R/c. The signal attenuation from the transmitter to the receiver is usually quantified by '''Path Loss''' defined as the ratio of the received signal power (P<sub>R</sub>) to the transmitted signal power (P<sub>T</sub>). Assuming isotropic transmitting and receiving radiators (i.e. radiating uniformly in all directions), the Path Loss in a free-space line-of-sight communication system is given by Friisâ formula:
:<math> \frac{P_R}{P_T} = \left( \frac{\lambda_0}{4\pi R} \right)^2 </math>