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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). Free-space line-of-sight channels are ideal scenarios that can typically be used to model aerial or space communication system applications.

Electromagnetic waves propagate in the form of spherical waves with a functional dependence of e^j(&omega;^t-k₀R)/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 &lambda;₀ 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/R², 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_R) to the transmitted signal power (P_T). 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 Friis’ formula: :<math> \frac{P_R}{P_T} = \left( \frac{\lambda_0}{4\pi R} \right)^2 </math><!--[[FileImage:friis1multi1_tn.png]]--> The above formula assumes that |thumb|400px|A multipath propagation scene showing all the receiving antenna is polarization-matched. Normally, there is rays arriving at a polarization mismatch between the transmitting and receiving antennasparticular receiver. In the case of directional transmitting and receiving antennas, Friis’ formula takes the following form: :<math> \frac{P_R}{P_T} = G_T G_R \left( \frac{\lambda_0}{4\pi R} \right)^2 ( \mathbf{ \hat{u}_T \cdot \hat{u}_R } )</math><!--[[File:friis2.png]]--> where '''u_T''' and '''u_R''' are the unit polarization vectors of the transmitting and receiving antennas, and G_T and G_R are their gains, respectively. [[File:los.png]] Figure: A Line-of-Sight (LOS) Propagation Scenario. === Multipath Propagation Channel === Free-space line-of-sight communications is an ideal scenario that is typically used to model aerial or space applications. In ground-based systems, the presence of the ground as a very large reflecting surface affects the signal propagation to a large extent. Along the path from a transmitter to a receiver, the signal may also encounter many obstacles and scatterers such as buildings, vegetation, etc. In an urban canyon environment with many buildings of different heights and other scatterers, a line of sight between the transmitter and receiver can hardly be established. In such cases, the propagating signals bounce back and forth among the building surfaces. It is these reflected or diffracted signals that are often received and detected by the receiver. Such environments are referred to as “multipath”. The group of rays arriving at a specific receiver location experience different attenuations and different time delays. This gives rise to constructive and destructive interference patterns that cause fast fading. As a receiver moves locally, the receiver power level fluctuates sizably due to these fading effects.