Editing Path loss (section)

==Prediction==
{{Unreferenced section|date=August 2017}}
Calculation of the path loss is usually called ''prediction''. Exact prediction is possible only for simpler cases, such as the above-mentioned ''free space'' propagation or the ''flat-earth model''. For practical cases the path loss is calculated using a variety of approximations.

''Statistical'' methods (also called ''stochastic'' or ''empirical'') are based on measured and averaged losses along typical classes of radio links. Among the most commonly used such methods are [[Hata Model for Urban Areas|Okumura–Hata]], the [[COST Hata model]], [[W.C.Y.Lee]], etc. These are also known as ''radio wave propagation models'' and are typically used in the design of [[cellular networks]] and [[public land mobile network]]s (PLMN). For wireless communications in the [[very high frequency]] (VHF) and [[ultra high frequency]] (UHF) frequency band (the bands used by walkie-talkies, police, taxis and cellular phones), one of the most commonly used methods is that of Okumura–Hata as refined by the [[Cost 231 Model|COST 231]] project. Other well-known models are those of Walfisch–Ikegami, [[W. C. Y. Lee]], and [[Erceg]]. For FM radio and TV broadcasting the path loss is most commonly predicted using the [[ITU]] model as described in [[P.1546]] (successor to [[P.370]]) recommendation.

Deterministic methods based on the physical laws of wave propagation are also used; [[ray tracing (physics)|ray tracing]] is one such method. These methods are expected to produce more accurate and reliable predictions of the path loss than the empirical methods; however, they are significantly more expensive in computational effort and depend on the detailed and accurate description of all objects in the propagation space, such as buildings, roofs, windows, doors, and walls. For these reasons they are used predominantly for short propagation paths. Among the most commonly used methods in the design of radio equipment such as antennas and feeds is the [[finite-difference time-domain method]].

The path loss in other frequency bands ([[medium wave]] (MW), [[shortwave]] (SW or HF), [[microwave]] (SHF)) is predicted with similar methods, though the concrete algorithms and formulas may be very different from those for VHF/UHF. Reliable prediction of the path loss in the SW/HF band is particularly difficult, and its accuracy is comparable to weather predictions.{{Citation needed|date=August 2007}}

Easy approximations for calculating the path loss over distances significantly shorter than the distance to the [[radio horizon]]:

* In free space the path loss increases with 20&nbsp;dB per ''decade'' (one decade is when the distance between the transmitter and the receiver increases ten times) or 6&nbsp;dB per ''octave'' (one octave is when the distance between the transmitter and the receiver doubles). This can be used as a very rough first-order approximation for (microwave) communication links;
* For signals in the UHF/VHF band propagating over the surface of the Earth the path loss increases with roughly 35–40&nbsp;dB per decade (10–12&nbsp;dB per octave). This can be used in cellular networks as a first guess.