Path loss

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Path loss is the attenuation undergone by an electromagnetic wave in transit from a transmitter to a receiver in a telecommunication system.

This term is commonly used in wireless communications and signal propagation. Path loss may be due to many effects such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption. Path loss is affected by factors such as terrain contours, different environments (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, the height and location of their antennas, etc.

In the study of wireless communications, path loss can be represented by the path loss exponent, whose value is normally in the range of 2 to 4 (where 2 is for propagation in free space, 4 is for relatively lossy environments and for the case of full specular reflection from the earth surface -- the so-called flat-earth model). In some environments, such as buildings, stadiums and other indoor environments, the path loss exponent can reach values in the range of 4 to 6. On the other hand in tunnels a waveguide type of propagation may occur with the path loss exponent dropping below 2.

Path loss is usually expressed in dB. In its simplest form the path loss can be calculated using the formula <math>P = 10\ n\ log(d) + C</math>, where <math>P</math> is the path loss in decibels, <math>n</math> is the path loss exponent, <math>d</math> is the distance between the transmitter and the receiver, usually measured in meters, and <math>C</math> is a constant which accounts for losses occurring due to penetration through the walls of the building, due to absorption in the human body, etc.

Path loss is a major component in the analysis and design of the link budget of a telecommunication system.

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 approximating methods.

For wireless communications in the VHF and UHF frequency band (e.g. the bands of walkie-talkies, police, taxis and cellular phones) one of the most commonly used method is that of Okumura-Hata as refined by the COST-231 project. Other well-known models are those of Walfisch-Ikegami, W.C.Y. Lee, Erceg, etc. For FM radio and TV broadcasting the path loss is most commonly predicted using the ITU model as descried in P.1546 (former P.370) recommendation. These methods are empirical (also called statistical) in nature because they have been developed after conducting a large number of path loss measurements in different environments and summarising their results in the form of approximation formulae and/or tables and curves.

Deterministic methods for predicting the path loss are also used, such as ray tracing. 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, external and internal walls, etc. etc.). For this reason they are used predominantly for short propagation paths.

The path loss in other frequency bands (e.g. MW, SW/HF is predicted with similar methods, though the concrete algorithms and formulae may be very different from those for VHF/UHF. Reliable prediction of the path loss in the SW/HF band is particularly difficult and in accuracy is comparable to weather predictions.

Synonym path attenuation.

Source: from Federal Standard 1037C and from MIL-STD-188fr:Path loss