Path loss

Path loss

Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system.

This term is commonly used in wireless communications and 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 also influenced by terrain contours, environment (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas.


Path loss normally includes "propagation losses" caused by the natural expansion of the radio wave front in free space (which usually takes the shape of an ever-increasing sphere), "absorption losses" (sometimes called penetration losses), when the signal passes through media not transparent to electromagnetic waves, "diffraction losses" when part of the radiowave front is obstructed by an opaque obstacle, and losses caused by other phenomena.

The signal radiated by a transmitter may also travel along many and different paths to a receiver simultaneously; this effect is called multipath. Multipath can either increase or decrease received signal strength, depending on whether the individual multipath wavefronts interfere constructively or destructively. The total power of interfering waves in a Rayleigh fading scenario vary quickly as a function of space (which is known as "small scale fading"), resulting in "fast fades" which are very sensitive to receiver position.

Loss exponent

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, a tunnel may act as a waveguide, resulting in a path loss exponent less than 2.

Path loss is usually expressed in dB. In its simplest form, the path loss can be calculated using the formula

:L = 10 n log_{10}(d) + C

where L is the path loss in decibels, n is the path loss exponent, d is the distance between the transmitter and the receiver, usually measured in meters, and C is a constant which accounts for system losses.


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 [ COST-231] , Okumura-Hata, 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 PLMN. For wireless communications in the VHF and UHF frequency band (the bands used 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 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 (former P.370) recommendation.

Deterministic methods based on the physical laws of wave propagation are also used; 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 (MW, SW, Microwave) 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.Fact|date=August 2007

Some easy to remember 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 dB per "decade" (one decade is when the distance between the transmitter and the receiver increases ten times) or 6 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 SHF (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 dB per decade (10 -- 12 dB per octave). This can be used in cellular networks as a first guess.


In cellular networks, such as UMTS and GSM, which operate in the UHF band, the value of the path loss in built-up areas can reach 110 -- 140 dB for the first kilometer of the link between the BTS and the mobile. The path loss for the first ten kilometers may be 150 -- 190 dB ("Note": These values are very approximate and are given here only as an illustration of the range in which the numbers used to express the path loss values "can eventually be", these are not definitive or binding figures -- the path loss may be very different for the same distance along two different paths and it can be different even along the same path if measured at different times.)

ee also

* Radio propagation model
* Professional Path Analysis Using a Spreadsheet:


*Federal Standard 1037C

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Path loss — Affaiblissement de propagation L’affaiblissement de propagation, aussi connu comme affaiblissement de parcours ou par son nom anglais de path loss, caractérise l affaiblissement que subit une onde électromagnétique lorsqu elle parcourt une… …   Wikipédia en Français

  • path loss — trasos nuostoliai statusas T sritis radioelektronika atitikmenys: angl. path loss vok. Streckenverluste, m rus. потери на трассе, f pranc. pertes sur le tracé, f …   Radioelektronikos terminų žodynas

  • Path loss — Die Artikel Pfadverlust und Dämpfung überschneiden sich thematisch. Hilf mit, die Artikel besser voneinander abzugrenzen oder zu vereinigen. Beteilige dich dazu an der Diskussion über diese Überschneidungen. Bitte entferne diesen Baustein erst… …   Deutsch Wikipedia

  • Free-space path loss — In telecommunication, free space path loss (FSPL) is the loss in signal strength of an electromagnetic wave that would result from a line of sight path through free space, with no obstacles nearby to cause reflection or diffraction. It does not… …   Wikipedia

  • Log-distance path loss model — The log distance path loss model is a radio propagation model that predicts the path loss a encounters inside a building or densely populated areas over distance.Applicable to / Under conditionsThe model is applicable to indoor propagation… …   Wikipedia

  • Loss — may refer to:*A negative difference between retail price and cost of production *An event in which the team or individual in question did not win. *Loss (baseball), a pitching statistic in baseball *Attenuation, a reduction in amplitude and… …   Wikipedia

  • Path — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Path ou PATH peut faire référence à : En informatique Le chemin d accès d un fichier ou d un répertoire, en informatique. PATH : La variable d… …   Wikipédia en Français

  • Path dependence — explains how the set of decisions one faces for any given circumstance is limited by the decisions one has made in the past, even though past circumstances may no longer be relevant. [Definition from [ Our Love Of …   Wikipedia

  • Loss and Gain — Infobox Book | name = Loss and Gain image caption = Title Page, 8th Ed., 1881 author = John Henry Newman country = England language = English genre = Philosophical novel, Campus novel publisher = Burns Oates pub date = 1848 Loss and Gain is a… …   Wikipedia

  • Propagation path obstruction — In telecommunication, a propagation path obstruction is a man made or natural physical feature that lies near enough to a radio path to cause a measurable effect on path loss, exclusive of reflection effects. An obstruction may lie to the side,… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.