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.

Causes

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.

Prediction

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 [http://www.lx.it.pt/cost231/ 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.

Examples

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: http://www.narte.org/n/212/Summer03Page10_13.pdf

References

*Federal Standard 1037C
*MIL-STD-188


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