Pathloss

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Pathloss is a term used in radio communications to denote the radio wave propagation losses taking place on a signal's path from the transmitter to the receiver. The value of the radio link's pathloss is one of the main input parameters for the design of radio communications equipment.

[edit] Measure

The pathloss is measured by the ratio of the signal's power at the transmitter's output to its power at the receiver's input. This ratio usually is expressed as logarithm and measured in decibels.

It 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, difraction 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. This effect is called multipath. Multipath can cause significant increase of the pathloss or it can contribute to some reduction of the pathloss. Whether the pathloss is increased or reduced by multipath depends on the phases of the different radio waves arriving along different paths.

Some easy to remember approximations for calculating the pathloss over distances significantly shorter than the distance to the radio horizon:

• In free space the pathloss 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 pathloss increases with roughly 35 -- 40 dB per decade (10 -- 12 dB per octave). This can be used in cellular networks as a first guess.

[edit] Predictions

Different methods can be used to predict the pathloss occurring at a given distance along a radio link. Very broadly these methods can be classified as:

• Deterministic methods, these are based on the physical laws of wave propagation. The most widely used such method for links in HF, VHF, UHF and SHF bands is ray tracing. Among the most commonly used methods in the design of radio equipment, such as antennas and feeders, is FDTD (Finite-Difference Time-Domain method, www.FDTD.org);
• Statistical methods (also called empirical), these are based on measured and averaged lossess along typical classes of radio links. Among 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;

[edit] Examples

In cellular networks, such as UMTS and GSM, which operate in the UHF band, the value of the pathloss in built-up areas can reach 110 -- 140 dB for the first kilometer of the link between the BTS and the mobile. The pathloss 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 pathloss values can eventually be, these are not definitive or binding figures -- the pathloss 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.)