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 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 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.
Overview In wireless communications, path loss is the reduction in signal strength as the signal travels from a transmitter to a receiver, and is an application for verifying the loss. There are several factors that affect this:
Free-space path loss: This is the fundamental loss that occurs due to the spreading of the radio wave as it propagates through space. It follows an inverse square law, meaning the signal strength decreases proportionally to the square of the distance between the transmitter and receiver. Diffraction: When a radio wave encounters an obstacle, it can be diffracted, or bent around the edge of the obstacle. This can cause additional signal loss, especially in urban environments with many buildings. Absorption: Certain atmospheric gases and obstacles like buildings and foliage can absorb radio waves, reducing their strength. Reflection and scattering: Radio waves can be reflected off surfaces like buildings and the ground, and scattered by objects like trees and lampposts. This can lead to multipath propagation, where the receiver receives multiple copies of the signal that may interfere with each other. In understanding path loss and minimizing it, there are four key factors to consider in designing a wireless communication system: 1) Determining the required transmitter power: The transmitter must have enough power to overcome the path loss in order for the signal to reach the receiver with sufficient strength. 2) Determine the appropriate antenna design and gain: Antennas with higher gain can focus the waves in a specific direction, reducing the path loss. 3) Optimize modulation scheme: The choice of modulation scheme can affect the robustness of the signal to path loss. 4) Set the receiver sensitivity appropriately: The receiver must be sensitive enough to detect weak signals.
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 waves combine at the receiver antenna, resulting in a received signal that may vary widely, depending on the distribution of the intensity and relative propagation time of the waves and bandwidth of the transmitted signal. The total power of interfering waves in a Rayleigh fading scenario varies quickly as a function of space (which is known as small scale fading). Small-scale fading refers to the rapid changes in radio signal amplitude in a short period of time or distance of travel.
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 {\displaystyle L=10n\log _{10}(d)+C}
where L {\displaystyle L} is the path loss in decibels, n {\displaystyle n} is the path loss exponent, d {\displaystyle d} is the distance between the transmitter and the receiver, usually measured in meters, and C {\displaystyle C} is a constant which accounts for system losses.
Radio engineer formula Radio and antenna engineers use the following simplified formula (derived from the Friis Transmission Formula) for the signal path loss between the feed points of two isotropic antennas in free space: Path loss in dB:
L = 20 log 10 ( 4 π d λ ) {\displaystyle L=20\log _{10}\left({\frac {4\pi d}{\lambda }}\right)}
where L {\displaystyle L} is the path loss in decibels, λ {\displaystyle \lambda } is the wavelength and d {\displaystyle d} is the transmitter-receiver distance in the same units as the wavelength. Note the power density in space has no dependency on λ {\displaystyle \lambda } ; The variable λ {\displaystyle \lambda } exists in the formula to account for the effective capture area of the isotropic receiving antenna.
Prediction
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