Link Budget
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A link budget is an accounting of all of the
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
gains and losses that a communication
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
experiences in a
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
system; from a transmitter, through a communication medium such as
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
s,
cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...
,
waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
, or
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
, to the receiver. It is an equation giving the received power from the transmitter power, after the attenuation of the transmitted signal due to propagation, as well as the
antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
s and
feedline In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from the ...
and other losses, and amplification of the signal in the receiver or any
repeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...
s it passes through. A link budget is a design aid, calculated during the design of a communication system to determine the received power, to ensure that the information is received intelligibly with an adequate
signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
. Randomly varying channel gains such as
fading In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...
are taken into account by adding some margin depending on the anticipated severity of its effects. The amount of margin required can be reduced by the use of mitigating techniques such as
antenna diversity Antenna diversity, also known as space diversity or spatial diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and ind ...
or
frequency hopping Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both tra ...
. A simple link budget equation looks like this: :Received power (dBm) = transmitted power (dBm) + gains (dB) − losses (dB) Power levels are expressed in (
dBm DBM or dbm may refer to: Science and technology * dBm, a unit for power measurement * DBM (computing), family of key-value database engines including dbm, ndbm, gdbm, and Berkeley DB * Database Manager (DBM), a component of 1987's ''Extended Edi ...
), Power gains and losses are expressed in
decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
s (dB), which is a
logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number  to the base  is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...
ic measurement, so adding decibels is equivalent to multiplying the actual power ratios.


In radio systems

For a line-of-sight
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
system, the primary source of loss is the decrease of the signal power due to uniform propagation, proportional to the inverse square of the distance (geometric spreading). * Transmitting antennas are for the most part neither isotropic (an imaginary class of antenna with uniform radiation in 3 dimensions) nor omnidirectional (a real class of antenna with uniform radiation in 2 dimensions). * The use of omnidirectional antennas is rare in telecommunication systems, so almost every link budget equation must consider antenna gain. * Transmitting antennas typically concentrate the signal power in a favoured direction, normally that in which the receiving antenna is placed. * Transmitter power is effectively increased (in the direction of highest antenna gain). This systemic gain is expressed by including the antenna gain in the link budget. * The receiving antenna is also typically directional, and when properly oriented collects more power than an isotropic antenna would; as a consequence, the receiving antenna gain (in decibels from isotropic, dBi) adds to the received power. * The antenna gains (transmitting or receiving) are scaled by the wavelength of the radiation in question. This step may not be required if adequate systemic link budgets are achieved.


Simplifications needed

Often link budget equations are messy and complex, so standard practices have evolved to simplify the
Friis transmission equation The Friis transmission formula is used in telecommunications engineering, equating the power at the terminals of a receive antenna as the product of power density of the incident wave and the effective aperture of the receiving antenna under i ...
into the link budget equation. It includes the transmit and receive antenna gain, the free space path loss and additional losses and gains, assuming line of sight between the transmitter and receiver. * The wavelength (or frequency) term is part of the free space loss part of the link budget. * The distance term is also considered in the free space loss.


Transmission line and polarization loss

In practical situations (deep space telecommunications, weak signal DXing etc.) other sources of signal loss must also be accounted for * The transmitting and receiving antennas may be partially cross-polarized. * The cabling between the radios and antennas may introduce significant additional loss. *
Fresnel zone A Fresnel zone ( ), named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver. The primary wave will travel in a relative straight line fro ...
losses due to a partially obstructed line of sight path. *
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
induced signal power losses in the receiver.


Endgame

If the estimated received power is sufficiently large (typically relative to the receiver sensitivity), which may be dependent on the communications protocol in use, the link will be useful for sending data. The amount by which the received power exceeds receiver sensitivity is called the
link margin In a wireless communication system, the link margin (LKM), measured in dB, is the difference between the minimum expected power received at the receiver's end, and the receiver's sensitivity (i.e., the received power at which the receiver will st ...
.


Equation

A link budget equation including all these effects, expressed logarithmically, might look like this: : P_\text = P_\text + G_\text - L_\text - L_\text - L_M + G_\text - L_\text \, where: :P_\text, received power (dBm) :P_\text, transmitter output power (dBm) :G_\text, transmitter
antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
(dBi) :L_\text, transmitter losses (coax, connectors...) (dB) :L_\text,
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. ...
, usually
free space loss In telecommunication, the free-space path loss (FSPL) (also known as Free Space Loss, FSL) is the attenuation of radio energy between the feedpoints of two antennas that results from the combination of the receiving antenna's capture area plus the ...
(dB) :L_\text, miscellaneous losses (
fading In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...
margin, body loss, polarization mismatch, other losses, ...) (dB) :G_\text, receiver
antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
(dBi) :L_\text, receiver losses (coax, connectors, ...) (dB) The loss due to propagation between the transmitting and receiving antennas, often called the path loss, can be written in dimensionless form by normalizing the distance to the wavelength: :L_\text\text = 20\log_\left(4\pi\right) (where distance and wavelength are in the same units) When substituted into the link budget equation above, the result is the logarithmic form of the
Friis transmission equation The Friis transmission formula is used in telecommunications engineering, equating the power at the terminals of a receive antenna as the product of power density of the incident wave and the effective aperture of the receiving antenna under i ...
. In some cases, it is convenient to consider the loss due to distance and wavelength separately, but in that case, it is important to keep track of which units are being used, as each choice involves a differing constant offset. Some examples are provided below. :L_\text (dB) ≈ 32.45 dB + 20 log10 requency (MHz)+ 20 log10 istance (km):L_\text (dB) ≈ −27.55 dB + 20 log10 requency (MHz)+ 20 log10 istance (m):L_\text (dB) ≈ 36.6 dB + 20 log10 requency (MHz)+ 20 log10 istance (miles) These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (''c'', approximately ) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and (1/s).


Non-line-of-sight radio

Because of building obstructions such as walls and ceilings, propagation losses indoors can be significantly higher. This occurs because of a combination of attenuation by walls and ceilings, and blockage due to equipment, furniture, and even people. * For example, a " 2 by 4" wood stud wall with
drywall Drywall (also called plasterboard, dry lining, wallboard, sheet rock, gypsum board, buster board, custard board, and gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thick ...
on both sides results in about 6 dB loss per wall at 2.4 GHz. * Older buildings may have even greater internal losses than new buildings due to materials and line of sight issues. Experience has shown that line-of-sight propagation holds only for about the first 3 meters. Beyond 3 meters propagation losses indoors can increase at up to 30 dB per 30 meters in dense office environments. This is a good rule-of-thumb, in that it is conservative (it overstates path loss in most cases). Actual propagation losses may vary significantly depending on building construction and layout. The attenuation of the signal is highly dependent on the frequency of the signal.


In waveguides and cables

Guided media such as coaxial and twisted pair electrical cable, radio frequency waveguide and optical fiber have losses that are exponential with distance. The
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. ...
will be in terms of dB per unit distance. This means that there is always a crossover distance beyond which the loss in a guided medium will exceed that of a line-of-sight path of the same length. Long distance
fiber-optic communication Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...
became practical only with the development of ultra-transparent glass fibers. A typical path loss for
single mode fiber In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode (electromagnetism), mode of light - the transverse mode. Modes are the possible s ...
is 0.2 dB/km, far lower than any other guided medium.


Earth–Moon–Earth communications

Link budgets are important in Earth–Moon–Earth communications. As the
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
of the Moon is very low (maximally 12% but usually closer to 7%), and the
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. ...
over the 770,000 kilometre return distance is extreme (around 250 to 310 dB depending on VHF-UHF band used,
modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
format and
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
effects), high power (more than 100 watts) and
high-gain antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance ...
s (more than 20 dB) must be used. * In practice, this limits the use of this technique to the spectrum at VHF and above. * The Moon must be above the horizon in order for EME communications to be possible.


Voyager program

The
Voyager program The Voyager program is an American scientific program that employs two robotic interstellar probes, ''Voyager 1'' and ''Voyager 2''. They were launched in 1977 to take advantage of a favorable alignment of Jupiter and Saturn, to Flyby (spacef ...
spacecraft have the highest known path loss (308dB as of 2002 ) and lowest link budgets of any telecommunications circuit. The
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide Telecommunications network, network of American spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that suppo ...
has been able to maintain the link at a higher than expected bitrate through a series of improvements, such as increasing the antenna size from 64m to 70m for a 1.2dB gain, and upgrading to low noise electronics for a 0.5dB gain in 2000–2001. During the
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
flyby, in addition to the 70-m antenna, two 34-m antennas and twenty-seven 25-m antennas were used to increase the gain by 5.6dB, providing additional link margin to be used for a 4× increase in bitrate.


See also

*
Friis transmission equation The Friis transmission formula is used in telecommunications engineering, equating the power at the terminals of a receive antenna as the product of power density of the incident wave and the effective aperture of the receiving antenna under i ...
*
Antenna gain-to-noise-temperature Antenna gain-to-noise-temperature (''G/T'') is a figure of merit in the characterization of antenna performance, where ''G'' is the antenna gain in decibels at the receive frequency, and ''T'' is the equivalent noise temperature of the receiving ...
*
Isotropic radiator An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a ...
*
Radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “A ...
*
Multipath propagation In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflec ...
*
RF planning In the context of mobile radio communication systems, RF planning is the process of assigning frequencies, transmitter locations and parameters to a wireless communications system to evaluate coverage and capacity. Coverage is the distance at wh ...


References

{{Reflist


External links


Link budget calculator for wireless LANLink budget tutorialPoint-to-point link budget calculatorMUOS Link budget calculator/plannerExample LTE, GSM and UMTS Link BudgetsPython link budget calculator for satellitesSmall satellites link budget (with python examples)
Budgets Telecommunications engineering Radio frequency propagation