Link budget
Encyclopedia
A link budget is the accounting of all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in a telecommunication
system. It accounts for the attenuation of
the transmitted signal due to propagation, as well as the antenna gain
s, feedline and miscellaneous losses. Randomly varying channel gains such as fading
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
or frequency hopping.
A simple link budget equation looks like this:
Note that decibels are logarithmic measurements, so adding decibels is equivalent to multiplying the actual numeric ratios.
radio
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.
Simplifications needed
Often link budget equations can become messy and complex, so there have evolved some standard practices to simplify the link budget equation
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
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.
where:
= received power (dBm)
= transmitter output power (dBm)
= transmitter antenna gain
(dBi)
= transmitter losses (coax, connectors...) (dB)
= free space loss or path loss
(dB)
= miscellaneous losses (fading
margin, body loss, polarization mismatch, other losses...) (dB)
= receiver antenna gain
(dBi)
= 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:
(dB) = 20*log[4*π*distance/wavelength] (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
.
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.
(dB) = 32.45 dB + 20*log[frequency(MHz)] + 20*log[distance(km)] http://people.deas.harvard.edu/~jones/es151/prop_models/propagation.html
(dB) = - 27.55 dB + 20*log[frequency(MHz)] + 20*log[distance(m)]
(dB) = 36.6 dB + 20*log[frequency(MHz)] + 20*log[distance(miles)]
These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (c, approximately 3 x 10^8 m/s) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and (1/sec).
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 30dB 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 path loss
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
became practical only with the development of ultra-transparent glass fibers. A typical path loss for
single mode fiber is 0.2 dB/km,
http://www.corningcablesystems.com/web/library/litindex.nsf/$ALL/EVO-412-EN/$FILE/EVO-412-EN.pdf
far lower than any other guided medium.
of the Moon is very low (maximally 12% but usually closer to 7%), and the path loss
over the 770,000 kilometre
return distance is extreme (around 250 to 310 dB
depending on VHF-UHF band used, modulation
format and Doppler shift effects), high power (more than 100 watts) and high-gain antenna
s (more than 20 dB) must be used.
spacecraft have the highest known path loss and lowest link budgets of any telecommunications circuit. Although the Deep Space Network
has been able to maintain the necessary technological advances to maintain the link, the received field strength is still many billions of times weaker than a battery powered wristwatch.
Practical examples
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
system. It accounts for the attenuation of
the transmitted signal due to propagation, as well as the antenna gain
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
s, feedline and miscellaneous losses. Randomly varying channel gains such as fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
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
Antenna diversity, also known as space 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 indoor environments, there is no clear line-of-sight between transmitter and...
or frequency hopping.
A simple link budget equation looks like this:
- Received Power (dBmDBmdBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
) = Transmitted Power (dBm) + Gains (dBDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
) − Losses (dB)
Note that decibels are logarithmic measurements, so adding decibels is equivalent to multiplying the actual numeric ratios.
Link budget: radio systems
For a line-of-sightLine-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
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.
- Transmitting antennas are for the most part not isotropic aka omnidirectional.
- Completely omnidirectional antennas are 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 can become messy and complex, so there have evolved some standard practices to simplify the link budget equation
- The wavelength term is often considered part of the free space loss equation. This complexity reduction is acceptable for terrestrial communication systems, where only line of sight is considered.
- Considering all carrier wave propagation to be wavelength-independent. This is justified by the conservation of energy law that requires that the electric field decrease in power as the square of the distance regardless of frequency (in free space propagation conditions).
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.
- Doppler shift induced signal power losses in the receiver.
Endgame
If the estimated received power is sufficiently large (typically relative to the receiver sensitivity
Sensitivity (electronics)
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
), 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.
The Link Budget Equation
A link budget equation including all these effects, expressed logarithmically, might look like this:where:
= received power (dBm) = transmitter output power (dBm) = transmitter antenna gainAntenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
(dBi)
= transmitter losses (coax, connectors...) (dB) = free space loss or path lossPath loss
Path loss is the reduction in power density 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....
(dB)
= miscellaneous losses (fadingFading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
margin, body loss, polarization mismatch, other losses...) (dB)
= receiver antenna gainAntenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
(dBi)
= 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:
(dB) = 20*log[4*π*distance/wavelength] (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
Friis transmission equation
The Friis transmission equation is used in telecommunications engineering, and gives the power received by one antenna under idealized conditions given another antenna some distance away transmitting a known amount of power. The formula was derived in 1945 by Danish-American radio engineer Harald T...
.
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.
(dB) = 32.45 dB + 20*log[frequency(MHz)] + 20*log[distance(km)] http://people.deas.harvard.edu/~jones/es151/prop_models/propagation.html (dB) = - 27.55 dB + 20*log[frequency(MHz)] + 20*log[distance(m)] (dB) = 36.6 dB + 20*log[frequency(MHz)] + 20*log[distance(miles)]These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (c, approximately 3 x 10^8 m/s) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and (1/sec).
Link budgets for 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 x 4” wood stud wall with drywall on both sides results in about 6dB loss per wall.
- 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 30dB 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.
Link budgets 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
Path loss is the reduction in power density 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
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information...
became practical only with the development of ultra-transparent glass fibers. A typical path loss for
single mode fiber is 0.2 dB/km,
http://www.corningcablesystems.com/web/library/litindex.nsf/$ALL/EVO-412-EN/$FILE/EVO-412-EN.pdf
far lower than any other guided medium.
Earth Moon Earth Communications
As the albedoAlbedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of the Moon is very low (maximally 12% but usually closer to 7%), and the path loss
Path loss
Path loss is the reduction in power density 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
1 E8 m
To help compare different orders of magnitude, this page lists lengths starting at 108 metres .Distances shorter than 108 metres* 102 Mm — Diameter of HD 149026 b, an unusually dense Jovian planet...
return distance is extreme (around 250 to 310 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
depending on VHF-UHF band used, modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
format and Doppler shift effects), high power (more than 100 watts) and high-gain antenna
High-gain antenna
A high-gain antenna is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows more precise targeting of the radio signal - also known as a directional antenna...
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 visible in order for EME communications to be possible.
Voyager Program
The Voyager ProgramVoyager program
The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
spacecraft have the highest known path loss and lowest link budgets of any telecommunications circuit. Although the Deep Space Network
Deep Space Network
The Deep Space Network, or DSN, is a world-wide network of large antennas and communication facilities that supports interplanetary spacecraft missions. It also performs radio and radar astronomy observations for the exploration of the solar system and the universe, and supports selected...
has been able to maintain the necessary technological advances to maintain the link, the received field strength is still many billions of times weaker than a battery powered wristwatch.
See also
- Friis transmission equationFriis transmission equationThe Friis transmission equation is used in telecommunications engineering, and gives the power received by one antenna under idealized conditions given another antenna some distance away transmitting a known amount of power. The formula was derived in 1945 by Danish-American radio engineer Harald T...
- DecibelDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
- Isotropic radiatorIsotropic radiatorAn 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 sphere centred on the source...
- Radiation patternRadiation patternIn the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
- Multipath propagation
- Free space loss
- RF PlanningRF PlanningIn the context of mobile and cellular communication systems, RF Planning is the process of assigning frequencies, transmitter locations and parameters of a wireless communications system to provide sufficient coverage and capacity for the services required...
Practical examples
- Earth Moon EarthEME (communications)Earth-Moon-Earth, also known as moon bounce, is a radio communications technique which relies on the propagation of radio waves from an Earth-based transmitter directed via reflection from the surface of the Moon back to an Earth-based receiver....
, telecommunications links using the Moon have a generalized 309db path loss - Voyager ProgramVoyager programThe Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s...
, the Voyager craft have the highest known path loss and lowest link budgets of any telecommunications circuit