Link budgetFrom Wikipedia, the free encyclopediaAlink budgetis 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 atelecommunicationsystem. It accounts for the attenuation of the transmitted signal due to propagation, as well as theantenna gains,feedlineand miscellaneous losses. Randomly varying channel gains such asfadingare 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 asantenna diversityorfrequency hopping.A simple link budget equation looks like this:Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) Losses (dB)Note thatdecibelsare logarithmic measurements, so adding decibels is equivalent to multiplying the actual numeric ratios.Contents[hide] 1In radio systems 1.1Simplifications needed 1.2Transmission line and polarization loss 1.3Endgame 1.4Equation 2Non-line-of-sight radio 3In waveguides and cables 4Examples 4.1EarthMoonEarth communications 4.2Voyager Program 5See also 6External links

In radio systemsFor aline-of-sightradiosystem, 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 neededOften 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 lossIn 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.

EndgameIf the estimated received power is sufficiently large (typically relative to thereceiver 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.

EquationA link budget equation including all these effects, expressed logarithmically, might look like this:

where:= received power (dBm)= transmitter output power (dBm)= transmitterantenna gain(dBi)= transmitter losses (coax, connectors...) (dB)=free space lossorpath loss(dB)= miscellaneous losses (fadingmargin, body loss, polarization mismatch, other losses...) (dB)= receiverantenna 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) = 20log[4distance/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 theFriis 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 + 20log[frequency(MHz)] + 20log[distance(km)][1](dB) = - 27.55 dB + 20log[frequency(MHz)] + 20log[distance(m)](dB) = 36.6 dB + 20log[frequency(MHz)] + 20log[distance(miles)]These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (c, approximately 310^8m/s) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and (1/sec).

Non-line-of-sight radioBecause 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.

In waveguides and cablesGuided media such as coaxial and twisted pair electrical cable, radio frequency waveguide and optical fiber have losses that are exponential with distance.Thepath losswill 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 distancefiber-optic communicationbecame practical only with the development of ultra-transparent glass fibers. A typical path loss forsingle mode fiberis 0.2 dB/km,[2]far lower than any other guided medium.

ExamplesEarthMoonEarth CommunicationsLink budgets are important inEarthMoonEarth communications. As thealbedoof the Moon is very low (maximally 12% but usually closer to 7%), and thepath lossover the770,000 kilometrereturn distance is extreme (around 250 to 310dBdepending on VHF-UHF band used,modulationformat andDoppler shifteffects), high power (more than 100 watts) andhigh-gain antennas(more than 20 dB) must be used. In practice, this limits the use of this technique to the spectrum atVHFand above. The Moon must be visible in order for EME communications to be possible.

Voyager ProgramTheVoyager Programspacecraft have the highest known path loss and lowest link budgets of any telecommunications circuit. Although theDeep Space Networkhas 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.