Link budget
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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 gains, 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.
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[edit] Link budget radio systems
For a line-of-sight radio system, a link budget equation might look like this:
where: PRX = received power (dBm) PTX = transmitter output power (dBm) GTX = transmitter antenna gain (dBi) LTX = transmitter losses (coax, connectors...) (dB) LFS = free space loss or path loss (dB) LM = miscellaneous losses (fading margin, body loss, polarization mismatch, other losses...) (dB) GRX = receiver antenna gain (dBi) LRX = receiver losses (coax, connectors...) (dB)
Communication links in free space have path losses that are the inverse square of the distance. The free space loss equation can be written in several equivalent ways depending on the units of measure. Here are some variations:
FSL (dB) = 20*log[4*π*distance/wavelength] (where distance and wavelength are in the same units)
FSL (dB) = 32.45 dB + 20*log[frequency(MHz)] + 20*log[distance(km)] [1]
FSL (dB) = -27.55 dB + 20*log[frequency(MHz)] + 20*log[distance(m)]
FSL (dB) = 36.6 dB + 20*log[frequency(MHz)] + 20*log[distance(miles)]
The first form is a variant of the Friis transmission equation, with the antenna gain factors removed (isotropic antennas assumed) and the loss factor converted to decibels by the 20* factor and the decimal logarithm operation. The other 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).
Reception is reliable when RxP > receiver sensitivity. The amount by which RxP exceeds receiver sensitivity is called link margin.
[edit] 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 sheetrock on both sides results in about 6dB loss per wall. Experience has shown that line-of-sight propagation holds only for about the first 20 feet. Beyond 20 feet, propagation losses indoors increase at up to 30dB per 100 feet 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.
[edit] Link budgets for other media
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 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, [2] far lower than any other guided medium.
