A link budget is an accounting of all of the power gains and losses that a communication signal experiences in a telecommunication system; from a transmitter, through a communication medium such as radio waves, cables, waveguides, or optical fibers, 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 gains and feedline and other losses, and amplification of the signal in the receiver or any repeaters 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. In most real-world systems the losses must be estimated to some degree, and may vary. A link margin is therefore specified as a safety margin between the received power and minimum power required by the receiver to accurately detect the signal. The link margin is chosen based on the anticipated severity of a communications drop out and can be reduced by the use of mitigating techniques such as antenna diversity or multiple-input and multiple-output (MIMO). A simple link budget equation looks like this:
Received power (dBm) = transmitted power (dBm) + gains (dB) − losses (dB) Power levels are expressed in (dBm), Power gains and losses are expressed in decibels (dB), which is a logarithmic measurement, so adding decibels is equivalent to multiplying the actual power ratios.
In radio systems A link budget equation including the key effects for a wireless radio transmission system, expressed logarithmically, might look like:
P RX = P TX + G TX − L TX − L FS − L M + G RX − L RX {\displaystyle P_{\text{RX}}=P_{\text{TX}}+G_{\text{TX}}-L_{\text{TX}}-L_{\text{FS}}-L_{M}+G_{\text{RX}}-L_{\text{RX}}\,}
where:
P RX {\displaystyle P_{\text{RX}}} , received power (dBm)
P TX {\displaystyle P_{\text{TX}}} , transmitter output power (dBm)
G TX {\displaystyle G_{\text{TX}}} , transmitter antenna gain (dBi)
L TX {\displaystyle L_{\text{TX}}} , transmitter losses (coax, connectors...) (dB)
L FS {\displaystyle L_{\text{FS}}} , path loss (dB)
L M {\displaystyle L_{\text{M}}} , miscellaneous losses (fading margin, body loss, polarization mismatch, other losses, ...) (dB)
G RX {\displaystyle G_{\text{RX}}} , receiver antenna gain (dBi)
L RX {\displaystyle L_{\text{RX}}} , receiver losses (coax, connectors, ...) (dB) The path loss is the loss due to propagation between the transmitting and receiving antennas and is usually the most significant contributor to the losses, and also the largest unknown. When transmitting through free space, it can be expressed in a dimensionless form by normalizing the distance to the wavelength:
L FS (dB) = 20 log 10 ( 4 π distance wavelength ) {\displaystyle L_{\text{FS}}{\text{(dB)}}=20\log _{10}\left(4\pi {{\text{distance}} \over {\text{wavelength}}}\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. 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 FS {\displaystyle L_{\text{FS}}} (dB) ≈ 32.45 dB + 20 log10[frequency (MHz)] + 20 log10[distance (km)]
L FS {\displaystyle L_{\text{FS}}} (dB) ≈ −27.55 dB + 20 log10[frequency (MHz)] + 20 log10[distance (m)]
L FS {\displaystyle L_{\text{FS}}} (dB) ≈ 36.6 dB + 20 log10[frequency (MHz)] + 20 log10[distance (miles)] These alternative forms can be derived by substituting wavelength with the ratio of propagation velocity (c, approximately 3×108 m/s) divided by frequency, and by inserting the proper conversion factors between km or miles and meters, and between MHz and Hz. The gain of both the transmitting and receiving antennas is affected by the antenna's directivity. For example, antennas can be isotropic, omnidirectional, directional, or sectorial, depending on the way in which the antenna power is oriented.
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