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, cable, waveguide, or optical fiber, 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. 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 MIMO (Multiple Input Multiple Output).
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
For a line-of-sight radio system, the primary source of loss is the decrease of the signal power as it spreads over an increasing area while it propagates, proportional to the square of the distance (geometric spreading). Transmitting antennas can be Omnidirectional, Directional, or Sectorial, depending on the way in which the antenna power is oriented. An omnidirectional antenna will distribute the power equally in every direction of a plane, so the radiation pattern has the shape of a sphere squeezed between two parallel flat surfaces. They are widely used in many applications, for instance in WiFi Access Points. Directional antennas concentrate the power in a specific direction, called the bore sight, and are widely used in point to point applications, like wireless bridges and satellite communications. Sectorial antennas concentrate the power in a wider region, typically embracing 45º, 60º, 90º or 120º. They are routinely deployed in Cellular towers.
Simplifications needed
The free space loss is easily calculated using Friis transmission equation which states that the loss is proportional to the square of the distance and the square of the frequency. Additionally losses are incurred in most radio links, including atmospheric attenuation by gases, rain, fog and clouds. Fading due to variations of the channel, multipath losses and antenna misalignment. In non line of sight links, diffraction and reflection losses are the most important since the direct path is not available.
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 losses due to a partially obstructed line of sight path. Doppler shift 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.
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 gain (dBi) , transmitter losses (coax, connectors...) (dB) , path loss, usually free space 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:
(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.