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Concept
Link budget
Summary
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 frequency hopping.
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.
For a line-of-sight 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 (geometric spreading).
Transmitting antennas are for the most part neither isotropic (an imaginary class of antenna with uniform radiation in 3 dimensions) nor omnidirectional (a real class of antenna with uniform radiation in 2 dimensions).
The use of omnidirectional antennas is 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.
Official source
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