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 for 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 inverse square law is independent of frequency, so one would expect path losses to also be constant with frequency. However, free space path loss is defined between isotropic antennas that have apertures that vary with the square of the wavelength. The apparent 6 dB increase of path loss with each octave (doubling) of frequency merely reflects this decrease in receive antenna aperture with increasing frequency. When a receive antenna of constant physical area receives a transmission from an isotropic antenna, the receive antenna gain increases 6 dB with each octave so the overall loss becomes independent of frequency. When antennas of constant physical area are used on both ends, the increase in total antenna gain is 12 dB per octave, so the net transmitter-to-receiver loss actually decreases 6 dB with each octave. This comes from the transmitting antenna being able to focus more of its power on the receive antenna.
Reception is reliable when RxP > receiver sensitivity
[edit] Link budgets for non-line of sight radio
Indoor deployments for example will refraction, reflection, multipath... etc.
[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.
