Link budget


Link budget

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:

:Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)

Note that decibels are logarithmic measurements, so adding decibels is equivalent to multiplying the actual numeric ratios.

Link budget radio systems

For a line of sight radio system, a link budget equation might look like this:

P_{RX} = P_{TX} + G_{TX} - L_{TX} - L_{FS} - L_M + G_{RX} - L_{RX} , where:
P_{RX} = received power (dBm) P_{TX} = transmitter output power (dBm) G_{TX} = transmitter antenna gain (dBi) L_{TX} = transmitter losses (coax, connectors...) (dB) L_{FS} = free space loss or path loss (dB) L_M = miscellaneous losses (fading margin, body loss, polarization mismatch, other losses...) (dB) G_{RX} = receiver antenna gain (dBi) L_{RX} = 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)] [http://people.deas.harvard.edu/~jones/es151/prop_models/propagation.html]

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.

Link budgets for non-line of sight radio

Indoor deployments for example will refraction, reflection, multipath... etc.

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 samelength.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, [http://www.corningcablesystems.com/web/library/litindex.nsf/$ALL/EVO-412-EN/$FILE/EVO-412-EN.pdf] far lower than any other guided medium.

ee also

*Friis transmission equation
*Decibel
*Isotropic radiator
*Radiation pattern
*Multipath
*Free space loss

External links

* [http://home.deds.nl/~pa0hoo/helix_wifi/linkbudgetcalc/wlan_budgetcalc.html Link budget calculator for Wireless LAN]
* [http://www.sss-mag.com/pdf/an9804.pdf Link budget tutorial]


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