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 takes into account the attenuation of the transmitted signal due to propagation, as well as the loss due to the antenna. Random attenuations such as fading are not taken into account in link budget calculations with the assumption that fading will be handled with diversity techniques.

A simple link budget equation looks like this:

Received Power (dB) = Transmitted Power (dBm) - Losses (dB)

Link budget for radio systems

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

 RxP = TxP + TxG - TxL - FSL - ML + RxG - RxL

 where: 	
   RxP = received power 
   TxP = transmitter output power (dBm) 
   TxG = transmitter antenna gain (dBi)
   TxL = transmitter losses (coax, connectors...) (dB) 
   FSL = free space loss or path loss (dB)
   ML  = miscellaneous losses (fading, body loss, polarization mismatch, other losses...) (dB)
   RxG = receiver antenna gain (dB)
   RxL = receiver losses (coax, connectors...)  (dB)

Line of sight deployments for example will have path losses that are related to the inverse square of the distance. The Free Space Loss equation can be written in several ways depending on the units of measure. Here are three examples:

   FSL (dB) = 32.45 dB + 20*log[frequency(MHz)] + 20*log[distance(km)] [1]

also

   FSL (dB) = 27.55 dB + 20*log[frequency(MHz)] + 20*log[distance(m)] 

and

   FSL (dB) = 36.6 dB + 20*log[frequency(MHz)] + 20*log[distance(miles)] 

Reception is reliable when RxP > receiver sensitivity

Link budgets for non-line of sight radio

Indoor deployments for example will have path losses that are related to the inverse cube of the distance. The link budget for an over the horizon radio path may include other path losses such as refraction, reflection, multipath... etc.

Link budgets for other media

Media such as cable, waveguide, fiber, etc. have losses that are linear over distance. The path loss will be in terms of dB per meter or dB per 100 feet. aa

See also

• Decibel
• Isotropic radiator
• Radiation pattern
• Multipath
• Free space loss

References

Frank Ohrtman Konrad Roeder Wi-Fi Handbook : Building 802.11b Wireless Networks McGraw-Hill, Inc. (2003) ISBN 0071412514 LOC TX-5-808-342 004.678 OHR