Example link budget calculation

As an example , we want to estimate the feasibility of a 5km link , with one access point and one client radio . The access point is connected to an omni directional antenna with 10 dBi gain , while the client is connected to a sectorial antenna with 14 dBi gain . The transmitting power of the Ap is 100m W ( or 20dBm ) and its sensitivity is -89 dBm . The trasmitting power of the client is 30mW ( or 15 dBm ) and its sensitiity is -82 dBm . The cable are short , with a loss of 2dB at each side .

Adding up all the gains and subtracting all the losses for the Ap to client link gives :

2- dBm ( TX Power Radio 1 )
+ 10 dBi ( Antenna Gain Radio 1 )
- 2 dB ( Cable Cables :psses Radio1  )
+ 14 dBi ( Antenna Gain Radio 2 )
= 40 dB = Total Gain

The path loss for a  5k m link , considering only the free space loss is : the same on the return trip . So our received signal elvel on the access point side is :

35dB - 113 dB = -78 dB

Since the receive sensitivity of the Ap is  89 dBm , this leaves us 11dB of fade margin ( 89dB - 78 dB ) . Overall , this link will probably work but couild use a bit more gain . By using a 24dBi dish on the client side rather than a 14 dBi sec torial antenna , you will get an additional 10 dBi of again on both directions of the link ( remember , antenna gain is reciprocal ) . A more expensive option would be to use higher power radios on both ends of the link , but note that adding an amplifier or higher powered card to one end generally des not help the overall quality of the link .

Online tools can be used to calculate the link budget . For example , the Green Bay Professional packet Radio's Wireless Network Link Analysis  is an excellent tool . The super Edition generates a PDF  file containing the Fresnel zone and radio path graphs . The calculation scripts can even be downloaded fromthe website and installed locally .

The Terabeam website also has excelllent calculators avaiable online .