link budget calculation

Example link budget calculation
As an example, we want to estimate the feasibility of a 5 km link, with one access point and one client radio. The access point is connected to an omnidirectional 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 100mW (or 20 dBm) and its sensitivity is -89 dBm. The transmitting power of the client is 30mW (or 15 dBm) and its sensitivity is -82 dBm. The cables 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:

20 dBm (TX Power Radio 1)
+ 10 dBi (Antenna Gain Radio 1)
- 2 dB (Cable Losses Radio 1)
+ 14 dBi (Antenna Gain Radio 2)
- 2 dB (Cable Losses Radio 2)
40 dB = Total Gain

The path loss for a 5 km link, considering only the free space loss is:

Path Loss = 40 + 20log(5000) = 113 dB

Subtracting the path loss from the total gain

40 dB - 113 dB = -73 dB
Since -73 dB is greater than the minimum receive sensitivity of the client radio (-82 dBm), the signal level is just enough for the client radio to be able to hear the access point. There is only 9 dB of margin (82 dB - 73 dB) which will likely work fine in fair weather, but may not be enough to protect against extreme weather conditions.

Next we calculate the link from the client back to the access point:

15 dBm (TX Power Radio 2)
+ 14 dBi (Antenna Gain Radio 2)
- 2 dB (Cable Losses Radio 2)
+ 10 dBi (Antenna Gain Radio 1)
- 2 dB (Cable Losses Radio 1)
35 dB = Total Gain

Obviously, the path loss is the same on the return trip. So our received signal level on the access point side is:

35 dB - 113 dB = -78 dB

Since the receive sensitivity of the AP is -89dBm, this leaves us 11dB of fade margin (89dB - 78dB). Overall, this link will probably work but could use a bit more gain. By using a 24dBi dish on the client side rather than a 14dBi sectorial antenna, you will get an additional 10dBi of gain 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 does 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(http://my.athenet.net/~multiplx/cgi-in/wireless.main.cgi) 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 from the website and installed locally.
The Terabeam website also has excellent calculators available online (http://www.terabeam.com/support/calculations/index.php).