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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 omnidirectional antenna with 10dBi gain, while the client is connected to a sectorial antenna with 14dBi gain. The transmitting power of the AP is 100mW (or 20dBm) and its sensitivity is -89dBm. The transmitting power of the client is 30mW (or 15dBm) and its sensitivity is -82dBm. 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 5km 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 -73dB is greater than the minimum receive sensitivity of the client radio (-82dBm), the signal level is just enough for the client radio to be able to hear the access point. There is only 9dB of margin (82dB -73dB) 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 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 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. We will look at one excellent online tool in more detail in the next section, Link planning software.

The Terabeam website also has excellent calculators available online.

Last Update: 2007-01-24